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The Elements of Geology

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Title: The Elements of Geology

Author: William Harmon Norton


        
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Language: English

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This etext was produced by Robert Rowe, Charles Franks
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THE ELEMENTS OF GEOLOGY

BY WILLIAM HARMON NORTON

PROFESSOR OF GEOLOGY IN CORNELL COLLEGE

PREFACE

Geology is a science of such rapid growth that no apology is
expected when from time to time a new text-book is added to those
already in the field. The present work, however, is the outcome of
the need of a text-book of very simple outline, in which causes
and their consequences should be knit together as closely as
possible,--a need long felt by the author in his teaching, and
perhaps by other teachers also. The author has ventured,
therefore, to depart from the common usage which subdivides
geology into a number of departments,--dynamical, structural,
physiographic, and historical,--and to treat in immediate
connection with each geological process the land forms and the
rock structures which it has produced.

It is hoped that the facts of geology and the inferences drawn
from them have been so presented as to afford an efficient
discipline in inductive reasoning. Typical examples have been used
to introduce many topics, and it has been the author's aim to give
due proportion to both the wide generalizations of our science and
to the concrete facts on which they rest.

There have been added a number of practical exercises such as the
author has used for several years in the class room. These are not
made so numerous as to displace the problems which no doubt many
teachers prefer to have their pupils solve impromptu during the
recitation, but may, it is hoped, suggest their use.

In historical geology a broad view is given of the development of
the North American continent and the evolution of life upon the
planet. Only the leading types of plants and animals are
mentioned, and special attention is given to those which mark the
lines of descent of forms now living.

By omitting much technical detail of a mineralogical and
paleontological nature, and by confining the field of view almost
wholly to our own continent, space has been obtained to give to
what are deemed for beginners the essentials of the science a
fuller treatment than perhaps is common.

It is assumed that field work will be introduced with the
commencement of the study. The common rocks are therefore briefly
described in the opening chapters. The drift also receives early
mention, and teachers in the northern states who begin geology in
the fall may prefer to take up the chapter on the Pleistocene
immediately after the chapter on glaciers.

Simple diagrams have been used freely, not only because they are
often clearer than any verbal statement, but also because they
readily lend themselves to reproduction on the blackboard by the
pupil. The text will suggest others which the pupil may invent. It
is hoped that the photographic views may also be used for
exercises in the class room.

The generous aid of many friends is recognized with special
pleasure. To Professor W. M. Davis of Harvard University there is
owing a large obligation for the broad conceptions and luminous
statements of geologic facts and principles with which he has
enriched the literature of our science, and for his stimulating
influence in education. It is hoped that both in subject-matter
and in method the book itself makes evident this debt. But besides
a general obligation shared by geologists everywhere, and in
varying degrees by perhaps all authors of recent American text-
books in earth science, there is owing a debt direct and personal.
The plan of the book, with its use of problems and treatment of
land forms and rock structures in immediate connection with the
processes which produce them, was submitted to Professor Davis,
and, receiving his approval, was carried into effect, although
without the sanction of precedent at the time. Professor Davis
also kindly consented to read the manuscript throughout, and his
many helpful criticisms and suggestions are acknowledged with
sincere gratitude.

Parts of the manuscript have been reviewed by Dr. Samuel Calvin
and Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W.
Beyer of the Iowa College of Agriculture and Mechanic Arts; Dr. U.
S. Grant of Northwestern University; Professor J. A. Udden of
Augustana College, Illinois; Dr. C. H. Gordon of the New Mexico
State School of Mines; Principal Maurice Ricker of the High
School, Burlington, Iowa; and the following former students of the
author who are engaged in the earth sciences: Dr. W. C. Alden of
the United States Geological Survey and the University of Chicago;
Mr. Joseph Sniffen, instructor in the Academy of the University of
Chicago, Morgan Park; Professor Martin Iorns, Fort Worth
University, Texas; Professor A. M. Jayne, Dakota University;
Professor G. H. Bretnall, Monmouth College, Illinois; Professor
Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,
instructor in the Iowa State Normal College; Principal C. C. Gray
of the High School, Fargo, North Dakota; and Mr. Charles Persons
of the High School, Hannibal, Missouri. A large number of the
diagrams of the book were drawn by Mr. W. W. White of the Art
School of Cornell College. To all these friends, and to the many
who have kindly supplied the illustrations of the text, whose
names are mentioned in an appended list, the writer returns his
heartfelt thanks.

WILLIAM HARMON NORTON

CORNELL COLLEGE, MOUNT VERNON, IOWA

JULY, 1905





INTRODUCTORY NOTE

During the preparation of this book Professor Norton has
frequently discussed its plan with me by correspondence, and we
have considered together the matters of scope, arrangement, and
presentation.

As to scope, the needs of the young student and not of the expert
have been our guide; the book is therefore a text-book, not a
reference volume.

In arrangement, the twofold division of the subject was chosen
because of its simplicity and effectiveness. The principles of
physical geology come first; the several chapters are arranged in
what is believed to be a natural order, appropriate to the
greatest part of our country, so that from a simple beginning a
logical sequence of topics leads through the whole subject. The
historical view of the science comes second, with many specific
illustrations of the physical processes previously studied, but
now set forth as part of the story of the earth, with its many
changes of aspect and its succession of inhabitants. Special
attention is here given to North America, and care is taken to
avoid overloading with details.

With respect to method of presentation, it must not be forgotten
that the text-book is only one factor in good teaching, and that
in geology, as in other sciences, the teacher, the laboratory, and
the local field are other factors, each of which should play an
appropriate part. The text suggests observational methods, but it
cannot replace observation in field or laboratory; it offers
certain exercises, but space cannot be taken to make it a
laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the
illustrative descriptions that a good and experienced teacher
should supply. Frequent use is made of induction and inference in
order that the student may come to see how reasonable a science is
geology, and that he may avoid the too common error of thinking
that the opinions of "authorities" are reached by a private road
that is closed to him. The further extension of this method of
presentation is urged upon the teacher, so that the young
geologist may always learn the evidence that leads to a
conclusion, and not only the conclusion itself.

W. M. DAVIS

HARVARD UNIVERSITY, CAMBRIDGE, MASS.

JULY, 1905





CONTENTS

INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY

PART I

EXTERNAL GEOLOGICAL AGENCIES

     I. THE WORK OF THE WEATHER
    II. THE WORK OF GROUND WATER
   III. RIVERS AND VALLEYS
    IV. RIVER DEPOSITS
     V. THE WORK OF GLACIERS
    VI. THE WORK OF THE WIND
   VII. THE SEA AND ITS SHORES
  VIII. OFFSHORE AND DEEP-SEA DEPOSITS

PART  II

INTERNAL GEOLOGICAL AGENCIES

    IX. MOVEMENTS OF THE EARTH'S CRUST
     X. EARTHQUAKES
    XI. VOLCANOES
   XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
  XIII. METAMORPHISM AND MINERAL VEINS

PART III

HISTORICAL GEOLOGY

   XIV. THE GEOLOGICAL RECORD
    XV. THE PRE-CAMBRIAN SYSTEMS
   XVI. THE CAMBRIAN
  XVII. THE ORDOVICIAN AND SILURIAN
 XVIII. THE DEVONIAN
   XIX. THE CARBONIFEROUS
    XX. THE MESOZOIC
   XXI. THE TERTIARY
  XXII. THE QUATERNARY
 INDEX





THE ELEMENTS OF GEOLOGY





INTRODUCTION

THE SCOPE AND AIM OF GEOLOGY


Geology deals with the rocks of the earth's crust. It learns from
their composition and structure how the rocks were made and how
they have been modified. It ascertains how they have been brought
to their present places and wrought to their various topographic
forms, such as hills and valleys, plains and mountains. It studies
the vestiges which the rocks preserve of ancient organisms which
once inhabited our planet. Geology is the history of the earth and
its inhabitants, as read in the rocks of the earth's crust.

To obtain a general idea of the nature and method of our science
before beginning its study in detail, we may visit some valley,
such as that illustrated in the frontispiece, on whose sides are
rocky ledges. Here the rocks lie in horizontal layers. Although
only their edges are exposed, we may infer that these layers run
into the upland on either side and underlie the entire district;
they are part of the foundation of solid rock which everywhere is
found beneath the loose materials of the surface.

The ledges of the valley of our illustration are of sandstone.
Looking closely at the rock we see that it is composed of myriads
of grains of sand cemented together. These grains have been worn
and rounded. They are sorted also, those of each layer being about
of a size. By some means they have been brought hither from some
more ancient source. Surely these grains have had a history before
they here found a resting place,--a history which we are to learn
to read.

The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses
of stone in a wall were laid before the courses which rest upon
them.

We have no reason to believe that the lowest layers which we see
here were the earliest ever formed. Indeed, some deep boring in
the vicinity may prove that the ledges rest upon other layers of
rock which extend downward for many hundreds of feet below the
valley floor. Nor may we conclude that the highest layers here
were the latest ever laid; for elsewhere we may find still later
layers lying upon them.

A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as
these are of kinds whose nearest living relatives now have their
home in the sea, we infer that it was on the flat sea floor that
the sandstone was laid. Its present position hundreds of feet
above sea level proves that it has since emerged to form part of
the land; while the flatness of the beds shows that the movement
was so uniform and gentle as not to break or strongly bend them
from their original attitude.

The surface of some of these layers is ripple-marked. Hence the
sand must once have been as loose as that of shallow sea bottoms
and sea beaches to-day, which is thrown into similar ripples by
movements of the water. In some way the grains have since become
cemented into firm rock.

Note that the layers on one side of the valley agree with those on
the other, each matching the one opposite at the same level. Once
they were continuous across the valley. Where the valley now is
was once a continuous upland built of horizontal layers; the
layers now show their edges, or OUTCROP, on the valley sides
because they have been cut by the valley trench.

The rock of the ledges is crumbling away. At the foot of each step
of rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in
the future.

Through the valley runs a stream. The waters of rains which have
fallen on the upper parts of the stream's basin are now on their
way to the river and the sea. Rock fragments and grains of sand
creeping down the valley slopes come within reach of the stream
and are washed along by the running water. Here and there they
lodge for a time in banks of sand and gravel, but sooner or later
they are taken up again and carried on. The grains of sand which
were brought from some ancient source to form these rocks are on
their way to some new goal. As they are washed along the rocky bed
of the stream they slowly rasp and wear it deeper. The valley will
be deeper in the future; it has been less deep in the past.

In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the
rocks, and the land forms to which they have been sculptured, the
record of a long succession of past changes involving the origin
of sand grains and their gathering and deposit upon the bottom of
some ancient sea, the cementation of their layers into solid rock,
the uplift of the rocks to form a land surface, and, last of all,
the carving of a valley in the upland. Everywhere, in the fields,
along the river, among the mountains, by the seashore, and in the
desert, we may discover slow changes now in progress and the
record of similar changes in the past. Everywhere we may catch
glimpses of a process of gradual change, which stretches backward
into the past and forward into the future, by which the forms and
structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long
process is geology. Geology treats of the natural changes now
taking place upon the earth and within it, the agencies which
produce them, and the land forms and rock structures which result.
It studies the changes of the present in order to be able to read
the history of the earth's changes in the past.

The various agencies which have fashioned the face of the earth
may. be divided into two general classes. In Part I we shall
consider those which work upon the earth from without, such as the
weather, running water, glaciers, the wind, and the sea. In Part
II we shall treat of those agencies whose sources are within the
earth, and among whose manifestations are volcanoes and
earthquakes and the various movements of the earth's crust. As we
study each agency we shall notice not only how it does its work,
but also the records which it leaves in the rock structures and
the land forms which it produces. With this preparation we shall
be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.





PART I

EXTERNAL GEOLOGICAL AGENCIES

CHAPTER I

THE WORK OF THE WEATHER


In our excursion to the valley with sandstone ledges we witnessed
a process which is going forward in all lands. Everywhere the
rocks are crumbling away; their fragments are creeping down
hillsides to the stream ways and are carried by the streams to the
sea, where they are rebuilt into rocky layers. When again the
rocks are lifted to form land the process will begin anew; again
they will crumble and creep down slopes and be washed by streams
to the sea. Let us begin our study of this long cycle of change at
the point where rocks disintegrate and decay under the action of
the weather. In studying now a few outcrops and quarries we shall
learn a little of some common rocks and how they weather away.

STRATIFICATION AND JOINTING. At the sandstone ledges we saw that
the rock was divided into parallel layers. The thicker layers are
known as STRATA, and the thin leaves into which each stratum may
sometimes be split are termed LAMINAE. To a greater or less degree
these layers differ from each other in fineness of grain, showing
that the material has been sorted. The planes which divide them
are called BEDDING PLANES.

Besides the bedding planes there are other division planes, which
cut across the strata from top to bottom. These are found in all
rocks and are known as joints. Two sets of joints,
running at about right angles to each other, together with the
bedding planes, divide the sandstone into quadrangular blocks.

SANDSTONE. Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of
the grains are of a clear glassy mineral called quartz. These
quartz grains are very hard and will scratch the steel of a knife
blade. They are not affected by acid, and their broken surfaces
are irregular like those of broken glass.

The grains of sandstone are held together by some cement. This may
be calcareous, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly ferruginous,--hydrated iron
oxide, or iron rust, forming the bond, somewhat as in the case of
iron nails which have rusted together. The strongest and most
lasting cement is siliceous, and sand rocks whose grains are
closely cemented by silica, the chemical substance of which quartz
is made, are known as quartzites.

We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view
its surface is more or less discolored and the grains are loose
and may be rubbed off with the finger. On gentle slopes the rock
is covered with a soil composed of sand, which evidently is
crumbled sandstone, and dark carbonaceous matter derived from the
decay of vegetation. Clearly it is by the dissolving of the cement
that the rock thus breaks down to loose sand. A piece of sandstone
with calcareous cement, or a bit of old mortar, which is really an
artificial stone also made of sand cemented by lime, may be
treated in a test tube with hydrochloric acid to illustrate the
process.

A LIMESTONE QUARRY. Here also we find the rock stratified and
jointed (Fig. 2). On the quarry face the rock is distinctly seen
to be altered for some distance from its upper surface. Below the
altered zone the rock is sound and is quarried for building; but
the altered upper layers are too soft and broken to be used for
this purpose. If the limestone is laminated, the laminae here have
split apart, although below they hold fast together. Near the
surface the stone has become rotten and crumbles at the touch,
while on the top it has completely broken down to a thin layer of
limestone meal, on which rests a fine reddish clay.

Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed
in a test tube with hydrochloric acid dissolves with brisk
effervescence, leaving the insoluble impurities, which were
disseminated through it, at the bottom of the tube as a little
clay.

We can now understand the changes in the upper layers of the
quarry. At the surface of the rock the limestone has completely
dissolved, leaving the insoluble residue as a layer of reddish
clay. Immediately below the clay the rock has disintegrated into
meal where the cement between the limestone grains has been
removed, while beneath this the laminae are split apart where the
cement has been dissolved only along the planes of lamination
where the stone is more porous. As these changes in the rock are
greatest at the surface and diminish downward, we infer that they
have been caused by agents working downward from the surface.

At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that
the rock is altered even down to the quarry floor.

A SHALE PIT. Let us now visit some pit where shale--a laminated
and somewhat hardened clay--is quarried for the manufacture of
brick. The laminae of this fine-grained rock may be as thin as
cardboard in places, and close joints may break the rock into
small rhombic blocks. On the upper surface we note that the shale
has weathered to a clayey soil in which all traces of structure
have been destroyed. The clay and the upper layers of the shale
beneath it are reddish or yellow, while in many cases the color of
the unaltered rock beneath is blue.

THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are
the leading types of the great group of stratified, or
sedimentary, rocks. This group includes all rocks made of
sediments, their materials having settled either in water upon the
bottoms of rivers, lakes, or seas, or on dry land, as in the case
of deposits made by the wind and by glaciers. Sedimentary rocks
are divided into the fragmental rocks--which are made of
fragments, either coarse or fine--and the far less common rocks
which are constituted of chemical precipitates.

The sedimentary rocks are divided according to their composition
into the following classes:

1. The arenaceous, or quartz rocks, including beds of loose sand
and gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).

2. The calcareous, or lime rocks, including limestone and a soft
white rock formed of calcareous powder known as chalk.

3. The argillaceous, or clay rocks, including muds, clays, and
shales. These three classes pass by mixture into one another. Thus
there are limy and clayey sandstones, sandy and clayey limestones,
and sandy and limy shales.

GRANITE. This familiar rock may be studied as an example of the
second great group of rocks,--the unstratified, or igneous rocks.
These are not made of cemented sedimentary grains, but of
interlocking crystals which have crystallized from a molten mass.
Examining a piece of granite, the most conspicuous crystals which
meet the eye are those of feldspar. They are commonly pink, white,
or yellow, and break along smooth cleavage planes which reflect
the light like tiny panes of glass. Mica may be recognized by its
glittering plates, which split into thin elastic scales. A third
mineral, harder than steel, breaking along irregular surfaces like
broken glass, we identify as quartz.

How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying
the loose formations of the surface, and in many parts of the
northern states where granite bowlders and pebbles more or less
decayed may be found in a surface sheet of stony clay called the
drift. Of the different minerals composing granite, quartz alone
remains unaltered. Mica weathers to detached flakes which have
lost their elasticity. The feldspar crystals have lost their
luster and hardness, and even have decayed to clay. Where long-
weathered granite forms the country rock, it often may be cut with
spade or trowel for several feet from the surface, so rotten is
the feldspar, and here the rock is seen to break down to a clayey
soil containing grains of quartz and flakes of mica.

These are a few simple illustrations of the surface changes which
some of the common kinds of rocks undergo. The agencies by which
these changes are brought about we will now take up under two
divisions,--CHEMICAL AGENCIES producing rock decay and MECHANICAL
AGENCIES producing rock disintegration.

THE CHEMICAL WORK OF WATER

As water falls on the earth in rain it has already absorbed from
the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks
into the ground and becomes what is termed ground water, it takes
into solution from the soil humus acids and carbon dioxide, both
of which are constantly being generated there by the decay of
organic matter. So both rain and ground water are charged with
active chemical agents, by the help of which they corrode and rust
and decompose all rocks to a greater or less degree. We notice now
three of the chief chemical processes concerned in weathering,--
solution, the formation of carbonates, and oxidation.

SOLUTION. Limestone, although so little affected by pure water
that five thousand gallons would be needed to dissolve a single
pound, is easily dissolved in water charged with carbon dioxide.
In limestone regions well water is therefore "hard." On boiling
the water for some time the carbon dioxide gas is expelled, the
whole of the lime carbonate can no longer be held in solution, and
much of it is thrown down to form a crust or "scale" in the kettle
or in the tubes of the steam boiler. All waters which flow over
limestone rocks or soak through them are constantly engaged in
dissolving them away, and in the course of time destroy beds of
vast extent and great thickness.

The upper surface of limestone rocks becomes deeply pitted, as we
saw in the limestone quarry, and where the mantle of waste has
been removed it may be found so intricately furrowed that it is
difficult to traverse.

Beds of rock salt buried among the strata are dissolved by seeping
water, which issues in salt springs. Gypsum, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched
with the finger nail, is readily taken up by water, giving to the
water of wells and springs a peculiar hardness difficult to
remove.

The dissolving action of moisture may be noted on marble
tombstones of some age, marble being a limestone altered by heat
and pressure and composed of crystalline grains. By assuming that
the date on each monument marks the year of its erection, one may
estimate how many years on the average it has taken for weathering
to loosen fine grains on the polished surface, so that they may be
rubbed off with the finger, to destroy the polish, to round the
sharp edges of tool marks in the lettering, and at last to open
cracks and seams and break down the stone. We may notice also
whether the gravestones weather more rapidly on the sunny or the
shady side, and on the sides or on the top.

The weathered surface of granular limestone containing shells
shows them standing in relief. As the shells are made of
crystalline carbonate of lime, we may infer whether the carbonate
of lime is less soluble in its granular or in its crystalline
condition.

THE FORMATION OF CARBONATES. In attacking minerals water does more
than merely take them into solution. It decomposes them, forming
new chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken
up by the action of water containing carbon dioxide, forming
alkaline carbonates. These carbonates are freely soluble and
contribute potash and soda to soils and river waters. By the
removal of the soluble ingredients of feldspar there is left the
silicate of alumina, united with water or hydrated, in the
condition of a fine plastic clay which, when white and pure, is
known as KAOLIN and is used in the manufacture of porcelain.
Feldspathic rocks which contain no iron compounds thus weather to
whitish crusts, and even apparently sound crystals of feldspar,
when ground to thin slices and placed under the microscope, may be
seen to be milky in color throughout because an internal change to
kaolin has begun.

OXIDATION. Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty
films. Oxygen and water have here united with the iron, forming
hydrated iron oxide. The effects of oxidation may be seen in the
alteration of many kinds of rocks and in red and yellow colors of
soils and subsoils.

Pyrite is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and
sulphur (iron sulphide). Under the attack of the weather it takes
up oxygen, forming iron sulphate (green vitriol), a soluble
compound, and insoluble hydrated iron oxide, which as a mineral is
known as limonite. Several large masses of iron sulphide were
placed some years ago on the lawn in front of the National Museum
at Washington. The mineral changed so rapidly to green vitriol
that enough of this poisonous compound was washed into the ground
to kill the roots of the surrounding grass.

AGENTS OF MECHANICAL DISINTEGRATION

HEAT AND COLD. Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool
and contract. When the difference in temperature between day and
night is considerable, the repeated strains of sudden expansion
and contraction at last become greater than the rocks can bear,
and they break, for the same reason that a glass cracks when
plunged into boiling water (Fig. 5).

Rocks are poor conductors of heat, and hence their surfaces may
become painfully hot under the full blaze of the sun, while the
interior remains comparatively cool. By day the surface shell
expands and tends to break loose from the mass of the stone. In
cooling in the evening the surface shell suddenly contracts on the
unyielding interior and in time is forced off in scales.

Many rocks, such as granite, are made up of grains of various
minerals which differ in color and in their capacity to absorb
heat, and which therefore contract and expand in different ratios.
In heating and cooling these grains crowd against their neighbors
and tear loose from them, so that finally the rock disintegrates
into sand.

The conditions for the destructive action of heat and cold are
most fully met in arid regions when vegetation is wanting for lack
of sufficient rain. The soil not being held together by the roots
of plants is blown away over large areas, leaving the rocks bare
to the blazing sun in a cloudless sky. The air is dry, and the
heat received by the earth by day is therefore rapidly radiated at
night into space. There is a sharp and sudden fall of temperature
after sunset, and the rocks, strongly heated by day, are now
chilled perhaps even to the freezing point.

In the Sahara the thermometer has been known to fall 131 degrees
F. within a few hours. In the light air of the Pamir plateau in
central Asia a rise of 90 degrees F. has been recorded from seven
o'clock in the morning to one o'clock in the afternoon. On the
mountains of southwestern Texas there are frequently heard
crackling noises as the rocks of that arid region throw off scales
from a fraction of an inch to four inches in thickness, and loud
reports are made as huge bowlders split apart. Desert pebbles
weakened by long exposure to heat and cold have been shivered to
fine sharp-pointed fragments on being placed in sand heated to 180
degrees F. Beds half a foot thick, forming the floor of limestone
quarries in Wisconsin, have been known to buckle and arch and
break to fragments under the heat of the summer sun.

FROST. By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more
or less porous and all contain more or less water in their pores.
Workers in stone call this "quarry water," and speak of a stone as
"green" before the quarry water has dried out. Water also seeps
along joints and bedding planes and gathers in all seams and
crevices. Water expands in freezing, ten cubic inches of water
freezing to about eleven cubic inches of ice. As water freezes in
the rifts and pores of rocks it expands with the irresistible
force illustrated in the freezing and breaking of water pipes in
winter. The first rift in the rock, perhaps too narrow to be seen,
is widened little by little by the wedges of successive frosts,
and finally the rock is broken into detached blocks, and these
into angular chip-stone by the same process.

It is on mountain tops and in high latitudes that the effects of
frost are most plainly seen. "Every summit" says Whymper, "amongst
the rock summits upon which I have stood has been nothing but a
piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen,
in Kamchatka, and in other frigid lands large areas are thickly
strewn with sharp-edged fragments into which the rock has been
shattered by frost.

ORGANIC AGENTS

We must reckon the roots of plants and trees among the agents
which break rocks into pieces. The tiny rootlet in its search for
food and moisture inserts itself into some minute rift, and as it
grows slowly wedges the rock apart. Moreover, the acids of the
root corrode the rocks with which they are in contact. One may
sometimes find in the soil a block of limestone wrapped in a mesh
of roots, each of which lies in a little furrow where it has eaten
into the stone.

Rootless plants called lichens often cover and corrode rocks as
yet bare of soil; but where lichens are destroying the rock less
rapidly than does the weather, they serve in a way as a
protection.

CONDITIONS FAVORING DISINTEGRATION AND DECAY. The
disintegration of rocks under frost and temperature changes
goes on most rapidly in cold and arid climates, and where
vegetation is scant or absent. On the contrary, the decay of rocks
under the chemical action of water is favored by a warm, moist
climate and abundant vegetation. Frost and heat and cold can only
act within the few feet from the surface to which the necessary
temperature changes are limited, while water penetrates and alters
the rocks to great depths.

The pupil may explain.

In what ways the presence of joints and bedding planes assists in
the breaking up and decay of rocks under the action of the
weather.

Why it is a good rule of stone masons never to lay stones on edge,
but always on their natural bedding planes.

Why stones fresh from the quarry sometimes go to pieces in early
winter, when stones which have been quarried for some months
remain uninjured.

Why quarrymen in the northern states often keep their quarry
floors flooded during winter.

Why laminated limestone should not be used for curbstone.

Why rocks composed of layers differing in fineness of grain and in
ratios of expansion do not make good building stone.

Fine-grained rocks with pores so small that capillary attraction
keeps the water which they contain from readily draining away are
more apt to hold their pores ten elevenths full of water than are
rocks whose pores are larger. Which, therefore, are more likely to
be injured by frost?

Which is subject to greater temperature changes, a dark rock or
one of a light color? the north side or the south side of a
valley?

THE MANTLE OF ROCK WASTE

We have seen that rocks are everywhere slowly wasting away. They
are broken in pieces by frost, by tree roots, and by heat and
cold. They dissolve and decompose under the chemical action of
water and the various corrosive substances which it contains,
leaving their insoluble residues as residual clays and sands upon
the surface. As a result there is everywhere forming a mantle of
rock waste which covers the land. It is well to imagine how the
country would appear were this mantle with its soil and vegetation
all scraped away or had it never been formed. The surface of the
land would then be everywhere of bare rock as unbroken as a quarry
floor.

THE THICKNESS OF THE MANTLE. In any locality the thickness of the
mantle of rock waste depends as much on the rate at which it is
constantly being removed as on the rate at which it is forming. On
the face of cliffs it is absent, for here waste is removed as fast
as it is made. Where waste is carried away more slowly than it is
produced, it accumulates in time to great depth.

The granite of Pikes Peak is disintegrated to a depth of twenty
feet. In the city of Washington granite rock is so softened to a
depth of eighty feet that it can be removed with pick and shovel.
About Atlanta, Georgia, the rocks are completely rotted for one
hundred feet from the surface, while the beginnings of decay may
be noticed at thrice that depth. In places in southern Brazil the
rock is decomposed to a depth of four hundred feet.

In southwestern Wisconsin a reddish residual clay has an average
depth of thirteen feet on broad uplands, where it has been removed
to the least extent. The country rock on which it rests is a
limestone with about ten per cent of insoluble impurities. At
least how thick, then, was that portion of the limestone which has
rotted down to the clay?

DISTINGUISHING CHARACTERISTICS OF RESIDUAL WASTE. We must learn to
distinguish waste formed in place by the action of the weather
from the products of other geological agencies. Residual waste is
unstratified. It contains no substances which have not been
derived from the weathering of the parent rock. There is a gradual
transition from residual waste into the unweathered rock beneath.
Waste resting on sound rock evidently has been shifted and was not
formed in place.

In certain regions of southern Missouri the land is covered with a
layer of broken flints and red clay, while the country rock is
limestone. The limestone contains nodules of flint, and we may
infer that it has been by the decay and removal of thick masses of
limestone that the residual layer of clay and flints has been left
upon the surface. Flint is a form of quartz, dull-lustered,
usually gray or blackish in color, and opaque except on thinnest
edges, where it is translucent.

Over much of the northern states there is spread an unstratified
stony clay called the drift. It often rests on sound rocks. It
contains grains of sand, pebbles, and bowlders composed of many
different minerals and rocks that the country rock cannot furnish.
Hence the drift cannot have been formed by the decay of the rock
of the region. A shale or limestone, for example, cannot waste to
a clay containing granite pebbles. The origin of the drift will be
explained in subsequent chapters.

The differences in rocks are due more to their soluble than to
their insoluble constituents. The latter are few in number and are
much the same in rocks of widely different nature, being chiefly
quartz, silicate of alumina, and iron oxide. By the removal of
their soluble parts very many and widely different rocks rot down
to a residual clay gritty with particles of quartz and colored red
or yellow with iron oxide.

In a broad way the changes which rocks undergo in weathering are
an adaptation to the environment in which they find themselves at
the earth's surface,--an environment different from that in which
they were formed under sea or under ground. In open air, where
they are attacked by various destructive agents, few of the rock-
making minerals are stable compounds except quartz, the iron
oxides, and the silicate of alumina; and so it is to one or more
of these comparatively insoluble substances that most rocks are
reduced by long decay.

Which produces a mantle of finer waste, frost or chemical decay?
which a thicker mantle? In what respects would you expect that the
mantle of waste would differ in warm humid lands like India, in
frozen countries like Alaska, and in deserts such as the Sahara?

THE SOIL. The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the
surface, where the waste has weathered longest, it is gradually
made fine enough to support the growth of plants, and is then
known as soil. The coarser waste beneath is sometimes spoken of as
subsoil. Soil usually contains more or less dark, carbonaceous,
decaying organic matter, called humus, and is then often termed
the humus layer. Soil forms not only on waste produced in place
from the rock beneath, but also on materials which have been
transported, such as sheets of glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather
is more rapid and effective on the fragments of the mantle of
waste than on the rocks from which waste is being formed. Why?

Any fresh excavation of cellar or cistern, or cut for road or
railway, will show the characteristics of the humus layer. It may
form only a gray film on the surface, or we may find it a layer a
foot or more thick, dark, or even black, above, and growing
gradually lighter in color as it passes by insensible gradations
into the subsoil. In some way the decaying vegetable matter
continually forming on the surface has become mingled with the
material beneath it.

HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and
the subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying
substance to fertilize it. Leaves and stems falling on the surface
are turned under by several agents. Earthworms and other animals
whose home is in the waste drag them into their burrows either for
food or to line their nests. Trees overthrown by the wind, roots
and all, turn over the soil and subsoil and mingle them together.
Bacteria also work in the waste and contribute to its enrichment.
The animals living in the mantle do much in other ways toward the
making of soil. They bring the coarser fragments from beneath to
the surface, where the waste weathers more rapidly. Their burrows
allow air and water to penetrate the waste more freely and to
affect it to greater depths.

ANTS. In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build
mounds which may reach as high above it. In some parts of Paraguay
and southern Brazil these mounds, like gigantic potato hills,
cover tracts of considerable area.

In search for its food--the dead wood of trees--the so-called
white ant constructs runways of earth about the size of gas pipes,
reaching from the base of the tree to the topmost branches. On the
plateaus of central Africa explorers have walked for miles through
forests every tree of which was plastered with these galleries of
mud. Each grain of earth used in their construction is moistened
and cemented by slime as it is laid in place by the ant, and is
thus acted on by organic chemical agents. Sooner or later these
galleries are beaten down by heavy rains, and their fertilizing
substances are scattered widely by the winds.

EARTHWORMS. In temperate regions the waste is worked over largely
by earthworms. In making their burrows worms swallow earth in
order to extract from it any nutritive organic matter which it may
contain. They treat it with their digestive acids, grind it in
their stony gizzards, and void it in castings on the surface of
the ground. It was estimated by Darwin that in many parts of
England each year, on every acre, more than ten tons of earth pass
through the bodies of earthworms and are brought to the surface,
and that every few years the entire soil layer is thus worked over
by them.

In all these ways the waste is made fine and stirred and enriched.
Grain by grain the subsoil with its fresh mineral ingredients is
brought to the surface, and the rich organic matter which plants
and animals have taken from the atmosphere is plowed under. Thus
Nature plows and harrows on "the great world's farm" to make ready
and ever to renew a soil fit for the endless succession of her
crops.

The world processes by which rocks are continually wasting away
are thus indispensable to the life of plants and animals. The
organic world is built on the ruins of the inorganic, and because
the solid rocks have been broken down into soil men are able to
live upon the earth.

SOLAR ENERGY. The source of the energy which accomplishes all this
necessary work is the sun. It is the radiant energy of the sun
which causes the disintegration of rocks, which lifts vapor into
the atmosphere to fall as rain, which gives life to plants and
animals. Considering the earth in a broad way, we may view it as a
globe of solid rock,--the lithosphere,--surrounded by two mobile
envelopes: the envelope of air,--THE ATMOSPHERE, and the envelope
of water,--THE HYDROSPHERE. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are
dissolved in all waters,--and both air and water enter and work
upon the solid earth. By their action upon the lithosphere they
have produced a third envelope,--the mantle of rock waste.

This envelope also is in movement, not indeed as a whole, but
particle by particle. The causes which set its particles in
motion, and the different forms which the mantle comes to assume,
we will now proceed to study.

MOVEMENTS OF THE MANTLE OF ROCK WASTE

At the sandstone ledges which we first visited we saw not only
that the rocks were crumbling away, but also that grains and
fragments of them were creeping down the slopes of the valley to
the stream and were carried by it onward toward the sea. This
process is going on everywhere. Slowly it may be, and with many
interruptions, but surely, the waste of the land moves downward to
the sea. We may divide its course into two parts,--the path to the
stream, which we will now consider, and its carriage onward by the
stream, which we will defer to a later chapter.

GRAVITY. The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull
of the earth's mass and follows it when free to do so. All
agencies which produce waste tend to set its particles free and in
motion, and therefore cooperate with gravity. On cliffs, rocks
fall when wedged off by frost or by roots of trees, and when
detached by any other agency. On slopes of waste, water freezes in
chinks between stones, and in pores between particles of soil, and
wedges them apart. Animals and plants stir the waste, heat expands
it, cold contracts it, the strokes of the raindrops drive loose
particles down the slope and the wind lifts and lets them fall. Of
all these movements, gravity assists those which are downhill and
retards those which are uphill. On the whole, therefore, the
downhill movements prevail, and the mantle of waste, block by
block and grain by grain, creeps along the downhill path.

A slab of sandstone laid on another of the same kind at an angle
of 17 degrees and left in the open air was found to creep down the
slope at the rate of a little more than a millimeter a month.
Explain why it did so.

RAIN. The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of
the blows of the falling drops, and washes them down all slopes to
within reach of permanent streams. On surfaces unprotected by
vegetation, as on plowed fields and in arid regions, the rain
wears furrows and gullies both in the mantle of waste and in
exposures of unaltered rock (Fig. 17).

At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the
hillside is steepest the soil may be exceedingly thin, or quite
absent, because removed about as fast as formed. Against the walls
of an abbey built on a slope in Wales seven hundred years ago, the
creeping waste has gathered on the uphill side to a depth of seven
feet. The slow-flowing sheet of waste is often dammed by fences
and walls, whose uphill side gathers waste in a few years so as to
show a distinctly higher surface than the downhill side,
especially in plowed fields where the movement is least checked by
vegetation.

TALUS. At the foot of cliffs there is usually to be found a slope
of rock fragments which clearly have fallen from above. Such a
heap of waste is known as talus. The amount of talus in any place
depends both on the rate of its formation and the rate of its
removal. Talus forms rapidly in climates where mechanical
disintegration is most effective, where rocks are readily broken
into blocks because closely jointed and thinly bedded rather than
massive, and where they are firm enough to be detached in
fragments of some size instead of in fine grains. Talus is removed
slowly where it decays slowly, either because of the climate or
the resistance of the rock. It may be rapidly removed by a stream
flowing along its base.

In a moist climate a soluble rock, such as massive limestone, may
form talus little if any faster than the talus weathers away. A
loose-textured sandstone breaks down into incoherent sand grains,
which in dry climates, where unprotected by vegetation, may be
blown away as fast as they fall, leaving the cliff bare to the
base. Cliffs of such slow-decaying rocks as quartzite and granite
when closely jointed accumulate talus in large amounts.

Talus slopes may be so steep as to reach THE ANGLE OF REPOSE, i.e.
the steepest angle at which the material will lie. This angle
varies with different materials, being greater with coarse and
angular fragments than with fine rounded grains. Sooner or later a
talus reaches that equilibrium where the amount removed from its
surface just equals that supplied from the cliff above. As the
talus is removed and weathers away its slope retreats together
with the retreat of the cliff, as seen in Figure 9.

GRADED SLOPES. Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges.
On the steeper slopes it is coarser and in more rapid movement
than on slopes more gentle, but mountain sides and hills and
plains alike come to be mantled with sheets of waste which
everywhere is creeping toward the streams. Such unbroken slopes,
worn or built to the least inclination at which the waste supplied
by weathering can be urged onward, are known as GRADED SLOPES.

Of far less importance than the silent, gradual creep of waste,
which is going on at all times everywhere about us, are the
startling local and spasmodic movements which we are now to
describe.

AVALANCHES. On steep mountain sides the accumulated snows of
winter often slip and slide in avalanches to the valleys below.
These rushing torrents of snow sweep their tracks clean of waste
and are one of Nature's normal methods of moving it along the
downhill path.

LANDSLIDES. Another common and abrupt method of delivering waste
to streams is by slips of the waste mantle in large masses. After
long rains and after winter frosts the cohesion between the waste
and the sound rock beneath is loosened by seeping water
underground. The waste slips on the rock surface thus lubricated
and plunges down the mountain side in a swift roaring torrent of
mud and stones.

We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved
in the sudden movement. Such slips occur when valleys have been
rapidly deepened by streams or glaciers and their sides have not
yet been graded. A favorable condition is where the strata dip
(i.e. incline downwards) towards the valley (Fig. 11), or are
broken by joint planes dipping in the same direction. The upper
layers, including perhaps the entire mountain side, have been cut
across by the valley trench and are left supported only on the
inclined surface of the underlying rocks. Water may percolate
underground along this surface and loosen the cohesion between the
upper and the underlying strata by converting the upper surface of
a shale to soft wet clay, by dissolving layers of a limestone, or
by removing the cement of a sandstone and converting it into loose
sand. When the inclined surface is thus lubricated the overlying
masses may be launched into the valley below. The solid rocks are
broken and crushed in sliding and converted into waste consisting,
like that of talus, of angular unsorted fragments, blocks of all
sizes being mingled pellmell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.

At Gohna, India, in 1893, the face of a spur four thousand feet
high, of the lower ranges of the Himalayas, slipped into the gorge
of the headwaters of the Ganges River in successive rock falls
which lasted for three days. Blocks of stone were projected for a
mile, and clouds of limestone dust were spread over the
surrounding country. The debris formed a dam one thousand feet
high, extending for two miles along the valley. A lake gathered
behind this barrier, gradually rising until it overtopped it in a
little less than a year. The upper portion of the dam then broke,
and a terrific rush of water swept down the valley in a wave
which, twenty miles away, rose one hundred and sixty feet in
height. A narrow lake is still held by the strong base of the dam.

In 1896, after forty days of incessant rain, a cliff of sandstone
slipped into the Yangtse River in China, reducing the width of the
channel to eighty yards and causing formidable rapids.

At Flims, in Switzerland, a prehistoric landslip flung a dam
eighteen hundred feet high across the headwaters of the Rhine. If
spread evenly over a surface of twenty-eight square miles, the
material would cover it to a depth of six hundred and sixty feet.
The barrier is not yet entirely cut away, and several lakes are
held in shallow basins on its hummocky surface.

A slide from the precipitous river front of the citadel hill of
Quebec, in 1889, dashed across Champlain Street, wrecking a number
of houses and causing the death of forty-five persons. The strata
here are composed of steeply dipping slate.

In lofty mountain ranges there may not be a single valley without
its traces of landslides, so common there is this method of the
movement of waste, and of building to grade over-steepened slopes.

ROCK SCULPTURE BY WEATHERING

We are now to consider a few of the forms into which rock masses
are carved by the weather.

BOWLDERS OF WEATHERING. In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers
have been broken along their joints and bedding planes are no
longer angular, as are those of the layers below. The edges and
corners of these blocks have been worn away by the weather. Such
rounded cores, known as bowlders of weathering, are often left to
strew the surface.

DIFFERENTIAL WEATHERING. This term covers all cases in which a
rock mass weathers differently in different portions. Any weaker
spots or layers are etched out on the surface, leaving the more
resistant in relief. Thus massive limestones become pitted where
the weather drills out the weaker portions. In these pits, when
once they are formed, moisture gathers, a little soil collects,
vegetation takes root, and thus they are further enlarged until
the limestone may be deeply honeycombed.

On the sides of canyons, and elsewhere where the edges of strata
are exposed, the harder layers project as cliffs, while the softer
weather back to slopes covered with the talus of the harder layers
above them. It is convenient to call the former cliff makers and
the latter slope makers.

Differential weathering plays a large part in the sculpture of the
land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in
the valleys and mountains of eastern Pennsylvania. But in such
instances the lowering of the surface of the weaker rock is also
due to the wear of streams, and especially to the removal by them
from the land of the waste which covers and protects the rocks
beneath.

Rocks owe their weakness to several different causes. Some, such
as beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are
closely divided by joints and bedding planes and are thus readily
broken up into blocks by mechanical agencies.

OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant
or where the attack for any reason is less severe. Such remnant
masses, if large, are known as outliers. When

Note the rain furrows on the slope at the foot of the monuments.
In the foreground are seen fragments of petrified trunks of trees,
composed of silica and extremely resistant to the weather. On the
removal of the rock layers in which these fragments were imbedded
they are left to strew the surface in the same way as are the
residual flints of southern Missouri. flat-topped, because of the
protection of a resistant horizontal capping layer, they are
termed mesas,--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are
finally consumed.

Monuments are smaller masses and may be but partially detached
from the cliff face. In the breaking down of sheets of horizontal
strata, outliers grow smaller and smaller and are reduced to
massive rectangular monuments resembling castles (Fig. 17). The
rock castle falls into ruin, leaving here and there an isolated
tower; the tower crumbles to a lonely pillar, soon to be
overthrown. The various and often picturesque shapes of monuments
depend on the kind of rock, the attitude of the strata, and the
agent by which they are chiefly carved. Thus pillars may have a
capital formed of a resistant stratum. Monuments may be undercut
and come to rest on narrow pedestals, wherever they weather more
rapidly near the ground, either because of the greater moisture
there, or--in arid climates--because worn at their base by
drifting sands.

Stony clays disintegrating under the rain often contain bowlders
which protect the softer material beneath from the vertical blows
of raindrops, and thus come to stand on pedestals of some height.
One may sometimes see on the ground beneath dripping eaves pebbles
left in the same way, protecting tiny pedestals of sand.

MOUNTAIN PEAKS AND RIDGES. Most mountains have been carved out of
great broadly uplifted folds and blocks of the earth's crust.
Running water and glacier ice have cut these folds and blocks into
masses divided by deep valleys; but it is by the weather, for the
most part, that the masses thus separated have been sculptured to
the present forms of the individual peaks and ridges.

Frost and heat and cold sculpture high mountains to sharp,
tusklike peaks and ragged, serrate crests, where their waste is
readily removed.

The Matterhorn of the Alps is a famous example of a mountain peak
whose carving by the frost and other agents is in active progress.
On its face "scarcely a rock anywhere is firmly attached," and the
fall of loosened stones is incessant. Mountain climbers who have
camped at its base tell how huge rocks from time to time come
leaping down its precipices, followed by trains of dislodged
smaller fragments and rock dust; and how at night one may trace
the course of the bowlders by the sparks which they strike from
the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles
the Matterhorn in form and has been carved by the same agencies.

"The Needles" of Arizona are examples of sharp mountain peaks in a
warm arid region sculptured chiefly by temperature changes.

Chemical decay, especially when carried on beneath a cover of
waste and vegetation, favors the production of rounded knobs and
dome-shaped mountains.

THE WEATHER CURVE. We have seen that weathering reduces the
angular block quarried by the frost to a rounded bowlder by
chipping off its corners and smoothing away its edges. In much the
same way weathering at last reduces to rounded hills the earth
blocks cut by streams or formed in any other way. High mountains
may at first be sculptured by the weather to savage peaks (Fig.
181), but toward the end of their life history they wear down to
rounded hills (Fig. 182). The weather curve, which may be seen on
the summits of low hills (Fig. 21), is convex upward.

In Figure 22, representing a cubic block of stone whose faces are
a yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner a; by one situated in the
middle of an edge b; by one in the center of a side c? How much
faster will a and b weather than c, and what will be the effect on
the shape of the block?

THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the
sake of clearness it is necessary to describe the work of each
geological agent separately. We must not forget, however, that in
Nature no agent works independently and alone; that every result
is the outcome of a long chain of causes. Thus, in order that the
mountain peak may be carved by the agents of disintegration, the
waste must be rapidly removed,--a work done by many agents,
including some which we are yet to study; and in order that the
waste may be removed as fast as formed, the region must first have
been raised well above the level of the sea, so that the agents of
transportation could do their work effectively. The sculpture of
the rocks is accomplished only by the cooperation of many forces.

The constant removal of waste from the surface by creep and wash
and carriage by streams is of the highest importance, because it
allows the destruction of the land by means of weathering to go on
as long as any land remains above sea level. If waste were not
removed, it would grow to be so thick as to protect the rock
beneath from further weathering, and the processes of destruction
which we have studied would be brought to an end. The very
presence of the mantle of waste over the land proves that on the
whole rocks weather more rapidly than their waste is removed. The
destruction of the land is going on as fast as the waste can be
carried away.

We have now learned to see in the mantle of waste the record of
the destructive action of the agencies of weathering on the rocks
of the land surface. Similar records we shall find buried deeply
among the rocks of the crust in old soils and in rocks pitted and
decayed, telling of old land surfaces long wasted by the weather.
Ever since the dry land appeared these agencies have been as now
quietly and unceasingly at work upon it, and have ever been the
chief means of the destruction of its rocks. The vast bulk of the
stratified rocks of the earth's crust is made up almost wholly of
the waste thus worn from ancient lands.

In studying the various geological agencies we must remember the
almost inconceivable times in which they work. The slowest process
when multiplied by the immense time in which it is carried on
produces great results. The geologist looks upon the land forms of
the earth's surface as monuments which record the slow action of
weathering and other agents during the ages of the past. The
mountain peak, the rounded hill, the wide plain which lies where
hills and mountains once stood, tell clearly of the great results
which slow processes will reach when given long time in which to
do their work. We should accustom ourselves also to think of the
results which weathering will sooner or later bring to pass. The
tombstone and the bowlder of the field, which each year lose from
their surfaces a few crystalline grains, must in time be wholly
destroyed. The hill whose rocks are slowly rotting underneath a
cover of waste must become lower and lower as the centuries and
millenniums come and go, and will finally disappear. Even the
mountains are crumbling away continually, and therefore are but
fleeting features of the landscape.





CHAPTER II

THE WORK OF GROUND WATER


LAND WATERS. We have seen how large is the part that water plays
at and near the surface of the land in the processes of weathering
and in the slow movement of waste down all slopes to the stream
ways. We now take up the work of water as it descends beneath the
ground,--a corrosive agent still, and carrying in solution as its
load the invisible waste of rocks derived from their soluble
parts.

Land waters have their immediate source in the rainfall. By the
heat of the sun water is evaporated from the reservoir of the
ocean and from moist surfaces everywhere. Mingled as vapor with
the air, it is carried by the winds over sea and land, and
condensed it returns to the earth as rain or snow. That part of
the rainfall which descends on the ocean does not concern us, but
that which falls on the land accomplishes, as it returns to the
sea, the most important work of all surface geological agencies.

The rainfall may be divided into three parts: the first DRIES UP,
being discharged into the air by evaporation either directly from
the soil or through vegetation; the second RUNS OFF over the
surface to flood the streams; the third SOAKS IN the ground and is
henceforth known as GROUND or UNDERGROUND WATER.

THE DESCENT OF GROUND WATER. Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying
rock. All rocks of the upper crust of the earth are more or less
porous, and all drink in water. IMPERVIOUS ROCKS, such as granite,
clay, and shale, have pores so minute that the water which they
take in is held fast within them by capillary attraction, and none
drains through. PERVIOUS ROCKS, on the other hand, such as many
sandstones, have pore spaces so large that water filters through
them more or less freely. Besides its seepage through the pores of
pervious rocks, water passes to lower levels through the joints
and cracks by which all rocks, near the surface are broken.

Even the closest-grained granite has a pore space of 1 in 400,
while sandstone may have a pore space of 1 in 4. Sand is so porous
that it may absorb a third of its volume of water, and a loose
loam even as much as one half.

THE GROUND-WATER SURFACE is the name given the upper surface of
ground water, the level below which all rocks are saturated. In
dry seasons the ground-water surface sinks. For ground water is
constantly seeping downward under gravity, it is evaporated in the
waste and its moisture is carried upward by capillarity and the
roots of plants to the surface to be evaporated in the air. In wet
seasons these constant losses are more than made good by fresh
supplies from that part of the rainfall which soaks into the
ground, and the ground-water surface rises.

In moist climates the ground-water surface (Fig. 24) lies, as a
rule, within a few feet of the land surface and conforms to it in
a general way, although with slopes of less inclination than those
of the hills and valleys. In dry climates permanent ground water
may be found only at depths of hundreds of feet. Ground water is
held at its height by the fact that its circulation is constantly
impeded by capillarity and friction. If it were as free to drain
away as are surface streams, it would sink soon after a rain to
the level of the deepest valleys of the region.

WELLS AND SPRINGS. Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells.
Where valleys cut this surface permanent streams are formed, the
water either oozing forth along ill-defined areas or issuing at
definite points called springs, where it is concentrated by the
structure of the rocks. A level tract where the ground-water
surface coincides with the surface of the ground is a swamp or
marsh.

By studying a spring one may learn much of the ways and work of
ground water. Spring water differs from that of the stream into
which it flows in several respects. If we test the spring with a
thermometer during successive months, we shall find that its
temperature remains much the same the year round. In summer it is
markedly cooler than the stream; in winter it is warmer and
remains unfrozen while the latter perhaps is locked in ice. This
means that its underground path must lie at such a distance from
the surface that it is little affected by summer's heat and
winter's cold.

While the stream is often turbid with surface waste washed into it
by rains, the spring remains clear; its water has been filtered
during its slow movement through many small underground passages
and the pores of rocks. Commonly the spring differs from the
stream in that it carries a far larger load of dissolved rock.
Chemical analysis proves that streams contain various minerals in
solution, but these are usually in quantities so small that they
are not perceptible to the taste or feel. But the water of springs
is often well charged with soluble minerals; in its slow, long
journey underground it has searched out the soluble parts of the
rocks through which it seeps and has dissolved as much of them as
it could. When spring water is boiled away, the invisible load
which it has carried is left behind, and in composition is found
to be practically identical with that of the soluble ingredients
of the country rock. Although to some extent the soluble waste of
rocks is washed down surface slopes by the rain, by far the larger
part is carried downward by ground water and is delivered to
streams by springs.

In limestone regions springs are charged with calcium carbonate
(the carbonate of lime), and where the limestone is magnesian they
contain magnesium carbonate also. Such waters are "hard"; when
used in washing, the minerals which they contain combine with the
fatty acids of soap to form insoluble curdy compounds. When
springs rise from rocks containing gypsum they are hard with
calcium sulphate. In granite regions they contain more or less
soda and potash from the decay of feldspar.

The flow of springs varies much less during the different seasons
of the year than does that of surface streams. So slow is the
movement of ground water through the rocks that even during long
droughts large amounts remain stored above the levels of surface
drainage.

MOVEMENTS OF GROUND WATER. Ground water is in constant movement
toward its outlets. Its rate varies according to many conditions,
but always is extremely slow. Even through loose sands beneath the
beds of rivers it sometimes does not exceed a fifth of a mile a
year.

In any region two zones of flow may be distinguished. The UPPER
ZONE OF FLOW extends from the ground-water surface downward
through the waste mantle and any permeable rocks on which the
mantle rests, as far as the first impermeable layer, where the
descending movement of the water is stopped. The DEEP ZONES OF
FLOW occupy any pervious rocks which may be found below the
impervious layer which lies nearest to the surface. The upper zone
is a vast sheet of water saturating the soil and rocks and slowly
seeping downward through their pores and interstices along the
slopes to the valleys, where in part it discharges in springs and
often unites also in a wide underflowing stream which supports and
feeds the river (Fig. 24).

A city in a region of copious rains, built on the narrow flood
plain of a river, overlooked by hills, depends for its water
supply on driven wells, within the city limits, sunk in the sand a
few yards from the edge of the stream. Are these wells fed by
water from the river percolating through the sand, or by ground
water on its way to the stream and possibly contaminated with the
sewage of the town?

At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to
go dry? It may be possible to get data from different wells and to
draw a diagram showing the ground-water surface as compared with
the surface of the ground.

FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in
pervious strata which are overlain by some impervious stratum.
Such layers are often carried by their dip to great depths, and
water may circulate in them to far below the level of the surface
streams and even of the sea. When a fissure crosses a water-
bearing stratum, or AQUIFIER, water is forced upward by the
pressure of the weight of the water contained in the higher parts
of the stratum, and may reach the surface as a fissure spring. A
boring which taps such an aquifer is known as an artesian well, a
name derived from a province in France where wells of this kind
have been long in use. The rise of the water in artesian wells,
and in fissure springs also, depends on the following conditions
illustrated in Figure 29. The aquifer dips toward the region of
the wells from higher ground, where it outcrops and receives its
water. It is inclosed between an impervious layer above and water-
tight or water-logged layers beneath. The weight of the column of
water thus inclosed in the aquifer causes water to rise in the
well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.

Which will supply the larger region with artesian wells, an
aquifer whose dip is steep or one whose dip is gentle? Which of
the two aquifers, their thickness being equal, will have the
larger outcrop and therefore be able to draw upon the larger
amount of water from the rainfall? Illustrate with diagrams.

THE ZONE OF SOLUTION. Near the surface, where the circulation of
ground water is most active, it oxidizes, corrodes, and dissolves
the rocks through which it passes. It leaches soils and subsoils
of their lime and other soluble minerals upon which plants depend
for their food. It takes away the soluble cements of rocks; it
widens fissures and joints and opens winding passages along the
bedding planes; it may even remove whole beds of soluble rocks,
such as rock salt, limestone, or gypsum. The work of ground water
in producing landslides has already been noticed. The zone in
which the work of ground water is thus for the most part
destructive we may call the zone of solution.

CAVES. In massive limestone rocks, ground water dissolves channels
which sometimes form large caves (Fig. 30). The necessary
conditions for the excavation of caves of great size are well
shown in central Kentucky, where an upland is built throughout of
thick horizontal beds of limestone. The absence of layers of
insoluble or impervious rock in its structure allows a free
circulation of ground water within it by the way of all natural
openings in the rock. These water ways have been gradually
enlarged by solution and wear until the upland is honeycombed with
caves. Five hundred open caverns are known in one county.

Mammoth Cave, the largest of these caverns, consists of a
labyrinth of chambers and winding galleries whose total length is
said to be as much as thirty miles. One passage four miles long
has an average width of about sixty feet and an average height of
forty feet. One of the great halls is three hundred feet in width
and is overhung by a solid arch of limestone one hundred feet
above the floor. Galleries at different levels are connected by
well-like pits, some of which measure two hundred and twenty-five
feet from top to bottom. Through some of the lowest of these
tunnels flows Echo River, still at work dissolving and wearing
away the rock while on its dark way to appear at the surface as a
great spring.

NATURAL BRIDGES. As a cavern enlarges and the surface of the land
above it is lowered by weathering, the roof at last breaks down
and the cave becomes an open ravine. A portion of the roof may for
a while remain, forming a "natural bridge."

SINK HOLES. In limestone regions channels under ground may become
so well developed that the water of rains rapidly drains away
through them. Ground water stands low and wells must be sunk deep
to find it. Little or no surface water is left to form brooks.

Thus across the limestone upland of central Kentucky one meets but
three surface streams in a hundred miles. Between their valleys
surface water finds its way underground by means of sink holes.
These are pits, commonly funnel shaped, formed by the enlargement
of crevice or joint by percolating water, or by the breakdown of
some portion of the roof of a cave. By clogging of the outlet a
sink hole may come to be filled by a pond.

Central Florida is a limestone region with its drainage largely
subterranean and in part below the level even of the sea. Sink
holes are common, and many of them are occupied by lakelets. Great
springs mark the point of issue of underground streams, while some
rise from beneath the sea. Silver Spring, one of the largest,
discharges from a basin eight hundred feet wide and thirty feet
deep a little river navigable for small steamers to its source.
About the spring there are no surface streams for sixty miles.

THE KARST. Along the eastern coast of the Adriatic, as far south
as Montenegro, lies a belt of limestone mountains singularly worn
and honeycombed by the solvent action of water. Where forests have
been cut from the mountain sides and the red soil has washed away,
the surface of the white limestone forms a pathless desert of rock
where each square rod has been corroded into an intricate branch
work of shallow furrows and sharp ridges. Great sink holes, some
of them six hundred feet deep and more, pockmark the surface of
the land. The drainage is chiefly subterranean. Surface streams
are rare and a portion of their courses is often under ground.
Fragmentary valleys come suddenly to an end at walls of rock where
the rivers which occupy the valleys plunge into dark tunnels to
reappear some miles away. Ground water stands so far below the
surface that it cannot be reached by wells, and the inhabitants
depend on rain water stored for household uses. The finest cavern
of Europe, the Adelsberg Grotto, is in this region. Karst, the
name of a part of this country, is now used to designate any
region or landscape thus sculptured by the chemical action of
surface and ground water. We must remember that Karst regions are
rare, and striking as is the work of their subterranean streams,
it is far less important than the work done by the sheets of
underground water slowly seeping through all subsoils and porous
rocks in other regions.

Even when gathered into definite channels, ground water does not
have the erosive power of surface streams, since it carries with
it little or no rock waste. Regions whose underground drainage is
so perfect that the development of surface streams has been
retarded or prevented escape to a large extent the leveling action
of surface running waters, and may therefore stand higher than the
surrounding country. The hill honeycombed by Luray Cavern,
Virginia, has been attributed to this cause.

CAVERN DEPOSITS. Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles
from the roof of caverns, the lime carbonate which it has taken
into solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called STALACTITES.
As the drops splash on the floor there are built up in the same
way thicker masses called STALAGMITES, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often
seals with rock the earth which accumulates in caverns, together
with whatever relics of cave dwellers, either animals or men, it
may contain.

Can you explain why slender stalactites formed by the drip of
single drops are often hollow pipes?

THE ZONE OF CEMENTATION. With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At
such depths it deposits these minerals in the pores of rocks,
cementing their grains together, and in crevices and fissures,
forming mineral veins. Thus below the zone of solution where the
work of water is to dissolve, lies the zone of cementation where
its work is chemical deposit. A part of the invisible load of
waste is thus transferred from rocks near the surface to those at
greater depths.

As the land surface is gradually lowered by weathering and the
work of rain and streams, rocks which have lain deep within the
zone of cementation are brought within the zone of solution. Thus
there are exposed to view limestones, whose cracks were filled
with calcite (crystallized carbonate of lime), with quartz or
other minerals, and sandstones whose grains were well cemented
many feet below the surface.

CAVITY FILLING. Small cavities in the rocks are often found more
or less completely filled with minerals deposited from solution by
water in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of
evaporating brine, but in the latter instance the solution is not
renewed as in the case of cavities in the rocks. A cavity thus
lined with inward-pointing crystals is called a GEODE.

CONCRETIONS. Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays
concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of REPLACEMENT. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock. Clays
and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or
shell, frequently forms the nucleus around which the concretion
grows.

Why are building stones more easily worked when "green" than after
their quarry water has dried out?

DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where
ground water is drawn by capillarity to the surface and there
evaporates, it leaves as surface incrustations the minerals held
in solution. White limy incrustations of this nature cover
considerable tracts in northern Mexico. Evaporating beneath the
surface, ground water may deposit a limy cement in beds of loose
sand and gravel. Such firmly cemented layers are not uncommon in
western Kansas and Nebraska, where they are known as "mortar
beds."

THERMAL SPRINGS. While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average
annual temperature of the region, and are then known as thermal
springs. In regions of present or recent volcanic activity, such
as the Yellowstone National Park, we may believe that the heat of
thermal springs is derived from uncooled lavas, perhaps not far
below the surface. But when hot springs occur at a distance of
hundreds of miles from any volcano, as in the case of the hot
springs of Bath, England, it is probable that their waters have
risen from the heated rocks of the earth's interior. The springs
of Bath have a temperature of 120 degrees F., 70 degrees above the
average annual temperature of the place. If we assume that the
rate of increase in the earth's internal heat is here the average
rate, 1 degree F. to every sixty feet of descent, we may conclude
that the springs of Bath rise from at least a depth of forty-two
hundred feet.

Water may descend to depths from which it can never be brought
back by hydrostatic pressure. It is absorbed by highly heated
rocks deep below the surface. From time to time some of this deep-
seated water may be returned to open air in the steam of volcanic
eruptions.

SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to
the surface highly charged with minerals in solution, on exposure
to the air it is commonly compelled to lay down much of its
invisible load in chemical deposits about the spring. These are
thrown down from solution either because of cooling, evaporation,
the loss of carbon dioxide, or the work of algae.

Many springs have been charged under pressure with carbon dioxide
from subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through
which they pass. On reaching the surface the pressure is relieved,
the gas escapes, and the lime carbonate is thrown down in deposits
called TRAVERTINE. The gas is sometimes withdrawn and the deposit
produced in large part by the action of algae and other humble
forms of plant life.

At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of
travertine are now building, chiefly by means of algae which cover
the bottoms, rims, and sides of the basins and deposit lime
carbonate upon them in successive sheets. The rock, snow-white
where dry, is coated with red and orange gelatinous mats where the
algae thrive in the over-flowing waters.

Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs
which formed these ancient deposits discharged near what was then
the bottom of the valley, and that as the valley has been deepened
by the river the ground water of the region has found lower and
lower points of issue.

In many parts of the country calcareous springs occur which coat
with lime carbonate mosses, twigs, and other objects over which
their waters flow. Such are popularly known as petrifying springs,
although they merely incrust the objects and do not convert them
into stone.

Silica is soluble in alkaline waters, especially when these are
hot. Hot springs rising through alkaline siliceous rocks, such as
lavas, often deposit silica in a white spongy formation known as
SILICEOUS SINTER, both by evaporation and by the action of algae
which secrete silica from the waters. It is in this way that the
cones and mounds of the geysers in the Yellowstone National Park
and in Iceland have been formed.

Where water oozes from the earth one may sometimes see a rusty
deposit on the ground, and perhaps an iridescent scum upon the
water. The scum is often mistaken for oil, but at a touch it
cracks and breaks, as oil would not do. It is a film of hydrated
iron oxide, or LIMONITE, and the spring is an iron, or chalybeate,
spring. Compounds of iron have been taken into solution by ground
water from soil and rocks, and are now changed to the insoluble
oxide on exposure to the oxygen of the air.

In wet ground iron compounds leached by ground water from the soil
often collect in reddish deposits a few feet below the surface,
where their downward progress is arrested by some impervious clay.
At the bottom of bogs and shallow lakes iron ores sometimes
accumulate to a depth of several feet.

Decaying organic matter plays a large part in these changes. In
its presence the insoluble iron oxides which give color to most
red and yellow rocks are decomposed, leaving the rocks of a gray
or bluish color, and the soluble iron compounds which result are
readily leached out,--effects seen where red or yellow clays have
been bleached about some decaying tree root.

The iron thus dissolved is laid down as limonite when oxidized, as
about a chalybeate spring; but out of contact with the air and in
the presence of carbon dioxide supplied by decaying vegetation, as
in a peat bog, it may be deposited as iron carbonate, or SIDERITE.

TOTAL AMOUNT OF UNDERGROUND WATERS. In order to realize the vast
work in solution and cementation which underground waters are now
doing and have done in all geological ages, we must gain some
conception of their amount. At a certain depth, estimated at about
six miles, the weight of the crust becomes greater than the rocks
can bear, and all cavities and pores in them must be completely
closed by the enormous pressure which they sustain. Below a depth
of even three or four miles it is believed that ground water
cannot circulate. Estimating the average pore spaces of the
different rocks of the earth's crust above this depth, and the
average per cents of their pore spaces occupied by water, it has
been recently computed that the total amount of ground water is
equal to a sheet of water one hundred feet deep, covering the
entire surface of the earth.





CHAPTER III

RIVERS AND VALLEYS


THE RUN-OFF. We have traced the history of that portion of the
rainfall which soaks into the ground; let us now return to that
part which washes along the surface and is known as the RUN-OFF.
Fed by rains and melting snows, the run-off gathers into courses,
perhaps but faintly marked at first, which join more definite and
deeply cut channels, as twigs their stems. In a humid climate the
larger ravines through which the run-off flows soon descend below
the ground-water surface. Here springs discharge along the sides
of the little valleys and permanent streams begin. The water
supplied by the run-off here joins that part of the rainfall which
had soaked into the soil, and both now proceed together by way of
the stream to the sea.

RIVER FLOODS. Streams vary greatly in volume during the year. At
stages of flood they fill their immediate banks, or overrun them
and inundate any low lands adjacent to the channel; at stages of
low water they diminish to but a fraction of their volume when at
flood.

At times of flood, rivers are fed chiefly by the run-off; at times
of low water, largely or even wholly by springs.

How, then, will the water of streams differ at these times in
turbidity and in the relative amount of solids carried in
solution?

In parts of England streams have been known to continue flowing
after eighteen months of local drought, so great is the volume of
water which in humid climates is stored in the rocks above the
drainage level, and so slowly is it given off in springs.

In Illinois and the states adjacent, rivers remain low in winter
and a "spring freshet" follows the melting of the winter's snows.
A "June rise" is produced by the heavy rains of early summer. Low
water follows in July and August, and streams are again swollen to
a moderate degree under the rains of autumn.

THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the
drainage area, the texture of the rocks, and other factors. With
an annual rainfall of fifty inches in an open country, about fifty
per cent is discharged; while with a rainfall of twenty inches
only fifteen per cent is discharged, part of the remainder being
evaporated and part passing underground beyond the drainage area.
Thus the Ohio discharges thirty per cent of the rainfall of its
basin, while the Missouri carries away but fifteen per cent. A
number of the streams of the semi-arid lands of the West do not
discharge more than five per cent of the rainfall.

Other things being equal, which will afford the larger proportion
of run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen
or unfrozen ground? Will there be a larger proportion of run-off
after long rains or after a season of drought? after long and
gentle rains, or after the same amount of precipitation in a
violent rain? during the months of growing vegetation, from June
to August, or during the autumn months?

DESERT STREAMS. In arid regions the ground-water surface lies so
low that for the most part stream ways do not intersect it.
Streams therefore are not fed by springs, but instead lose volume
as their waters soak into the thirsty rocks over which they flow.
They contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they
wither at times of drought to a mere trickle of water, to a chain
of pools, or go wholly dry, while at long intervals rains fill
their dusty beds with sudden raging torrents. Desert rivers
therefore periodically shorten and lengthen their courses,
withering back at times of drought for scores of miles, or even
for a hundred miles from the point reached by their waters during
seasons of rain.

THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams TRANSPORT
the waste of the land; they wear, or ERODE, their channels both on
bed and banks; and they DEPOSIT portions of their load from time
to time along their courses, finally laying it down in the sea.
Most of the work of streams is done at times of flood.

TRANSPORTATION

THE INVISIBLE LOAD OF STREAMS. Of the waste which a river
transports we may consider first the invisible load which it
carries in solution, supplied chiefly by springs but also in part
by the run-off and from the solution of the rocks of its bed. More
than half the dissolved solids in the water of the average river
consists of the carbonates of lime and magnesia; other substances
are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate
(Epsom salts), sodium chloride (common salt), and even silica, the
least soluble of the common rock-making minerals. The amount of
this invisible load is surprisingly large. The Mississippi, for
example, transports each year 113,000,000 tons of dissolved rock
to the Gulf.

THE VISIBLE LOAD OF STREAMS. This consists of the silt which the
stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood
one may note the muddy water, its silt being kept from settling by
the rolling, eddying currents; and often by placing his ear close
to the bottom of a boat one may hear the clatter of pebbles as
they are hurried along. In mountain torrents the rumble of
bowlders as they clash together may be heard some distance away.
The amount of the load which a stream can transport depends on its
velocity. A current of two thirds of a mile per hour can move fine
sand, while one of four miles per hour sweeps along pebbles as
large as hen's eggs. The transporting power of a stream varies as
the sixth power of its velocity. If its velocity is multiplied by
two, its transporting power is multiplied by the sixth power of
two: it can now move stones sixty-four times as large as it could
before.

Stones weigh from two to three times as much as water, and in
water lose the weight of the volume of water which they displace.
What proportion, then, of their weight in air do stones lose when
submerged?

MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream
by its velocity per second, giving the discharge per second in
cubic feet. The amount of silt to a cubic foot of water is found
by filtering samples of the water taken from different parts of
the stream and at different times in the year, and drying and
weighing the residues. The average amount of silt to the cubic
foot of water, multiplied by the number of cubic feet of water
discharged per year, gives the total load carried in suspension
during that time. Adding to this the estimated amount of sand and
gravel rolled along the bed, which in many swift rivers greatly
exceeds the lighter material held in suspension, and adding also
the total amount of dissolved solids, we reach the exceedingly
important result of the total load of waste discharged by the
river. Dividing the volume of this load by the area of the river
basin gives another result of the greatest geological interest,--
the rate at which the region is being lowered by the combined
action of weathering and erosion, or the rate of denudation.

THE RATE OF DENUDATION OF RIVER BASINS. This rate varies widely.
The Mississippi basin may be taken as a representative land
surface because of the varieties of surface, altitude and slope,
climate, and underlying rocks which are included in its great
extent. Careful measurements show that the Mississippi basin is
now being lowered at a rate of one four-thousandth of a foot a
year, or one foot in four thousand years. Taking this as the
average rate of denudation for the land surfaces of the globe,
estimates have been made of the length of time required at this
rate to wash and wear the continents to the level of the sea. As
the average elevation of the lands of the globe is reckoned at
2411 feet, this result would occur in nine or ten million years,
if the present rate of denudation should remain unchanged. But
even if no movements of the earth's crust should lift or depress
the continents, the rate of wear and the removal of waste from
their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their
slopes become more gentle. The length of time required to wear
them away is therefore far in excess of that just stated.

The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to
the depth of four feet. At what rate is the Potomac basin being
lowered from this cause alone?

It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years.
Why so much faster than the Potomac and the Mississippi?

HOW STREAMS GET THEIR LOADS. The load of streams is derived from a
number of sources, the larger part being supplied by the
weathering of valley slopes. We have noticed how the mantle of
waste creeps and washes to the stream ways. Watching the run-off
during a rain, as it hurries muddy with waste along the gutter or
washes down the hillside, we may see the beginning of the route by
which the larger part of their load is delivered to rivers.
Streams also secure some of their load by wearing it from their
beds and banks,--a process called erosion.

EROSION

Streams erode their beds chiefly by means of their bottom load,--
the stones of various sizes and the sand and even the fine mud
which they sweep along. With these tools they smooth, grind, and
rasp the rock of their beds, using them in much the fashion of
sandpaper or a file.

WEATHERING OF RIVER BEDS. The erosion of stream beds is greatly
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and
cold, joints are opened, rocks are pried loose and broken up and
made ready to be swept away by the stream at time of flood.

POTHOLES. In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which
gather in it are whirled round and round, and, acting like the bit
of an auger, bore out a cylindrical pit called a pothole. Potholes
sometimes reach a depth of a score of feet. Where they are
numerous they aid materially in deepening the channel, as the
walls between them are worn away and they coalesce.

WATERFALLS. One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges
stones from the face of the ledge over which it pours, and often
undermines it by excavating a deep pit at its base. Slice after
slice is thus thrown down from the front of the cliff, and the
cataract cuts its way upstream leaving a gorge behind it.

NIAGARA FALLS. The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of
the region. Some thirteen miles from the outlet it plunges over a
ledge one hundred and seventy feet high into the head of a narrow
gorge which extends for seven miles to the escarpment of the
upland in which the gorge is cut. The strata which compose the
upland dip gently upstream and consist at top of a massive
limestone, at the Falls about eighty feet thick, and below of soft
and easily weathered shale. Beneath the Falls the underlying shale
is cut and washed away by the descending water and retreats also
because of weathering, while the overhanging limestone breaks down
in huge blocks from time to time.

Niagara is divided by Goat Island into the Horseshoe Falls and the
American Falls. The former is supplied by the main current of the
river, and from the semicircular sweep of its rim a sheet of water
in places at least fifteen or twenty feet deep plunges into a pool
a little less than two hundred feet in depth. Here the force of
the falling water is sufficient to move about the fallen blocks of
limestone and use them in the excavation of the shale of the bed.
At the American Falls the lesser branch of the river, which flows
along the American side of Goat Island, pours over the side of the
gorge and breaks upon a high talus of limestone blocks which its
smaller volume of water is unable to grind to pieces and remove.

A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while
the American Falls retreated at the rate of 0.64 feet in the same
period. We cannot doubt that the same agency which is now
lengthening the gorge at this rapid rate has cut it back its
entire length of seven miles.

While Niagara Falls have been cutting back a gorge seven miles
long and from two hundred to three hundred feet deep, the river
above the Falls has eroded its bed scarcely below the level of the
upland on which it flows. Like all streams which are the outlets
of lakes, the Niagara flows out of Lake Erie clear of sediment, as
from a settling basin, and carries no tools with which to abrade
its bed. We may infer from this instance how slight is the erosive
power of clear water on hard rock.

Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat
Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST,
how long is it since the Niagara River fell over the edge of the
escarpment where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39)
shows alternating deeps and shallows which cannot be accounted
for, except in a single instance, by the relative hardness of the
rocks of the river bed. The deeps do not exceed that at the foot
of the Horseshoe Falls at the present time. When the gorge was
being cut along the shallows, how did the Falls compare in
excavating power, in force, and volume with the Niagara of to-day?
How did the rate of recession at those times compare with the
present rate? Is the assumption made above that the rate of
recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-
five feet. From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will
be the fate of the Falls left behind when the other has passed
beyond the head of the island?

The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-
jointed rocks? on horizontal strata or on strata steeply inclined?

DEPOSITION

While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to
many delays en route. Now and again it is laid aside, to be picked
up later and carried some distance farther on its way. One of the
most striking features of the river therefore is the waste
accumulated along its course, in bars and islands in the channel,
beneath its bed, and in flood plains along its banks. All this
alluvium, to use a general term for river deposits, with which the
valley is cumbered is really en route to the sea; it is only
temporarily laid aside to resume its journey later on. Constantly
the river is destroying and rebuilding its alluvial deposits, here
cutting and there depositing along its banks, here eroding and
there building a bar, here excavating its bed and there filling it
up, and at all times carrying the material picked up at one point
some distance on downstream before depositing it at another.

These deposits are laid down by slackening currents where the
velocity of the stream is checked, as on the inner side of curves,
and where the slope of the bed is diminished, and in the lee of
islands, bridge piers and projecting points of land. How slight is
the check required to cause a current to drop a large part of its
load may be inferred from the law of the relation of the
transporting power to the velocity. If the velocity is decreased
one half, the current can move fragments but one sixty-fourth the
size of those which it could move before, and must drop all those
of larger size.

Will a river deposit more at low water or at flood? when rising or
when falling?

STRATIFICATION. River deposits are stratified, as may be seen in
any fresh cut in banks or bars. The waste of which they are built
has been sorted and deposited in layers, one above another; some
of finer and some of coarser material. The sorting action of
running water depends on the fact that its transporting power
varies with the velocity. A current whose diminishing velocity
compels it to drop coarse gravel, for example, is still able to
move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower
currents may deposit on the gravel bed layers of sand, and, still
later, slack water may leave on these a layer of mud. In case of
materials lighter than water the transporting power does not
depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.

CROSS BEDDING. A section of a bar exposed at low water may show
that it is formed of layers of sand, or coarser stuff, inclined
downstream as steeply often as the angle of repose of the
material. From a boat anchored over the lower end of a submerged
sand bar we may observe the way in which this structure, called
cross bedding, is produced. Sand is continually pushed over the
edge of the bar at b (Fig. 42) and comes to rest in successive
layers on the sloping surface. At the same time the bar may be
worn away at the upper end, a, and thus slowly advance down
stream. While the deposit is thus cross bedded, it constitutes as
a whole a stratum whose upper and lower surfaces are about
horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.

WATER WEAR. The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are WATER
WORN, or rounded, except when near their source. Rolling along the
bottom they have been worn round by impact and friction as they
rubbed against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose
nearly half their weight and become well rounded after traveling
fifteen miles in rotating cylinders partly filled with water.
Marbles are cheaply made in Germany out of small limestone cubes
set revolving in a current of water between a rotating bed of
stone and a block of oak, the process requiring but about fifteen
minutes. It has been found that in the upper reaches of mountain
streams a descent of less than a mile is sufficient to round
pebbles of granite.

LAND FORMS DUE TO RIVER EROSION

RIVER VALLEYS. In their courses to the sea, rivers follow valleys
of various forms, some shallow and some deep, some narrow and some
wide. Since rivers are known to erode their beds and banks, it is
a fair presumption that, aided by the weather, they have excavated
the valleys in which they flow.

Moreover, a bird's-eye view or a map of a region shows the
significant fact that the valleys of a system unite with one
another in a branch work, as twigs meet their stems and the
branches of a tree its trunk. Each valley, from that of the
smallest rivulet to that of the master stream, is proportionate to
the size of the stream which occupies it. With a few explainable
exceptions the valleys of tributaries join that of the trunk
stream at a level; there is no sudden descent or break in the bed
at the point of juncture. These are the natural consequences which
must follow if the land has long been worked upon by streams, and
no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been
formed by the river systems which drain them, aided by the work of
the weather; they are not gaping fissures in the earth's crust, as
early observers imagined, but are the furrows which running water
has drawn upon the land.

As valleys are made by the slow wear of streams and the action of
the weather, they pass in their development through successive
stages, each of which has its own characteristic features. We may
therefore classify rivers and valleys according to the stage which
they have reached in their life history from infancy to old age.

YOUNG RIVER VALLEYS

INFANCY. The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba
was so recently covered by the waters of an extinct lake, known as
Lake Agassiz, that the surface remains much as it was left when
the lake was drained away. The flat floor, spread smooth with
lake-laid silts, is still a plain, to the eye as level as the sea.
Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not
exceeding sixty feet in depth, their gradients differing little
from the general slopes of the region. The trunk streams have but
few tributaries; the river system, like a sapling with few limbs,
is still undeveloped. Along the banks of the trunk streams short
gullies are slowly lengthening headwards, like growing twigs which
are sometime to become large branches.

The flat interstream areas are as yet but little scored by
drainage lines, and in wet weather water lingers in ponds in any
initial depressions on the plain.

CONTOURS. In order to read the topographic maps of the text-book
and the laboratory the student should know that contours are lines
drawn on maps to represent relief, all points on any given contour
being of equal height above sea level. The CONTOUR INTERVAL is the
uniform vertical distance between two adjacent contours and varies
on different maps.

To express regions of faint relief a contour interval of ten or
twenty feet is commonly selected; while in mountainous regions a
contour interval of two hundred and fifty, five hundred, or even
one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or
on a spread of glacial drift left by the retreat of continental
ice sheets, such as covers much of Canada and the northeastern
parts of the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground
water stands high, and, exuding in the undrained initial
depressions, forms marshes and lakes.

LAKES. Lakes are perhaps the most obvious of these fleeting
features of infancy. They are short-lived, for their destruction
is soon accomplished by several means. As a river system advances
toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the
undrained depressions of the region. Lakes having outlets are
drained away as their basin rims are cut down by the outflowing
streams,--a slow process where the rim is of hard rock, but a
rapid one where it is of soft material such as glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the
shore and strew the debris worn from it over the lake bed. Shallow
lakes are often filled with organic matter from decaying
vegetation.

Does the outflowing stream, from a lake carry sediment? How does
this fact affect its erosive power on hard rock? on loose
material?

Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters
of the lake for a length of twenty miles with the waste brought
down from the high Alps. For this distance there extends up the
Rhone Valley an alluvial plain, which has grown lakeward at the
rate of a mile and a half since Roman times, as proved by the
distance inland at which a Roman port now stands.

How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the
great dam across the Colorado River at Austin, Texas, sediments
thirty-nine feet deep gathered in seven years.

Lake Mendota, one of the many beautiful lakes of southern
Wisconsin, is rapidly cutting back the soft glacial drift of its
shores by means of the abrasion of its waves. While the shallow
basin is thus broadened, it is also being filled with the waste;
and the time is brought nearer when it will be so shoaled that
vegetation can complete the work of its effacement.

Along the margin of a shallow lake mosses, water lilies, grasses,
and other water-loving plants grow luxuriantly. As their decaying
remains accumulate on the bottom, the ring of marsh broadens
inwards, the lake narrows gradually to a small pond set in the
midst of a wide bog, and finally disappears. All stages in this
process of extinction may be seen among the countless lakelets
which occupy sags in the recent sheets of glacial drift in the
northern states; and more numerous than the lakes which still
remain are those already thus filled with carbonaceous matter
derived from the carbon dioxide of the atmosphere. Such fossil
lakes are marked by swamps or level meadows underlain with muck.

THE ADVANCE TO MATURITY. The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins
of its area, are gradually effaced. By the furrowing action of the
rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire
area, and not an acre is left on which the fallen raindrop does
not find already cut for it an uninterrupted downward path which
leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is
now wholly destroyed; the region is all reduced to valley slopes.

THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds
with the initial surface of the region on which the stream begins
to flow, although its way may lead through basins and down steep
descents. The successive profiles to which it reduces its bed are
illustrated in Figure 51. As the gradient, or rate of descent of
its bed, is lowered, the velocity of the river is decreased until
its lessening energy is wholly consumed in carrying its load and
it can no longer erode its bed. The river is now AT GRADE, and its
capacity is just equal to its load. If now its load is increased
the stream deposits, and thus builds up, or AGGRADES, its bed. On
the other hand, if its load is diminished it has energy to spare,
and resuming its work of erosion, DEGRADES its bed. In either case
the stream continues aggrading or degrading until a new gradient
is found where the velocity is just sufficient to move the load,
and here again it reaches grade.

V-VALLEYS. Vigorous rivers well armed with waste make short work
of cutting their beds to grade, and thus erode narrow, steep-sided
gorges only wide enough at the base to accommodate the stream. The
steepness of the valley slopes depends on the relative rates at
which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream
may saw out a gorge whose sides are nearly or even quite vertical,
but as a rule young valleys whose streams have not yet reached
grade are V-shaped; their sides flare at the top because here the
rocks have longest been opened up to the action of the weather.
Some of the deepest canyons may be found where a rising land mass,
either mountain range or plateau, has long maintained by its
continued uplift the rivers of the region above grade.

In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you
suggest a reason?

THE GRAND CANYON OF THE COLORADO RIVER IN ARIZONA. The Colorado
River trenches the high plateau of northern Arizona with a
colossal canyon two hundred and eighteen miles long and more than
a mile in greatest depth. The rocks in which the canyon is cut are
for the most part flat-lying, massive beds of limestones and
sandstones, with some shales, beneath which in places harder
crystalline rocks are disclosed. Where the canyon is deepest its
walls have been profoundly dissected. Lateral ravines have widened
into immense amphitheaters, leaving between them long ridges of
mountain height, buttressed and rebuttressed with flanking spurs
and carved into majestic architectural forms. From the extremity
of one of these promontories it is two miles or more across the
gulf to the point of the one opposite, and the heads of the
amphitheaters are thirteen miles apart.

The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the
river, a swift, powerful stream a few hundred feet wide, turbid
with reddish silt, by means of which it continually rasps its
rocky bed as it hurries on. The Colorado is still deepening its
gorge. In the Grand Canyon its gradient is seven and one half feet
to the mile, but, as in all ungraded rivers, the descent is far
from uniform. Graded reaches in soft rock alternate with steeper
declivities in hard rock, forming rapids such as, for example, a
stretch of ten miles where the fall averages twenty-one feet to
the mile. Because of these dangerous rapids the few exploring
parties who have traversed the Colorado canyon have done so at the
hazard of their lives.

The canyon has been shaped by several agencies. Its depth is due
to the river which has sawed its way far toward the base of a
lofty rising plateau. Acting alone this would have produced a
slitlike gorge little wider than the breadth of the stream. The
impressive width of the canyon and the magnificent architectural
masses which fill it are owing to two causes.: Running water has
gulched the walls and weathering has everywhere attacked and
driven them back. The horizontal harder beds stand out in long
lines of vertical cliffs, often hundreds of feet in height, at
whose feet talus slopes conceal the outcrop of the weaker strata.
As the upper cliffs have been sapped and driven back by the
weather, broad platforms are left at their bases and the sides of
the canyon descend to the river by gigantic steps. Far up and down
the canyon the eye traces these horizontal layers, like the
flutings of an elaborate molding, distinguishing each by its
contour as well as by its color and thickness.

The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no
more than well begun. It has not yet wholly reached grade, and the
great task of the river and its tributaries--the task of leveling
the lofty plateau to a low plain and of transporting it grain by
grain to the sea--still lies almost entirely in the future.

WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to
the initial surface over which it began its course; still more
commonly are they developed in the rock mass through which it is
cutting its valley. Thus, wherever a stream leaves harder rocks to
flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or
rapid, results.

At time of flood young tributaries with steeper courses than that
of the trunk stream may bring down stones and finer waste, which
the gentler current cannot move along, and throw them as a dam
across its way. The rapids thus formed are also ephemeral, for as
the gradient of the tributaries is lowered the main stream becomes
able to handle the smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in
the erosion of their beds because of their smaller volume, and
thus join it by plunging over the side of its gorge. But as the
river approaches grade and slackens its down cutting, the
tributaries sooner or later overtake it, and effacing their falls,
unite with it on a level.

Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have
been obliterated already.

LOCAL BASELEVELS. That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks,
ungraded outcrops of hard rocks remaining as barriers which give
rise to rapids or falls. Until these barriers are worn away they
constitute local baselevels, below which level the stream, up
valley from them, cannot cut. They are eroded to grade one after
another, beginning with the least strong, or the one nearest the
mouth of the stream. In a similar way the surface of a lake in a
river's course constitutes for all inflowing streams a local
baselevel, which disappears when the basin is filled or drained.

MATURE AND OLD RIVERS

Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great
task of wearing down the land mass which it drains and carrying it
particle by particle to the sea. The relief of the land is now at
its greatest; for the main channels have been sunk to grade, while
the divides remain but little worn below their initial altitudes.
Ground water now stands low. The run-off washes directly to the
streams, with the least delay and loss by evaporation in ponds and
marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley
slopes is now largest and sheds to the streams a heavier load of
waste than ever before. At maturity the river system is doing its
greatest amount of work both in erosion and in the carriage of
water and of waste to the sea.

LATERAL EROSION. On reaching grade a river ceases to scour its
bed, and it does not again begin to do so until some change in
load or volume enables it to find grade at a lower level. On the
other hand, a stream erodes its banks at all stages in its
history, and with graded rivers this process, called lateral
erosion, or PLANATION, is specially important. The current of a
stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and
continually wears and saps its banks. On the inner side deposition
takes place in the more shallow and slower-moving water. The inner
bank of bends is thus built out while the outer bank is worn away.
By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is
thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle
slopes. While widening their valleys streams maintain a constant
width of channel, so that a wide-floored valley does not signify
that it ever was occupied by a river of equal width.

THE GRADIENT. The gradients of graded rivers differ widely. A
large river with a light load reaches grade on a faint slope,
while a smaller stream heavily burdened with waste requires a
steep slope to give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the
Great Plains and surcharged with the waste brought down both by
its branches in the mountains and by those whose tracks lie over
the soft rocks of the plains. It is compelled to maintain a
gradient of eight feet to the mile in western Nebraska. The Ohio
reaches grade with a slope of less than four inches to the mile
from Cincinnati to its mouth, and the powerful Mississippi washes
along its load with a fall of but three inches per mile from Cairo
to the Gulf.

Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream
supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the
load should be increased in amount or in coarseness?

THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed
over the gentler slopes of the lowering hills. With smaller loads
to carry, the rivers now deepen their valleys and find grade with
fainter declivities nearer the level of the sea. This limit of the
level of the sea beneath which they cannot erode is known as
baselevel. [Footnote: The term "baselevel" is also used to
designate the close approximation to sea level to which streams
are able to subdue the land.] As streams grow old they approach
more and more closely to baselevel, although they are never able
to attain it. Some slight slope is needed that water may flow and
waste be transported over the land. Meanwhile the relief of the
land has ever lessened. The master streams and their main
tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion
of their innumerable branches and the wear of the weather the
divides everywhere are lowered and subdued to more and more gentle
slopes. Mountains and high plateaus are thus reduced to rolling
hills, and at last to plains, surmounted only by such hills as may
still be unreduced to the common level, because of the harder
rocks of which they are composed or because of their distance from
the main erosion channels. Such regions of faint relief, worn down
to near base level by subaerial agencies, are known as PENEPLAINS
(almost plains). Any residual masses which rise above them are
called MONADNOCKS, from the name of a conical peak of New
Hampshire which overlooks the now uplifted peneplain of southern
New England.

In its old age a region becomes mantled with thick sheets of fine
and weathered waste, slowly moving over the faint slopes toward
the water ways and unbroken by ledges of bare rock. In other
words, the waste mantle also is now graded, and as waterfalls have
been effaced in the river beds, so now any ledges in the wide
streams of waste are worn away and covered beneath smooth slopes
of fine soil. Ground water stands high and may exude in areas of
swamp. In youth the land mass was roughhewn and cut deep by stream
erosion. In old age the faint reliefs of the land dissolve away,
chiefly under the action of the weather, beneath their cloak of
waste.

THE CYCLE OF EROSION. The successive stages through which a land
mass passes while it is being leveled to the sea constitute
together a cycle of erosion. Each stage of the cycle from infancy
to old age leaves, as we have seen, its characteristic records in
the forms sculptured on the land, such as the shapes of valleys
and the contours of hills and plains. The geologist is thus able
to determine by the land forms of any region the stage in the
erosion cycle to which it now belongs, and knowing what are the
earlier stages of the cycle, to read something of the geological
history of the region.

INTERRUPTED CYCLES. So long a time is needed to reduce a land mass
to baselevel that the process is seldom if ever completed during a
single uninterrupted cycle of erosion. Of all the various
interruptions which may occur the most important are gradual
movements of the earth's crust, by which a region is either
depressed or elevated relative to sea level.

The DEPRESSION of a region hastens its old age by decreasing the
gradient of streams, by destroying their power to excavate their
beds and carry their loads to a degree corresponding to the amount
of the depression, and by lessening the amount of work they have
to do. The slackened river currents deposit their waste in Hood
plains which increase in height as the subsidence continues. The
lower courses of the rivers are invaded by the sea and become
estuaries, while the lower tributaries are cut off from the trunk
stream.

ELEVATION, on the other hand, increases the activity of all
agencies of weathering, erosion, and transportation, restores the
region to its youth, and inaugurates a new cycle of erosion.
Streams are given a steeper gradient, greater velocity, and
increased energy to carry their loads and wear their beds. They
cut through the alluvium of their flood plains, leaving it on
either bank as successive terraces, and intrench themselves in the
underlying rock. In their older and wider valleys they cut narrow,
steep-walled inner gorges, in which they flow swiftly over rocky
floors, broken here and there by falls and rapids where a harder
layer of rock has been discovered. Winding streams on plains may
thus incise their meanders in solid rock as the plains are
gradually uplifted. Streams which are thus restored to their youth
are said to be REVIVED.

As streams cut deeper and the valley slopes are steepened, the
mantle of waste of the region undergoing elevation is set in more
rapid movement. It is now removed particle by particle faster than
it forms. As the waste mantle thins, weathering attacks the rocks
of the region more energetically until an equilibrium is reached
again; the rocks waste rapidly and their waste is as rapidly
removed.

DISSECTED PENEPLAINS. When a rise of the land brings one cycle to
an end and begins another, the characteristic land forms of each
cycle are found together and the topography of the region is
composite until the second cycle is so far advanced that the land
forms of the first cycle are entirely destroyed. The contrast
between the land surfaces of the later and the earlier cycles is
most striking when the earlier had advanced to age and the later
is still in youth. Thus many peneplains which have been elevated
and dissected have been recognized by the remnants of their
ancient erosion surfaces, and the length of time which has elapsed
since their uplift has been measured by the stage to which the new
cycle has advanced.

THE PIEDMONT BELT. As an example of an ancient peneplain uplifted
and dissected we may cite the Piedmont Belt, a broad upland lying
between the Appalachian Mountains and the Atlantic coastal plain.
The surface of the Piedmont is gently rolling. The divides, which
are often smooth areas of considerable width, rise to a common
plane, and from them one sees in every direction an even sky line
except where in places some lone hill or ridge may lift itself
above the general level (Fig. 62). The surface is an ancient one,
for the mantle of residual waste lies deep upon it, soils are
reddened by long oxidation, and the rocks are rotted to a depth of
scores of feet.

At present, however, the waste mantle is not forming so rapidly as
it is being removed. The streams of the upland are actively
engaged in its destruction. They flow swiftly in narrow, rock-
walled valleys over rocky beds. This contrast between the young
streams and the aged surface which they are now so vigorously
dissecting can only be explained by the theory that the region
once stood lower than at present and has recently been upraised.
If now we imagine the valleys refilled with the waste which the
streams have swept away, and the upland lowered, we restore the
Piedmont region to the condition in which it stood before its
uplift and dissection,--a gently rolling plain, surmounted here
and there by isolated hills and ridges.

The surface of the ancient Piedmont plain, as it may be restored
from the remnants of it found on the divides, is not in accordance
with the structures of the country rocks. Where these are exposed
to view they are seen to be far from horizontal. On the walls of
river gorges they dip steeply and in various directions and the
streams flow over their upturned edges. As shown in Figure 67, the
rocks of the Piedmont have been folded and broken and tilted.

It is not reasonable to believe that when the rocks of the
Piedmont were thus folded and otherwise deformed the surface of
the region was a plain. The upturned layers have not always
stopped abruptly at the even surface of the Piedmont plain which
now cuts across them. They are the bases of great folds and tilted
blocks which must once have risen high in air. The complex and
disorderly structures of the Piedmont rocks are those seen in
great mountain ranges, and there is every reason to believe that
these rocks after their deformation rose to mountain height.

The ancient Piedmont plain cuts across these upturned rocks as
independently of their structure as the even surface of the sawed
stump of some great tree is independent of the direction of its
fibers. Hence the Piedmont plain as it was before its uplift was
not a coastal plain formed of strata spread in horizontal sheets
beneath the sea and then uplifted; nor was it a structural plain,
due to the resistance to erosion of some hard, flat-lying layer of
rock. Even surfaces developed on rocks of discordant structure,
such as the Piedmont shows, are produced by long denudation, and
we may consider the Piedmont as a peneplain formed by the wearing
down of mountain ranges, and recently uplifted.

THE LAURENTIAN PENEPLAIN. This is the name given to a denuded
surface on very ancient rocks which extends from the Arctic Ocean
to the St. Lawrence River and Lake Superior, with small areas also
in northern Wisconsin and New York. Throughout this U-shaped area,
which incloses Hudson Bay within its arms, the country rocks have
the complicated and contorted structures which characterize
mountain ranges. But the surface of the area is by no means
mountainous. The sky line when viewed from the divides is unbroken
by mountain peaks or rugged hills. The surface of the arm west of
Hudson Bay is gently undulating and that of the eastern arm has
been roughened to low-rolling hills and dissected in places by
such deep river gorges as those of the Ottawa and Saguenay. This
immense area may be regarded as an ancient peneplain truncating
the bases of long-vanished mountains and dissected after
elevation.

In the examples cited the uplift has been a broad one and to
comparatively little height. Where peneplains have been uplifted
to great height and have since been well dissected, and where they
have been upfolded and broken and uptilted, their recognition
becomes more difficult. Yet recent observers have found evidences
of ancient lowland surfaces of erosion on the summits of the
Allegheny ridges, the Cascade Mountains (Fig. 69), and the western
slope of the Sierra Nevadas.

THE SOUTHERN APPALACHIAN REGION. We have here an example of an
area the latter part of whose geological history may be deciphered
by means of its land forms. The generalized section of Figure 70,
which passes from west to east across a portion of the region in
eastern Tennessee, shows on the west a part of the broad
Cumberland plateau. On the east is a roughened upland platform,
from which rise in the distance the peaks of the Great Smoky
Mountains. The plateau, consisting of strata but little changed
from their original flat-lying attitude, and the platform,
developed on rocks of disordered structure made crystalline by
heat and pressure, both stand at the common level of the line AB.
They are separated by the Appalachian valley, forty miles wide,
cut in strata which have been folded and broken into long narrow
blocks. The valley is traversed lengthwise by long, low ridges,
the outcropping edges of the harder strata, which rise to about
the same level,--that of the line cd. Between these ridges stretch
valley lowlands at the level ef excavated in the weaker rocks,
while somewhat below them lie the channels of the present streams
now busily engaged in deepening their beds.

THE VALLEY LOWLANDS. Were they planed by graded or ungraded
streams? Have the present streams reached grade? Why did the
streams cease widening the floors of the valley lowlands? How long
since? When will they begin anew the work of lateral planation?
What effect will this have on the ridges if the present cycle of
erosion continues long uninterrupted?

THE RIDGES OF THE APPALACHIAN VALLEY. Why do they stand above the
valley lowlands? Why do their summits lie in about the same plane?
Refilling the valleys intervening between these ridges with the
material removed by the streams, what is the nature of the surface
thus restored? Does this surface cd accord with the rock
structures on which' it has been developed? How may it have been
made? At what height did the land stand then, compared with its
present height? What elevations stood above the surface cd? Why?
What name may you use to designate them? How does the length of
time needed to develop the surface cd compare with that needed to
develop the valley lowlands?

THE PLATFORM AND PLATEAU. Why do they stand at a common level ab?
Of what surface may they be remnants? Is it accordant with the
rock structure? How was it produced? What unconsumed masses
overlooked it? Did the rocks of the Appalachian valley stand above
this surface when it was produced? Did they then stand below it?
Compare the time needed to develop this surface with that needed
to develop cd. Which surface is the older?

How many cycles of erosion are represented here? Give the erosion
history of the region by cycles, beginning with the oldest, the
work done in each and the work left undone, what brought each
cycle to a close, and how long relatively it continued.





CHAPTER IV

RIVER DEPOSITS


The characteristic features of river deposits and the forms which
they assume may be treated under three heads: (1) valley deposits,
(2) basin deposits, and (3) deltas.

VALLEY DEPOSITS

FLOOD PLAINS are the surfaces of the alluvial deposits which
streams build along their courses at times of flood. A swift
current then sweeps along the channel, while a shallow sheet of
water moves slowly over the flood plain, spreading upon it a thin
layer of sediment. It has been estimated that each inundation of
the Nile leaves a layer of fertilizing silt three hundredths of an
inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the
flat rock floor of a valley which is being widened by the lateral
erosion of a graded stream (Fig. 60). Flood-plain deposits of
great thickness may be built by aggrading rivers even in valleys
whose rock floors have never been thus widened.

A cross section of a flood plain shows that it is highest next the
river, sloping gradually thence to the valley sides. These wide
natural embankments are due to the fact that the river deposit is
heavier near the bank, where the velocity of the silt-laden
channel current is first checked by contact with the slower-moving
overflow.

Thus banked off from the stream, the outer portions of a flood
plain are often ill-drained and swampy, and here vegetal deposits,
such as peat, may be interbedded with river silts.

A map of a wide flood plain, such as that of the Mississippi or
the Missouri (Fig. 77), shows that the courses of the tributaries
on entering it are deflected downstream. Why?

The aggrading streams by which flood plains are constructed
gradually build their immediate banks and beds to higher and
higher levels, and therefore find it easy at times of great floods
to break their natural embankments and take new courses over the
plain. In this way they aggrade each portion of it in turn by
means of their shifting channels,

BRAIDED CHANNELS. A river actively engaged in aggrading its valley
with coarse waste builds a flood plain of comparatively steep
gradient and often flows down it in a fairly direct course and
through a network of braided channels. From time to time a channel
becomes choked with waste, and the water no longer finding room in
it breaks out and cuts and builds itself a new way which reunites
down valley with the other channels. Thus there becomes
established a network of ever-changing channels inclosing low
islands of sand and gravel.

TERRACES. While aggrading streams thus tend to shift their
channels, degrading streams, on the contrary, become more and more
deeply intrenched in their valleys. It often occurs that a stream,
after having built a flood plain, ceases to aggrade its bed
because of a lessened load or for other reasons, such as an uplift
of the region, and begins instead to degrade it. It leaves the
original flood plain out of reach of even the highest floods. When
again it reaches grade at a lower level it produces a new flood
plain by lateral erosion in the older deposits, remnants of which
stand as terraces on one or both sides of the valley. In this way
a valley may be lined with a succession of terraces at different
levels, each level representing an abandoned flood plain.

MEANDERS. Valleys aggraded with fine waste form well-nigh level
plains over which streams wind from side to side of a direct
course in symmetric bends known as meanders, from the name of a
winding river of Asia Minor. The giant Mississippi has developed
meanders with a radius of one and one half miles, but a little
creek may display on its meadow as perfect curves only a rod or so
in radius. On the flood plain of either river or creek we may find
examples of the successive stages in the development of the
meander, from its beginning in the slight initial bend sufficient
to deflect the current against the outer side. Eroding here and
depositing on the inner side of the bend, it gradually reaches
first the open bend whose width and length are not far from equal,
and later that of the horseshoe meander whose diameter transverse
to the course of the stream is much greater than that parallel
with it. Little by little the neck of land projecting into the
bend is narrowed, until at last it is cut through and a "cut-off"
is established. The old channel is now silted up at both ends and
becomes a crescentic lagoon, or oxbow lake, which fills gradually
to an arc-shaped shallow depression.

FLOOD PLAINS CHARACTERISTIC OF MATURE RIVERS. On reaching grade a
stream planes a flat floor for its continually widening valley.
Ever cutting on the outer bank of its curves, it deposits on the
inner bank scroll-like flood-plain patches. For a while the valley
bluffs do not give its growing meanders room to develop to their
normal size, but as planation goes on, the bluffs are driven back
to the full width of the meander belt and still later to a width
which gives room for broad stretches of flood plain on either
side.

Usually a river first attains grade near its mouth, and here first
sinks its bed to near baselevel. Extending its graded course
upstream by cutting away barrier after barrier, it comes to have a
widened and mature valley over its lower course, while its young
headwaters are still busily eroding their beds. Its ungraded
branches may thus bring down to its lower course more waste than
it is competent to carry on to the sea, and here it aggrades its
bed and builds a flood plain in order to gain a steeper gradient
and velocity enough to transport its load.

As maturity is past and the relief of the land is lessened, a
smaller and smaller load of waste is delivered to the river. It
now has energy to spare and again degrades its valley, excavating
its former flood plains and leaving them in terraces on either
side, and at last in its old age sweeping them away.

ALLUVIAL CONES AND FANS. In hilly and mountainous countries one
often sees on a valley side a conical or fan-shaped deposit of
waste at the mouth of a lateral stream. The cause is obvious: the
young branch has not been able as yet to wear its bed to accordant
level with the already deepened valley of the master stream. It
therefore builds its bed to grade at the point of juncture by
depositing here its load of waste,--a load too heavy to be carried
along the more gentle profile of the trunk valley.

Where rivers descend from a mountainous region upon the plain they
may build alluvial fans of exceedingly gentle slope. Thus the
rivers of the western side of the Sierra Nevada Mountains have
spread fans with a radius of as much as forty miles and a slope
too slight to be detected without instruments, where they leave
the rock-cut canyons in the mountains and descend upon the broad
central valley of California.

As a river flows over its fan it commonly divides into a
branchwork of shifting channels called DISTRIBUTARIES, since they
lead off the water from the main stream. In this way each part of
the fan is aggraded and its symmetric form is preserved.

PIEDMONT PLAINS. Mountain streams may build their confluent fans
into widespread piedmont (foot of the mountain) alluvial plains.
These are especially characteristic of arid lands, where the
streams wither as they flow out upon the thirsty lowlands and are
therefore compelled to lay down a large portion of their load. In
humid climates mountain-born streams are usually competent to
carry their loads of waste on to the sea, and have energy to spare
to cut the lower mountain slopes into foothills. In arid regions
foothills are commonly absent and the ranges rise, as from
pedestals, above broad, sloping plains of stream-laid waste.

THE HIGH PLAINS. The rivers which flow eastward from the Rocky
Mountains have united their fans in a continuous sheet of waste
which stretches forward from the base of the mountains for
hundreds of miles and in places is five hundred feet thick (Fig.
80). That the deposit was made in ancient times on land and not in
the sea is proved by the remains which it contains of land animals
and plants of species now extinct. That it was laid by rivers and
not by fresh-water lakes is shown by its structure. Wide stretches
of flat-lying, clays and sands are interrupted by long, narrow
belts of gravel which mark the channels of the ancient streams.
Gravels, and sands are often cross bedded, and their well worn
pebbles may be identified with the rocks of the mountains. After
building this sheet of waste the streams ceased to aggrade and
began the work of destruction. Large uneroded remnants, their
surfaces flat as a floor, remain as the High Plains of western
Kansas and Nebraska.

RIVER DEPOSITS IN SUBSIDING TROUGHS. To a geologist the most
important river deposits are those which gather in areas of
gradual subsidence; they are often of vast extent and immense
thickness, and such deposits of past geological ages have not
infrequently been preserved, with all their records of the times
in which they were built, by being carried below the level of the
sea, to be brought to light by a later uplift. On the other hand,
river deposits which remain above baselevels of erosion are swept
away comparatively soon.

THE GREAT VALLEY OF CALIFORNIA is a monotonously level plain of
great fertility, four hundred miles in length and fifty miles in
average width, built of waste swept down by streams from the
mountain ranges which inclose it,--the Sierra Nevada on the east
and the Coast Range on the west. On the waste slopes at the foot
of the bordering hills coarse gravels and even bowlders are left,
while over the interior the slow-flowing streams at times of
flood spread wide sheets of silt. Organic deposits are now forming
by the decay of vegetation in swampy tule (reed) lands and in
shallow lakes which occupy depressions left by the aggrading
streams.

Deep borings show that this great trough is filled to a depth of
at least two thousand feet below sea level with recent
unconsolidated sands and silts containing logs of wood and fresh-
water shells. These are land deposits, and the absence of any
marine deposits among them proves that the region has not been
invaded by the sea since the accumulation began. It has therefore
been slowly subsiding and its streams, although continually
carried below grade, have yet been able to aggrade the surface as
rapidly as the region sank, and have maintained it, as at present,
slightly above sea level.

THE INDO-GANGETIC PLAIN, spread by the Brahmaputra, the Ganges,
and the Indus river systems, stretches for sixteen hundred miles
along the southern base of the Himalaya Mountains and occupies an
area of three hundred thousand square miles (Fig.342). It consists
of the flood plains of the master streams and the confluent fans
of the tributaries which issue from the mountains on the north.
Large areas are subject to overflow each season of flood, and
still larger tracts mark abandoned flood plains below which the
rivers have now cut their beds. The plain is built of far-
stretching beds of clay, penetrated by streaks of sand, and also
of gravel near the mountains. Beds of impure peat occur in it, and
it contains fresh-water shells and the bones of land animals of
species now living in northern India. At Lucknow an artesian well
was sunk to one thousand feet below sea level without reaching the
bottom of these river-laid sands and silts, proving a slow
subsidence with which the aggrading rivers have kept pace.

WARPED VALLEYS. It is not necessary that an area should sink below
sea level in order to be filled with stream-swept waste. High
valleys among growing mountain ranges may suffer warping, or may
be blockaded by rising mountain folds athwart them. Where the
deformation is rapid enough, the river may be ponded and the
valley filled with lake-laid sediments. Even when the river is
able to maintain its right of way it may yet have its declivity so
lessened that it is compelled to aggrade its course continually,
filling the valley with river deposits which may grow to an
enormous thickness.

Behind the outer ranges of the Himalaya Mountains lie several
waste-filled valleys, the largest of which are Kashmir and Nepal,
the former being an alluvial plain about as large as the state of
Delaware. The rivers which drain these plains have already cut
down their outlet gorges sufficiently to begin the task of the
removal of the broad accumulations which they have brought in from
the surrounding mountains. Their present flood plains lie as much
as some hundreds of feet below wide alluvial terraces which mark
their former levels. Indeed, the horizontal beds of the Hundes
Valley have been trenched to the depth of nearly three thousand
feet by the Sutlej River. These deposits are recent or subrecent,
for there have been found at various levels the remains of land
plants and land and fresh-water shells, and in some the bones of
such animals as the hyena and the goat, of species or of genera
now living. Such soft deposits cannot be expected to endure
through any considerable length of future time the rapid erosion
to which their great height above the level of the sea will
subject them.

CHARACTERISTICS OF RIVER DEPOSITS. The examples just cited teach
clearly the characteristic features of extensive river deposits.
These deposits consist of broad, flat-lying sheets of clay and
fine sand left by the overflow at time of flood, and traversed
here and there by long, narrow strips of coarse, cross-bedded
sands and gravels thrown down by the swifter currents of the
shifting channels. Occasional beds of muck mark the sites of
shallow lakelets or fresh-water swamps. The various strata also
contain some remains of the countless myriads of animals and
plants which live upon the surface of the plain as it is in
process of building. River shells such as the mussel, land shells
such as those of snails, the bones of fishes and of such land
animals as suffer drowning at times of flood or are mired in
swampy places, logs of wood, and the stems and leaves of plants
are examples of the variety of the remains of land and fresh-water
organisms which are entombed in river deposits and sealed away as
a record of the life of the time, and as proof that the deposits
were laid by streams and not beneath the sea.

BASIN DEPOSITS

DEPOSITS IN DRY BASINS. On desert areas without outlet to the sea,
as on the Great Basin of the United States and the deserts of
central Asia, stream-swept waste accumulates indefinitely. The
rivers of the surrounding mountains, fed by the rains and melting
snows of these comparatively moist elevations, dry and soak away
as they come down upon the arid plains. They are compelled to lay
aside their entire load of waste eroded from the mountain valleys,
in fans which grow to enormous size, reaching in some instances
thousands of feet in thickness.

The monotonous levels of Turkestan include vast alluvial tracts
now in process of building by the floods of the frequently
shifting channels of the Oxus and other rivers of the region. For
about seven hundred miles from its mouth in Aral Lake the Oxus
receives no tributaries, since even the larger branches of its
system are lost in a network of distributaries and choked with
desert sands before they reach their master stream. These
aggrading rivers, which have channels but no valleys, spread their
muddy floods--which in the case of the Oxus sometimes equal the
average volume of the Mississippi--far and wide over the plain,
washing the bases of the desert dunes.

PLAYAS. In arid interior basins the central depressions may be
occupied by playas,--plains of fine mud washed forward from the
margins. In the wet season the playa is covered with a thin sheet
of muddy water, a playa lake, supplied usually by some stream at
flood. In the dry season the lake evaporates, the river which fed
it retreats, and there is left to view a hard, smooth, level floor
of sun-baked and sun-cracked yellow clay utterly devoid of
vegetation.

In the Black Rock desert of Nevada a playa lake spreads over an
area fifty miles long and twenty miles wide. In summer it
disappears; the Quinn River, which feeds it, shrinks back one
hundred miles toward its source, leaving an absolutely barren
floor of clay, level as the sea.

LAKE DEPOSITS. Regarding lakes as parts of river systems, we may
now notice the characteristic features of the deposits in lake
basins. Soundings in lakes of considerable size and depth show
that their bottoms are being covered with tine clays. Sand and
gravel are found along; their margins, being brought in by streams
and worn by waves from the shore, but there are no tidal or other
strong currents to sweep coarse waste out from shore to any
considerable distance. Where fine clays are now found on the land
in even, horizontal layers containing the remains of fresh-water
animals and plants, uncut by channels tilled with cross-bedded
gravels and sands and bordered by beach deposits of coarse waste,
we may safely infer the existence of ancient lakes.

MARL. Marl is a soft, whitish deposit of carbonate of lime,
mingled often with more or less of clay, accumulated in small
lakes whose feeding springs are charged with carbonate of lime and
into which little waste is washed from the land. Such lakelets are
not infrequent on the surface of the younger drift sheets of
Michigan and northern Indiana, where their beds of marl--sometimes
as much as forty feet thick--are utilized in the manufacture of
Portland cement. The deposit results from the decay of certain
aquatic plants which secrete lime carbonate from the water, from
the decomposition of the calcareous shells of tiny mollusks which
live in countless numbers on the lake floor, and in some cases
apparently from chemical precipitation.

PEAT. We have seen how lakelets are extinguished by the decaying
remains of the vegetation which they support. A section of such a
fossil lake shows that below the growing mosses and other plants
of the surface of the bog lies a spongy mass composed of dead
vegetable tissue, which passes downward gradually into PEAT,--a
dense, dark brown carbonaceous deposit in which, to the unaided
eye, little or no trace of vegetable structure remains. When
dried, peat forms a fuel of some value and is used either cut into
slabs and dried or pressed into bricks by machinery.

When vegetation decays in open air the carbon of its tissues,
taken from the atmosphere by the leaves, is oxidized and returned
to it in its original form of carbon dioxide. But decomposing in
the presence of water, as in a bog, where the oxygen of the air is
excluded, the carbonaceous matter of plants accumulates in
deposits of peat.

Peat bogs are numerous in regions lately abandoned by glacier ice,
where river systems are so immature that the initial depressions
left in the sheet of drift spread over the country have not yet
been drained. One tenth of the surface of Ireland is said to be
covered with peat, and small bogs abound in the drift-covered area
of New England and the states lying as far west as the Missouri
River. In Massachusetts alone it has been reckoned that there are
fifteen billion cubic feet of peat, the largest bog occupying
several thousand acres.

Much larger swamps occur on the young coastal plain of the
Atlantic from New Jersey to Florida. The Dismal Swamp, for
example, in Virginia and North Carolina is forty miles across. It
is covered with a dense growth of water-loving trees such as the
cypress and black gum. The center of the swamp is occupied by Lake
Drummond, a shallow lake seven miles in diameter, with banks of
pure-peat, and still narrowing from the encroachment of vegetation
along its borders.

SALT LAKES. In arid climates a lake rarely receives sufficient
inflow to enable it to rise to the basin rim and find an outlet.
Before this height is reached its surface becomes large enough to
discharge by evaporation into the dry air the amount of water that
is supplied by streams. As such a lake has no outlet, the minerals
in solution brought into it by its streams cannot escape from the
basin. The lake water becomes more and more heavily charged with
such substances as common salt and the sulphates and carbonates of
lime, of soda, and of potash, and these are thrown down from
solution one after another as the point of saturation for each
mineral is reached. Carbonate of lime, the least soluble and often
the most abundant mineral brought in, is the first to be
precipitated. As concentration goes on, gypsum, which is insoluble
in a strong brine, is deposited, and afterwards common salt. As
the saltness of the lake varies with the seasons and with climatic
changes, gypsum and salt are laid in alternate beds and are
interleaved with sedimentary clays spread from the waste brought
in by streams at times of flood. Few forms of life can live in
bodies of salt water so concentrated that chemical deposits take
place, and hence the beds of salt, gypsum, and silt of such lakes
are quite barren of the remains of life. Similar deposits are
precipitated by the concentration of sea water in lagoons and arms
of the sea cut off from the ocean.

LAKES BONNEVILLE AND LAHONTAN. These names are given to extinct
lakes which once occupied large areas in the Great Basin, the
former in Utah, the latter in northwestern Nevada. Their records
remain in old horizontal beach lines which they drew along their
mountainous shores at the different levels at which they stood,
and in the deposits of their beds. At its highest stage Lake
Bonneville, then one thousand feet deep, overflowed to the north
and was a fresh-water lake. As it shrank below the outlet it
became more and more salty, and the Great Salt Lake, its withered
residue, is now depositing salt along its shores. In its strong
brine lime carbonate is insoluble, and that brought in by streams
is thrown down at once in the form of travertine.

Lake Lahontan never had an outlet. The first chemical deposits to
be made along its shores were deposits of travertine, in places
eighty feet thick. Its floor is spread with fine clays, which must
have been laid in deep, still water, and which are charged with
the salts absorbed by them as the briny water of the lake dried
away. These sedimentary clays are in two divisions, the upper and
lower, each being about one hundred feet thick. They are separated
by heavy deposits of well-rounded, cross-bedded gravels and sands,
similar to those spread at the present time by the intermittent
streams of arid regions. A similar record is shown in the old
floors of Lake Bonneville. What conclusions do you draw from these
facts as to the history of these ancient lakes?

DELTAS

In the river deposits which are left above sea level particles of
waste are allowed to linger only for a time. From alluvial fans
and flood plains they are constantly being taken up and swept
farther on downstream. Although these land forms may long persist,
the particles which compose them are ever changing. We may
therefore think of the alluvial deposits of a valley as a stream
of waste fed by the waste mantle as it creeps and washes down the
valley sides, and slowly moving onwards to the sea.

In basins waste finds a longer rest, but sooner or later lakes and
dry basins are drained or filled, and their deposits, if above sea
level, resume their journey to their final goal. It is only when
carried below the level of the sea that they are indefinitely
preserved.

On reaching this terminus, rivers deliver their load to the ocean.
In some cases the ocean is able to take it up by means of strong
tidal and other currents, and to dispose of it in ways which we
shall study later. But often the load is so large, or the tides
are so weak, that much of the waste which the river brings in
settles at its mouth, there building up a deposit called the
DELTA, from the Greek letter of that name, whose shape it
sometimes resembles.

Deltas and alluvial fans have many common characteristics. Both
owe their origin to a sudden check in the velocity of the river,
compelling a deposit of the load; both are triangular in outline,
the apex pointing upstream; and both are traversed by
distributaries which build up all parts in turn.

In a delta we may distinguish deposits of two distinct kinds,--
the submarine and the subaerial. In part a delta is built of waste
brought down by the river and redistributed and spread by waves
and tides over the sea bottom adjacent to the river's mouth. The
origin of these deposits is recorded in the remains of marine
animals and plants which they contain.

As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaerial deposits
altogether similar to those of the flood plain, of which indeed
the subaerial delta is the prolongation. Here extended deposits of
peat may accumulate in swamps, and the remains of land and fresh-
water animals and plants swept down by the stream are imbedded in
the silts laid at times of flood.

Borings made in the deltas of great rivers such as the
Mississippi, the Ganges, and the Nile, show that the subaerial
portion often reaches a surprising thickness. Layers of peat, old
soils, and forest grounds with the stumps of trees are discovered
hundreds of feet below sea level. In the Nile delta some eight
layers of coarse gravel were found interbedded with river silts,
and in the Ganges delta at Calcutta a boring nearly five hundred
feet in depth stopped in such a layer.

The Mississippi has built a delta of twelve thousand three hundred
square miles, and is pushing the natural embankments of its chief
distributaries into the Gulf at a maximum rate of a mile in
sixteen years. Muddy shoals surround its front, shallow lakes,
e.g. lakes Pontchartrain and Borgne, are formed between the
growing delta and the old shore line, and elongate lakes and
swamps are inclosed between the natural embankments of the
distributaries.

The delta of the Indus River, India, lies so low along shore that
a broad tract of country is overflowed by the highest tides. The
submarine portion of the delta has been built to near sea level
over so wide a belt offshore that in many places large vessels
cannot come even within sight of land because of the shallow
water.

A former arm of the sea, the Rann of Cutch, adjoining the delta on
the east has been silted up and is now an immense barren flat of
sandy mud two hundred miles in length and one hundred miles in
greatest breadth. Each summer it is flooded with salt water when
the sea is brought in by strong southwesterly monsoon winds, and
the climate during the remainder of the year is hot and dry. By
the evaporation of sea water the soil is thus left so salty that
no vegetation can grow upon it, and in places beds of salt several
feet in thickness have accumulated. Under like conditions salt
beds of great thickness have been formed in the past and are now
found buried among the deposits of ancient deltas.

SUBSIDENCE OF GREAT DELTAS. As a rule great deltas are slowly
sinking. In some instances upbuilding by river deposits has gone
on as rapidly as the region has subsided. The entire thickness of
the Ganges delta, for example, so far as it has been sounded,
consists of deposits laid in open air. In other cases interbedded
limestones and other sedimentary rocks containing marine fossils
prove that at times subsidence has gained on the upbuilding and
the delta has been covered with the sea.

It is by gradual depression that delta deposits attain enormous
thickness, and, being lowered beneath the level of the sea, are
safely preserved from erosion until a movement of the earth's
crust in the opposite direction lifts them to form part of the
land. We shall read later in the hard rocks of our continent the
records of such ancient deltas, and we shall not be surprised to
find them as thick as are those now building at the mouths of
great rivers.

LAKE DELTAS. Deltas are also formed where streams lose their
velocity on entering the still waters of lakes. The shore lines of
extinct lakes, such as Lake Agassiz and Lakes Bonneville and
Lahontan, may be traced by the heavy deposits at the mouths of
their tributary streams.

We have seen that the work of streams is to drain the lands of the
water poured upon them by the rainfall, to wear them down, and to
carry their waste away to the sea, there to be rebuilt by other
agents into sedimentary rocks. The ancient strata of which the
continents are largely made are composed chiefly of material thus
worn from still more ancient lands--lands with their hills and
valleys like those of to-day--and carried by their rivers to the
ocean. In all geological times, as at the present, the work of
streams has been to destroy the lands, and in so doing to furnish
to the ocean the materials from which the lands of future ages
were to be made. Before we consider how the waste of the land
brought in by streams is rebuilt upon the ocean floor, we must
proceed to study the work of two agents, glacier ice and the wind,
which cooperate with rivers in the denudation of the land.





CHAPTER V

THE WORK OF GLACIERS


THE DRIFT. The surface of northeastern North America, as far south
as the Ohio and Missouri rivers, is generally covered by the
drift,--a formation which is quite unlike any which we have so far
studied. A section of it, such as that illustrated in Figure 87,
shows that for the most part it is unstratified, consisting of
clay, sand, pebbles, and even large bowlders, all mingled pell-
mell together. The agent which laid the drift is one which can
carry a load of material of all sizes, from the largest bowlder to
the finest clay, and deposit it without sorting.

The stones of the drift are of many kinds. The region from which
it was gathered may well have been large in order to supply these
many different varieties of rocks. Pebbles and bowlders have been
left far from their original homes, as may be seen in southern
Iowa, where the drift contains nuggets of copper brought from the
region about Lake Superior. The agent which laid the drift is one
able to gather its load over a large area and carry it a long way.

The pebbles of the drift are unlike those rounded by running water
or by waves. They are marked with scratches. Some are angular,
many have had their edges blunted, while others have been ground
flat and smooth on one or more sides, like gems which have been
faceted by being held firmly against the lapidary's wheel. In many
places the upper surface of the country rock beneath the drift has
been swept clean of residual clays and other waste. All rock
rotten has been planed away, and the ledges of sound rock to which
the surface has been cut down have been rubbed smooth and
scratched with long, straight, parallel lines. The agent which
laid the drift can hold sand and pebbles firmly in its grasp and
can grind them against the rock beneath, thus planing it down and
scoring it, while faceting the pebbles also.

Neither water nor wind can do these things. Indeed, nothing like
the drift is being formed by any process now at work anywhere in
the eastern United States. To find the agent which has laid this
extensive formation we must go to a region of different climatic
conditions.

THE INLAND ICE OF GREENLAND. Greenland is about fifteen hundred
miles long and nearly seven hundred miles in greatest width. With
the exception of a narrow fringe of mountainous coast land, it is
completely buried beneath a sheet of ice, in shape like a vast
white shield, whose convex surface rises to a height of nine
thousand feet above the sea. The few explorers who have crossed
the ice cap found it a trackless desert destitute of all life save
such lowly forms as the microscopic plant which produces the so-
called "red snow." On the smooth plain of the interior no rock
waste relieves the snow's dazzling whiteness; no streams of
running water are seen; the silence is broken only by howling
storm winds and the rustle of the surface snow which they drive
before them. Sounding with long poles, explorers find that below
the powdery snow of the latest snowfall lie successive layers of
earlier snows, which grow more and more compact downward, and at
last have altered to impenetrable ice. The ice cap formed by the
accumulated snows of uncounted centuries may well be more than a
mile in depth. Ice thus formed by the compacting of snow is
distinguished when in motion as GLACIER ICE.

The inland ice of Greenland moves. It flows with imperceptible
slowness under its own weight, like, a mass of some viscous or
plastic substance, such as pitch or molasses candy, in all
directions outward toward the sea. Near the edge it has so thinned
that mountain peaks are laid bare, these islands in the sea of ice
being known as NUNATAKS. Down the valleys of the coastal belt it
drains in separate streams of ice, or GLACIERS. The largest of
these reach the sea at the head of inlets, and are therefore
called TIDE GLACIERS. Their fronts stand so deep in sea water that
there is visible seldom more than three hundred feet of the wall
of ice, which in many glaciers must be two thousand and more feet
high. From the sea walls of tide glaciers great fragments break
off and float away as icebergs. Thus snows which fell in the
interior of this northern land, perhaps many thousands of years
ago, are carried in the form of icebergs to melt at last in the
North Atlantic.

Greenland, then, is being modeled over the vast extent of its
interior not by streams of running water, as are regions in warm
and humid climates, nor by currents of air, as are deserts to a
large extent, but by a sheet of flowing ice. What the ice sheet is
doing in the interior we may infer from a study of the separate
glaciers into which it breaks at its edge.

THE SMALLER GREENLAND GLACIERS. Many of the smaller glaciers of
Greenland do not reach the sea, but deploy on plains of sand and
gravel. The edges of these ice tongues are often as abrupt as if
sliced away with a knife (Fig. 92), and their structure is thus
readily seen. They are stratified, their layers representing in
part the successive snowfalls of the interior of the country. The
upper layers are commonly white and free from stones; but the
lower layers, to the height of a hundred feet or more, are dark
with debris which is being slowly carried on. So thickly studded
with stones is the base of the ice that it is sometimes difficult
to distinguish it from the rock waste which has been slowly
dragged beneath the glacier or left about its edges. The waste
beneath and about the glacier is unsorted. The stones are of many
kinds, and numbers of them have been ground to flat faces. Where
the front of the ice has retreated the rock surface is seen to be
planed and scored in places by the stones frozen fast in the sole
of the glacier.

We have now found in glacier ice an agent able to produce the
drift of North America. The ice sheet of Greenland is now doing
what we have seen was done in the recent past in our own land. It
is carrying for long distances rocks of many kinds gathered, we
may infer, over a large extent of country. It is laying down its
load without assortment in unstratified deposits. It grinds down
and scores the rock over which it moves, and in the process many
of the pebbles of its load are themselves also ground smooth and
scratched. Since this work can be done by no other agent, we must
conclude that the northeastern part of our own continent was
covered in the recent past by glacier ice, as Greenland is to-day.

VALLEY GLACIERS

The work of glacier ice can be most conveniently studied in the
separate ice streams which creep down mountain valleys in many
regions such as Alaska, the western mountains of the United States
and Canada, the Himalayas, and the Alps. As the glaciers of the
Alps have been studied longer and more thoroughly than any others,
we shall describe them in some detail as examples of valley
glaciers in all parts of the world.

CONDITIONS OF GLACIER FORMATION. The condition of the great
accumulation of snow to which glaciers are due--that more or less
of each winter's snow should be left over unmelted and
unevaporated to the next--is fully met in the Alps. There is
abundant moisture brought by the winds from neighboring seas. The
currents of moist air driven up the mountain slopes are cooled by
their own expansion as they rise, and the moisture which they
contain is condensed at a temperature at or below 32 degrees F.,
and therefore is precipitated in the form of snow. The summers are
cool and their heat does not suffice to completely melt the heavy
snow of the preceding winter. On the Alps the SNOW LINE--the lower
limit of permanent snow--is drawn at about eight thousand five
hundred feet above sea level. Above the snow line on the slopes
and crests, where these are not too steep, the snow lies the year
round and gathers in valley heads to a depth of hundreds of feet.

This is but a small fraction of the thickness to which snow would
be piled on the Alps were it not constantly being drained away.
Below the snow fields which mantle the heights the mountain
valleys are occupied by glaciers which extend as much as a
vertical mile below the snow line. The presence in the midst of
forests and meadows and cultivated fields of these tongues of ice,
ever melting and yet from year to year losing none of their bulk,
proves that their loss is made good in the only possible way. They
are fed by snow fields above, whose surplus of snow they drain
away in the form of ice. The presence of glaciers below the snow
line is a clear proof that, rigid and motionless as they appear,
glaciers really are in constant motion down valley.

THE NEVE FIELD. The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow. Great
peaks tower above it, and snowy slopes rise on either side on the
flanks of mountain spurs. From these heights fierce winds drift
the snows into the amphitheater, and avalanches pour in their
torrents of snow and waste. The snow of the amphitheater is like
that of drifts in late winter after many successive thaws and
freezings. It is made of hard grains and pellets and is called
NEVE. Beneath the surface of the neve field and at its outlet the
granular neve has been compacted to a mass of porous crystalline
ice. Snow has been changed to neve, and neve to glacial ice, both
by pressure, which drives the air from the interspaces of the
snowflakes, and also by successive meltings and freezings, much as
a snowball is packed in the warm hand and becomes frozen to a ball
of ice.

THE BERGSCHRUND. The neve is in slow motion. It breaks itself
loose from the thinner snows about it, too shallow to share its
motion, and from the rock rim which surrounds it, forming a deep
fissure called the bergschrund, sometimes a score and more feet
wide.

SIZE OF GLACIERS. The ice streams of the Alps vary in size
according to the amount of precipitation and the area of the neve
fields which they drain. The largest of Alpine glaciers, the
Aletsch, is nearly ten miles long and has an average width of
about a mile. The thickness of some of the glaciers of the Alps is
as much as a thousand feet. Giant glaciers more than twice the
length of the longest in the Alps occur on the south slope of the
Himalaya Mountains, which receive frequent precipitations of snow
from moist winds from the Indian Ocean. The best known of the many
immense glaciers of Alaska, the Muir, has an area of about eight
hundred square miles (Fig. 95).

GLACIER MOTION. The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the
enormous pressure of their weight and because of less marginal
resistance, move faster than small ones. The Muir advances at the
rate of seven feet a day, and some of the larger tide glaciers of
Greenland are reported to move at the exceptional rate of fifty
feet and more in the same time. Glaciers move faster by day than
by night, and in summer than in winter. Other laws of glacier
motion may be discovered by a study of Figures 96 and 97. It is
important to remember that glaciers do not slide bodily over their
beds, but urged by gravity move slowly down valley in somewhat the
same way as would a stream of thick mud. Although small pieces of
ice are brittle, the large mass of granular ice which composes a
glacier acts as a viscous substance.

CREVASSES. Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions
under which the ice cracks and opens in great fissures called
crevasses. At an abrupt descent in the bed the ice is shattered
into great fragments, which unite again below the icefall.
Crevasses are opened on lines at right angles to the direction of
the tension. TRANSVERSE CREVASSES are due to a convexity in the
bed which stretches the ice lengthwise (Fig. 99). MARGINAL
CREVASSES are directed upstream and inwards; RADIAL CREVASSES are
found where the ice stream deploys from some narrow valley and
spreads upon some more open space. What is the direction of the
tension which causes each and to what is it due?

LATERAL AND MEDIAL MORAINES. The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center
are called the medial moraines. The one on either margin is a
lateral moraine, and is clearly formed of waste which has fallen
on the edge of the ice from the valley slopes. A medial moraine
cannot be formed in this way, since no rock fragments can fall so
far out from the sides. But following it up the glacial stream,
one finds that a medial moraine takes its beginning at the
junction of the glacier and some tributary and is formed by the
union of their two adjacent lateral moraines. Each branch thus
adds a medial moraine, and by counting the number of medial
moraines of a trunk stream one may learn of how many branches it
is composed.

Surface moraines appear in the lower course of the glacier as
ridges, which may reach the exceptional height of one hundred
feet. The bulk of such a ridge is ice. It has been protected from
the sun by the veneer of moraine stuff; while the glacier surface
on either side has melted down at least the distance of the height
of the ridge. In summer the lowering of the glacial surface by
melting goes on rapidly. In Swiss glaciers it has been estimated
that the average lowering of the surface by melting and
evaporation amounts to ten feet a year. As a moraine ridge grows
higher and more steep by the lowering of the surface of the
surrounding ice, the stones of its cover tend to slip down its
sides. Thus moraines broaden, until near the terminus of a glacier
they may coalesce in a wide field of stony waste.

ENGLACIAL DRIFT. This name is applied to whatever debris is
carried within the glacier. It consists of rock waste fallen on
the neve and there buried by accumulations of snow, and of that
engulfed in the glacier where crevasses have opened beneath a
surface moraine. As the surface of the glacier is lowered by
melting, more or less englacial drift is brought again to open
air, and near the terminus it may help to bury the ice from view
beneath a sheet of debris.

THE GROUND MORAINE. The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been
torn and worn from the glacier's bed and banks. While the stones
of the surface moraines remain as angular as when they lodged on
the ice, many of those of the ground moraine have been blunted on
the edges and faceted and scratched by being ground against one
another and the rocky bed.

In glaciers such as those of Greenland, whose basal layers are
well loaded with drift and whose surface layers are nearly clean,
different layers have different rates of motion, according to the
amount of drift with which they are clogged. One layer glides over
another, and the stones inset in each are ground and smoothed and
scratched. Usually the sides of glaciated pebbles are more worn
than the ends, and the scratches upon them run with the longer
axis of the stone. Why?

THE TERMINAL MORAINE. As a glacier is in constant motion, it
brings to its end all of its load except such parts of the ground
moraine as may find permanent lodgment beneath the ice. Where the
glacier front remains for some time at one place, there is formed
an accumulation of drift known as the terminal moraine. In valley
glaciers it is shaped by the ice front to a crescent whose convex
side is downstream. Some of the pebbles of the terminal moraine
are angular, and some are faceted and scored, the latter having
come by the hard road of the ground moraine. The material of the
dump is for the most part unsorted, though the water of the
melting ice may find opportunity to leave patches of stratified
sands and gravels in the midst of the unstratified mass of drift,
and the finer material is in places washed away.

GLACIER DRAINAGE. The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as VALLEY DRIFT.

The sand brought out by streams from beneath a glacier differs
from river sand in that it consists of freshly broken angular
grains. Why?

The stream derives its water chiefly from the surface melting of
the glacier. As the ice is touched by the rays of the morning sun
in summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells
cut by their whirling waters where some crevasse has begun to open
across their path. These wells lead into chambers and tunnels by
which sooner or later their waters find way to the rock floor of
the valley and there unite in a subglacial stream.

THE LOWER LIMIT OF GLACIERS. The glaciers of a region do not by
any means end at a uniform height above sea level. Each terminates
where its supply is balanced by melting. Those therefore which are
fed by the largest and deepest neves and those also which are best
protected from the sun by a northward exposure or by the depth of
their inclosing valleys flow to lower levels than those whose
supply is less and whose exposure to the sun is greater.

A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years
causes them to wither and melt back. The variation in glaciers is
now carefully observed in many parts of the world. The Muir
glacier has retreated two miles in twenty years. The glaciers of
the Swiss Alps are now for the most part melting back, although a
well-known glacier of the eastern Alps, the Vernagt, advanced five
hundred feet in the year 1900, and was then plowing up its
terminal moraine.

How soon would you expect a glacier to advance after its neve
fields have been swollen with unusually heavy snows, as compared
with the time needed for the flood of a large river to reach its
mouth after heavy rains upon its headwaters?

On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig.
108). These "glacier tables" commonly slope more or less strongly
to the south, and thus may be used to indicate roughly the points
of the compass. Can you explain their formation and the direction
of their slope? On the other hand, a small and thin stone, or a
patch of dust, lying on the ice, tends to sink a few inches into
it. Why?

In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?

Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove
this fact?

What effect would you expect the laws of glacier motion to have on
the slant of the sides of transverse crevasses?

A trunk glacier has four medial moraines. Of how many tributaries
is it composed? Illustrate by diagram.

State all the evidences which you have found that glaciers move.

If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?

PIEDMONT GLACIERS

THE MALASPINA GLACIER. Piedmont (foot of the mountain) glaciers
are, as the name implies, ice fields formed at the foot of
mountains by the confluence of valley glaciers. The Malaspina
glacier of Alaska, the typical glacier of this kind, is seventy
miles wide and stretches for thirty miles from the foot of the
Mount Saint Elias range to the shore of the Pacific Ocean. The
valley glaciers which unite and spread to form this lake of ice
lie above the snow line and their moraines are concealed beneath
neve. The central area of the Malaspina is also free from debris;
but on the outer edge large quantities of englacial drift are
exposed by surface melting and form a belt of morainic waste a few
feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand
feet in depth. The glacier here is practically stagnant, and lakes
a few hundred yards across, which could not exist were the ice in
motion and broken with crevasses, gather on their beds sorted
waste from the moraine. The streams which drain the glacier have
cut their courses in englacial and subglacial tunnels; none flow
for any distance on the surface. The largest, the Yahtse River,
issues from a high archway in the ice,--a muddy torrent one
hundred feet wide and twenty feet deep, loaded with sand and
stones which it deposits in a broad outwash plain (Fig. 110).
Where the ice has retreated from the sea there is left a hummocky
drift sheet with hollows filled with lakelets. These deposits help
to explain similar hummocky regions of drift and similar plains of
coarse, water-laid material often found in the drift-covered area
of the northeastern United States.

THE GEOLOGICAL WORK OF GLACIER ICE

The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its
channel occupies but a small portion of the valley. The glacier is
slow and big; its rate of motion may be less than a millionth of
that of running water over the same declivity, and its bulk is
proportionately large and fills the valley to great depth.
Moreover, glacier ice is a solid body plastic under slowly applied
stresses, while the water of rivers is a nimble fluid.

TRANSPORTATION. Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand;
while streams push and roll much of their load along their beds,
and their power of transporting waste depends solely upon their
velocity. The amount of the surface load of glaciers is limited
only by the amount of waste received from the mountain slopes
above them. The moving floor of ice stretched high across a valley
sweeps along as lateral moraines much of the waste which a
mountain stream would let accumulate in talus and alluvial cones.

While a valley glacier carries much of its load on top, an ice
sheet, such as that of Greenland, is free from surface debris,
except where moraines trail away from some nunatak. If at its edge
it breaks into separate glaciers which drain down mountain
valleys, these tongues of ice will carry the selvages of waste
common to valley glaciers. Both ice sheets and valley glaciers
drag on large quantities of rock waste in their ground moraines.

Stones transported by glaciers are sometimes called erratics. Such
are the bowlders of the drift of our northern states. Erratics may
be set down in an insecure position on the melting of the ice.

DEPOSIT. Little need be added here to what has already been said
of ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged
beneath the glacier as ground moraine is often an extremely dense,
stony clay, having been compacted under the pressure of the
overriding ice.

EROSION. A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.

The rock bed over which a glacier has moved is seen in places to
have been abraded, or ground away, to smooth surfaces which are
marked by long, straight, parallel scorings aligned with the line
of movement of the ice and varying in size from hair lines and
coarse scratches to exceptional furrows several feet deep. Clearly
this work has been accomplished by means of the sharp sand, the
pebbles, and the larger stones with which the base of the glacier
is inset, and which it holds in a firm grasp as running water
cannot. Hard and fine-grained rocks, such as granite and
quartzite, are often not only ground down to a smooth surface but
are also highly polished by means of fine rock flour worn from the
glacier bed.

In other places the bed of the glacier is rough and torn. The
rocks have been disrupted and their fragments have been carried
away,--a process known as PLUCKING. Moving under immense pressure
the ice shatters the rock, breaks off projections, presses into
crevices and wedges the rocks apart, dislodges the blocks into
which the rock is divided by joints and bedding planes, and
freezing fast to the fragments drags them on. In this work the
freezing and thawing of subglacial waters in any cracks and
crevices of the rock no doubt play an important part. Plucking
occurs especially where the bed rock is weak because of close
jointing. The product of plucking is bowlders, while the product
of abrasion is fine rock flour and sand.

Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?

ROCHES MOUTONNEES AND ROUNDED HILLS. The prominences left between
the hollows due to plucking are commonly ground down and rounded
on the stoss side,--the side from which the ice advances,--and
sometimes on the opposite, the lee side, as well. In this way the
bed rock often comes to have a billowy surface known as roches
moutonnees (sheep rocks). Hills overridden by an ice sheet often
have similarly rounded contours on the stoss side, while on the
lee side they may be craggy, either because of plucking or because
here they have been less worn from their initial profile.

THE DIRECTION OF GLACIER MOVEMENT. The direction of the flow of
vanished glaciers and ice sheets is recorded both in the
differences just mentioned in the profiles of overridden hills and
also in the minute details of the glacier trail.

Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities,
on the other hand, have their edges worn on the lee side and left
sharp upon the stoss.

Surfaces worn and torn in the ways which we have mentioned are
said to be glaciated. But it must not be supposed that a glacier
everywhere glaciates its bed. Although in places it acts as a rasp
or as a pick, in others, and especially where its pressure is
least, as near the terminus, it moves over its bed in the manner
of a sled. Instances are known where glaciers have advanced over
deposits of sand and gravel without disturbing them to any notable
degree. Like a river, a glacier does not everywhere erode. In
places it leaves its bed undisturbed and in places aggrades it by
deposits of the ground moraine.

CIRQUES. Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet
waters reflect the sheltering ramparts of rugged rock which tower
far above them. Such mountain hollows are termed CIRQUES. As a
powerful spring wears back a recess in the valley side where it
discharges, so the fountain head of a glacier gradually wears back
a cirque. In its slow movement the neve field broadly scours its
bed to a flat or basined floor. Meanwhile the sides of the valley
head are steepened and driven back to precipitous walls. For in
winter the crevasse of the bergschrund which surrounds the neve
field is filled with snow and the neve is frozen fast to the rocky
sides of the valley. In early summer the neve tears itself free,
dislodging and removing any loosened blocks, and the open fissure
of the bergschrund allows frost and other agencies of weathering
to attack the unprotected rock. As cirques are thus formed and
enlarged the peaks beneath which they lie are sharpened, and the
mountain crests are scalloped and cut back from either side to
knife-edged ridges.

In the western mountains of the United States many cirques, now
empty of neve and glacier ice, and known locally as "basins,"
testify to the fact that in recent times the snow line stood
beneath the levels of their floors, and thus far below its present
altitude.

GLACIER TROUGHS. The channel worn to accommodate the big and
clumsy glacier differs markedly from the river valley cut as with
a saw by the narrow and flexible stream and widened by the weather
and the wash of rains. The valley glacier may easily be from one
thousand to three thousand feet deep and from one to three miles
wide. Such a ponderous bulk of slowly moving ice does not readily
adapt itself to sharp turns and a narrow bed. By scouring and
plucking all resisting edges it develops a fitting channel with a
wide, flat floor, and steep, smooth sides, above which are seen
the weathered slopes of stream-worn mountain valleys. Since the
trunk glacier requires a deeper channel than do its branches, the
bed of a branch glacier enters the main trough at some distance
above the floor of the latter, although the surface of the two ice
streams may be accordant. Glacier troughs can be studied best
where large glaciers have recently melted completely away, as is
the case in many valleys of the mountains of the western United
States and of central and northern Europe (Fig. 114). The typical
glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little
broken by projecting spurs and lateral ravines. It is as if a V-
valley cut by a river had afterwards been gouged deeper with a
gigantic chisel, widening the floor to the width of the chisel
blade, cutting back the spurs, and smoothing and steepening the
sides. A river valley could only be as wide-floored as this after
it had long been worn down to grade.

The floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a
thousand feet in height, over which the stream that now drains the
valley tumbles in waterfalls. Reaches between the steps are often
basined. Lakelets may occupy hollows excavated in solid rock, and
other lakes may be held behind terminal moraines left as dams
across the valley at pauses in the retreat of the glacier.

FJORDS are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow
bay with steep rock walls and basined floor. Fjords are found only
in regions which have suffered glaciation, such as Norway and
Alaska.

HANGING VALLEYS. These are lateral valleys which open on their
main valley some distance above its floor. They are conspicuous
features of glacier troughs from which the ice has vanished; for
the trunk glacier in widening and deepening its channel cut its
bed below the bottoms of the lateral valleys.

Since the mouths of hanging valleys are suspended on the walls of
the glacier trough, their streams are compelled to plunge down its
s

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