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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*** START OF THE PROJECT GUTENBERG EBOOK THE ELEMENTS OF GEOLOGY ***
[Transcriber's Note:
_Text_ and =Text= represent italic and bold text respectively.
Subscripts are displayed as an underscore followed by the number
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* * * * *
[Illustration: A Valley with Rocky Ledges cut in the Horizontal
Strata, Scotland]
THE ELEMENTS OF GEOLOGY
BY
WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
GINN & COMPANY
BOSTON * NEW YORK * CHICAGO * LONDON
Copyright, 1905, 1921, by
WILLIAM HARMON NORTON
ALL RIGHTS RESERVED
5511
The Atheneum Press
GINN & COMPANY PROPRIETORS
BOSTON * U.S.A.
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
palæontological 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
ACKNOWLEDGMENT OF ILLUSTRATIONS
Adams, Professor F. D., McGill University, Canada, 241.
Alden, Dr. W. C., Washington, D.C., 353.
American Museum of Natural History, New York, 344.
Ash, H. C., Galesburg, Ill., 133.
Beyer, Dr. S. W., Iowa College of Agriculture, 363.
Calvin, Dr. Samuel, Iowa State University, 45, 295, 317, 325, 371.
Carney, Frank, Ithaca, N.Y., 356.
Clark, Dr. Wm. B., Maryland Geological Survey, 43.
Borne, Dr. Georg v. d., Jena, Germany, 5, 6.
Daly, Dr. R. A., Ottawa, Canada, 164.
Defieux, C. A., Liverpool, England, 154.
* Detroit Photographic Co., 235, 236.
* Ellis, W. M., Edna, Kan., 13.
Fairchild, Professor H. L., University of Rochester, 141, 357.
Field Columbian Museum, Chicago, 87.
Forster, Dr. A. E., University of Vienna, 32.
Gardner, J. L., Boston, 12, 140, 352.
Geological Survey of Canada, 256.
Gilbert, Dr. G. K., by courtesy of the American Book Company, 39.
* Haines, Ben, New Albany, Ind., 33.
* Haynes, F. J., St. Paul, Minn., 52, 95, 233.
Henderson, Judge Julius, Boulder, Col., 94.
James, George Wharton, Pasadena, Cal., 16, 127, 215, 229.
Johnston-Lavis, Professor H. J., Beaulieu, France, 216.
King, J. Harding, Stourbridge, England, 119.
Lawson, Dr. Andrew C., University of California, 113.
Le Conte, Professor J. N., University of California, 8.
Libbey, Dr. William, Princeton University, 92.
* McAllister, T. H., New York, 242.
* Meyers, H. C., Boise, Id., 19.
Mills, Professor H. A., Cornell College, 208, 304.
Norton, Professor W. H., Cornell College, 14, 35, 59, 88, 128,
183, 226, 234, 255, 340, 364, 367.
* Notman, Wm. & Son, Montreal, Canada, 98, 181.
Obrutschew, Dr. W., Tomsk Technological Institute, Siberia, 73.
Oldham, Dr. R. D., Geological Survey of India, 120.
* Peabody, H. C., Pasadena, Cal., 54.
* Pierce, C. C. & Co., Los Angeles, Cal., 15.
Pillsbury, Arthur, San Francisco, Cal., .115.
* Rau, Wm., Philadelphia, 18, 21, 122, 123, 218.
Reusch, Dr. Hans, Geological Survey of Norway, 112.
Reynolds, Professor S. H., University College, Bristol, England, 202.
Ricker, Principal Maurice, Burlington, Iowa, 48, 89.
* Shepard, E. A., Minneapolis, Minn., 105.
Smith, W. S. Tangier, Los Gatos, Cal., 186.
* Soule Photographic Co., Boston, 131.
U. S. Geological Survey, 3, 4, 23, 25, 34, 41, 63, 69, 78, 79,
80, 110, 111, 114, 125, 126, 129, 130, 142, 151, 153, 169,
172,177, 178, 188, 211, 212, 214, 228, 237, 238, 239, 243, 244,
254, 257, 340, 341, 353, 355.
U. S. National Museum, 149, 220, 221, 222, 225, 332.
* Valentine & Sons, Dundee, Scotland, 40, 136, 227.
Vroman, A. C., Pasadena, Cal., 17.
* Ward's Natural Science Establishment, Rochester, N.Y., 152.
* Welch, R., Belfast, Ireland, 1, 37.
* Westgate, Dr. L. G., Ohio Wesleyan University, 66.
Whymper, Edward, London, England, 106.
* Wilcox, W. D., Washington, D.C., 20.
* Wilson, Dr. A. W. G., McGill University, Canada, 68.
* Wilson, G. W., & Co., Aberdeen, Scotland, 82, 213.
* Worsley-Benison, F. H., Cheapstow, England, 170.
* Dealer in photographs or lantern slides.
CONTENTS
Page
Introduction.--The Scope And Aim Of Geology 1
PART I
EXTERNAL GEOLOGICAL AGENCIES
Chapter
I. The Work Of The Weather 5
II. The Work Of Ground Water 39
III. Rivers And Valleys 54
IV. River Deposits 93
V. The Work Of Glaciers 113
VI. The Work Of The Wind 144
VII. The Sea And Its Shores 155
VIII. Offshore And Deep-Sea Deposits 174
PART II
INTERNAL GEOLOGICAL AGENCIES
IX. Movements Of The Earth's Crust 195
X. Earthquakes 233
XI. Volcanoes 238
XII. Underground Structures Of Igneous Origin 265
XIII. Metamorphism And Mineral Veins 281
PART III
HISTORICAL GEOLOGY
XIV. The Geological Record 291
XV. The Pre-Cambrian Systems 304
XVI. The Cambrian 315
XVII. The Ordovician And Silurian 327
XVIII. The Devonian 341
XIX. The Carboniferous 350
XX. The Mesozoic 368
XXI. The Tertiary 394
XXII. The Quaternary 416
INDEX 451
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 _laminæ_. 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_ (Fig. 1). Two sets of joints, running at about
right angles to each other, together with the bedding planes, divide
the sandstone into quadrangular blocks.
[Illustration: Fig. 1. Cliff of Sandstone, Ireland]
=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.
[Illustration: Fig. 2. Section of Limestone Quarry
Scale, 1 in. = 30 ft. _a_, red residual clay; _mn_, pitted
surface of rotted limestone; _bb_, limestone divided into thin
layers; _c_, thick layers of laminated limestone, the laminæ
being firmly cemented together; _j_, _j_, _j_, joints. Is _bb_
thin-layered because originally so laid, or because it has been
broken up by weathering, although once like _c_ thick-layered?]
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.
[Illustration: Fig. 3. Conglomerate]
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.
[Illustration: Fig. 4. Surface of Limestone furrowed by
Weathering, Montana]
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 (Fig. 4).
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.
[Illustration: Fig. 5. Bowlder split by Heat and Cold,
Western Texas]
=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 (Fig. 6).
[Illustration: Fig. 6. Bowlders scaling off under Heat and Cold,
Western Texas]
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° F. within a
few hours. In the light air of the Pamir plateau in central Asia a
rise of 90° 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 Spitzbergen, 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.
[Illustration: Fig. 7. Rocks broken by Frost, Summit of the
Eggischhorn, Switzerland]
=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 coöperate 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° 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 (Fig. 8). 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.
[Illustration: Fig. 8. Talus at Foot of Granite Cliffs, Sierra
Nevada Mountains]
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.
[Illustration: Fig. 9. Diagram Illustrating Retreat of Cliff,
_c_, and Talus, _t_]
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.
[Illustration: Fig. 10. A Landslide, Quebec]
=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.
[Illustration: Fig. 11. Diagram Illustrating Conditions favorable
to a Landslide
_lm_, limestone dipping toward valley of river, _r_; _sh_, shale]
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 pell-mell 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 débris
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.
[Illustration: Fig. 12. Bowlders of Weathering, Granite Quarry,
Cape Ann, Massachusetts]
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 (Fig. 10).
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.
[Illustration: Fig. 13. Differential Weathering on a Monument,
Colorado]
=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.
[Illustration: Fig. 14. Honeycombed Limestone, Iowa]
[Illustration: Fig. 15. Cliffs and Slopes on North Wall of the
Grand Canyon of the Colorado River, Arizona]
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_ (Fig. 15).
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.
[Illustration: Fig. 16. Taverlone Mesa, New Mexico]
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.
[Illustration: Fig. 17. Monuments, Arizona
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.]
=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 flat-topped, because of the
protection of a resistant horizontal capping layer, they are termed
_mesas_ (Fig. 16),--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.
[Illustration: Fig. 18. Undercut Monuments, Colorado]
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
(Fig. 19). 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.
[Illustration: Fig. 19. Roosevelt Column, Idaho
An erosion pillar 70 feet high. How was it produced? Why
quadrangular? What does it show as to the recent height of the
hillside surface?]
Frost and heat and cold sculpture high mountains to sharp, tusklike
peaks and ragged, serrate crests, where their waste is readily removed
(Fig. 8).
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.
[Illustration: Fig. 20. Mount Assiniboine, Canada]
[Illustration: Fig. 21. Big Round Top and Little Round Top,
Gettysburg, Pennsylvania]
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?
[Illustration: Fig. 22]
=The coöperation 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
coöperation 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.
[Illustration: Fig. 23. Mount Sneffels, Colorado
Describe and account for what you see in this view. What
changes may the mountain be expected to undergo in the future
from the agencies now at work upon it?]
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.
[Illustration: Fig. 24. Diagram Illustrating the Relation of the
Ground-Water Surface to the Surface of the Ground
The dotted line represents the ground-water surface, and the
arrows indicate the direction of the movements of ground-water.
_m_, marsh; _w_, well; _r_, river]
=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.
[Illustration: Fig. 26. Geological Conditions favorable to
Strong Springs
_a_, limestone; _b_, shale; _c_, coarse sandstone; _d_,
limestone; _e_, sandstone; _f_, fissure. The strata dip toward
the South, _S_. Redraw the diagram, marking the points at which
strong springs (_ss_) may be expected.]
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).
[Illustration: Fig. 27. Diagram of Well which goes dry in
Drought, _a_, and of of Unfailing Well, _b_
Redraw the diagram, showing by dotted line the normal
ground-water surface and by broken line the ground-water
surface at times of drought]
[Illustration: Fig. 28. Diagram of Wet Weather Stream, _a_, and
of Permanent Stream, _b_
Redraw the diagram, showing ground-water surface by dotted line]
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
_aquifer, 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.
[Illustration: Fig. 29. Section across South Dakota from the
Black Hills to Sioux Falls (S), Illustrating the Conditions
of Artesian Wells
_a_, crystalline impervious rocks; _b_, sedimentary rocks,
shales, limestones, and sandstones; _c_, pervious sandstone,
the aquifer; _d_, impervious shales; _w_, _w_, _w_, artesian wells.]
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.
[Illustration: Fig. 30. Diagram of Caverns and Sink Holes]
=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.
[Illustration: Fig. 31. Sink Holes in the Karst, Austria]
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.
[Illustration: Fig. 32. Underground Stream Issuing from Base of
Cliff, the Karst, Austria]
=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.
[Illustration: Fig. 33. Stalactites and Stalagmites, Marengo
Cavern, Indiana]
=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 (Fig. 34).
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?
[Illustration: Fig. 34. Concretions in Sandstone, Wyoming]
=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° F., 70° 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° 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.
[Illustration: Fig. 35. Calcareous Deposits from Hot Springs,
Yellowstone National Park]
=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
(Fig. 35) 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 (Fig. 234).
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, therefore, water cannot go. Above it all
rocks are water-soaked, up to the limit of their capacity, to within a
few feet of the surface. Estimating the average pore space of the rocks
above a depth of six miles at from two and a half per cent to five per
cent of their volume, it is found that the total amount of ground water
may be great enough to cover the entire surface of the earth to a depth
of from eight hundred to sixteen hundred feet.
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?
[Illustration: Fig. 36. Rise of Ground-Water Surface (broken
line) beneath Valley (_V_) in Arid Region]
=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.
[Illustration: Fig. 37. Pothole in Bed of Stream, Ireland]
=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.
[Illustration: Fig. 38. Map of the Gorge of the Niagara River]
=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.
[Illustration: Fig. 39. Longitudinal Section of Niagara Gorge
Black, water; _F_, falls; _R_, rapids; _W_, whirlpool;
_E_, escarpment; _N_, north; _S_, south]
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?
[Illustration: Fig. 40. A Stream in Scotland
In what ways is the bed now being deepened?]
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.
[Illustration: Fig. 41. Sand Bar deposited by Stream, showing
Cross Bedding]
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.
[Illustration: Fig. 42. Longitudinal Section of a River Bar]
=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.
[Illustration: Fig. 43. Water-Worn Pebbles, Upper Potomac River,
Maryland]
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.
[Illustration: Fig. 44. A Young Lacustrine Plain; the Red River
of the North
Scale 5 inches = about 11 miles. Contour interval, 20 feet]
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.
[Illustration: Fig. 45. A Young River, Iowa
Note that it has hardly begun to cut in the plain of glacial
drift on which it flows]
=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.
[Illustration: Fig. 46. A Young Drift Region in Wisconsin
Describe this area. How high are the hills? Are they such in form
and position as would be left by stream erosion? Consult a map of
the entire state and notice that the Fox River finds its way to Lake
Michigan, while the Wisconsin empties into the Mississippi. Describe
that portion of the divide here shown between the Mississippi and
the St. Lawrence systems. Which is the larger river, the Wisconsin
or the Fox? Other things being equal, which may be expected to
deepen its bed the more rapidly? What changes are likely to occur
when one of these rivers comes to flow at a lower level than the
other? Why have not these changes occurred already?]
=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 débris 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.
[Illustration: Fig. 47. A Small Lake being broadened and shoaled
by Wave Wear
_ls_, lake surface; dotted line, initial shore;
_b_, fill made of material taken from _a_]
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.
[Illustration: Fig. 48. A Lake well-nigh effaced, Montana
By what means is the lake bed being filled?]
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.
[Illustration: Fig. 49. A Level Meadow, Scotland
Explain its origin. What will be its future?]
=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.
[Illustration: Fig. 50. Drainage Maps
_A_, an area in its infancy, Buena Vista County, Iowa;
_B_, an area in its maturity, Ringgold County, Iowa]
[Illustration: Fig. 51. Successive Longitudinal Profiles of a
Stream
_am_, initial profile, with waterfall at _w_, and basins at _l_
and _l´_, which at first are occupied by lakes and later are
filled or drained; _b_, _c_, _d_, and _e_, profiles established
in succession as the stream advances from infancy toward old
age. Note that these profiles are concave toward the sky. This
is the _erosion curve_. What contrasting form has the weather
weather curve (p. 34)?]
=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.
[Illustration: Fig. 52. A V-Valley,--the Canyon of the
Yellowstone
Note the steep sides. What processes are at work upon them? How
wide is the valley at the base compared with the width of the
stream? Do you see any river deposits along the banks? Is the
stream flowing swiftly over a rock bed, or quietly over a bed
which it has built up? Is it graded or ungraded? Note that the
canyon walls project in interlocking spurs]
=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.
[Illustration: Fig. 53. Section of the Yellowstone Canyon
This canyon is 100 feet deep, 2500 feet wide at the top, and
about 250 feet wide at the bottom. Neglecting any cutting of the
river against the banks, estimate what part of the excavation
of the canyon is due to the vertical erosion of its bed by the
river and what to weathering and rain wash on the canyon sides]
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 (Fig. 15). 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.
[Illustration: Fig. 54. Grand Canyon of the Colorado River,
Arizona]
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 (Fig. 15). 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.
[Illustration: Fig. 55. Diagrams illustrating Conditions which
produce Falls or Rapids
_A_, vertical succession of harder and softer rocks;
_B_, horizontal succession of the same. In _A_ the stream _ab_
in sinking its bed through a mass of strata of different degrees
of hardness has discovered the weak layer _s_ beneath the hard
layer _h_. It rapidly cuts its way in _s_, while in _A_ its
work is delayed. Thus the profile _afb´_ is soon reached, with
falls at _f_. In _B_ the initial profile is shown by dotted
line.]
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.
[Illustration: Fig. 56. Longitudinal Section of Yellowstone
River at Lower Fall, _F_, and Upper Fall, _F´_, Yellowstone
National Park
_la_, lava deeply decayed through action of thermal waters; _m_
and _m´_, masses of decayed lavas to whose hardness the falls
are due. Which fall will be worn away the sooner? How far
upstream will each fall migrate? Draw profile of the river when
one fall has disappeared]
[Illustration: Fig. 57. Diagram illustrating Migration of a
Fall due to a Hard Layer _H_, in the Midst of Soft Layers
_S_ and _S_, all dipping upstream
_a_, _b_, _c_, _d_, and _e_, successive positions of the fall;
_r_, rapid to which the fall is reduced. Draw diagram showing
migration of fall in strata dipping _downstream_. Under what
conditions of inclination of the strata will a fall migrate the
farthest and have the longest life? Under what conditions will
it migrate the least distance and soonest be destroyed?]
=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.
[Illustration: Fig. 58. Maturely Dissected Plateau near
Charleston, West Virginia
Compare the number of streams in any given number of square
miles with the number on an area of the same size in the Red
River Valley (Fig. 44). What is the shape of the ridges? Are
their summits broad or narrow? Are their crests even or broken
by knobs and cols (the depressions on the crest line)? If the
latter, how deeply have the cols been worn beneath the summits
of the knobs?]
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.
[Illustration: Fig. 59. A Maturity Dissected Region of Slight
Relief, Iowa]
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.
[Illustration: Fig. 60. Successive Stages, _A_, _B_, _C_, and
_D_, in Valley-Widening by Planation
Describe valley _A_. What changes have taken place in _B_, _C_,
and _D_? Do the river bends remain stationary or move up or
down valley? With what effect on the projecting spurs of the
valley sides? Draw diagrams showing a still later stage than _D_]
=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?
[Illustration: Fig. 61. Successive Cross Sections of a Region as
it advances from Infancy _a_, to Old Age _e_]
_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_.[1] 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 subaërial 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.
[1] The term "baselevel" is also used to designate the close
approximation to sea level to which streams are able to
subdue the land.
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.
[Illustration: Fig. 62. Peneplain surrounded by Monadnocks,
Piedmont Belt, Virginia
From Davis' _Elementary Physical Geography]
=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.
[Illustration: Fig. 63. Young Inner Gorge in Wide Older Valley,
Alaska]
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_.
[Illustration: Fig. 64. Incised Meanders of Oneota River, Iowa]
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.
[Illustration: Fig. 65.
Describe the valley of stream _a_. Is it young or old? How does
the valley of _b_ differ from that of _a_? Compare as to form
and age the inner valley of _b_ with the outer valley and with
the valley of _a_. Account for the inner valley. Why does it
not extend to the upper portion of the course of _b_? Will it
ever do so? Draw longitudinal profile of _b_, showing the
different gradient of upper and lower portions of its course
not here seen. As the inner valley of tributary _c_ extends
headward it may invade the valley of _a_ before the inner
valley of _a_ has worked upstream to the area seen in the
diagram. With what results?]
=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.
[Illustration: Fig. 66. Dissected Peneplain of Southern New
England]
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.
[Illustration: Fig. 67. Section in Piedmont Belt
_M_, a monadnock]
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.
[Illustration: Fig. 68. The area of the Laurentian Peneplain
(shaded)]
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 (see Fig. 179, P. 211). 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.
[Illustration: Fig. 69. View in the Cascade Mountains, Washington
The general level to which these ridges rise may be accounted
for by the uplift and dissection of a once low-lying peneplain]
=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?
[Illustration: Fig. 70. Generalized Section of the Southern
Appalachian Region in Eastern Tennessee]
_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.= The deposits which streams build along their courses
at times of flood are known as flood plains. 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 (Fig. 368).
[Illustration: Fig. 71. Cross Section of a Flood Plain]
A cross section of a flood plain (Fig. 71) 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.
[Illustration: Fig. 72. Waste-filled Valley and Braided
Channels of the Upper Mississippi]
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.
[Illustration: Fig. 73. Terraced Valley of River in Central Asia]
[Illustration: Fig. 74. Terraces carved in Alluvial Deposits]
Which is older, the rock floor of the valley or the river
deposits which fill it? What are the relative ages of terraces
_a_, _b_, _c_, and _e_? It will be noted that the remnants of
the higher flood plains have not been swept away by the
meandering river, as it swung from side to side of the valley
at lower levels, because they have been defended by ledges of
hard rock in the projecting spurs of the initial valley. The
stream has encountered such defending ledges at the point
marked _d_]
[Illustration: Fig. 75. River Terraces of Rock covered with
Alluvium
_c_, recent flood plain of the river. To what processes is it
due? Account for the alluvium at _a_ and _b_ and on the
opposite side of the valley at the same levels. Which is the
older? Account for the flat rock floors on which these deposits
of alluvium rest. Give the entire history which may be read in
the section]
=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.
[Illustration: Fig. 76. Development of a Meander
The dotted line in _a_, _b_, and _c_ shows the stage preceding that indicate by the unbroken line]
=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 (Fig. 76, _a_) whose width
and length are not far from equal, and later that of the horseshoe
meander (Fig. 76, _b_) 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 (Fig. 76,
_c_), or oxbow lake, which fills gradually to an arc-shaped shallow
depression.
[Illustration: Fig. 77. Map of a portion of the Flood Plain of
the Missouri River
Each small square represents one square mile. How wide is the
flood plain of the Missouri? How wide is the flood plain of the
Big Sioux? Why is the latter river deflected down valley on
entering the flood plain of the master stream? How do the
meanders of the two rivers compare in size? How does the width
of each flood plain compare with the width of the belt occupied
by the meanders of the river? Do you find traces of any former
channels?]
=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 (Fig 60). 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 (Fig. 77).
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.
[Illustration: Fig. 78. Alluvial Cones, Wyoming]
=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.
[Illustration: Fig. 79. Tributaries and Distributaries of a
Fan-Building Stream]
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.
[Illustration: Fig. 80. Section from the Rocky Mountains Eastward
River deposits dotted]
=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.
[Illustration: Fig. 81. Cross Section of Aggraded Valley,
showing Structure of River Deposits]
=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.
[Illustration: Fig. 82. Digging Peat, Scotland]
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.
[Illustration: Fig. 83. Map of Lake Bonneville and Lahontan
From Davis' _Physical Geography_]
=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.
[Illustration: Fig. 84. Section of Deposits in Beds of Lakes
Bonneville and Lahontan]
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 (_a_ and _c_, Fig. 84). They are
separated by heavy deposits of well-rounded, cross-bedded gravels and
sands (_b_, Fig. 84), 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
(D) 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 subaërial. 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.
[Illustration: Fig. 85. Delta of the Mississippi River]
As the submarine delta grows near to the level of the sea the
distributaries of the river cover it with subaërial deposits
altogether similar to those of the flood plain, of which indeed the
subaërial 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 subaërial 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.
[Illustration: Fig. 86. Radial Section of a Delta
This section of a delta illustrates the structure of the
platform which swift streams well loaded with coarse waste
build in the water bodies into which they empty. Three members
may be distinguished: the _bottom set beds_, _a_: the _fore set
beds_, _b_; and the _top set beds_, _c_. Account for the slope
of each of these. Why are the bottom set beds of the finer
material and why do they extend beyond the others? How does the
profile of this delta differ from that of an alluvial cone and
why?]
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 coöperate with rivers in the denudation of the
land.
[Illustration: Fig. 87. Section of Undifferentiated Drift near
Chicago]
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.
[Illustration: Fig. 88. Characteristic Pebbles from the Drift
No. 1 has six facets; No. 4, originally a rounded river
pebble, has been nibbled down to one flat face; Nos. 3
and 5 are battered subangular fragments on one side only]
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.
[Illustration: Fig. 89. Smoothed and Scored Rock Surface exposed
to View by the Removal of Overlying Drift, Iowa]
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 (Fig. 88). 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 (Fig. 89). 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.
[Illustration: Fig. 90. Map of Greenland
Glacier ice covers all but the areas shaded]
=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_.
[Illustration: Fig. 91. Hypothetical Cross Section of Greenland]
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.
[Illustration: Fig. 92. A Greenland 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° 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.
[Illustration: Fig. 93. Glaciers heading in Snow-filled
Amphitheaters, the Alps]
[Illustration: Fig. 94. Bergschrund of a Glacier in Colorado]
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 névé field.= The head of an Alpine valley occupied by a glacier
is commonly a broad amphitheater deeply filled with snow (Fig. 93).
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 _névé_. Beneath the surface
of the névé field and at its outlet the granular névé has been
compacted to a mass of porous crystalline ice. Snow has been changed
to névé, and névé 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.
[Illustration: Fig. 95. Sea Wall of the Muir Glacier, Alaska]
=The bergschrund.= The névé 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 (Fig. 94).
=Size of glaciers.= Th
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