Geology: The Science of the Earth's Crust
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Title: Geology: The Science of the Earth's Crust
Author: William J. Miller
Release date: December 12, 2014 [eBook #47648]
Most recently updated: October 24, 2024
Language: English
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Transcriber's Note
Italic text is denoted by _underscores_ and bold text by =equal signs=.
Whole and partial numbers are displayed as 2-1/2.
[Illustration: Photo, Ridge Studio; Courtesy Ogden Chamber of
Commerce]
_A great ledge in Ogden Canyon near Ogden, Utah. The rock, still
retaining its stratification, was deposited layer upon layer
horizontally mostly as sand upon the floor of a sea which covered the
region fully 25,000,000 years ago. That the sea was of very early
Paleozoic (i.e., Cambrian) age has been proved by fossils in associated
strata. Long after their deep burial and consolidation within the
earth, the strata were subjected to tremendous mountain-making
pressure, notably altered to a rock called "Quartzite," raised high
above sea level, and tilted almost vertically. Then through long ages
(millions of years) overlying rocks of great thickness have been cut
away (eroded) by weathering and stream action, laying bare the ledge as
we see it to-day._
Popular Science Library
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration]
VOLUME THREE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
By P. F. Collier & Son Company
MANUFACTURED IN U. S. A.
GEOLOGY
The Science of the Earth's Crust
BY
WILLIAM J. MILLER
Professor of Geology, Smith College
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
In the preparation of this book the author has attempted to present, in
popular form, the salient points of a general survey of the whole great
science of geology, the science which deals with the history of the
earth and its inhabitants as revealed in the rocks.
The use of technical and unusual terms has been reduced to a minimum
compatible with a reasonable understanding of the subject by the
layman. Each of the relatively few scientific terms is explained where
first used in the text, and a glossary of common geological terms has
been appended.
The matter of illustrations has received very careful attention, and
only pictures, maps, and diagrams are used which actually illustrate
important features of the text. A special point has been made to
introduce only cuts of simple construction comparatively free from
technicalities. Nearly every illustration is accompanied by a really
explanatory title.
A number of the pictures are from the author's collection of
photographs, and many of the line-cuts have either been made or
considerably modified by the author. Among the numerous sources of
illustrations, special mention should be made of the United States
Geological Survey, the New York State Museum, the American Museum
of Natural History, the University of Chicago Press, and various
individuals, full credit being given wherever due.
William J. Miller.
Northampton, Mass.
CONTENTS
CHAPTER PAGE
I. Introduction 9
II. Weathering and Erosion 22
III. Stream Work 30
IV. The Sea and Its Work 51
V. Glaciers and Their Work 59
VI. The Action of Wind 71
VII. Instability of the Earth's Crust 76
VIII. Volcanoes and Igneous Rocks 99
IX. Waters Within the Earth 113
X. How Mountains Come and Go 130
XI. A Study of Lakes 142
XII. How the Earth May Have Originated 158
XIII. Very Ancient Earth History 164
XIV. Ancient Earth History 179
XV. Medieval Earth History 208
XVI. Modern Earth History 221
XVII. Evolution of Plants 249
XVIII. Geological History of Animals (Excluding
Vertebrates) 259
XIX. Geological History of Vertebrate Animals
(Including Man) 281
XX. Mineralogy 307
XXI. Economic Geology 342
Glossary of Common Geological Terms 377
LIST OF ILLUSTRATIONS
Ledge of Rock with Nearly Vertical Strata _Frontispiece_
PLATE FACING PAGE
1 Granite Weathering to Soil Leaving Residual
Cores of Joint Blocks (_Upper picture_) 32
Looking-Glass Rock, Utah. Stratified Sandstone
Sculptured by Wind Erosion (_Lower picture_) 32
2 Grand Canyon of the Yellowstone River. A
Channel Worn by Erosion 33
3 Gorge of the Niagara River Below the Falls.
A Sample of Recent Erosion 64
4 Winding Stream in the St. Lawrence Valley
with Flood Plain (_Upper picture_) 65
Davidson Glacier, Alaska, Showing Action on
the Valley's Walls and Floor (_Lower picture_) 65
5 Swift Current Valley in Glacier National
Park, Formed by Stream and Glacier Action 96
6 Yosemite Valley from Western Entrance. The
Result of Glacial Action 97
7 An Upbend Fold or Anticline in Maryland 128
8 Ledge of Igneous Rock Showing Joints
(_Upper picture_) 129
Fault Fracture in Limestone Formations
(_Lower pictures_) 129
9 Molten Lava Flowing Over a Cliff in Hawaii
(_Upper picture_) 160
Dikes of Granite Cutting Old Rock
(_Lower picture_) 160
10 Lassen Peak, California, in Eruption
(_Left picture_) 161
Devil's Tower, Wyoming, Once a Mass of Molten
Rock Forced Through Strata (_Right picture_) 161
11 Mammoth Hot Springs Terrace in Yellowstone
National Park (_Upper picture_) 224
Crater Lake, Oregon, Formed by the Subsidence
of a Volcano (_Lower picture_) 224
12 Archeozoic Rock, Oldest Known Rock Formation
on Earth (_Upper picture_) 225
Twisted Mass of Stratified Limestone, Surrounded
by Granite (_Lower picture_) 225
13 Paleozoic Rock, Covered with Oldest Known
Animal Remains 256
14 "Stone-Lily" Stems in Ordovician Strata (_Upper
left picture_) 257
Various Brachiopods in Ordovician Strata (_Upper
right picture_) 257
Stratified Limestone of Middle Ordovician Era
Containing Fossils (_Lower picture_) 257
15 A Landscape of the Coal Age (_Upper picture_) 288
Fossil Fern on a Piece of Shale (_Lower picture_) 288
16 Restoration of Huge Animals of the Mesozoic Era 289
17 Skeleton of Two-Legged Carnivorous Dinosaur
(_Large picture_) 320
Earliest Known Bird Form (_Insert_) 320
18 Skeleton of Large Flying Reptile (_Upper picture_) 321
Skeleton of a Swimming Reptile of the Mesozoic
Era (_Lower picture_) 321
19 Appalachians Along New River, Peneplain Upraised Again 352
20 Glacial Bowlder Left by Ice Sheet (_Upper picture_) 353
Esker, Deposited by a Stream in the Great
Glacier (_Lower picture_) 353
CHAPTER I
INTRODUCTION
Earth features are not fixed. The person of ordinary intelligence,
surrounded as he is by a great variety of physical features, is, unless
he has devoted some study to the subject, very likely to regard those
features as practically unchangeable, and to think that they are now
essentially as they were in the beginning of the earth's history. Some
of the most fundamental ideas taught in this book are that the physical
features of the earth, as we behold them to-day, represent but a single
phase of a very long-continued history; that significant changes
are now going on all around us; and that we are able to interpret
present-day earth features only by an understanding of earth changes in
the past.
_Geology_, meaning literally "earth science," deals with the history of
the earth and its inhabitants as revealed in the rocks. The science is
very broad in its scope. It treats of the processes by which the earth
has been, and is now being, changed; the structure of the earth; the
stages through which it has passed; and the evolution of the organisms
which have lived upon it.
_Geography_ deals with the distribution of the earth's physical
features, in their relation to one another, to the life of sea and
land, and human life and culture. It is the present and outward
expression of geological effects.
As a result of the work of many able students of geology during the
past century and a quarter, it is now well established that our planet
has a definitely recorded history of many millions of years, and that
during the lapse of those eons, revolutionary changes in earth features
have occurred, and also that there has been a vast succession of
living things which, from very early times, have gradually passed from
simple into more and more complex forms. The physical changes and the
organisms of past ages have left abundant evidence of their character,
and the study of the rock formations has shown that within them we have
a fairly complete record of the earth's history. Although very much yet
remains to be learned about this old earth, it is a remarkable fact
that man, through the exercise of his highest faculty, has come to know
so much concerning it.
The following words, by the late Professor Barrell, admirably summarize
the significance of geological history. "The great lesson taught by the
study of the outer crust is that the earth mother, like her children,
has attained her present form through ceaseless change, which marks
the pulse of life and which shall cease only when her internal forces
slumber and the cloudy air and surf-bound ocean no more are moving
garments. The flowing landscapes of geologic time may be likened to a
kinetoscopic panorama. The scenes transform from age to age, as from
act to act; seas and plains and mountains of different types follow and
replace each other through time, as the traveler sees them succeed each
other in space. At times the drama hastens, and unusual rapidity of
geologic action has, in fact, marked those epochs since man has been
a spectator upon the earth. Science demonstrates that mountains are
transitory forms, but the eye of man through all his lifetime sees no
change, and his reason is appalled at the conception of a duration so
vast that the milleniums of written history have not accomplished the
shifting of even one of the fleeting views which blend into the moving
picture."[A]
[A] Central Connecticut in the Geologic Past, pp. 1-2.
Or in the words of Tennyson:
There rolls the deep where grew the tree.
O, earth, what changes hast thou seen!
There where the long street roars, hath been
The stillness of the central sea.
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mist, the solid lands,
Like clouds they shape themselves and go.
The following statement of some of the more definite important
conclusions regarding earth changes may serve to make still clearer the
general scope of the science of geology. The evidences upon which these
conclusions are based are discussed in various parts of this book. For
untold millions of years the rocks at and near the earth's surface
have been crumbling; streams have been incessantly sawing into the
lands; the sea has been eating into continental masses; the winds have
been sculpturing desert lands; and, more intermittently and locally,
glaciers have plowed through mountain valleys, and even great sheets of
ice have spread over considerable portions of continents. Throughout
geologic time, the crust of the earth has shown marked instability.
Slow upward and downward movements of the lands relative to sea level
have been very common, in many cases amounting to even thousands of
feet. Various parts of the earth have been notably affected by sudden
movements (resulting in earthquakes) along fractures in the outer
crust. During millions of years molten materials have, at various
times, been forced into the earth's crust, and in many cases to its
surface. Mountain ranges have been brought forth and cut down. The site
of the Appalachian Mountains was, millions of years ago, the bottom of
a shallow sea. Lakes have come and gone. The Great Lakes have come into
existence very recently (geologically), that is to say, since the great
Ice Age. A study of stratified rocks of marine origin shows that all,
or nearly all, of the earth's surface has at some time, or times, been
covered by sea water. Over certain districts the sea has transgressed
and retrogressed repeatedly. Organisms have inhabited the earth for
many millions of years. In earlier known geologic time, the plants and
animals were comparatively simple and low in the scale of organization,
and through the succeeding ages higher and more complex types were
gradually evolved until the highly organized forms of the present time,
including the human race, were produced.
The rocks of the earth constitute the special field of study for the
geologist because they contain the records of events through which the
earth and its inhabitants have passed during the millions of years of
time until their present conditions have been reached. All the rocks of
the earth's crust may be divided into three great classes: _igneous_,
_sedimentary_, and _metamorphic_.
_Igneous_ rocks comprise all those which have ever been in a molten
condition, and of these we have the _volcanic_ rocks (for example,
lavas), which have cooled at or near the surface; _plutonic_ rocks (for
example, granites), which have cooled in great masses at considerable
depths below the surface; and the _dike_ rocks which, when molten, have
been forced into fissures in the earth's crust and there cooled.
_Sedimentary_ rocks comprise all those which have been deposited under
water, except some wind-blown deposits, and they are nearly always
arranged in layers (stratified). Such rocks are called strata. They may
be of mechanical origin such as clay or mud which hardens to _shale_;
sand, which consolidates into _sandstone_; and gravel, which when
cemented becomes _conglomerate_. They may be of organic origin such as
limestone, most of which is formed by the accumulation of calcareous
shells; _flint_ and _chert_, which are accumulations of siliceous
shells; or _coal_, which is formed by the accumulation of partly
decayed organic matter. Or, finally, they may be formed by chemical
precipitation, as beds of _salt_, _gypsum_, _bog iron ore_, etc.
_Metamorphic_ rocks include both sedimentary and igneous rocks which
have been notably changed from their original condition. Traces or
remains of plants and animals preserved in the rocks are known as
fossils. The term originally meant anything dug out of the earth,
whether organic or inorganic, but for many years it has been strictly
applied to organic remains. Many thousands of species of fossils are
known from rocks of all ages except the oldest, and more are constantly
being brought to light, but these represent only a small part of
the life of past ages because relatively few organic remains were
deposited under conditions favorable for preservation in fossil form.
The fossils in the rocks are, however, a fair average of the groups of
organisms to which they belong. It is really remarkable that such a
vast number of fossils are imbedded in the rocks, and from a study of
these many fundamental conclusions regarding the history of life on our
planet may be drawn.
As early as the fifth century B. C., Xenophanes is said to have
observed fossil shells and plants in the rocks of Paros, and to
have attributed their presence to incursions of the sea over the
land. Herodotus, about a century later, came to a similar conclusion
regarding fossil shells in the mountains of Egypt. None of the
ancients, however, seemed to have the slightest conception of the
significance of fossils as time markers in the history of the earth.
(See discussion below.)
In the Middle Ages, distinguished writers held curious views regarding
fossils. Thus Avicenna (980-1037) believed that fossils represented
unsuccessful attempts on the part of nature to change inorganic
materials into organisms within the earth by a peculiar creative force
(_vis plastica_). About two centuries later, Albertus Magnus held
a somewhat similar view. Leonardo da Vinci (1452-1519), the famous
artist, architect, and engineer, while engaged in canal building in
northern Italy, saw fossils imbedded in the rocks, and concluded
that these were the remains of organisms which actually lived in
sea water which spread over the region. During the seventeenth and
eighteenth centuries, many correctly held that fossils were really
of organic origin, but it was commonly taught that all fossils
represented remains of organisms of an earlier creation which were
buried in the rocks during the great Deluge (Noah's Flood). William
Smith (1769-1839), of England, was, however, the first to recognize
the fundamental significance of fossils for determining the relative
ages of sedimentary rocks. This discovery laid the foundation for the
determination of earth chronology which is of great importance in the
study of the history of the earth. (See discussions below.)
Organic remains, dating as far back as tens of millions of years, have
been preserved in the rocks of the earth in various ways. A very common
kind of fossilization is the preservation of only the hard parts of
organisms. Thus the soft parts have disappeared by decomposition, while
the hard parts, such as bones, shells, etc., remain. In many cases
practically complete skeletons of large and small animals which lived
millions of years ago have been found intact in the rocks. Fossils
which show none of the original material, but only the shape or form,
are also very abundant. When sediment hardens around an imbedded
organism, and the organism then decomposes or dissolves away, a cavity
or fossil mold only is left. Casts of organisms or parts of them are
formed by filling shells or molds with sediment or with mineral matter
carried in solution by underground water. Only rarely have casts of
wholly soft animals been found in ancient rocks. In other cases both
original form and structure are preserved, but none of the original
material. This is known as petrifaction which takes place when a plant
or hard part of an animal has been replaced, particle by particle,
by mineral matter from solution in underground water. Not uncommonly
organic matter, such as wood, or inorganic matter, such as carbonate
of lime shells, has been so perfectly replaced that the original
structures are preserved almost as in life. The popular idea that
petrified wood is wood which has been changed into stone is, of course,
incorrect. It is doubtful if flesh has ever been truly petrified. In
many cases mainly the carbon only of organisms has been preserved.
This is also true of plants where, under conditions of slow chemical
change or decomposition, the hydrogen and oxygen mostly disappear,
leaving much of the carbon with original structures often remarkably
preserved. Fine examples are fossil plants in the great coal-bearing
strata. Much more rarely entire organisms have been preserved either
by freezing or by natural embalmment. Most remarkable are the species
of mammoths and rhinoceroses, extinct for thousands of years, bodies
of which, with flesh, hide, and hair still intact, have been held in
cold storage in the frozen soils of Siberia, or other cases. Insects
have been perfectly preserved in amber, as, for example, in the Baltic
region. This amber is a hardened resin in which the insects were
caught while it was still soft and exuding from the trees. Finally, we
should mention the preservation of tracks and trails of land and water
animals. Thousands of tracks of long-extinct great reptiles occur in
the sandstones and shales of the Connecticut Valley of Massachusetts.
The footprints were made in soft sandy mud which hardened and then
became covered with more sediment.
Few fossils occur in other than the sedimentary rocks. Most numerous,
by far, are fossils in rocks of marine origin, because on relatively
shallow sea bottoms, where sediments of the geologic ages have
largely accumulated, the conditions for fossilization have been
most favorable. Among the many conditions which have produced great
diversity in numbers and distribution of marine organisms during
geologic time are temperature, depth of water, clearness of water,
nature of sea bottom, degree of salinity, and food supply. River
and lake deposits also not uncommonly contain remains of organisms
which inhabited the waters, but also others which were carried in.
"Surrounding trees drop their leaves, flowers, and fruit upon the mud
flats, insects fall into the quiet waters, while quadrupeds are mired
in mud or quicksand and soon buried out of sight. Flooded streams bring
in quantities of vegetable debris, together with carcasses of land
animals drowned by the sudden rise of the flood" (W. B. Scott).
In the study of the many changes which have taken place in the history
of the earth, a fundamental consideration is the determination of
the relative ages of the rocks, especially the strata. How can the
geologist assign a rock formation of any part of the earth to a
particular age in the history of the earth? How can it be proved that
certain rock formations in various parts of the earth originated
practically at the same time? There are two important criteria.
First, in any region where the strata have not been disturbed from
their normal order, the older strata underlie the younger because the
underlying sediments must have been deposited first. Now, the total
thickness of the stratified series of the earth has been estimated to
be no less than 200,000 feet and only a small part of this is actually
present in any given locality or region. It is, therefore, evident
that the order of superposition of strata is in itself not sufficient
for the determination of the relative ages of all the strata in even
a considerable portion of a single continent, not to mention its utter
inadequacy in building up the geological column of the whole earth.
When, however, the second criterion, namely, the fossil content of the
strata, is used in direct connection with the order of superposition,
we have the real basis for determining the relative ages of strata for
all parts of the earth. The discovery of this method was very largely
due to the painstaking field work in England by William Smith about the
beginning of the nineteenth century.
It is a well-established fact that organisms have inhabited the earth
for many millions of years and that, through the geologic ages,
they have continuously changed, with gradual development of higher
and higher types. Tens of thousands of species have come and gone.
Accepting this fact, it is then clear that strata which were formed
at notably different times must contain notably different fossils,
while strata which accumulated at practically the same time contain
similar fossils, allowing, of course, for reasonable differences in
geographical distribution of organisms as at the present time. Each
epoch of earth history or series of strata has its characteristic
assemblage of organisms. In short, "a geological chronology is
constructed by carefully determining, first of all, the order of
superposition of the stratified rocks, and next by learning the fossils
characteristic of each group of strata.... The order of succession
among the fossils is determined from the order of superposition of
the strata in which they occur. When that succession has been thus
established, it may be employed as a general standard" (W. B. Scott).
It should, however, be borne in mind that precise contemporaneity of
strata in widely separated districts can rarely, if ever, be determined
because of the very great length of geologic time and the general
slowness of the evolution of organisms. Rocks carrying remarkably
similar fossils may really be several thousand years different in age;
but this is, indeed, a very small limit of error when one considers the
vast antiquity of the earth. Much very accurate and satisfactory work
has been done, especially in Europe and North America, in correlating
strata and assigning them to their places in the geological time table
(see below), but a vast amount of work yet remains to be done before
the task is complete.
Certain types or species of organisms are much more useful than others
in the determination of earth chronology. Best of all for world-wide
correlations are species which were widely distributed and which
persisted for relatively short times. Thus any species which lived in
the surface waters of the ocean and was easily distributed over wide
areas, while, at the same time, it existed as such only a short time,
is the best type of chronologic indicator.
The known history of the earth has been more or less definitely divided
into great eras and lesser periods and epochs, constituting what may
be called the geologic time scale. In the accompanying table the era
and period names, except those representing earlier time, are mostly
world-wide in their usage. Epoch names, being more or less locally
applied, are omitted from the table. Very conservative estimates of
the length of time represented by the eras and the most characteristic
general features of the life of the main divisions are also given.
PRINCIPAL DIVISIONS OF GEOLOGIC TIME
(Modified after U. S. Geological Survey.)
=========================================================================
| | |Millions
Era. | Period. | Characteristic life. |of years
| | |estimated
-----------+--------------+------------------------------------+---------
| |"Age of man." Animals and plants |
|Quaternary. |of modern types. |
+--------------+------------------------------------+
Cenozoic | |"Age of mammals." Rise of highest |3 to 5.
|Tertiary. |animals except man. Rise and |
| |development of highest orders of |
| |plants. |
-----------+--------------+------------------------------------+---------
| |"Age of reptiles." Rise and |
|Cretaceous. |culmination of huge land reptiles |
| |(dinosaurs), of shellfish with |
| |complexly partitioned coiled |
+--------------+shells (ammonites), and of great |
| |flying reptiles. First appearance |
Mesozoic |Jurassic. |(in Jurassic) of birds and mammals; |5 to 10.
| |of cycads, an order of palm-like |
+--------------|plants (in Triassic); and of |
| |angiospermous plants, among |
|Triassic. |which are palms and hardwood |
| |trees (in Cretaceous). |
-----------+--------------+------------------------------------+---------
| |"Age of amphibians." Dominance of |
|Permian. |club mosses (lycopods) and plants |
+--------------+of horsetail and fern types. |
| |Primitive flowering plants and |
|Pennsylvanian.|earliest cone-bearing trees. |
| |Beginnings of back-boned land |
+--------------+animals (land vertebrates). Insects.|
|Mississippian.|Animals with nautiluslike coiled |
| |shells (ammonites) and sharks |
| |abundant. |
+--------------+------------------------------------+
| |"Age of fishes." Shellfish |
|Devonian. |(mollusks) also abundant. Rise of |
| |amphibians and land plants. |
+--------------+------------------------------------|
| A {|Shell-forming sea animals dominant, |
| g {|especially those related to |
| e {|the nautilus (cephalopods). Rise |
Paleozoic | {|and culmination of the marine |17 to 25.
|Silurian. o {|animals sometimes known as sea |
| f {|lilies (crinoids) and of giant |
| {|scorpionlike crustaceans |
| I {|(eurypterids). Rise of fishes and |
| n {|of reef-building corals. |
+-----------n-{+------------------------------------+
| v {|Shell-forming sea animals, |
| e {|especially cephalopods and |
|Ordovician.r {|mollusk-like brachiopods, abundant. |
| t {|Culmination of the buglike marine |
| e {|crustaceans known as trilobites. |
+-----------b-{+------------------------------------+
| r {|Trilobites and brachiopods most |
|Cambrian. a {|characteristic animals. Seaweeds |
| t {|(algæ) abundant. No trace of |
| e {|land animals found. |
-----------+-----------s-{+------------------------------------+---------
| |First life that has left distinct |
Proterozoic|Algonkian. |record. Crustaceans, brachiopods, |
| |and seaweeds. |
-----------+--------------+------------------------------------+
| |Organic matter in form of graphite |25 to 50+
Archeozoic |Archean. |(black lead), but no determinable |
| |fossils found. |
-----------+--------------+------------------------------------+---------
The length of time represented by human history is very short compared
to the vast time of known geological history. The one is measured
by thousands of years, while the other must be measured by tens of
millions of years. Just as we may roughly divide human history into
certain ages according to some notable person, nation, principle, or
force as, for example, the "Age of Pericles," the "Roman Period,"
the "Age of the French Revolution," or the "Age of Electricity," so
geologic history may be subdivided according to great predominant
physical or organic phenomena, such as "the Appalachian Mountain
Revolution" (toward the end of the Paleozoic era), the "Age of Fishes"
(Devonian period), or the "Age of Reptiles" (Mesozoic era).
In the study of earth history, as in the study of human history, it
is important to distinguish between events and records of events.
Historical events are continuous, but they are by no means all
recorded. Records of events are often interrupted and seemingly sharply
separated from each other.
CHAPTER II
WEATHERING AND EROSION
All rocks at and near the surface of the earth crumble or decay.
The term "weathering" includes all the processes whereby rocks are
broken up, decomposed, or dissolved. A mass of very hard and seemingly
indestructible granite, taken from a quarry, will, in a very short
time, geologically considered, crumble (Plate 1). During the short
span of the ordinary human life weathering effects are generally of
very little consequence, but during the long ages of geologic time the
various processes of weathering have been slowly and ceaselessly at
work upon the outer crust of the earth, and such tremendous quantities
of rock material have been broken up that the lands of the earth have
everywhere been profoundly affected.
Most of us have noticed buildings and monuments in which the stones
show marked effects of weathering. A good case in point is Westminster
Abbey, London, in which many of the stones are badly weathering, some
of the more ornamental parts having crumbled beyond recognition since
the building was erected in the thirteenth century. In many countries,
tombstones and monuments only one or two centuries old are so badly
weathered that the inscriptions are scarcely if at all legible.
What are some of the processes of nature whereby rocks are weathered?
In cold countries, and often in mountains of generally mild climate
regions, the alternate freezing and thawing of water is a potent agency
in breaking up rocks where the soils are thin or absent. On freezing,
water expands about one-tenth of its volume and exerts the enormous
pressure of over 2,000 pounds per square inch. Nearly all relatively
hard rock formations are separated into more or less distinct blocks by
natural cracks called "joints" (Plate 8). Very commonly the rocks also
contain minute crevices, fissures, and pores. Repeated freezing and
thawing of water which finds its way into such openings finally causes
even the most resistant rocks to break up into smaller and smaller
fragments. A very striking example of difference in climatic effect
upon a given rock mass is the obelisk in Central Park, New York. For
many centuries this famous monument stood practically without change in
the dry, frostless climate of Egypt, but very soon after its removal to
the moist, frosty climate of New York, it began to crumble so rapidly
that it was necessary to cover it with a coating of glaze to protect it
from the atmosphere.
Temperature change, especially in dry regions, is also an important
agency for mechanical breaking up of rocks. On high mountains and on
deserts, a daily range of temperature of from 70 degrees to 80 degrees
is frequent. Due to heat absorption, rocks in desert regions, during
the day, not uncommonly reach temperatures of fully 120 degrees, while
during the night, due to heat radiation, their temperature falls
greatly. During the heating of the outer portion of the rock, the
various minerals each expand differently, thus setting up a series
of stresses and strains tending to cause the minerals to pull apart.
The outer portions of the rocks which are subjected to unstable and
relatively rapid temperature changes, often crack or peel off in slabs
or flakes, this process being called exfoliation. Stone Mountain in
Georgia, and some of the mountains of the southern Sierra Nevada range
in California, are excellent examples of mountains which are being
rounded off by exfoliation. The principle is the same as that which
causes the "spalling" of stones in buildings during fires.
Masses of débris consisting of more or less angular rock fragments of
all sizes commonly occur at the bases of cliffs and mountains. They
represent materials which have weathered off the ledges mainly by frost
action and temperature changes.
Where electrical storms are frequent, lightning often shatters
portions of rock ledges. Many such cases have come under the writer's
observation in the Adirondack Mountains of New York. The total effect
of lightning as a weathering agency is, however, relatively small.
Another minor weathering effect is the mechanical action of plants. The
principle is well illustrated by the breaking or tilting of sidewalks
by the wedging action of the growing roots of trees. In many places the
roots of plants growing in cracks in rocks, exert powerful pressure
causing the rocks or blocks of rocks to wedge apart.
Let us now briefly consider some of the chemical processes of
weathering. The solvent effect of perfectly pure water upon rocks is
very slight and slow. But such water is not found in nature because
certain atmospheric gases, especially oxygen and carbonic acid gas, are
always present in it, and they notably increase the solvent power of
the water. Such water has the power to slowly but completely dissolve
the common rock called limestone which consists of carbonate of lime.
This material is then carried away by the streams. Rocks, like certain
sandstones which contain carbonate of lime cementing material, are
caused to crumble due to removal of the cement in solution. Carbonic
acid gas in water also has the power to chemically alter various
minerals in many common rocks and thus the rocks fall apart and the
carbonates which result from the action usually are carried away in
solution. One of the most important changes of this kind takes place
when the very common mineral feldspar is attacked by water containing
carbonic acid gas and the mineral alters to a soluble carbonate, kaolin
(or clay) and silica.
The oxygen, both of the air and that which is contained in water, is
a very important chemical agent of decomposition of many rocks. Water
at the surface and the upper part of the crust of the earth as well
as moisture in the air are also important chemical agents which bring
about rock decay. We are all familiar with the rusting of iron which
is due to the chemical union of the iron with oxygen, thus forming an
iron oxide which in turn commonly unites with water from air or earth.
Now, many rocks contain iron, not as such, but held in combination with
other substances in the form of various minerals, and this iron of the
rocks, where subjected to the oxygen and moisture of air or water,
slowly unites with the oxygen and water to form a hydrated iron oxide
which is essentially iron-rust. The minerals containing considerable
iron are, therefore, decomposed and the rocks crumble. There are
various iron oxides, usually more or less hydrated, ranging in color
from red through brown to yellow, and these constitute probably
the most common and striking colors of the rocks of the earth. The
gorgeously colored Grand Canyon of the Yellowstone River is a very fine
example of large scale coloring due to development of much hydrated
oxide of iron during the weathering of lava rock, the process having
been aided by the action of heated underground waters.
Most of the soils of the earth are the direct result of weathering.
Important exceptions are soils which have been transported by the
action of water, ice, or wind. Although the process of weathering is
very slow and relatively superficial, it is, nevertheless, true that
in many places, the products of weathering form faster than they can
be carried away. Such weathered materials accumulate in their place
of origin to form soils. The upper few hundred feet of the earth's
crust is everywhere more or less fractured and porous and the rocks
are there affected in varying degrees by most of the ordinary agents
of weathering. In such cases, outside the areas which were recently
covered by ice during the great Ice Age, it is common to find the loose
soil grading downward into rotten rock, and this in turn into the fresh
practically unaltered bedrock. Soils of this kind are generally not
more than ten or twenty feet deep, though under exceptional conditions,
as in parts of Brazil, they attain depths of several hundred feet.
In order to make still clearer some of the above principles of
weathering and also to give the reader some understanding of the most
common types of residual soils, we shall consider what happens to a
few rather definite types of ordinary rocks when they are subjected to
weathering. A very simple case is that of a sandstone, the mineral
grains (mostly quartz) of which are held together by carbonate of
lime. The lime simply dissolves and is carried away, while many of
the mineral grains may remain to form a soil of nearly pure sand.
Where oxide of iron forms the cementing material, the rock yields less
readily to weathering, and the sandy soil will be yellowish brown or
red according to the climate. Another simple case is that of limestone
which when perfectly pure yields no soil because it is all soluble.
Pure limestone is, however, rare, and the various mineral impurities
in it, being to a considerable degree insoluble, tend to remain to
form a residual soil which may vary from sandy to clayey, and usually
brown or red due to the setting free of oxides of iron. According to
one estimate a thickness of about 100 feet of a certain fairly impure
limestone formation in Virginia must weather to yield a layer of soil
one foot thick. Soils of this kind, which are usually rich, are common
in many limestone valleys of the Appalachian Mountains. In the case of
shale rock, which is hardened mud, the cementing materials are removed,
some chemical changes in the minerals may take place, and the rock
crumbles to a claylike soil. What happens to a very hard, resistant
igneous rock like granite when attacked by the weather? Such a rock
always consists mainly of the two very common minerals feldspar and
quartz, usually with smaller amounts of other minerals such as mica,
hornblende, augite, or magnetite. The feldspar, which when fresh is
harder than steel, slowly yields when attacked by water containing
carbonic acid gas and crumbles or decays to a mixture of kaolin (clay),
carbonate of potash, and silica (quartz). Clay is an important
constituent of most good soils, while the carbonate of potash is
essential as a food for most plants. Due to yielding of the grains
or crystals of feldspar, the granite falls apart (see Plate 1). The
grains of quartz remain chemically unchanged, though they may be more
or less broken by changes of temperature, and the other minerals, which
are mostly iron-bearing, yield more or less to weathering, resulting
in a variety of products, among which are oxides of iron. A typical
granite, therefore, gives rise to a good heavy soil which is yellow,
brown or red according to climate. Such granite soils are common in
many parts of the Piedmont Plateau from Maryland to Georgia. Most of
the dark-colored igneous rocks, like ordinary basaltic lava, contain
much feldspar, various iron-bearing minerals, and little or no quartz.
Such rocks yield to the weather like granite but, because of lack of
quartz, the soils are more clayey. Rich soils of this kind occur in the
great lava fields of the northwestern United States and in the Hawaiian
Islands.
The importance of the breaking down of feldspar under the influence of
the weather, as above described, not only from the standpoint of soil
development, but also as regards the wearing down of the lands of the
earth, is difficult to overemphasize because that mineral is by far the
most abundant constituent of the earth's crust.
The term "erosion" is one of the most important in geologic science.
It comprises all the processes whereby the lands of the earth are
worn down. It involves the breaking up of earth material, and its
transportation through the agency of water, ice, or wind. Weathering,
including the various subprocesses as above described, is a very
important process of erosion. By this process much rock material is
got into condition for transportation. Another process of erosion,
called "corrasion," consists in the rubbing or bumping of rocks
fragments of all sizes carried by water, ice, or wind against the
general country rock, thus causing the latter to be gradually worn
away. A fine illustration of exceedingly rapid corrasion of very hard
rock was that of the Sill tunnel in Austria, which was paved with
granite blocks several feet thick. Water carrying large quantities of
rock fragments over the pavement at high velocity caused the granite
blocks to be worn through in only one year. Ordinarily in nature,
however, the rate of wear is much slower than this. Pressure exerted
upon the country rock by any agency of transportation may cause
relatively loose joint blocks, into which most rock formations are
separated, to be pushed away. This process, called "plucking," is
especially effective in the case of flowing ice.
CHAPTER III
STREAM WORK
Most streams are incessantly at work cutting or eroding their way into
the earth's crust and carrying off the products of weathering. By this
means the general level of lands is gradually being reduced to nearer
and nearer sea level. Base level of erosion is reached when any stream
has eroded to its greatest possible depth, and a whole region is said
to be base-leveled when, by the action of streams, it has been reduced
to a practically flat condition. A region of this kind is known as a
"peneplain."
To one who has not seriously considered the matter, the power of even
moderately swift water to transport rock débris seems incredible.
A well-established law of transportation by running water is that
the transporting power of a current varies as the sixth power of its
velocity. For example, a current which is just able to move a rock
fragment of a given size will, when its velocity is merely doubled, be
able to move along a piece of similar rock sixty-four times as large!
That this must be the case may be readily proved as follows: A current
of given velocity is just able to move a block of rock, say, of one
cubic inch in the form of a cube. A cubic block sixty-four times as
large has a face of sixteen square inches. By doubling the velocity of
the current, therefore, twice as much water must strike each of the
sixteen square inches of the face of the larger block with twice the
force, thus exerting sixty-four times the power against the face of the
larger block, or enough to move it along. This surprising law accounts
for the fact that in certain floods, like the one which rushed over
Johnstown, Pennsylvania, in 1889, locomotives, massive iron bridges,
and great bowlders were swept along with great velocity. It is obvious,
then, that ordinarily swift rivers in time of flood accomplish far more
work of erosion (especially transportation) than during many days or
even some months of low water.
Few people have the slightest idea as to the enormous amount of earth
material which the rivers are carrying into the sea each year. The
burden carried by the Mississippi River has been carefully studied for
many years. Each year this river discharges about 400,000,000 tons of
material in suspension; 120,000,000 tons in solution; and 40,000,000
tons rolled along the bottom. This all represents earth material
eroded from the drainage basin of the river. It is sufficient to cover
a square mile 325 feet deep, or if placed in ordinary freight cars
it would require a train reaching around the earth several times to
contain it. Since the drainage basin of the Mississippi covers about
1,250,000 square miles, it is, therefore, evident that this drainage
area is being worn down at the average rate of about one foot in
3,840 years, and this is perhaps, a fair average for the rivers of
the earth. The Ganges River, being unusually favorably situated for
rapid erosion, wears down its drainage basin about one foot in 1,750
years. It has been estimated that nearly 800,000,000 tons of material
are annually carried into the sea by the rivers of the United States.
According to this the country, as a whole, is being cut down at the
rate of about one foot in 9,000 years. In arriving at this figure it
should, of course, be borne in mind that the average level of hundreds
of thousands of square miles of the western United States, particularly
the so-called Great Basin, is practically not being reduced at all
because none of the streams there reach the sea.
Deposition of sediment is an important natural consequence of erosion.
The destination of most streams is the sea, and where tides are
relatively slight the sediments discharged mostly accumulate relatively
near the mouths of the rivers in the form of flat, fan-shaped delta
deposits. Some rivers, like the Ganges, which carry such unusual
quantities of sediment, are able to construct deltas in spite of
considerable tides. Deltas also form in lakes. In most cases, however,
rivers enter the sea where there are considerable tides and their
loads are more widely spread over the marginal sea bottom. But in many
cases some of the sediment does not reach the mouth of the stream. It
is, instead, deposited along its course either where the velocity is
sufficiently checked, as is the case over many flood-plain areas of
rivers, or where a heavily loaded, relatively swift stream has its
general velocity notably diminished. An excellent example of the latter
type of stream is the Platte River, which is swift and loaded with
sediment in its descent from the Rocky Mountains, but, on reaching
the relatively more nearly level Nebraska country, it has its current
sufficiently checked to force it to deposit sediment and build up its
channel along many miles of its course, and this in spite of the fact
that it still maintains a considerable current. In a mountainous arid
region a more or less intermittent stream at times of flood becomes
heavily loaded with rock débris and rushes down the mountain side.
On reaching the valley floor the velocity is greatly checked and most
of the load is deposited at the base of the mountain, successive
accumulations of such materials, called alluvial cones or fans, having
not uncommonly built up to depths of hundreds, or even several thousand
feet.
[Illustration: Plate 1.--(_a_) Granite Weathering to Soil near
Northampton, Mass. Under the action of weathering all of the once hard,
fresh, mass of granite has crumbled to soil except the fairly fresh
rounded masses which are residual cores of "joint blocks." (_Photo by
the author._)]
[Illustration: Plate 1.--(_b_) Looking-Glass Rock, Utah. The rock is
stratified sandstone sculptured mainly by wind erosion, that is, by
the wind driving particles of sand against it. (_Photo by Cross, U. S.
Geological Survey._)]
[Illustration: Plate 2.--Grand Canyon of the Yellowstone River in
Yellowstone National Park. The great waterfall 308 feet high is shown.
The large swift river has here sunk its channel (by erosion) to a
maximum depth of 1,200 feet during very recent geological time, and the
process is still going on. The wonderful coloring is due to iron oxides
set free during weathering of the lava rock. (_Photo by Hillers, U. S.
Geological Survey._)]
Any newly formed land surface, like a recently drained lake bed or
part of the marginal sea bottom which has been raised into land, has
a drainage system developed upon it. In the early or youthful stage
of such a new land area lying well above sea level, under ordinary
climatic conditions a few streams only form and these tend to follow
the natural or initial slope of the land. These streams carve out
narrow, steep-sided valleys, and all of them are actively engaged
in cutting down their channels, or, in other words, none of them
have reached base level, and flood plains and meandering curves are
therefore lacking. During this youthful stage there are no sharp
drainage divides; gorges and waterfalls are not uncommonly present;
and the relief of the land in general is not rugged. A good example of
youthful topography is the region around Fargo, North Dakota, which is
part of the bed of a great recently drained lake. The Grand Canyon of
the Yellowstone River is an excellent illustration of a youthful valley
cut in a high plateau of geologically recent origin. (Plate 2.)
As time goes on, a region in youth gradually gives way to a region in
maturity, during which stage the maximum number (usually a network)
of streams in broader V-shaped valleys have developed; divisions of
drainage are sharp; the maximum ruggedness of relief has developed;
the larger streams only have cut down so near base level that winding
(meandering) courses and flood plains are well developed along them;
and waterfalls and gorges are rarely present. An almost perfect example
of a region in maturity is that around Charleston, West Virginia.
The old-age stage develops next in the history of the region, during
which only a moderate number of streams remain, most of these being
at or close to base level so that sweeping curves or meanders (Plate
4) and cut-off meanders or "ox bows" and wide flood plains are
characteristic and common. The relief is greatly subdued and the term
"rolling country" might be applied to the moderately hilly region.
Divisions of drainage are, of course, not at all sharp and the valleys
are wide and shallow. Oxbow lakes are common, but gorges and waterfalls
are absent. A region typical of old-age topography is that around
Caldwell, Kansas.
Finally, after the remaining low hills have been cut down, the region
is in the condition of a broad monotonous plain, practically devoid
of relief, over which the sluggish streams pursue very winding, more
or less shifting or indefinite courses. For the attainment of this
final stage (called a "peneplain") in the normal cycle of erosion
a proportionately very long time is necessary, because the rate of
erosion becomes slower and slower as the region is being cut down.
Then, too, some change of level between the land and the sea is very
likely to take place before the peneplain stage is reached. It is
doubtful if any extensive region was ever brought to the condition of
a perfect peneplain. Some masses of more resistant or more favorably
situated rocks are almost sure to maintain at least moderate heights
above the general plain level. Geologically recently upraised, fairly
well developed peneplains are southern New England and the great
region of eastern Canada. The remarkably even sky lines of these
regions mark the peneplain level before the uplift took place, and
occasionally masses, called "monadnocks" from Mount Monadnock in
southern New Hampshire, rise above the general level. The valleys in
such an uplifted peneplain region have been carved out by streams
since the uplift began. We have positive evidence that more or less
well-developed peneplains of considerable extent existed in various
parts of the earth at various times during the many millions of years
of known earth history.
The normal cycle of erosion which, as outlined above, tends toward the
peneplain condition may be interrupted at any stage by other processes.
An excellent case in point is the upper Mississippi Valley, which had
reached the old-age stage, even approximating a peneplain, just before
the great Ice Age. Then, during the withdrawal of the vast sheet of ice
from the region toward the close of the Ice Age, extensive deposits
(moraines, etc.) of glacial débris were left irregularly strewn over
the country, giving rise to many low hills, lake basins, and altered
drainage lines, in some cases with resultant gorge development. Some
distinct features of a youthful topography are, therefore, plastered
over what was otherwise a region well along in old age. The general
district around the Dells of Wisconsin River well illustrates this
principle.
Changes in level between land and sea which take place during the
erosion of a region may also disturb the normal cycle of erosion. For
example, a region in old age may be considerably upraised so that
the streams have their velocities notably increased. Such a region
is said to be "rejuvenated" and the streams, which are revived in
activity, begin to cut youthful valleys in the bottom of the old ones
and, after a time, the general surface of the region is subjected to
vigorous erosion and a new cycle of erosion will be carried out unless
interfered with in some way, as by relative change of level between the
land and the sea. In this connection the history of the topography in
the general vicinity of Harrisburg, Pennsylvania, may be of interest by
way of illustration of the principle just described. The long, narrow,
parallel Appalachian Mountain ridges there rise to about the same
level, causing a remarkably even sky line as viewed from one of the
summits. This even sky line marks approximately the surface of what was
a peneplain late in the Mesozoic era. Early in the succeeding Cenozoic
era, the broad peneplain was notably upraised to nearly the present
altitudes of the ridge tops. The revived Susquehanna River left the
old course which it had on the peneplain surface, and began to carve
out its present valley, while tributaries (subsequent streams) to it
developed along belts of weaker rock and thus they formed the present
parallel valleys separated by belts of more resistant rocks which stand
out as ridges. In this way, the mature stage of topography was reached.
Very recently, geologically, the region has been rejuvenated enough to
cause the larger streams to appreciably sink their channels below the
general valley floors. The reader will find a general discussion of
movements of the earth's crust in a succeeding chapter.
[Illustration: Fig. 1.--The submerged Hudson River channel is clearly
shown by the contour lines on the sea floor. Figures indicate depth of
water in fathoms. Geologically recent sinking of the land has caused
the "drowning" of the river valley. (Coast and Geodetic Survey).]
If, for example, a region along the seaboard has reached the mature
stage of erosion, and the land notably subsides relative to sea level,
the tidewater will enter the lower valleys to form estuaries and the
valleys are said to be "drowned." The large streams, or at least their
lower courses, are thus obliterated and also the general erosion of the
region is distinctly diminished. The recently sunken coast of Maine
well illustrates the idea of "drowned valleys." The drowned valley of
the lower Hudson River is another fine example.
[Illustration: Fig. 2.--Sketch maps showing how the Shenandoah River
captured the upper waters of Beaverdam Creek in Virginia. The abandoned
valley of the creek across Blue Ridge is now called a "wind gap."
(After B. Willis.)]
What is termed stream "piracy" is of special interest in connection
with stream work. By this is meant the stealing of one stream or
part of a stream by another. We shall here explain only one of the
various ways by which stream capture may be effected. One of two
fairly active streams, flowing roughly parallel to each other, is more
favorably situated and has cut its channel deeper. Its tributaries
are, therefore, more favorable to extension of headwaters and, in
time, one of its tributaries eats back far enough to tap a branch of
the less favorably situated stream so that the waters of this branch
are diverted into the more favorably situated stream. The Shenandoah
River of Virginia has been such a pirate. This river developed as a
tributary of the Potomac. By headward extension toward the south, the
Shenandoah finally tapped and diverted the upper waters of the smaller,
less favorably situated Beaverdam Creek. The notch or so-called "wind
gap" through which the upper waters of Beaverdam Creek formerly flowed
across the Blue Ridge is still plainly visible. Such abandoned water
gaps, known as "wind gaps," are common in the central Appalachian
Mountain region.
A remarkable type of river is one which has been able to maintain
its course through a barrier, even a mountain range, which has been
built across it. Thus, the Columbia River, after flowing many miles
across the great lava plateau, has maintained its course right across
the growing Cascade Range by cutting a deep canyon while the mountain
uplift has been in progress. In a similar manner the Ogden River of
Utah has kept its westward course by cutting a deep canyon into the
Wasatch Range which has geologically recently, though slowly, risen
across its path. In no other way can we possibly explain the fact that
such a river, rising on one side of a high mountain range, cuts right
across it.
A feature of minor though considerable popular interest is the
development of "potholes" by stream action. Where eddies occur, in
rather active streams, rock fragments of varying sizes may be whirled
around in such manner as to corrode or grind the bedrock, resulting
in the development of cylinder-shaped "potholes." Such holes vary in
diameter up to fifty feet or more in very exceptional cases. In the
production of large "potholes" many rock fragments are worn away and
new ones supplied to continue the work. Locally, some stream beds are
honeycombed with "potholes."
[Illustration: Fig. 3.--Grand Canyon, Arizona. (From Darton's "Story of
the Grand Canyon.")]
Strikingly narrow and deep valleys, called gorges and canyons, are
rather exceptional features of stream action. Most wonderful of all
features of this kind is the Grand Canyon of the Colorado River in
Arizona. In fact, this canyon takes high rank among the most remarkable
works of nature. The canyon is over 200 miles long, from 4,000 to 6,000
feet deep, and from 8 to 15 miles wide. Contrary to popular opinion,
this mighty canyon is not a result of some violent process, such as
volcanic action, or the sudden sinking of part of the earth's crust.
Nor is it the result of the scouring action of a great glacier. It is
simply a result of the operation of the ordinary processes of erosion
where the conditions have been exceptionally favorable. Some of the
favorable conditions have been, and are, a large volume of very swift
water (Colorado River) continually charged with an abundance of rock
fragments for the work of corrasion, and a great thickness of rock
which the river must cut through before reaching base-level. Aridity of
climate also tends to preserve the canyon form. The whole work has been
accomplished in very late geological time, and the tremendous volume of
rock which has been weathered and eroded to produce the canyon has all
been carried away by the Colorado River and accumulated in the great
delta deposit near where the river empties into the Gulf of California.
Even now the canyon is growing deeper and wider because the very active
Colorado River is still from 2,000 to 3,000 feet above sea level.
Standing on the southern rim near Grand Canyon station at an altitude
of nearly 7,000 feet, and looking down into the canyon, one beholds a
vast maze of side canyons, high, vertical rock walls which follow very
sinuous courses, giving rise to a steplike topography, and countless
rock pinnacles, towers, and mesas often of mountain-like proportions.
The side canyons are the result of erosion by tributaries to the main
river which have gradually developed and worked headward as the main
river has cut down. The mountain-like sculptured forms which rise out
of the canyon are erosion remnants, or, in other words, masses of rock
which were more favorably situated against erosion by either the main
river or any of its tributaries. All of the rocks of the broader, main
portions of the canyon are strata of Paleozoic age, arranged as a vast
pile of almost horizontal layers, including sandstone, limestone,
and shale. Some of these layers, being distinctly more resistant
than others, stand out in the canyon wall in the form of conspicuous
cliffs, in some cases hundreds of feet high. The very striking color
bands (mostly light gray, red, and greenish gray), which may be traced
in and out along the canyon sides, represent the outcropping edges
of variously colored rock layers. Far down in the canyon lies the
steep-sided, V-shaped inner gorge, or canyon which is fully 1,000 feet
deep. The rocks are there not ordinary strata, but rather metamorphic
and igneous rocks, mostly dark gray, not in layers, and about uniformly
resistant to erosion. There is reason to believe that this inner gorge
has developed mainly since a distinct renewed uplift (rejuvenation)
of the Colorado Plateau after the river began its canyon cutting. The
narrow, steep-sided inner gorge may thus be readily accounted for and
the general lack of steplike forms on its sides is due to essential
uniformity of the rock material as regards resistance to erosion.
[Illustration: Fig. 4.--Profile and structure section across the
line A-A in Fig. 3. Length of section 10 miles, vertical scale not
exaggerated. The main relief features, and the relations of the rocks
below the surface are shown. The granite and gneiss are of Archeozoic
age, and the overlying nearly horizontal strata are of Paleozoic age.
(After Darton, U. S. Geological Survey.)]
The wonderful King's River Canyon of the southern Sierras in California
is remarkable for its combined narrowness and depth. It is a steep
V-shaped canyon whose maximum depth is 6,900 feet, carved out in
mostly solid granite by the action of weathering and running water.
Some idea of the vast antiquity of the earth may be gleaned from the
fact that this tremendously deep canyon has been produced by erosion
in one of the most resistant of all known rocks in very late geologic
time! Conditions favorable for cutting this canyon have been volume and
swiftness of water and a liberal supply of grinding tools.
Among the many other great canyons of the western United States brief
mention may be made of the Grand Canyon of the Yellowstone River
in the National Park. The plateau into which the river has cut its
steep-sided, narrow, V-shaped canyon, with a maximum depth of 1,200
feet, has been geologically recently built up by outpourings of
vast sheets of lava. The large volume of very swift water, aided by
decomposition and weakening of the ordinarily very hard rock by the
action of the hot springs, has been able to carve out this deep canyon
practically within the last period of earth history. The deepening
process is still vigorously in progress. The wonderful coloring of the
rock, mostly in tones of yellow and brown, is due to the hydrated iron
oxides developed during the decay of the iron-bearing minerals of the
lava, the chemical action having been greatly aided by the action of
the hot waters. (See Plate 2.)
In regard to its origin, the marvelous Yosemite Valley, or canyon,
falls in a somewhat different category, and it is discussed beyond in
connection with the work of ice. Suffice it to say here that running
water has been a very important factor in its origin.
In New York and New England there are many gorges which have developed
by the action of running water since the Great Ice Age. Famous among
these are Ausable Chasm and Watkins Glen of New York, and the Flume in
the White Mountains of New Hampshire.
[Illustration: Fig. 5.--Sketch map showing the retreat of the crest
of Niagara Falls from 1842 to 1905, based upon actual surveys. The
retreat of the inner part of the Horseshoe Fall was more than 300 feet.
(Modified by the author after Gilbert, U. S. Geological Survey.)]
Before leaving our discussion of the work of running water, we should
briefly consider waterfalls. True waterfalls originate in a number of
ways. Most common of all is what may be termed the "Niagara type"
of waterfall. Niagara Falls merit more than passing mention not only
because of their scenic grandeur, but also because of the unusual
number of geologic principles which their origin and history so clearly
illustrate. Niagara Falls are divided into two main portions, the
Canadian, or so-called "Horseshoe Fall," and the "American Fall,"
separated by a large island. The crest of the American Fall is about
1,000 feet long and nearly straight, while the crest of the Canadian
Fall is notably curved inward upstream, and it is about 3,000 feet
long. The height of the Falls is 167 feet. Downstream from the Falls
there is a very steep-sided gorge about 200 feet deep and seven miles
long. The exposed rocks of the region are nearly horizontal layers of
limestone underlain with shales. Relatively more resistant limestone
forms the crest of the falls, and directly underneath are the much
weaker shales. Herein lies the principle of this type of waterfall
because, due to weathering and the swirling action of the water, the
weaker underlying rocks erode faster, thus causing the overlying rock
to overhang so that from time to time blocks of it already more or less
separated by cracks (joints), fall down and are mostly carried away
in the swift current. Thus the waterfall maintains itself while it
steadily retreats upstream. Careful estimates based upon observations
made between 1827 and 1905 show that the Canadian Fall retreated at
the rate of from three to five feet per year, while the American Fall
retreated during the same time at the rate of only several inches
per year. It has been well established that Niagara Falls came into
existence soon after the ice of the great Ice Age had retreated
from the district. The falls started by plunging over a limestone
escarpment, situated at what is now the mouth of the gorge seven miles
downstream from the present falls. If we consider the rate of recession
of the falls to have been always five feet per year, the length of time
required to cut the gorge would be something over 7,000 years. But
the problem is not so simple, because we know that, at the time of,
or shortly after, the beginning of the falls, the upper Great Lakes
drained farther north and not over the falls; and that this continued
for a considerable, though unknown, length of time. During this
interval the volume of water in Niagara River was notably diminished,
and hence the recession of the falls must have been slower. On the
other hand, judging by the width of the gorge, the length of the crest
of the falls has generally been considerably less than at present,
which in turn means greater concentration of water over the crest
and more rapid wear. Various factors considered, the best estimates
for the age of the falls vary from 7,000 to 50,000 years, an average
being about 25,000 years. Although this figure is not precise, it is,
nevertheless, of considerable geologic interest because it shows that
the age of Niagara Falls (and gorge) is to be reckoned in some tens of
thousands of years, rather than hundreds of thousands or millions of
years. Although waterfalls of the Niagara type are the most common of
all, it is by no means necessary that the particular rocks should be
limestone and shale.
Another common kind of waterfall may be termed the "Yosemite type,"
so named from the high falls in the Yosemite Valley of California. At
the great falls of the Yosemite, the rock is a homogeneous granite and
the undermining process does not operate. Yosemite Creek first plunges
vertically over a granite cliff for 1,430 feet to form the Upper Falls,
which must rank among the very highest of all true water falls. The
water then descends about 800 feet by cascading through a narrow gorge,
after which it makes a final vertical plunge of over 300 feet. A brief
history of the falls is about as follows. A great steep-sided V-shaped
canyon several thousand feet deep had been carved out by the action
of the Merced River which now flows through the valley. Then, during
the Ice Age, a mighty glacier plowed through the canyon, filling it
to overflowing. The granite of this district having been unusually
highly fractured by great vertical joint cracks was relatively easy
prey for ice erosion. Due to its great weight, the erosive power of
the ice was most potent toward the bottom, successive joint blocks
were removed, and the valley was thus widened and the sides steepened
or even commonly made practically vertical. (See Plate 6.) After the
melting of the ice, certain of the streams, like Yosemite Creek and
Bridal Veil Creek, were forced to enter the valley by plunging over
great perpendicular, granite cliffs which are in reality joint faces.
This type of waterfall does not retreat, but it constantly diminishes
in height by cutting into the crest. A number of other high falls of
this kind occur in the Yosemite region, and also in other mountain
valleys which formerly contained glaciers, as in the Canadian Rockies,
the Alps, and Norway, the rocks in these regions being of various kinds.
In the case of the "Yellowstone type" of waterfall a different
principle is involved, namely, a distinctly harder or more resistant
mass of rock which extends vertically across the channel of the stream.
At the Great Falls of the Yellowstone a mass of relatively fresh, hard
lava lies athwart the course of the river, while just below it the lava
has been much weakened by decomposition. The harder rock therefore
acts as a barrier, while, in the course of time, the weak rock on the
downstream side has been worn away until a waterfall 308 feet high has
developed. This waterfall does not retreat very appreciably, but it is
probably increasing in height, due both to the scouring action of the
water at the base of the fall and the unusual clearness of the river
water here, thus causing little wear at the crest. It should be noted,
in this connection, that the channel just on the upstream side of the
barrier cannot be cut down faster than the top of the barrier itself.
The famous Victoria Falls of the Zambezi River in South Africa,
represents a relatively uncommon type of waterfall. Considering height
of the fall, length of crest, and volume of water, this is perhaps
the greatest waterfall in the world. The Zambezi River, a mile wide,
plunges over 400 feet vertically into a chasm only a few hundred feet
wide and at right angles to the main course of the stream. The general
country rock is hard lava, but locally a narrow belt of the rock has
been highly fractured vertically, due to earth movements or faulting
(see explanation beyond) and therefore weakened and more subject
to weathering than the general body of the lava rock. This belt of
weakened rock has been easy prey for erosion by the Zambezi River
and the chasm has there developed. In fact the chasm is still being
increased in depth. Leaving the chasm toward one end, the river flows
through a narrow zig-zag gorge whose position has been determined by
big joint cracks. The mile-wide crest of the falls is interrupted by
a good many ledges and even small islands. The thundering noise of
this great waterfall is most impressive, but a good complete view is
impossible because most of the chasm is constantly filled with dense
spray.
Still another type of waterfall develops by the removal of joint blocks
by the action of running water. Falls of this type are fairly common
though they seldom attain really great heights. Where the rock in the
bed of a stream is traversed by well-developed vertical joint cracks,
slabs of rock cleaved by the joints may fall away due to weathering or
they may be pushed away by pressure of the water. Such a fall retreats
upstream by removal of joint blocks even in comparatively homogeneous
rocks. Taughannock Falls, 215 feet high, in southern central New York,
has developed by this manner in a shaly sandstone. The several falls
(one 50 feet high) in the famous gorge at Trenton Falls, in central New
York, have developed in this way in limestone.
CHAPTER IV
THE SEA AND ITS WORK
It is well known that the waters of the sea cover nearly three-fourths
of the surface of the earth. We think of the United States as being a
large piece of land of over three million square miles--but the sea
is about forty-five times as large, that is, it covers approximately
140,000,000 square miles. It is a remarkable fact that the average
depth of the great oceans of the earth is nearly two and one-half
miles. If the sea were universally present everywhere with the same
depth, it would be almost two miles deep. Yet this vast body of water
is an extremely thin layer when compared with the earth's diameter
of 8,000 miles. The Pacific is the deepest of the oceans with an
average depth of about two and three-fourths miles. The deepest ocean
water ever sounded is 32,114 feet (over six miles), not far from
the Philippine Islands. This is known as the Planet Deep, and was
discovered in 1912. Second deepest is 30,930 feet near the island of
Guam. In the Pacific Ocean there are five places where the water is
over five miles deep and eleven places were it is over four miles deep.
The deepest sounding ever made in the Atlantic Ocean was 27,972 feet,
not far from Porto Rico.
Many substances are known to be in solution in sea water, but in spite
of this the composition is remarkably uniform. The most abundant
substance by far in solution is common salt. In every 100 pounds of
sea water, there are 3.5 pounds of mineral matter of various kinds
dissolved. Nearly 78 per cent of the dissolved matter is common salt.
The principal other constituents in solution are chloride and sulphate
of magnesia, and the sulphates of lime and potash. All other dissolved
mineral substances together make up less than one per cent of the
total. It has been estimated that if all the dissolved mineral matter
should be brought together, it would form a layer 175 feet thick over
the whole sea bottom. The salts of the sea have been mostly supplied by
the rivers, which in turn have derived them from the disintegration and
chemical decay of the rocks.
If we make a general comparison with the surface of the land, the
floor of the ocean is a vast monotonous plain. None of the sea bottom
compares with the ruggedness of mountains, and even the more level
portions of land surface show many sharp minor irregularities such as
stream trenches. But the sea bottom is characterized by its smoothness
of surface. There are under the sea, however, mountain-like ridges,
plateaus, submarine volcanoes and valleys known as "deeps." But these
rarely show ruggedness of relief like similar features on land.
One of the most remarkable relief features of the ocean bottom is the
so-called "continental shelf." This is a relatively narrow platform
covered by shallow water bordering nearly all the lands of the earth.
Seaward, the depth of water is greatest, and it is seldom over 600 or
800 feet. The continental shelves of the world cover about 10,000,000
square miles or about one-fourteenth of the area of the sea.
Viewed in a broad way, there are two great classes of marine deposits;
first, those laid down comparatively near the borders of the land,
that is, on the continental shelf and continental slope, and second,
the abysmal deposits laid down on the bottom of the deep ocean. Those
found along and near the continental borders are largely land-derived
materials, that is to say, they are mostly sediments carried from the
land into the sea by rivers, and to a lesser extent rock material
broken up by waves along many shores. Practically all such land-derived
material is deposited within 100 to 300 miles of the shores. The
continental border deposits are extremely variable. Near shore they
are chiefly gravel and sands, while farther out they become gradually
finer, and on the continental slope only very fine muds are deposited.
These deposits usually contain more or less organic materials and
shells or skeletons of organisms. In some cases the shells or skeletons
of organisms predominate or even exist to the exclusion of nearly
all other material, as is true of the coral deposits or reefs which
form only in shallow water. Deposits like those just described as
accumulating on the bottom of the shallow sea, comparatively near the
lands, are of great significance to the geologist because just such
marine deposits now consolidated into sandstone, conglomerate, shale,
and limestone, are so widely exposed over the various continents.
A knowledge of the conditions under which shallow sea deposits are
now forming, is, therefore, of great value in interpreting events of
earth history as they are recorded in similar rocks which have been
accumulating through millions of years of time. One specific instance
will make this matter clearer. Using the method outlined in Chapter I
for the determination of earth chronology, and our knowledge of present
conditions under which shallow sea deposits are formed, it has been
well established that a shallow sea spread over fully four-fifths of
the area of North America during the middle Ordovician period of the
early Paleozoic era. Beyond this main conclusion, a careful study of
these rocks has revealed many important facts regarding the physical
geography, life, and climate of that time. The importance of this whole
matter is still further emphasized by the statement that five-sixths of
the exposed rocks of the earth are strata--mostly of shallow sea origin.
The deposits on the deep sea bottom are very largely either organic or
the shells and skeletons of organisms which have fallen to the bottom
from near the surface as already explained. Most common of these are
the deep sea "oozes" which are made up of the remains and shells of
tiny organisms called "foraminifers." These "oozes" cover about 50
million square miles of the sea bottom down to depths of from two to
three miles.
At depths greater than from two to three miles, a peculiar red clay is
the prevailing deposit. This is most extensive of all, covering an area
of 55 million square miles, or nearly the total area of lands of the
earth. Some remains of organisms are mixed with this clay, but since
most of the shells are of carbonate of lime and very thin, they are
dissolved without reaching the bottom in the deep sea water which is
under great pressure and rich in carbonic acid gas.
The deep sea deposits, both "oozes" and red clay, do, however, contain
some land-derived and other materials. Thus off the west coast of
Africa some dust carried by the prevailing winds from the Sahara
Desert, is known to fall in the deep sea several hundred miles from
shore. Volcanic dust is carried for many miles and deposited in the
deep sea--particularly in the South Pacific Ocean. Bits of porous
volcanic rock called "pumice" sometimes float long distances out over
the deep sea, before becoming water soaked. Icebergs often drift far
out from the polar regions over the deep sea, and on melting the rock
débris which they carry is dropped to the sea bottom. Also, particles
of iron and dust from meteorites ("shooting stars") have been dredged
from the deep sea.
One important geological significance of the deep sea deposits is
the proof which they furnish that, from at least as far back as the
beginning of the Paleozoic era, fully twenty-five million years ago, to
the present time, the two great deep ocean basins--the Atlantic and the
Pacific--have maintained essentially the same positions on the earth.
This is proved by the fact that nowhere, on any continent among the
rocks of all ages, as old at least as the early Paleozoic, do we find
any really typical deep-sea deposits. There is then no evidence that a
deep sea ever spread over any considerable part of any continent, and
this in spite of the fact that marine deposits of shallow water origin
furnish abundant evidence of former sea extensions. The shallow seas
have at various times spread over large portions of the continents.
On many rocky coasts the waves are incessantly pounding and wearing
away the rocks. In such places the sea, like a mighty horizontal saw,
is cutting into the borders of the lands. The finer materials produced
by the grinding up of the rocks are carried seaward by the undertow.
But, if the land remains stationary with reference to the sea, this
landward cutting by the waves reaches a limit. Since even big waves
have very little effect in water 100 or 200 feet deep, a shelf is cut
by the waves and this shelf, not many miles wide, is covered by shallow
water. The finer ground-up rock materials carried out by the undertow
are dumped just beyond the edge of the shelf which is thus built out
seaward as a terrace. In traveling over this shelf and terrace, the
waves, due to friction, lose their power. With gradually sinking land,
a much wider shelf may be cut, because the power of the waves is then
allowed to continue.
It might be of interest to cite a few cases of relatively rapid coast
destruction by the waves which have come under human observation. A
remarkable example is the island of Heligoland on which is (or was)
located the powerful German fort which guards the entrance to the Kiel
Canal. In the year 800 A. D. this island had 120 miles of shore line;
in 1300 it had 45 miles of shore; in 1649 only 8 miles; and in 1900
but 3 miles of shore line remained. In southeastern England "whole
farms and villages have been washed away in the last few centuries,
the sea cliffs retreating from 7 to 15 feet a year." A church located
a mile from the sea shore near the mouth of the Thames river, in the
sixteenth century, now stands on a cliff overlooking the sea. An island
in Chesapeake Bay covered over 400 acres in 1848, and the waves have
since reduced it to about fifty acres. Study showed that the relatively
soft unconsolidated strata of the Nashaquitsa Cliffs on the island of
Martha's Vineyard, were cut back at the rate of 5-1/2 feet per year,
between 1846 and 1886.
If part of the relatively smooth sea bottom should be raised into land,
the resulting shore line would of course, be regular and free from
indentations or sharp embayments. Examples of such coast which are
very young are at Cape Nome, Alaska; the northern coast of Spain; and
the west coast of northern South America. Soon, however, such a shore
line is attacked, and, either where the waves are greatest or the rocks
are weakest, indentations will result and the whole coast is gradually
eaten back until the power of the waves is largely spent in traveling
across the shallow water shelf. Sand bars are then built across the
mouths of the bays or indentations which later the rivers gradually
fill up with sediment. The result is a relatively straight or regular
old shore line. The coast of Texas has about reached this stage.
If a portion of the relatively rugged land surface should become
submerged under the sea, a very irregular, deeply indented shore line
would result, due to the entrance of tidewater into the valleys. The
deeply indented coast of Maine is a fine example of a very irregular
youthful shore line produced by geologically recent sinking of a
rugged, hilly region so that tidewater backs for miles into the
lower reaches of the river valleys. The promontories and islands are
undergoing rapid wear, and the development of bars across the inlets
has scarcely begun. Other excellent examples are the coasts of Norway
and southern Alaska. Such a coast is then attacked by the ocean waves
and the promontories are cut back until the broad shallow water shelf
is formed, after which sand bars are built across the remaining
embayments and the shore line becomes relatively regular.
It is, then, a remarkable fact that, whether shore lines originate
by emergence of sea bottom, or by sinking of land, there is a very
strong tendency on the part of nature to develop regular shore lines.
It should be stated that the principles of wave work and shore-form
development just outlined apply almost equally well to lakes,
especially large ones.
Before leaving this subject of shore-line development, mention should
be made of the fact that bars and beaches are often built part way
or wholly across embayments of the coast with surprising rapidity.
To illustrate, Sandy Hook, New Jersey, is advancing northward, while
Rockaway Beach, New York, is extending westward, the tendency being
to close up the entrance to New York harbor and to make the line of
seashore more nearly regular. Records show that Rockaway Beach actually
advanced westward more than three miles between the years 1835 and
1908.
CHAPTER V
GLACIERS AND THEIR WORK
A glacier may be defined as a mass of flowing ice. The motion may
not be that of flowage in the usually accepted sense of the term. A
discussion of the various theories of glacier motion will not here be
attempted. Glaciers form only in regions of perpetual snow, but they
commonly move down far below the line of perpetual snow of any given
region. In the polar regions they may form near sea level, while in the
tropics they form at altitudes of two to three miles, and there only
rarely. In southern Alaska, the lower limit of perpetual snow is about
5,000 feet above sea level, and many of the glaciers come down to sea
(Plate 4), while in the Alps, the lower limit of perpetual snow is at
about 9,000 feet, and the glaciers descend as much as 5,000 feet below
it.
In regions of perpetual snow there is a tendency for more or less snow
to accumulate faster than it can be removed by evaporation or melting.
As such snow accumulates it gradually undergoes a change, especially in
its lower parts, first into granulated snow (so-called "névé") and then
into solid ice. Snow drifts in the northern United States often undergo
similar transformation, after a few months first to névé, and then to
ice. This transformation seems to be brought about mainly by weight of
overlying snow which compacts the snow crystals; by rain or melting
snow percolating into the snow to freeze and fill spaces between the
snow crystals; and by the actual growth of the crystals themselves.
When ice of sufficient thickness has accumulated (probably at best
several hundred feet), the spreading action or flowage begins and a
glacier has developed. Renewed snowfalls over the gathering ground keep
up the supply of ice.
There are several types of glaciers: valley or alpine glaciers; cliff
or hanging glaciers; piedmont glaciers; ice caps; and continental ice
sheets. A valley or alpine glacier consists essentially of a stream
of ice slowly flowing down a valley and fed from a catchment basin of
snow within a region of perpetual snow. In the Alps, where glaciers of
this sort are very typically shown, they vary in length up to eight or
nine miles. Perhaps the grandest display of great valley glaciers is in
southern Alaska where they attain lengths up to forty or fifty miles
and widths of one or two miles (Plate 4).
Hanging or cliff glaciers are in many ways like valley glaciers, but
they are generally smaller; they develop in snow-filled basins above
the snow line usually on steep mountain sides; and they do not reach
down into well-defined valleys. Most of the glaciers of the Glacier
National Park in Montana and many of those in the Cascade Mountains are
of this type. Mount Rainier in Washington is one of the most remarkable
single large mountain peaks in the world, in regard to development of
glaciers over it. Great tongues of ice, starting mostly at 8,000 to
10,000 feet above sea level, flow down the sides of the mountain for
distances of to four and even six miles. The total area of ice in this
remarkable system of radiating glaciers on this one mountain is over
forty square miles. These Mount Rainier glaciers are in general best
classified as intermediate in type between valley and hanging glaciers.
[Illustration: Fig. 6.--Map of Mount Rainier, Washington, showing its
wonderful system of glaciers which covers more than 40 square miles.
Dotted portions represent moraines. (U. S. Geological Survey.)]
In some high latitude areas, as in Iceland and Spitzbergen, snow and
ice may accumulate on relatively level plains or plateaus and slowly
spread or flow radially from their centers. These are called ice caps.
Ordinary ice caps usually do not cover more than some hundreds of
square miles.
Continental glaciers or ice sheets are, in principle, much like ice
caps, but they are larger. Greenland is buried under an ice sheet of
moderate size (about 500,000 square miles), the motion being outward in
all directions toward the sea. Tongues of ice, like valley glaciers,
are commonly sent off from the main body of ice across the land
border of Greenland into the sea. The size of the great ice sheet of
Antarctica is not definitely known, but it covers probably at least
several million square miles. Two continental ice sheets of special
interest to the geologist are those which existed during the great
Ice Age of the Quaternary period. One of these then covered nearly
4,000,000 square miles of North America, while the other covered about
600,000 square miles of northern Europe. The main facts regarding
the Ice Age are given in a succeeding chapter. The facts brought out
in the present discussion of existing glaciers will greatly aid in
understanding the Ice Age.
How fast do glaciers flow? Based upon many observations, we may say
that an average rate of flow for the glaciers of the world is not more
than a few feet per day. A very exceptional case is a large glacier,
branching off as a tongue from the ice sheet of Greenland, which is
said to move sixty to seventy-five feet per day. Some of the great
Alaskan glaciers have been found to flow from four to forty feet per
day. Most glaciers of the Alps move only one to two feet per day. A
glacier advances only when the rate of motion is greater than the rate
of melting of its lower end and vice versa in the case of retreat. Thus
it is true, though seemingly paradoxical, to assert that a glacier has
a constant forward motion even when it is retreating by melting.
By watching the changing position of marked objects placed in the ice,
it has been proved that, in a valley glacier, the top moves faster
than the bottom; the middle moves faster than the sides; the rate of
motion increases with thickness of ice, slope of floor over which it
moves, and temperature.
Ice, like molasses candy, tends to crack when subjected to a relatively
sudden force, and where the ice rides over a salient on the bed of
the glacier, transverse cracks or fissures often develop. Due to more
rapid motion of the central part of a valley glacier, stresses and
strains are set up and crevasses are formed, usually pointing obliquely
upstream. Where the ice tends to spread laterally in a broad portion
of a valley, longitudinal cracks may develop. Crevasses vary in size
up to several feet in width and hundreds of feet in depth. Owing to
the forward motion of the ice, old fissures tend to close up and new
ones form, and, aided by uneven melting, the surface of a glacier is
generally very rough.
Like running water, ice may have considerable erosive power when it is
properly supplied with tools. The total erosive effect which has been,
and is now being, accomplished by ice compared with that of running
water is, however, slight. One of the main processes by which ice
erosion is accomplished is "corrasion" due to the rubbing or grinding
action of hard rock fragments frozen into the bottom and sides of the
glacier. Thick ice, shod with hard rock fragments and flowing through a
deep, narrow valley of soft rock, is especially powerful as an erosive
agent because the abrasive tools are supplied; the work to be done is
easy; and the deep ice causes great pressure on the bottom and lower
sides of the valley. Rock surfaces which have been thus subjected to
ice erosion are characteristically smoothed and more or less scratched,
striated, or ground due to the corrosive effects of small and large
rock fragments. This affords one of the best means of proving the
former presence of a glacier over a region or in a valley. A typical
V-shaped stream cut (eroded) valley is changed into one with a U-shaped
profile or cross section by glacier erosion (Plate 5).
Another important process of ice erosion is "plucking," which consists
in pushing among already more or less loosened joint blocks by the
pressure of the moving ice. The pressure thus exerted, especially by
a deep valley glacier, may be enormous. This process was an important
factor in the development of the famous Yosemite Valley, a very brief
account of whose history it will now be instructive to give.
[Illustration: Plate 3.--The Gorge of Niagara River Below the Great
Falls. The strata (containing fossils) were accumulated on the bottom
of the Silurian sea which overspread the region at least 18,000,000
years ago. Since the Ice Age or within 20,000 to 40,000 years, the
river has carved out the gorge. (_Courtesy of the Haines Photo Company,
Conneaut, Ohio._)]
[Illustration: Plate 4.--(_a_) A Winding Stream in the St. Lawrence
Valley of New York. Due to its low velocity the stream cuts its channel
down very little, but it swings or "meanders" slowly from one side of
its valley to the other, developing a wide flood plain. The stream once
flowed against the valley wall shown at the middle left. (_Photo by the
author._)]
[Illustration: Plate 4.--(_b_) Davidson Glacier, Alaska. This glacier
is at work slowly grinding down the valley floor and cutting back its
walls, thus changing the original stream-cut, V-shaped profile, like
that of Plate 5. (_Photo by Wright, U. S. Geological Survey._)]
The Yosemite Valley, about 7 miles long, less than one mile wide,
and from 2,000 to 4,000 feet deep, lies on the western slope of the
Sierra Nevada Mountains of California. Great cliffs of granite,
mostly from 1,000 to over 3,000 feet high, bound the valley on either
side. The floor of the valley is wide and remarkably flat (Plate 6).
Just prior to the Ice Age, by the processes of erosion already set
forth, the Merced River had carved out a great steep-sided V-shaped
canyon commonly from 1,000 to 3,000 feet deep. During the Ice Age,
two glaciers joined to form an extra deep powerful glacier, which
flowed through a deep part of the Merced Canyon and modified it into
the Yosemite Valley, essentially as we see it to-day. Because the ice
was shod with many fragments of hard rock (granite), and the pressure
at the bottom and lower sides of the glacier (several thousand feet
thick) was so great, the V-shaped stream-cut canyon was changed to a
U-shaped canyon with very steep to even vertical walls. A factor of
great importance which notably aided the erosive power of the glacier
in this case was the existence of an unusual number of large vertical
joint cracks in the granite in this local region. The plucking action
of the ice was thus very greatly facilitated and great slabs of rock,
separated by the vertical joints, especially toward the lower sides
and bottom of the valley, were pushed away one after another by the
ice. When the ice disappeared, great precipitous joint faces from 1,000
to 3,000 feet high were left along the valley sides. At its lower
end the glacier left a dam of glacial débris (moraine) across the
valley, thus causing a lake to form over the valley floor. The wide
flat bottom of the valley was caused by filling up of the lake with
sediment. The uniqueness of the Yosemite Valley is, then, due to a
remarkable combination of several main factors; one, the presence of
a large swift river well supplied with tools which carved out a deep
V-shaped canyon; two, a mighty glacier which plowed its way through
this canyon and converted it by erosion into a U-shaped canyon;
three, the weakening of the rock by many joint cracks, thus greatly
facilitating the ice erosion; and four, a postglacial lake covering
the valley floor which became filled with sediment. As a result of the
ice work, several streams, tributary to the main stream (Merced River)
which flows through the bottom of the valley, were forced to plunge
over great vertical rock walls (joint faces), thus producing high and
beautiful true waterfalls, including the very high Upper Yosemite Fall
where Yosemite Creek makes a straight drop of 1,430 feet. A tributary
valley like that of Yosemite Creek, which ends abruptly well above
the main valley, is known as a "hanging" valley. The valley of Bridal
Veil Creek is another good example. (See Plate 6.) Valleys which were
once occupied by active glaciers are generally characterized by their
U-shaped cross sections and their hanging (tributary) valleys, but
the great height and steepness of the valley walls in Yosemite are
exceptional.
A type of glacial erosion which is of special interest is the
sculpturing of so-called "cirques" or "amphitheaters" in mountains
within the region of perpetual snow. Where the main mass of snow and
ice in the catchment basin or gathering ground of a valley glacier
pulls away from the snow and névé on the upper slopes, the rock wall
is more or less exposed in the deep crevasse. During warm days water
fills the joint cracks in the rocks down in this crevasse (so-called
"Bergschrund"), and during cold nights the water freezes and forces
the blocks of rock apart. This is greatest toward the bottom of the
crevasse and so, by this excavating or quarrying process, vertical or
very steep walls are developed around a great bowlike basin or cirque.
Such cirques, now free from glacial ice, with precipitous walls 500 to
2,000 feet high and one-fourth of a mile to one-half of a mile across,
are common in the Sierra Nevada and Cascade Ranges and in the Rocky
Mountains.
What becomes of the materials eroded by the ice? An answer to this
question involves at least a brief discussion of the deposition of
glacial débris, this constituting an important feature of the work
of ice. The débris transported by a glacier is carried either on
its surface or within it, or pushed along under it. It is generally
heterogeneous material ranging from the finest clay through sand and
gravel, to bowlders of many tons' weight. Various types of glacial
deposits are abundantly illustrated by débris left strewn over much
of the northeastern United States and some reference to these will be
made.
Most valley glaciers carry considerable débris on their surfaces, this
representing material which falls or is carried down from the valley
walls upon the margins of the ice, thus forming marginal moraines.
When two glaciers flow together, one marginal moraine from each will
coalesce to form a medial moraine. The material carried along at the
bottom of a glacier is called the ground moraine. Where it contains
much very fine grained material with pebbles or bowlders scattered
through its mass, it is called "till" or "bowlder clay." The pebbles or
bowlders of the ground moraine are commonly facetted and striated as a
result of having been rubbed against the bedrock on which the glacier
moved. Ground moraine material is the most extensively developed of
all glacial deposits. It is so widely scattered over the glaciated
northeastern portion of the United States that most of the soils
consist of it, having been left strewn over the country during the
melting of the vast ice sheet.
When a glacier remains practically stationary for some time, more or
less material which it carries is piled up at its lower end to form
a terminal moraine. Repeated pauses during general glacier retreat
permit the accumulations of so-called recessional moraines. A wonderful
display of recessional moraines occurs from the Great Lakes south,
where they are festooned one within another and remain almost exactly
as they were formed during pauses in retreat of great lobes of ice
during the closing stages of the Ice Age. A great terminal moraine
marks the southernmost limit of the ice sheet during the Ice Age,
a very fine illustration being the ridge of low irregular hills
extending the whole length of Long Island. Some of the material in that
morainic ridge was transported by the ice from northern New England.
Considerable rock débris is transported within the ice, and such
"englacial" material in part results from rock débris which falls
on the surface in the catchment basin and becomes buried under new
snowfalls which change to ice, and in part from material which falls
into the crevasses in the glacier farther down the valley. Marked
objects thrown into the catchment basin have, after many years, emerged
at or near the end of the glacier; thus the rate of motion can be very
accurately told. A very remarkable case of transportation through the
body of a glacier is the following: In 1820, three men were buried
under an avalanche in the catchment basin of the Bossons Glacier in
the Alps. Forty-one years later several parts of the bodies, including
the three heads together with some pieces of clothing, emerged at the
foot of the glacier after traveling most of its length at the rate of
eight inches per day. The heads were so perfectly preserved after their
remarkable journey in cold storage that they were clearly recognized by
former friends!
Where a valley floor slopes downward away from the end of a glacier,
waters emerging from the ice, heavily loaded with rock débris, cause
more or less deposition of the débris on the valley floor often for
miles beyond the ice front. Such a deposit is called a "valley train."
When the ice front pauses for a considerable time upon a rather flat
surface, the débris-laden waters emerging from the ice develop an
"outwash plain" by deposition of sediment rather uniformly over the
flat surface. A very fine example is the plain which constitutes most
of the southern half of Long Island just beyond the southern limit of
the great terminal moraine ridge.
A type of glacial deposit of particular interest is the "drumlin"
which is, in reality, only a special form of ground moraine material
(commonly till), and, therefore, essentially unstratified. Typical
drumlins are low, rounded mounds of till with roughly elliptical bases
and steeper fronts facing the direction from which the ice flowed.
Their long axes are always parallel to the direction of ice movement.
In height they commonly range from 50 to 200 feet. Their mode of origin
is not yet definitely known, but they form near the margins of broad
lobes of ice either by erosion of earlier glacial deposits, or by
accumulation beneath the ice under peculiarly favorable conditions,
as perhaps in the longitudinal crevasses. One of the finest and most
extensive exhibitions of drumlins in the world is in western New York
between Syracuse and Rochester. Thousands of drumlins there rise above
the general level of the Ontario plain, the New York Central Railroad
passing through the very midst of them. Drumlins are also abundant in
eastern Wisconsin.
Another type of glacial deposit in the form of low hills is the "kame"
which, unlike the drumlin, always consists of more or less stratified
material. Kames are seldom over 200 feet high, and they are of various
shapes. In many cases they form irregular groups of hills, and in
other cases fairly well defined kame ridges. Kames form as deposits
from débris-laden streams emerging from the margins of glaciers, the
water sometimes rising as great fountains because of the pressure. Such
deposits are now actually in process of formation along the edge of
the great Malaspina Glacier of Alaska. Kames are commonly associated
with terminal and recessional moraines. "Eskers" are similar except
that they are long winding low ridges of stratified material deposited
by débris-laden streams, probably in longitudinal fissures in the ice
near its margin. (See Plate 20.)
Glacial bowlders, or "erratics" are blocks of rock or bowlders left
strewn over the country during the melting of the ice. They vary in
size from small pebbles to those of many tons of weight, and most of
them were derived from ledges of relatively hard, resistant rocks. (See
Plate 20.) Erratics have very commonly been carried a few miles from
their parent ledges, while more rarely they have traveled even hundreds
of miles. They are extremely abundant in New York and New England,
many occurring even high up on the mountains. In some cases erratics
of ten or more tons' weight have been left in such remarkably balanced
positions on bedrock that a child can cause one of them to swing back
and forth slightly. Such a bowlder is literally a "rocking stone."
In the Adirondack Mountains the writer recently observed a rounded
erratic of very hard rock fourteen feet in diameter resting in a very
remarkably balanced position on top of another large round glacial
bowlder.
CHAPTER VI
THE ACTION OF WIND
Only during the last quarter of a century have geologists come to
properly appreciate the really important geological work of the wind.
One reason for this is the fact that people live mostly in humid
regions where the soils are largely effectually protected against
wind action by the vegetation. But even in such regions, wind action
is by no means negligible. One has but to observe the great clouds of
dust raised by strong wind from freshly cultivated fields during a
little dry weather in the late spring. Much of this dust is carried
considerable distances, often miles, and in some cases young crops
are injured by removal of soil from around the roots, while in other
cases young plants are buried by deposition of the wind-blown material
over them. In humid regions, the action of the wind is perhaps most
strikingly exhibited along and near shores of sea and lakes, where
loose dry sands are picked up and transported in great quantities,
often depositing them as sand dunes, which may form groups of hills
covering considerable areas. Very conspicuous examples are the sand
dunes of Dune Park in northern Indiana, and the dunes along the coast
of New Jersey.
But the action of wind is most strikingly effective in desert and
semiarid regions. The importance of the work of wind is made more
impressive when we realize that about one-fifth of the land of the
earth is desert.
In deserts some of the ordinary agents of weathering and erosion
are either absent or notably reduced in effectiveness. Thus, stream
action is, in general, reduced to a minimum; weathering effects due
to moisture in the air are notably reduced, and either frost action,
or wedge work of ice, is relatively unimportant due to lack of water.
Change of temperature between night and day is, however, unusually
important as a process whereby rocks are broken up due to relatively
rapid expansion and contraction in deserts because such temperature
changes are exceptionally great, and rocks and soils are almost
everywhere directly exposed, being free from vegetation.
The finer grained materials, especially sand grains, in deserts
are picked up by the wind and driven, often with great velocity,
against barren rock ledges and large and small rock fragments. By
this process (corrasion) the rocks are worn and often polished by the
materials blown against them. The principle is that of the artificial
sand-blast, used in etching glass, or cleaning and polishing building
and decorative stones. Under favorable conditions wind-driven sand
accomplishes noticeable erosion in a surprisingly short time. Thus, in
a hard wind storm, a plate glass window in a lighthouse on Cape Cod was
worn to opaqueness, while in a few weeks or months the directly exposed
window glass may there be worn through.
The great erosive effect of wind-driven sand is relatively close to
the ground because the larger and heavier fragments are not lifted to
very considerable heights. For this reason ordinary telegraph poles
are difficult to maintain in desert regions because, unless they are
specially protected, they are soon cut down by sand swept against
their bases. In the desert regions of our Southwestern States cliffs
rising above the general level of the country are often undercut by
wind erosion, sometimes with the development of large caverns. (See
Plate 1.) Even the high portions of great ledges are there more or less
fantastically sculptured by wind erosion, the softer portions being
more deeply cut into than the harder. The famous sphinx of Egypt has
been notably roughened by action of this kind.
The enormous power of high winds to transport rock material in desert
regions is strikingly illustrated by the great sand storms of the
Sahara Desert, where sand and dust, forming clouds with cubic miles of
volume, sweep for many miles across the country. Some one has estimated
that every cubic mile of air in such a storm contains more than
100,000 tons of rock material. It is said that an army of 50,000 men
under Cambyses was buried under the sands of a storm in the desert of
northern Africa.
Dust from some of these storms is known to be driven hundreds of miles
out over the Atlantic Ocean, there to settle in the sea. In mountainous
desert regions, like the Great Basin of our Western States, the general
tendency is for the rock materials wind-eroded from the mountains to
be carried into the intermontane basins or valleys. Some basins of
this sort are believed to contain depths of 1,000 to 2,000 feet of
wind-blown material.
A special kind of wind-blown material called "loess," is a sort
of fine-grained yellow, or brown loam which, though relatively
unconsolidated, has a remarkable property of standing out as high
steep cliffs or bluffs along the banks of streams. Many thousands of
square miles of northern China are covered with loess. Among many other
regions, thousands of square miles of parts of the States of Iowa,
Nebraska, and Kansas are covered with loess, which, in this case, is
believed to be fine material gathered by winds from the region just
after the retreat of one of the ice sheets of the great Ice Age, when
there was very little vegetation to hold down the loose soils of
glacial origin.
Much as snowdrifts are formed, so, in many places, the wind-driven
sands are built up into sand hills or so-called "dunes." Dunes are very
common in many places, as for example, along our middle Atlantic coast;
in Dune Park of northern Indiana; and in the great arid and semiarid
regions of the Western States. Where there is a distinctly prevailing
direction of wind, the sand is blown to the leeward side from the
windward side, and the dunes are caused to migrate in the direction of
the wind. The burial and destruction of forests, and the uncovering of
the dead trees is not uncommonly caused by migration of sand dunes, all
stages of this phenomenon being well exhibited in Dune Park, Indiana.
The rate of dune migration is very variable, but study in a number of
places has shown a rate of from a few feet to more than 100 feet per
year. Arable lands, buildings, and even towns have been encroached upon
and buried under drifting sand. An interesting example is a church in
the village of Kunzen, on the Baltic seashore which, in a period of
sixty years, became completely buried under a dune and then completely
uncovered by migration of the dune. Much destruction has been wrought
by shifting sands on the Bay of Biscay, where farms and even villages
have been overwhelmed. The ruins of the ancient cities of Babylon and
Nineveh are buried mostly under wind-blown sand and dust. There is
good reason to believe that the climate of central and western Asia is
now notably drier than it was a few thousand years ago, and this may
help to explain the burial of many old cities and villages there under
wind-blown deposits.
CHAPTER VII
INSTABILITY OF THE EARTH'S CRUST
The crust of the earth is unstable. To the modern student of
geology the old notion of a "terra firma" is outworn. The idea of
an unshakable, immovable earth could never have emanated from the
inhabitants of an earthquake country. In general we may recognize
two types of crustal movements--slow and sudden. To most people the
sudden movements accompanied by earthquakes are more significant and
impressive because they are more localized and evident, and often
accompanied by destruction of property, or quick, though minor, changes
in the landscape. But movements which take place slowly and quietly are
often of far greater significance in the interpretation of the profound
physical changes which have affected the earth during its millions of
years of known history.
[Illustration: Fig. 7.--Structure section across the Hudson River
Valley near West Point, New York. The shafts and tunnel, 1,200 feet
below sea level, in solid rock, show the position of the New York City
aqueduct from the Catskills. The Preglacial valley has been submerged
and filled with Postglacial sediment to a depth of nearly 800 feet.
(Redrawn by the author after Berkey, from New York State Museum
Bulletin.)]
A few well-known examples will serve to prove that upward, downward,
and differential movements of the earth's crust have actually taken
place not only in the remote ages of geologic time, but also that
such movements have geologically recently taken place, and that
similar movements are still going on. It is very important that the
reader thoroughly appreciate the fact that crustal disturbances,
often profound ones, do take place, because this is one of the most
fundamental tenets of geologic science. Let us consider the case of the
Hudson-Champlain-St. Lawrence Valley region. That the whole region was
once notably higher (at least 1,000 feet) than at present is proved by
the drowned character of the Hudson Valley, in which tidewater extends
northward for 150 miles to near Troy. Where the New York City Aqueduct
passes under the Hudson River near Newburgh, the bedrock bottom of the
old river channel is now about 800 feet below sea level as determined
by drilling. This old channel is there filled up nearly to sea level
with glacial and postglacial rock débris, which shows that the old
channel must have been cut before the oncoming of the ice of the great
Ice Age. Before the Ice Age, then, the lower Hudson Valley must have
been considerably more than 800 feet higher than at present, because it
then contained a river with sufficient current to be an active agent
of erosion, carving out the canyonlike valley in the vicinity of West
Point. This conclusion is strongly reenforced by the fact that the old
valley of the Hudson River has been definitely traced as a distinct
trench across the shallow sea bottom for about 100 miles eastward from
the entrance to New York harbor. Toward the eastern end of this trench
the depth of water is now considerably over 1,000 feet, and thus it is
obvious that, preceding the Ice Age, the earth's crust in the vicinity
of New York City must have been much higher than at present, so that
the Hudson River was able to erode its now completely drowned channel.
Somewhat similar evidence has also established the fact that the lower
St. Lawrence Valley region was much higher before the Ice Age. It is
evident, therefore, that the general Hudson-St. Lawrence Valley region
is now notably lower with reference to sea level than it was before the
Ice Age. That this was caused by actual sinking of the earth's crust
rather than by a rise of sea level is proved by the fact that similar
changes of level between land and sea did not take place at the same
time even along the Atlantic and Gulf coast of our Southern States.
We shall now proceed to the next step in the geologically recent
history of earth-crust movements in the Hudson-Champlain-St. Lawrence
Valley region by asserting that, since the Ice Age, the land was
actually notably lower than at present. In fact, the land was enough
lower to allow tidewater to extend up the St. Lawrence Valley into
the Ontario basin, and all through the Champlain-Hudson Valley. Many
beaches, bars, and delta deposits formed in these arms of the sea are
still plainly preserved, in some cases with shells and bones of marine
animals in them, now hundreds of feet above sea level. These marine
deposits are highest above sea level in the northern portion of the
Champlain Valley, where they lie at an altitude of 700 feet or more and
their altitude steadily diminishes southward to about 300 to 400 feet
in the general vicinity of Albany, and to near sea level in the general
vicinity of New York City. Obviously, then, the land stood lower during
part or all of the interval of not more than a few tens of thousands
of years since the Ice Age than at present. This leads us to the third
important conclusion regarding earth movements in this region, namely,
that still later the land has undergone a differential uplift, the rate
having steadily increased toward the north where the total uplift is
many hundreds of feet. We have discussed this region somewhat in detail
because the principles of slow up and down movements of the earth's
crust are there so plainly recorded.
Among many other regions where earth movements similar to those above
described have taken place, brief mention may be made of Norway. The
great fjords of Norway were, just before the Ice Age, stream-cut
valleys which were then more or less modified by glacial erosion,
and after the Ice Age the rivers in them were drowned due to land
subsidence. The kind of evidence is like that above given for the lower
Hudson River. Since the subsidence there has been partial reelevation,
as proved by the fact that along the sides of the larger fjords marine
terraces and beaches may be traced with gradually increasing altitude
for many miles (150 or more) back into the country where they are
hundreds of feet above tidewater.
Scandinavia is of still further special interest because very
appreciable earth movements have there come under human observation.
Marks carefully placed along the shores of Sweden by the government
have proved that during the last 150 years the southern end of the
country has actually subsided several feet, while from Stockholm north
the land has risen in increasing amount, reaching a maximum of seven or
eight feet. In southern Sweden, at Malmo, a certain street now at times
becomes covered by wind-driven high water, and during excavations made
some years ago an older street eight feet below the present one was
found.
A theory which appears to be in perfect harmony with the facts to
account for the subsidence and partial reelevation of central eastern
North America and Scandinavia since the beginning of the Ice Age is
that the great weight of ice during the Ice Age pressed the land down,
and that since the removal of the ice there has been an appreciable
tendency for the land to spring back.
Certain crustal movements which have occurred about the Bay of Naples
are of very special interest because actual human history dates can be
placed upon them. Most remarkable are the records in connection with
the temple of Jupiter Serapis which was built near the shore before
the Christian era. The land sank about five feet and a new pavement
had to be constructed; then, by the middle of the third century A. D.,
the temple rose to well above sea level. By about the ninth century
the land had subsided fully thirty feet, so that marble columns of
the temple were bored full of holes as high as twenty-one feet above
their bases by marine-shelled animals, species of which still live in
the bay. Then a slow uplift of twenty-three feet began, bringing the
bases of the columns two feet above sea level by 1749. Since that time
a slight sinking has taken place and this seems to be still going on.
Three of the marble columns with the borings still stand in upright
position.
While the movements just described were taking place, the island of
Capri, twenty miles across the Bay of Naples, has slowly sunk to an
amount estimated at thirty or forty feet as proved by evidence from the
famous Blue Grotto. About the beginning of the Christian era a large
ancient wave-cut cave, part of which is now called the Blue Grotto, had
its floor above sea level, and it was used by certain Romans as a cool
place to retire to from the heat. In order to obtain better light an
opening was cut through its upper portion. The land has sunk so much
that at the present time even part of the artificial opening (through
which tourists pass) is now under water.
By way of illustrating remarkable contrasts in direction of crustal
movements on very considerable scales in a given region, we shall
briefly mention some facts regarding part of the coast of southern
California and the neighboring islands of Santa Catalina and San
Clemente, respectively twenty-five and fifty miles offshore. Those
movements were not, however, checked up by human history records.
The mainland at San Pedro has clearly risen 1,240 feet, as proved by
the presence of unusually perfect coast terraces (so-called "raised
beaches"), while San Clemente has risen 1,500 feet as proved by the
raised beaches into which deep, youthful V-shaped stream-cut valleys
have been sunk, and a shore line characteristic of recent notable
uplift. It is a remarkable fact that at the same time the intervening
island (Santa Catalina) has notably sunk, as proved by the nature
of its shore line, and the distinctly more mature character of its
topography.
We are, however, by no means dependent upon lands along sea shores
for evidences of slow rising and sinking of land. Thus, by careful
measurements it has been shown that the general region of the Great
Lakes is now differentially rising toward the northeast at the rate
of about five inches per 100 miles per century. At Chicago the rise
of water is estimated at about nine inches per century, which means
increase of flowage through the Chicago Canal. At this rate the upper
lakes would, in some thousands of years, drain through this canal to
the Mississippi. A well-preserved shore line of the large ancestor
of Lake Ontario shows a steady increase in altitude at the rate of
several feet per mile toward the northeast from near Niagara to the St.
Lawrence Valley, thus proving a tilting of the land since the shore
line was formed.
Shore lines of the great ancestor of Great Salt Lake also show warping
of the earth's crust, some parts of a definite shore line being several
hundred feet higher than others.
Very significant evidence pointing to profound crustal movements
consist in the finding of fossil remains of marine animals in the
strata high above sea level, very commonly from one to three miles, in
many parts of the world, especially in the high mountains. In Wyoming,
nearly horizontal strata of the Mesozoic Age carrying marine fossils
lie two miles or more above sea level. The fact that given formations,
carrying marine fossils representing certain definite portions of
geologic time, are found at various altitudes up to several miles in
many parts of the world, shows that the land in those places has
really risen relative to sea level.
It should not be presumed from the above discussion that the sea level
itself has never changed. Thus, the vast areas of thick ice sheets in
both North America and Europe during the Great Ice Age represented
sufficient water withdrawn from the sea to very appreciably lower its
level. All land-derived materials, carried into the sea mainly by
rivers, displace sea water, with consequent rise of its level. If all
existing lands were worn down and carried into the sea, its level would
be raised some hundreds of feet. Subsidence of any part of the ocean
bottom would cause a lowering of sea level. There is a strong reason
to believe that some such shiftings of sea level have occurred during
the vast lapse of geologic time. During certain periods erosion of the
land predominated, and during other periods building up of the land
predominated, as pointed out in the chapters on geologic history. It is
not thought that shifting of sea level has ever amounted to more than a
few hundred feet, at least not during the millions of years of the more
clearly recorded earth history.
We have thus far considered slow upward and downward movements of the
earth's crust without notable structural changes in the rocks. Another
type of crustal disturbance causes more or less profound changes in
the structures of the rocks themselves. Just how the earth originated
is a matter of uncertainty, but we can be sure that for many millions
of years it has been a shrinking body. The outer, or crustal, portion
of the earth, in adjusting itself to the contracting interior, has had
many pressures, stresses, and strains set up within it. As results of
such forces the rocks at and near the earth's surface have in various
places, and at various times, been broken (faulted) and subjected to
sudden movements (see discussion beyond), while those well within the
crustal portion, that is to say a few miles or more down, have, in
many cases, been bent (folded), or even crumpled. For these reasons
the surface and near-surface crustal portions are called the "zone of
fracture," while the more deeply buried portions comprise the "zone
of flowage." In the zone of flowage the rocks, where subjected to
great lateral pressure, act like plastic materials and therefore bend
rather than break, because of the great weight of overlying materials.
Laboratory experiments have confirmed the findings of geologists in
this regard. Small masses of rocks properly inclosed in nickel-steel
cylinders have been subjected to slow differential pressures equivalent
to those which obtain twenty to forty miles within the earth. Under
such conditions rocks have been made to change shape very notably
without fracturing. Both geological observations and experiments have
led us to conclude that not even small fractures or crevices can remain
open at a depth greater than ten or twelve miles even in the hardest
rocks.
From time to time, during the long history of the earth, forces of
lateral pressure have been slowly exerted along more or less localized
zones or belts within the earth's crust, and the rocks have been
deformed chiefly by bending or folding, especially in those regions
where mountains of the folded type have developed. Movements of this
type are considered beyond in the chapter on mountains. Rock folds vary
in size from microscopic to miles across, and they exhibit many shapes.
Plate 7 will give the reader a good idea of actual rock folds of
common sizes and shapes in various places. Folded structures are most
clearly discernible in sedimentary rocks, because of their stratified
(layered) arrangement. Since folds in hard rocks rarely, if ever,
develop except at a depth of some miles within the earth, they show at
the surface only where great thicknesses of overlying materials have
been stripped off by erosion.
[Illustration: Fig. 8.--An outcrop of stratified crystalline limestone
(or marble) exhibiting two small sharp folds--a syncline on the
left and an anticline on the right--near Lenox, Mass, These folds
developed during the great mountain-making disturbance at the end of
the Ordovician period fully 20,000,000 years ago. (After Dale, U. S.
Geological Survey.)]
From the standpoint of our consideration of slow earth-crust movements,
it is important to bear in mind that lateral pressure in the zone of
flowage has not only notably deformed rocks, but that, as a result
of the buckling forces, given rock masses have, in many cases, been
notably shifted downward or upward--mainly upward--from their original
positions. Folded strata carrying shells of sea animals are commonly
found thousands of feet above sea level in many of the great mountain
ranges of the world. During the process of folding on a large scale the
crust of the earth is very appreciably shortened at right angles to
the direction of applied pressure, due to squeezing or bending of the
strata. In the case of the Appalachian mountains of Pennsylvania it has
been estimated that such shortening amounts to about twenty-six miles
or, in other words, that the strata originally spread out horizontally
across an area whose width was about 100 miles have been squeezed or
folded into an area whose width is twenty-six miles less.
[Illustration: Fig. 9.--Structure section showing the profile of the
mountains and relations of rocks below the surface near Livingston,
Montana. The strata were crowded together until they bent into great
sharply defined folds at the time of the Rocky Mountain Revolution
several million years ago. Then the rocks broke along the fault
fracture and the mass on the right was shoved over upon the mass on the
left. (After U. S. Geological Survey.)]
We shall now turn to a consideration of sudden earth movements and some
of their effects, including earthquakes. Mention has already been made
of the fact that, when pressures and strains are set up in the outer
portion ("zone of fracture") of the earth's crust, the rocks yield
mainly by breaking or fracturing because the rocks not being under a
great load of overlying material are there brittle. The earth's crust
has been fractured on small and large scales in many places during
the long space of geologic time. Where one block of earth's crust
has slipped or moved past another along a fracture we have what is
called a "fault." Such displacements of rock masses vary in amount
from less than an inch to some miles, and they constitute one of the
most important features of the architecture of the outer portion of
the earth. There are two types of faults fundamentally different as
to cause. In one type (so-called "normal fault") the rocks suddenly
yield to a force of tension; a fracture develops and the earth block on
one side of the fracture or fault drops with reference to that on the
other. In the other type (so-called "thrust faults") the rocks yield
suddenly to a force of compression or lateral thrust, and one block
of earth is pushed or thrust partly over another along the surface of
fracture or fault. (See Plate 8.)
Faults range in length up to hundreds of miles, those from one to
twenty miles in length being very common. Where an earth block has been
displaced thousands of feet along a fault surface, it is not to be
understood that the whole displacement resulted from a single movement,
but rather from a series of sudden movements separated by greater or
less intervals of time. Each sudden movement along a fault surface
produces a vibration of the earth near by. Many such sudden movements
are known to have caused violent earthquakes. Displacements of twenty
to fifty feet, as a result of single movements, are definitely known
to have taken place in various regions during the last fifty years;
and rarely, if ever, has any sudden displacement of as much as several
hundred feet occurred. Cliffs and steep slopes very commonly result
from faulting, but, because of the long lapse of time required for the
repeated movements in the case of great faults, the cliffs or steep
slopes begin to wear back and become more or less subdued long before
the last of the movements take place. In regions where movements along
great faults have long since ceased, the original steep slopes may be
completely obliterated by erosion.
[Illustration: Fig. 10.--Vertical sections through strata illustrating
common kinds of faults: a, "normal faults" where one mass simply sinks
below another; b, a "thrust fault" where one mass is shoved over
another. (After U. S. Geological Survey.)]
How does the geologist determine the actual amount of displacement,
especially in the case of a large fault in stratified rocks? First, the
various formations of the region, where unaffected by faulting, are
carefully studied, especially in regard to the character and thickness
of each, and their relative geologic ages or normal order as they
were deposited one layer above the other. Then, in the simple case of
a normal-fault surface at right angles to horizontal strata, it is
only necessary to find out what two formations or parts of formations
come together along the fault fracture, and the actual amount of
displacement is readily determined. Where strata and normal fault
surfaces lie at various angles, and also in thrust faults, those angles
must be determined in addition to the data above named. In many mining
regions, where valuable deposits are affected by faulting, accurate
knowledge of the direction and amount of displacements of faults is of
great economic importance.
A few examples of normal faults from well-known districts will now be
briefly described. The whole eastern front of the central and southern
Sierra Nevada Range of California is a great, steep fault slope, from
a few thousand to ten thousand or more feet high and hundreds of miles
long, of such recent geologic age that it has been only moderately
affected by erosion. In fact, it is well known that the southern
two-thirds of the range is a great tilted fault block, the total
displacement having resulted from repeated sudden movements since about
the middle of the present geologic era. A great fault also extends
along the eastern base of the great Wasatch Range of Utah and the steep
slope thousands of feet high is a fault scarp only slightly modified
by erosion. Renewed movements along this profound fault have very
recently taken place as proved by the presence of fresh fault scarps
in loose deposits which have accumulated across the mouths of some of
the canyons, as, for example, near Ogden. In fact, practically all of
the north-south ranges of the Great Basin from Utah to California are
essentially a series of tilted fault blocks. Another great fault, less
conspicuous from the topographic standpoint, is hundreds of miles long
in the Coast Range Mountains of California. At the time of the San
Francisco earthquake of 1906 there was a renewed sudden movement along
this great fracture. The eastern one-half of the Adirondack Mountains
of New York is literally a mosaic of hundreds of fault blocks. Many
of these faults are from two to thirty miles long and they commonly
show displacements of from a few hundred to 2,000 or more feet. A
glance at the geological map (in colors) of the vicinity of the great
copper mines at Bisbee, Arizona, shows most of that region to contain
a network of normal faults which separate it into a mosaic of fault
blocks. In each of the examples of faults just given a block of earth
has sunk relative to the other, or in other words, each is a "normal
fault."
We shall now turn to some large scale cases of faults in which great
masses of earth have been pushed one over another--so-called "thrust
faults." In the southern Appalachian Range, and especially well
exhibited in the vicinity of Rome, Georgia, one portion of the mountain
mass has literally been shoved over another, at a low angle over a
fault surface many miles long, for fully seven miles westward. Both the
tremendous weight of rock material actually translated and the number
of sudden movements required in the operation stagger the imagination.
It is safe to say that during the long time of this great operation
violent earthquakes were not uncommon. In the Rocky Mountains of the
northern United States and southern Canada there is the greatest known
thrust fault on the continent. It is hundreds of miles long, and the
actual displacement is commonly at least several miles. In the Glacier
National Park of Montana it has been established that the front range
portion of the Rockies has actually been pushed at least seven miles,
and possibly as much as twenty miles, eastward over a fault surface,
and out upon the Great Plains. In some cases rocks of the Prepaleozoic
Age have there been pushed upon rocks of the late Mesozoic Age, thus
locally upsetting the geologic column.
[Illustration: Fig. 11.--East-west profile and vertical structure
sections fifty-two miles long in the Mohawk Valley region of New York,
showing numerous tilted fault blocks which notably influence the
topography. Vertical scale exaggerated. The rocks are Prepaleozoic and
early Paleozoic in age. (Modified by the author after Darton, New York
State Museum.)]
The Wasatch Range of Utah, in addition to the great normal fault along
its western base, contains a remarkable system of thrust faults. In
the region now occupied by the Wasatch Mountains a number of parallel
(thrust) faults were developed close together and the broken pieces of
the earth's crust between them were pushed up, the rocks on one side
of each crack riding up over those on the other side until a great
mountain range was formed where once lay a plain. In the Ogden Canyon
one great earth block of Prepaleozoic (Algonkian) Age has been shoved
thousands of feet over late Paleozoic (Carboniferous) rock, which
latter has in turn been thrust over early Paleozoic (Cambrian) rock.
This thrust faulting was accomplished before the development of the
geologically recent normal fault along the western base of the range.
[Illustration: Fig. 12.--Vertical (structure) section through a part
of the earth's crust several miles long in Ogden Canyon, Utah, showing
the system of great thrust faults. Prepaleozoic (Algonkian) rocks have
been pushed far over upon late Paleozoic (Carboniferous) strata, which
latter have in turn been shoved over early Paleozoic (Cambrian) strata,
etc. (After U. S. Geological Survey.)]
Any sudden movement of part of the crust of the earth, due to a natural
cause, produces a trembling or shaking called an earthquake. Though
earthquakes are generally classed among the most terrifying of all
natural phenomena, those which have occurred during human historic
times have had scarcely any geological or topographical effects of
real consequence on the face of the earth. Locally, the effects may
be notable and the destruction of life and property may be great. The
earth may be locally cracked and rent, small fault scarps may develop,
landslides and avalanches may result from the shaking of the earth,
buildings may be demolished, and sea waves may be rolled upon the
land. On the other hand, many earthquakes, called "tremors," are too
slight to be noticed by people, though they are recorded by specially
constructed instruments called "seismographs." We have already stated
that actual sudden displacements causing earthquakes have amounted to
twenty or even fifty feet right along fault fractures, but during the
vibrations or quakings, which are often so destructively sent out into
the neighboring country, the earth's surface rarely actually moves
more than a small fraction of an inch. Because of the suddenness of
the movement objects on the surface may be moved inches or even feet.
Violent shocks may last one or two minutes and cause the whole earth
to tremble, though at distant points only seismographs record the
movement. It is probably true that some part of the earth is shaking
all the time.
Studies during the last fifty years have made it certain that the main
cause of earthquakes is the sudden slipping of earth blocks past each
other along fault fractures, the sudden slipping furnishing the impulse
which sends out the vibrations into the surrounding more or less
elastic crust of the earth. The low rumbling to roaring sound, which
sometimes immediately precedes an earthquake, is probably due to the
grinding of the rocks together below the surface.
Earthquakes generally accompany volcanic outbursts of the violent
or explosive type, and in such cases subterranean explosions cause
both the eruptions and the quakings of the earth. It is well known
that the principal volcanic districts or belts of the earth are also
the belts of most frequent earthquakes, but this does not mean that
volcanic action causes most of the earthquakes. Active volcanoes and
earthquakes are so commonly associated in the same belts because those
belts no doubt represent portions of the crust which are now most
actively yielding to the forces directly resulting from the shrinkage
of the earth. Within the volcanic belts many earthquakes take place
unaccompanied by any volcanic action, and many others take place
far from volcanoes. Some earthquakes have been caused by the impact
of great landslides or avalanches, or by the sudden caving in of
underground openings.
Brief descriptions of a few typical carefully studied earthquakes
during recent years will serve to make the main features of earthquakes
still clearer to the reader.
The violent Japanese earthquake of 1891 was caused by the sinking of
a block of earth forty miles long from two to thirty feet below that
on the other side of a fault fracture. There was also considerable
horizontal shifting, and cracks developed in the adjacent region. A
distinct fault scarp, fifteen to twenty feet high, developed, and in
some cases extended right across cultivated fields.
[Illustration: Fig. 13.--Map of the United States, showing the large
areas over which three of the greatest of our earthquakes were actually
felt by people. These earthquakes were recorded in many parts of the
world by delicate instruments: New Madrid, 1811; Charleston, 1886; San
Francisco, 1906.]
[Illustration: Fig. 14.--Sketch map showing the trace of the great
fault fracture along which a renewed sudden movement of as much as
twenty feet took place to cause the San Francisco earthquake of 1906.
(After U. S. Geological Survey.)]
The San Francisco earthquake of 1906 was produced by renewed movement
along the great fault which extends lengthwise through the Coast Range
Mountains for several hundred miles. It is literally correct to say
that, for 250 miles along this great earth fracture, one part of the
Coast Range instantaneously slipped from two to twenty-two feet past
the other. More or less of the movement extended at least several
thousand feet down into the earth. In this case both sides slipped and
the movement was very largely horizontal rather than vertical. The
land on the east side of the fault moved south and that on the west
side moved north, the amount diminishing away from the fault on each
side so that some miles out the actual crustal movement was only a few
inches. When one thinks of the tremendous volumes of earth material
involved in this shifting of the earth's crust, is it any wonder
that such destructive earthquake waves were produced? Many buildings
were wrecked, several hundred people were killed, the disastrous San
Francisco fire resulted, water mains were broken, and fences and roads
crossed by the fault were dislocated as much as fifteen to twenty feet.
[Illustration: Plate 5.--Swift Current Valley in Glacier National Park,
Montana. This was once a deep V-shaped canyon carved out (eroded)
by stream action. Then a great valley glacier slowly plowed its way
through it during the Ice Age and, by ice erosion, the present nearly
straight U-shaped canyon has resulted. (_Photo by Campbell, U. S.
Geological Survey._)]
[Illustration: Plate 6.--View in the Yosemite Valley from Near the
Western Entrance. The great rock called "El Capitan," on the left rises
3,500 feet above the river, and Bridal Veil Falls on the right is 620
feet high. All the rock is granite, the nearly vertical walls of which
have resulted from the action of a great glacier which plowed its way
through the valley during the Ice Age; the valley walls have been cut
back by the removal of large vertical joint blocks. The flat bottom of
the valley has resulted from the filling with sediment of a postglacial
lake in the valley. (_Photo by F. N. Kneeland, Northampton, Mass._)]
During the great earthquake on the coast of Alaska in 1899 notable
changes took place along the shore for some miles, one portion having
suddenly risen as much as forty-seven feet, while another portion sank
below sea level.
[Illustration: Fig. 15.--Map showing the principal earthquake regions
of the world.]
In 1886 the earthquake centering near Charleston, S. C., was preceded
by rumbling and roaring noises and the slight quaking increased to
violent shaking which lasted more than a minute. Eight minutes later
a rather violent earthquake shock took place, followed during the
next ten or twelve hours by less severe shocks. Most buildings in the
city were wrecked or more or less badly damaged, and some people
were killed. The shocks were so violent that the quaking was actually
felt by people over an area of more than 2,000,000 square miles, the
disturbance having spread at the rate of about 150 miles per minute.
Near Charleston openings and fissures were formed through which sand
and muddy water were ejected, but the cause of the disturbance was most
likely slipping of the old very hard rocks below the loose deposits of
the Coastal Plain.
From 1811 to 1813 a series of violent earthquakes developed in the
general vicinity of New Madrid, Missouri. In an area of over 2,000
square miles, now called the "sunk country," many portions suddenly
sank giving rise to small fault scarps or cliffs, and various lake
basins were formed. Development of a fissure caused a local change in
the course of the Mississippi River.
In 1897, Assam, India, was shaken by an earthquake of unusual
magnitude, which lasted 2-1/2 minutes. An area of 150,000 square miles
was disastrously shaken, and the shocks were distinctly felt over
an area of 750,000 square miles. A number of notable fault scarps
developed, the movement on one having been thirty-five feet.
CHAPTER VIII
VOLCANOES AND IGNEOUS ROCKS
Not only because of the great power and terrifying grandeur of violent
eruptions, but also because of their destruction of life and property,
volcanoes stand out in the popular mind as among the most real and
important of all geological phenomena. But great volcanic outbursts,
like violent earthquakes, are in truth only outward, sensible,
relatively minor manifestations of the tremendous earth-changing forces
below the surface. They are far less important as geological agencies
than the mighty interior forces which cause parts of continents to
be slowly upraised and the rocks folded, or even than the incessant
action of streams whereby the lands are cut down. Even as an igneous
agency, volcanoes are notably less important than the development and
shifting of molten materials within the earth's crust. Volcanic action
is, however, not only conspicuous, but it is also of real significance
as a means of changing the earth, such action having taken place since
very early known geologic time. After bringing out the main facts and
principles of volcanoes, aided by descriptions of specific eruptions,
we shall turn to a consideration of igneous activity within the earth's
crust.
Mount Vesuvius in Italy is perhaps the most famous active volcano in
the world. Its eruptions have been more or less carefully studied for
a longer time than any other. The eruption in the year 79 A. D. was
really a tremendous explosion causing a large part of the old crater to
be blown away, and sending immense volumes of rock fragments, mostly
finely divided (so-called "ashes") into the air which completely buried
the small city of Pompeii. Water from the great clouds of condensing
steam, mixed with "ashes," formed muddy floods which overwhelmed
Herculaneum. Little or no lava was erupted. Since that time the crater
has been more or less active and the present cone, 4,000 feet high, has
been built up. During the last fifty years the greatest eruptions took
place in 1872 and 1906, when, streams of molten rock flowed down the
sides of the mountain.
[Illustration: Fig. 16.--Map showing the distribution of active and
recently active volcanoes of the world.]
One of the greatest volcanic explosions ever recorded was that of the
island of Krakatoa, between Sumatra and Java, in 1883. The greater part
of the island was blown away and there was water 1,000 feet deep,
where just before the island stood hundreds of feet high. About a cubic
mile of rock material was sent into the air mostly in the form of fine
dust--some of it for seventeen miles--and completely hid the sun,
causing total darkness during the eruption. Dust fell over an area of
several hundred thousand square miles. Several days after the explosion
ships more than 1,000 miles away were dust covered. Such enormous
quantities of a light porous lava called "pumice" fell and floated
upon the sea that navigation was badly obstructed many miles from the
volcano. Extremely fine dust gradually spread through the whole earth's
atmosphere, causing the extraordinary red sunsets for several months.
The sound of the explosion was heard for hundreds of miles. Great
sea waves 50 to 100 feet high were stirred up and they swept inland
for several miles over the low-lying coast lands of neighboring Java
and Sumatra, overwhelming hundreds of villages and drowning tens of
thousands of people.
[Illustration: Fig. 17.--The great hole left after the top of Mt.
Katmai in southern Alaska was blown off in 1912 by one of the most
tremendous volcanic explosions in the annals of human history.
The water in the lake is hot. (After Griggs, National Geographic
Magazine.)]
One of the greatest explosions on record was that of Katmai volcano,
several thousand feet high, on the coast of Alaska, in June, 1912.
Not only was the top of the mountain completely blown off, but also a
great crater pit, three miles wide across the top and several thousand
feet deep, was developed in the stump of the former mountain. Volcanic
dust fell to a depth of several feet within twenty-five to fifty miles
of the mountain. Dust accumulated to a depth of nearly a foot in the
village of Kodiak, 100 miles east of the mountain, where total darkness
prevailed for more than two days. A lake of very hot water now occupies
the bottom of the great new crater. The noise of the explosion was
heard for at least 750 miles.
[Illustration: Fig. 18.--Diagrammatic vertical or structure section
through a portion of the earth illustrating the common modes of
occurrence of igneous rocks. P, deep-seated (plutonic) igneous rock; S,
strata; D, dikes; M, mass of igneous rock forced between strata bending
them upward; F, feeding channel of volcano; V, volcano; L, lava sheets.
(By the author.)]
One of the most frightful volcanic catastrophes of recent years was the
eruption of Mont Pelée, island of Martinique, West Indies, in 1902.
In this case, also, no lava was poured out, but violent explosions
sent great clouds of very highly heated gases and vapors, mingled with
incandescent dust, thousands of feet into the air. One of these great
clouds rushed down the mountain at hurricane speed and destroyed the
city of St. Pierre with its 30,000 inhabitants. After the main eruption
a spine or core of hard rock began to rise out of the crater and it
slowly grew to a height of 1,000 feet in several months, after which it
began to crumble away. This spine probably represented nearly frozen
lava which solidified as it was gradually forced out of the mountain.
Of special interest to us, though not of great importance is the only
active volcano in the United States. In May, 1914, Mount Lassen (or
Lassen Peak), a long inactive volcano in northern California, suddenly
burst forth explosively and during the next several years hundreds of
eruptions occurred. Little or no lava appeared, but great clouds of
steam and dust often shot into the air from one to three miles above
the top of the mountain, which lies over 10,000 feet above sea level.
(Plate 10.) Great quantities of dust have accumulated for miles around
the mountain. At this writing (October, 1920) the mountain is again
active.
It should not be presumed, however, that all, or nearly all, volcanoes
are of the explosive type. Others of the more quiet type are well
exemplified by the two great Hawaiian volcanoes, Mauna Loa and
Kilauea. Any but relatively very minor explosions rarely, if ever,
occur, the product of such volcanoes being almost wholly lava, which
flows down the mountainsides in molten streams. The Hawaiian Islands
have, in fact, been almost entirely built up by successive eruptions
of lava, the building-up process having begun well below sea level.
Mauna Loa rises to nearly 14,000 feet above the sea, but, due to the
fact that the streams of lava have spread so far, the mountain has an
exceptionally low angle of slope which makes it difficult to realize
that it is so high. Considering its submarine portion, Mauna Loa
really rises nearly 30,000 feet above the sea floor. Although Kilauea
lies nearly 4,000 feet above sea level on the flank of Mauna Loa, and
only twenty miles distant from it, the two volcanoes are singularly
independent in regard to their eruptions. Each mountain has a crater
irregularly oval in shape, nearly three miles long, bounded by almost
vertical walls of hard lava, in some cases arranged in terraces. The
floors of the great crater pits are relatively level, and consist of
black lava in which are lakes of molten and even boiling lava. The
black lava floor is, in each case, only a frozen or hardened crust upon
a great column of molten lava extending down into the mountain. Prior
to an eruption of Mauna Loa the lava column rises hundreds of feet in
the crater, but during recent years the lava seldom, if ever, flows out
over the crater rim. Instead, it breaks through the mountainsides at
various altitudes, the great flow of 1919 having started at an altitude
of about 8,000 feet. This stream of liquid rock, fully one-half of a
mile wide, flowed for weeks down the mountainside and into the ocean,
the waters of which, in contact with the highly heated lava, were
thrown into terrific commotion. In 1885 a stream of lava several miles
wide flowed forty-five miles. In one case, lava traveled the first
fifteen miles in two hours, but this is an unusually great rate of
speed. Lava streams in general seldom move faster than one or two miles
per hour, and as the liquid rock gradually cools and becomes more and
more viscous, the speed diminishes to zero. Almost incredible volumes
of steam emanate from streams of molten lava.
In 1840 an outflow of lava took place from the side of Kilauea Mountain
and ran into the sea. Since that time the floor of the great crater
pit (quoting Professor W. H. Hobbs) "has been essentially a movable
platform of frozen lava of unknown and doubtless variable thickness
which has risen and descended (hundreds of feet) like the floor of an
elevator car between its guiding ways. The floor has, however, never
been complete, for one or more open lakes are always to be seen, that
of Halemaumau, located near the southwestern margin, having been
much the most persistent. Within the open lakes the boiling lava is
apparently white hot at a depth of but a few inches below the surface,
and in the overturnings of the mass these hotter portions are brought
to the surface and appear as white streaks marking the redder surface
portions. From time to time the surface freezes over, the cracks open
and erupt at favored points along the fissures, sending up jets and
fountains of lava, the material of which falls in pasty fragments
that build up driblet cones. Small fluid clots are shot out, carrying
threadlike lines of lava glass behind them, the well-known 'Pelée's
hair.' Sometimes the open lakes build up congealed walls, rising above
the general level of the pit, and from their rim the lava spills over
in cascades to spread out upon the frozen floor."
In some regions, like the Columbian Plateau of the northwestern United
States and the Deccan of India, each covering about 200,000 square
miles, vast quantities of lava have been poured out layer upon layer to
depths of even thousands of feet. Distinct volcanic cones or mountains
in those regions are either absent or too scarce to look to as sources
of so much lava. Such lava floods were probably mostly erupted from
great fissures in the earth's crust, the fluidity to spread many miles.
Some idea of the quantitative geological importance of volcanism may be
conveyed to the reader when we assert that, according to a conservative
estimate, fully one-half of a million cubic miles of molten rocks have
been poured out upon the surface of the earth through volcanic action
in relatively recent geological time! The Cascade Range with its lofty
peaks, including Mount Shasta and Mount Rainier, each rising more than
14,000 feet above the sea, has been built up very largely by volcanic
action during the last era of geologic time. Many other mountain peaks
and various ranges have been similarly developed either wholly or in
part. The great chain of Aleutian Islands extending hundreds of miles
into the sea, is the scene of much volcanic activity where a great
mountain range is now literally being born out of the sea by the
processes of vulcanism.
Before this the reader has more than likely wondered about the source
of the heat, vapors (mainly water), and power involved in volcanic
action. Answers to these questions are closely tied up with the precise
cause (or causes) of volcanic action which remains one of the most
uncertain of the larger problems of geologic science. Before briefly
discussing the causes, a few additional facts should be stated. First,
in regard to the heat, a careful determination of the temperature
of the molten lava of Kilauea in 1911 showed it to be 1,260 degrees
Centigrade, or 2,300 degrees F. This is, however, a relatively low
temperature, because many lavas from other regions show melting points
all the way up to at least 2,000 degrees Centigrade (3,600 degrees
F.). Water in the form of steam is quantitatively one of the greatest
products of volcanoes. A fair idea of the tremendous volumes of water
involved may be gained from the statement that a careful estimate shows
that fully 460,000,000 gallons of water in the form of steam erupted
from a single secondary cone of Mount Etna during a period of 100
days. Among other gases which are given off in greater or less amounts
during volcanic activity are carbonic acid gas, sulphureted hydrogen,
sulphur dioxide, and hydrochloric acid. Some idea of the power back of
volcanoes may be gained not only from the tremendous explosions such
as those above described, but also from the fact that the pressure
necessary to raise the column of lava from sea level to the top of
Mauna Loa (nearly 14,000 feet) is about 1,150 atmospheres, or about
17,000 pounds per square inch. The actual pressure must there be much
greater because the lava is forced up from far below sea level.
A long-held idea that a relatively thin crust covers a molten interior,
and that downward pressure of this crust due to earth contraction
causes molten rocks to be forced out, has been too thoroughly disproved
to now be at all seriously entertained. The fact that near-by volcanoes
commonly erupt entirely independently, as in the case of Mauna Loa
and Kilauea, shows that there can be no universal liquid beneath a
relatively thin crust. Other arguments against liquidity of the earth's
interior are that the earth acts like a body nearly as rigid as steel
against the powerful tide-producing forces, and that earthquake waves
which pass through the earth to a depth of at least 2,000 miles are the
kind which require a solid medium for transmission.
Let us then briefly consider more plausible views regarding the cause
of volcanic action. First of all we may be sure that the earth is
highly heated inside. Measurements in many deep borings show that the
temperature increases at the rate of about 1 degree F. for each 50 to
60 feet downward, to depths greater than a mile. Accordingly, on the
basis of 1 degree rise in 50 feet, at depths of 20 to 35 miles, the
temperature must be great enough (2,120 degrees to 3,590 degrees F.),
to cause all ordinary rocks to melt _if they were at the surface_. At
such depths, however, the downward pressure upon the rocks is so great
that their melting points are notably raised, and there is every reason
to believe that under ordinary conditions the rocks 20 to 35 miles down
are not molten. If we adhere to the older (nebular) hypothesis of earth
origin, the interior heat of the earth is left over from the cooling,
once molten, earth. On the basis of another (planetesimal) hypothesis,
the earth's heat is due to the steady, powerful action of gravity
causing the earth to contract. In any case, the earth is hot inside as
proved by deep well records and igneous phenomena in general, and it is
a contracting or shrinking body as proved by the many large scale zones
of wrinkling or folding of rocks. If, then, highly heated solid rocks
at reasonable distances down in any part of the earth are subjected
to relief of pressure by an earth movement such as upward crumpling
of the crust, or by readjustment of large fault blocks, such heated
solid rocks would become molten. The very earth movement which brings
about relief of pressure and melting may very reasonably be regarded
as the power which forces some of the newly formed molten material
higher up into the earth's crust, and even out upon the surface. This
view harmonizes with the well-known fact, already mentioned, that the
main belts of active volcanoes are also the main belts of active earth
movements, such as earthquakes.
Another source of power behind volcanic action is steam pressure. We
have already mentioned the fact that vast amounts of water in the form
of steam escape from volcanoes or even from streams of molten lava. The
violent volcanic explosions are quite certainly all, or nearly all,
direct results of sudden giving way of volcanoes to steam pressure
which accumulates during greater or less periods of time, and with
little or no possibility of escape, without rupturing the mountain.
Steam alone, or combined with some of the other gases so common as
volcanic products, may also aid in forcing out molten rock. What is
the source of the steam and other gases or vapors? According to one
view they were originally in the earth, while according to another view
the water at least has been absorbed by the molten rocks from surface
waters which worked their way downward. At least two arguments oppose
the second hypothesis: first, that not a few volcanoes are really many
miles from the sea or other bodies of water, while downward percolation
of rain water would fall far short of supplying the tremendous
quantities of water ejected, and second, any water taken up by molten
rock must be absorbed within a very few miles of the surface because,
as we have learned, farther down there are no openings large enough to
permit the downward passage of water, but, as a matter of fact, the
very upper part of the earth's crust is just the place where molten
rocks begin to give up their water, often with terrific violence.
We may now turn to a consideration of the other very important kind of
igneous activity, namely, the rise and transfer of molten materials
within the earth's crust, but not to the surface. The quantity of such
deep-seated (so-called "plutonic") igneous rock material which has
been intruded into the earth's crust within known geologic time, is
far greater than that which has been forced to surface, that is the
so-called "volcanic" material. The plutonic rocks are always thoroughly
crystallized, and they are generally coarser grained than the volcanic
rocks.
Where molten materials have been forced into cracks or fissures in the
crust of the earth and there congealed, we have a very common mode of
occurrence called "dikes" (Plate 9). In many regions often one set of
dikes was formed, after which one or more succeeding injections from
the same or different deep-seated bodies of molten rock took place,
and some of the later dikes were forced to cut across earlier ones.
Dikes of all lengths up to at least thirty miles, and of all widths up
to many hundreds of feet, are known, but they are generally less than
a mile long and not more than a few feet or rods wide. They have been
intruded into all kinds of rock formations--igneous, sedimentary, and
metamorphic. Dikes are common in many parts of the world and they often
excite the interest of lay-men. They are wonderfully displayed along
the southern coast of Maine. Plate 9 shows small dikes where the molten
material was forced from a larger mass into a body of older dark
rock. The Palisades of the Hudson River, just north of New York City,
consists of a layer of igneous rock several hundred feet thick which,
in the molten condition, was forced nearly horizontally between layers
of sandstone millions of years ago, that is in the early Mesozoic era.
The palisade or columnar structure was caused by cracking of the rock
during the cooling and contraction. This is the explanation of most
columnar structures of igneous rocks, exceptionally fine exhibitions
being at the Giant's Causeway in Ireland, and Devil's Tower, Wyoming
(Plate 10).
A type of occurrence not so common, but of special interest, is where
a body of molten rock rising in nearly horizontal strata becomes
cooler and therefore stiffer or more viscous and, losing its power to
penetrate, forces its way between the layers causing the strata to be
arched or domed over it. Sufficient removal of overlying material by
erosion has revealed many fine examples of this type of occurrence.
Another type of interest is the volcanic neck, which is the core or
plug filling the feeding channel of a volcano. In certain regions, like
parts of Arizona and New Mexico, extinct volcanic mountains may be all
cut away by erosion, except the central cores or necks which, both
because they are more resistant and are last to be reached by erosion,
stand out conspicuously as great towers on the landscape (Plate 9).
Most important of all from the quantitative standpoint, however, are
the great bodies of igneous rocks, ranging up to many miles across,
which, in a molten condition, were forced irregularly into the earth's
crust from unknown depths.
The common rock called granite belongs in this category of rocks, which
are the best and most extensively developed of all igneous types.
The roots or cores of great mountain ranges often consist of such
rocks which are exposed to view only after removal of great thickness
of overlying material. Immense areas of granite and other plutonic
rocks of extra deep-seated origin are exposed, because of removal of
overlying material by erosion, in southeastern Canada, the Adirondack
Mountains, New England, the Piedmont Plateau of the Atlantic Coast, and
in the Sierra Nevada Mountains. All the rock forming the lofty walls of
Yosemite Valley is granite, which was forced into the earth's crust in
relatively late Mesozoic time, and which has since been laid bare by
erosion.
CHAPTER IX
WATERS WITHIN THE EARTH
It has been estimated that approximately 1,500 cubic miles of water
fall upon the surface of the United States each year. About one-half
of this goes back into the atmosphere by evaporation; about one-third
of it flows away in surface streams; and the remaining one-sixth
enters the crust of the earth. Considerable water which enters the
earth returns to the surface as springs, by capillarity of soils and
rocks, or by being drawn up into plants and evaporated. Some idea of
the amount of ground water may be gleaned from the statement, based
upon a careful estimate, that all the water in the rocks and soils of
the first 100 feet below the surface of the United States would make a
layer seventeen feet thick. In most humid regions the soils and loose
rock formations are saturated with water at greater or less depths
(usually less than 100 feet) below the surface. The surface of this
saturated layer is called the ground-water level, or more familiarly
the "water table." The water table shifts up and down more or less
according to variation in rainfall.
In addition to the water held in the loose rocks and soils near the
earth's surface, large quantities occur in definite layers (usually
strata) of porous rocks which very commonly extend at various angles,
hundreds or even thousands of feet into the earth. A very fine
illustration of this principle is the case of the Dakota sandstone
formation of Nebraska. Almost anywhere across the State a well drilled
through a bed of clay and into the porous sandstone layer encounters
water. (Figure 19.) Another principle is also well illustrated,
namely, that water in such a porous layer may actually travel hundreds
of miles, water obtained from a well sunk to the Dakota sandstone
having actually traveled under the surface of the State all the way
from the eastern face of the Rocky Mountains, where rain and melting
snow entered the upturned and exposed porous rock layer. Another good
example is Iowa, where certain porous rock layers outcropping in the
northwestern and northeastern corners of that and adjacent States
gradually bend down under the State, reaching the greatest depths (up
to 3,000 feet) far in the interior. From wells 3,000 feet deep near
Boone, Iowa, it is, therefore, a fact that some of the water pumped
out of the earth actually traveled underground all the way from beyond
the corners of the State. This sort of travel of underground water is
common in many parts of the world. It should be clearly understood
that such water does not flow freely as in a pipe along subterranean
passageways, but rather it slowly works its way between the grains of
porous rock. Where such water moves distinctly downward, and the porous
layer has both above and below it an impervious rock layer like shale
or clay, it gradually gets under greater and greater pressure. In some
cases such pressure has actually been found by deep drilling to be
equivalent to that of a column of water several thousand feet high. The
rate of motion of water in porous underground rock layers is very slow,
data from various sources indicating a rate of speed of not more than
one-fifth of a mile a year in coarse porous sandstone, while in many
rocks it cannot be more than ten to fifty feet per year.
[Illustration: Fig. 19.--Vertical (structure) section from the Rocky
Mountains to Omaha, Nebraska, illustrating a widespread underground
porous rock layer (the Dakota sandstone) charged with water under
pressure, the clay formation acting as a cap rock. (After Darton, U. S.
Geological Survey.)]
We still have to consider a third mode of occurrence of waters within
the earth. Many formations, like granite and other types of crystalline
rocks are neither in definite layers, nor are they sufficiently porous
to allow water to really flow through them. Where such rocks extend
far down from or near the surface, how does rain water descend? It
does so along cracks or fractures (both joints and faults) which we
have learned are almost universally abundantly present in all hard
rocks in the upper (or zone of fracture) portion of the earth's crust.
Joint cracks are generally very irregular in direction and spacing,
while fault fractures are usually fairly regular and straight. Many
cracks are not wide enough to allow anything like good passageways for
water, while others are sufficiently open to allow water to travel
along them for hundreds, or even thousands of feet. In canyons of the
West, springs not rarely emerge from the bottoms of great, nearly
vertical ledges of granite and other hard crystalline rocks, the waters
certainly having entered the rocks hundreds, or even some thousands of
feet, higher. In rocks of the kind here considered it is evident, then,
that the movements of subterranean waters must be mostly exceedingly
irregular and usually not in great quantities. In many deep mines of
the world, underground water causes little or no trouble except often
near the surface. Occasionally a shaft or tunnel strikes a prominent
joint or fault fracture filled with water.
What we might really call underground streams may occur only under
exceptional conditions in rocks other than limestone, but in limestone
they are not uncommon because the slow solubility of the rock allows
underground waters to slowly enlarge the passageways to form distinct
channels. Echo River, which flows through Mammoth Cave, is a fine case
in point.
Most water by far which emerges as springs, was at one time surface
water. A simple, but common case is where rain water soaking through
porous soil (e.g., sand) or rock, sinks to the top of an underlying
impervious layer (e.g., clay) along whose surface it flows until it
reaches the side of a valley where a spring results. In fact, wherever
the water table is crossed by the surface of the ground, water must
either seep or flow out. Where underground streams which are common in
limestone regions reach the surface on hill or valley sides, springs
result. Another source of springs is where under proper conditions of
slope a porous rock layer, charged with water well below the surface,
appears at a lower level than its source of water. Still another type
of spring is where a fissure or fracture crosses a water-bearing layer
in which the pressure is great enough to cause the water to rise to the
surface along the relatively open fissure or fracture.
In various localities we hear of springs in seemingly paradoxical
situations on tops of hills and even mountains. Such a mystery is not
difficult to clear up. In the first place, such springs are rarely at
the summit of the hill or mountain. A case well known to many persons
is the small, but never-failing spring a little below the summit of
Mount Whiteface, a peak in the Adirondacks, rising 3,000 feet above the
general level of the immediately surrounding country. In this case, a
mass of highly fractured rock, subjected to much rainfall and lying
above the level of the spring, is sufficiently large easily to contain
and give forth enough water to account for several such springs. In
rare cases, however, springs or flowing wells are located on summits,
and in such places it is only necessary to bear in mind some of the
principles above set forth, but mainly the facts that water may travel
under pressure long distances underground, and that the point of
emergence may be on a hill which is actually lower than the source of
the water far away.
The economic significance of underground waters is forcibly brought
to our attention when we realize that 75 per cent of the people of
the United States depend upon wells for their water supply. Many city
supplies, most farm supplies, and much irrigation water come from
wells. The 3,000,000 people of Iowa, for example, are dependent upon
underground waters from wells varying in depth from a few feet to
several thousand feet.
[Illustration: Fig. 20.--Ideal section illustrating the chief requisite
conditions of artesian wells. A, a porous stratum; B and C, impervious
beds below and above A, acting as confining strata; F, the height of
the water level in the porous bed A, or, in other words, the height of
the reservoir or fountainhead; D and E, flowing wells springing from
the porous water-filled bed A. (After U. S. Geological Survey.)]
Most wells are simply dug to depths a little below the water table. In
humid climate regions the depths seldom exceed fifty feet. The water
encountered in such wells is rarely under pressure. In some regions of
deep soils or loose formations, wells are actually bored with an auger
to depths of as great as 200 feet. Deep wells in relatively hard rocks
are always drilled to depths of even thousands of feet. In such cases
the purpose is to strike either a porous rock layer charged with water,
or a crack or fissure filled with water, the water almost always being
under pressure (sometimes very great), under such conditions. These are
called artesian wells whether the water under pressure actually flows
out at the surface or not.
We may now inquire as to the necessary conditions for artesian wells.
This may best be done by the aid of diagrams. Figure 20 illustrates a
very common case where a porous layer, lying between impervious layers,
passing under a valley, comes to the surface of the hills on each side
where the water enters the porous layer. On sinking a well to the
water-charged layer, the water rushes through the hole to a greater or
less distance above the surface. In Figure 21 the porous and impervious
layers are simply tilted, and the water under pressure rises through
the free opening to the surface. Wells of this kind are also common
in the Atlantic Coastal Plain of the United States. In another case,
less comprehensible to the layman, the porous water-bearing stratum
curves downward under a hill or mountain, water entering it where it
is exposed on each side. Under such conditions a flowing artesian well
cannot be drilled at or near the summit, but since the water is under
pressure it will rise in the hole to a level approaching that of the
lowest part of the outcrop of the porous layer on either side of the
hill or mountain. This is essentially the condition of things toward
the interior of Iowa, where water from the deeper wells rises 2,000
feet or more in the holes, but does not reach the surface.
[Illustration: Fig. 21.--Section illustrating the thinning out of a
porous water-bearing bed. A, inclosed between impervious beds B and C,
thus furnishing the necessary conditions for an artesian fountain at D.
(After U. S. Geological Survey.)]
The drilling of deep wells, where records, including samples of rock
materials brought up, have been kept, has been a great aid to the
geologist in determining, or rendering more precise, the knowledge of
not only the kinds of rocks underground, but also the thicknesses and
structural relations of the formations.
In yet another way deep wells are of special significance, that is in
regard to the light which they throw upon the subterranean temperature
of the earth. Very recently the deepest well in the world was drilled
near Fairmont, West Virginia, to a depth of 7,579 feet, in quest of
oil or gas. At a depth of 7,500 feet, the temperature was found to be
168 degrees F. Allowing for a near-surface temperature 50 degrees,
this means an average rate of increase downward of 1 degree in 62
feet. The second deepest well is near Clarksburg, West Virginia, sunk
to a depth of 7,386 feet, with a temperature of 172 degrees at the
7,000-foot level, or at the rate Of 1 degree in 57 feet, allowing for
a near-surface temperature of 50 degrees. It is a remarkable fact,
that little or no water was encountered all the way down. A well 7,348
feet deep in southeastern Germany gave a temperature of 186 degrees
at the bottom, or a rate of increase of 1 degree in 54 feet. These
three records are about the average for the deep holes of the world.
Next to the deepest mining shafts in the world are in the copper
mining region of northern Michigan, where over 5,000 feet (counted
vertically, not down the slope) down the temperature is nearly 90
degrees the year round. The rate of increase is here less than in most
wells of such depth, because of the cooling air currents. Many years
ago a rather remarkable experiment in well drilling was tried by the
city of Budapest, Hungary, the attempt being to get a supply of water
at the brewing temperature of 176 degrees in order to encourage the
manufacture of beer. After getting a good supply of water at a depth
of 3,120 feet and a temperature of 158 degrees, work was stopped. In
building the two great tunnels (St. Gotthard and Simplon) through
portions of the Alps, such high temperatures were encountered that work
was continued only under great difficulties. In the famous Comstock
gold and silver mine of Nevada, over forty years ago, temperatures as
high as 157 degrees were encountered in the shafts at a depth of only
2,000 feet or a little over, the exceptional temperature for such depth
no doubt being due to occurrence of the ores in geologically recent
igneous rocks which have not yet cooled to the normal temperature for
the depth of 2,000 feet.
From the sanitary standpoint, wells are of very great significance,
especially in view of the fact that such a large proportion of people
depend upon well water. It is generally understood that typhoid
fever is more common in the country than in cities, in spite of what
might reasonably be expected. What are some of the causes leading
up to such a situation? The idea that water purifies itself after
flowing a relatively short distance is, in many cases, far from being
true, especially when we are dealing with underground water. Actual
observations prove that germ-laden water may travel surprisingly far
underground. Germ-laden water from barns, cesspools, or outhouses
spreads notably on sinking to the water table and it is easy to see
how so many wells become contaminated. On general principles, a
geologist is especially wary of water from a well in a barn yard.
The well for human use at least should be located out of reasonable
range of such contamination. Under the condition of the diagram a
well or spring some distance down the side of the hill may actually
be unfit for use, though a serious situation is much less likely to
develop there. Nor should one assume that by locating the well on
the uphill side of the house, and the outhouses or cesspool on the
downhill side, safety is assured. From what we have learned in regard
to earth movements, and the tilting of strata from their original
positions, we know how the movement of water in the saturated zone
near the surface may be downhill roughly following the hill slope,
while in a tilted porous layer of rock farther below the surface the
movement of water may be in just the opposite direction. A well drilled
into the solid rock for safety on the uphill side of a house might
derive its water from this very same porous layer, whose water has
been contaminated from a cesspool or other source down the side of
the hill. Such a case is by no means a theory or a rarity. There is
also real danger of contamination in cases where the water flows more
like streams underground through cracks or fissures in hard or dense
rocks, or through channels developed by solution in limestone. It may
happen that water becoming contaminated from barn sites, cesspools, or
outhouses finds its way along such a channel to the side or bottom of
a well. The author well remembers the case of a farmer whose house,
barn, and well were close together on a little limestone terrace and
who continued to use the well water although he complained of its
disagreeable taste, especially after a rain when he could "taste the
barn in it."
[Illustration: Fig. 22.--Diagram illustrating a danger of contamination
of wells by impure underground water. S, soil; B, bedrock; P, porous
stratum. Impure water from cesspool moves through porous layer to
bottom of well. (By the author.)]
Finally, in this connection, it may be said that wells should be
located in the light of the principles above explained, best of all
upon the advice of some one with geological training, and that, to
insure safety to health from the well water, sanitary analyses (at
small cost) should be made once or twice a year. A bad well should be
abandoned and a new one sunk.
A large amount of money has been wasted upon, and much mystery and
superstition has surrounded so-called "water witches," or those who
claim some special or supernatural power of locating supplies of
underground water. Most common of all devices used is the so-called
"divining rod," which is a forked stick of willow, witch-hazel, or
other wood, according to the seemingly special requirement of the
operator. Certain mechanical and electrical devices are also employed.
With one fork of the divining rod grasped in each hand and the main
part of the stick upright, the operator walks about until, due to
some "mysterious" influence, a place is found where underground
water pulls the upright portion of the stick downward in spite of
the grasp of the holder. Some operators even claim to know just how
deep a well must be sunk. Without any attempt to question the honesty
of all operators, geologists are in full accord with the following
quotation from a paper published by M. L. Fuller, for the United States
Geological Survey: "The uselessness of the divining rod is indicated
by the facts that it may be worked at will by the operator, that he
fails to detect strong water currents in tunnels and other channels
that afford no surface indications of water, and that his locations
in limestone regions where water flows in well-defined channels are
no more successful than those dependent upon mere guesses. In fact,
its operators are successful only in regions in which ground water
occurs in definite sheets in porous material, or in more or less
clayey deposits, such as pebbly clay or till. In such regions (which
are extremely common) few failures can occur, for wells can get water
almost anywhere. Ground water occurs under certain definite conditions,
and just as surface streams may be expected wherever there is a valley,
so ground water may be found where certain rocks and conditions exist.
No appliance, either mechanical or electrical, has yet been devised
that will detect water in places where plain common sense will not show
its presence just as well. The only advantage of employing a 'water
witch,' as the operator of the divining rod is sometimes called, is
that crudely skilled services are thus occasionally obtained, since
the men so employed, if endowed with any natural shrewdness, become,
through their experience in locating wells, better observers of the
occurrences and movements of ground water than the average person."
It should not be assumed, however, from the above statement that the
location or foretelling of underground water is mostly hopeless from
a scientific point of view. In most regions the kinds of rocks which
would be pierced by wells can be more or less accurately foretold by
careful studies of the rocks exposed at the surface. But foretelling
the underground water is often much more uncertain. Where the geologic
structure or arrangement of rocks in a region is fairly regular, as
in the case of most sedimentary rocks, and a few scattering deep
wells have been drilled, with records preserved, the geologist, by
combining such data with his surface studies, can do much toward
putting the facts regarding the underground waters of the region on a
scientific basis. There are many such regions, an excellent case in
point being Iowa, regarding which State the United States Geological
Survey has published a report containing data by the use of which
it is possible to foretell almost exactly what formations would be
pierced by drilling from 1,000 to 3,000 feet or more, the thickness
of each, which ones are water-bearing and, in many cases, even the
character of the mineralization of the water for almost any part of
the State. Such knowledge, through the years, is worth untold millions
of dollars to the State. Where the rocks are igneous and rather
uniformly dense, usually little or nothing can be accurately foretold
about the underground water supplies, because in such rocks the water
follows exceedingly variable and irregular cracks and fissures. In
metamorphic rocks the difficulties are usually about as great. In
limestone regions, with humid climate, much water travels in channels
underground, but these are so exceedingly irregular that there is no
way of locating them by surface studies. In humid climates it seldom
happens, however, that a well does not reach at least a fair supply of
water within a few thousand feet even in rock formations in which the
water travels along irregular cracks and channels.
Certain other important features of the geological work of underground
water should be brought to the attention of the reader. One of these
is its power to dissolve mineral substances of many kinds more or less
rapidly. As already pointed out, limestone is especially susceptible to
solution in water, both surface and underground.
The carbonate of lime taken into solution from limestone is the
principal substance which causes so-called "hard water." Most of the
solution takes place in the upper part of the zone of fracture of the
earth's crust and the dissolved substances are carried along generally
to the lower levels where they tend to deposit (and crystallize),
filling fissures, cracks, and even tiny spaces between mineral grains.
Cracks and fissures thus filled by mineral matter from solution
are called "veins." In many mining regions valuable ores and other
substances have been deposited from underground water solutions and
concentrated in veins. In many places underground waters with certain
substances in solution travel through various rocks or encounter
solutions of other substances and, as a result of chemical action, many
new mineral combinations result. Such actions through the millions
of years of geologic time have effected great changes in many rock
formations. In the case of petrification, like that of petrified wood,
the buried organism slowly decomposes cell by cell, and particle by
particle it is replaced by mineral matter from underground water
solutions. In this manner the remarkable so-called petrified forests
(not really forests) of Arizona and the Yellowstone Park were formed,
the petrifying material there having been the very common substance
called silica which is the same in composition as the familiar mineral
quartz. Mineral matter carried in solution in surface streams is
derived from ground waters which reach the surface. An idea of the
tremendous quantity of mineral matter thus removed may be gained from
the statement that by careful determination the Mississippi River
carries 120,000,000 tons in solution into the Gulf of Mexico each year.
[Illustration: Fig. 23.--Structure section and part of landscape in a
limestone region showing how caves and natural bridges are formed by
the dissolving action of underground water. AA, limestone; BB, sink
holes; DD, caves and galleries; and an arch (natural bridge) which is
the remnant of a large cave. (After Shaler, U. S. Geological Survey.)]
One interesting effect of the dissolving power of underground water
in limestone regions is the development of caves or caverns. Most
remarkable of all is Mammoth Cave, Kentucky, with its hundreds of
miles of passageways and galleries. This marvelous work of nature is
all a result of the action of underground water which has dissolved
and carried away vast quantities of limestone. Echo River, which flows
through the cavern, is still carrying on the work aided by various
underground tributaries. The stalactites and stalagmites, which are
so strikingly displayed in many caves, as at Luray, Virginia, in
which water with carbonate of lime drips or oozes from the roof and,
due mainly to evaporation, deposits the lime. Many wonderful and
fantastic effects are thus produced. Where part of the roof of a cave
is dissolved out, or falls in, a "sink hole" results. Where all but a
portion of the roof of a cave or underground channel has fallen in,
a natural bridge, like the famous one in Virginia, results, though
natural bridges are also formed by other means.
In concluding this chapter we shall briefly discuss hot underground
waters, hot springs, and geysers. There are two well-known ways by
which underground waters may become heated. One is by the movement of
water downward into the normally heated portion of the earth, the rate
of increase downward being, as above stated, 1 degree F. for about
50 to 60 feet. Water descending two miles would, therefore, attain a
temperature of about 200 degrees F. In some regions such a temperature
may be reached at depths considerably less. Such water (under pressure)
taking a short course to the surface (forming springs) at a lower
level would retain much of its heat taken up far below the surface.
In regions where there are great down-folds of the strata (i.e.,
synclines), as in the central to southern Appalachians, conditions
appear to be favorable for such warm or hot springs, as, for example,
at Hot Springs, Virginia. A second cause of the heating of underground
water is by the descent of surface waters into contact with masses of
still hot igneous rock of relatively recent geologic age. In some such
cases the water does not go more than some hundreds of feet down and
when, under proper conditions, it returns to the surface hot and even
boiling springs may result.
[Illustration: Plate 7.--An Upbend Fold (_anticline_) in the
Appalachian Mountain Strata Near Hancock, Maryland. The strata were
deposited in horizontal layers upon the sea bottom, covering the region
many millions of years ago in middle Paleozoic time. At the time of
the Appalachian Mountain revolution, near the end of Paleozoic time,
this and many other folds developed well below the surface. Removal
of overlying material by erosion has laid bare the fold as we see it
to-day. (_Photo by Russell, U. S. Geological Survey._)]
[Illustration: Plate 8.--(_a_) A Ledge of Igneous Rock (Granite) in
Northern New York. This illustrates so-called "joints" or natural
cracks, commonly separating most hard rock masses into more or less
prismatic blocks. (_Photo by the author._)]
[Illustration: Plate 8.--(_b_) A Fault Fracture in a Ledge at East
Canada Creek in the Mohawk Valley, New York. The Ordovician limestone
formation in thin layers on the right has sunk hundreds of feet along
vertical fault to the left of middle, bringing it sharply against the
older (Cambrian) massive formation on the left. The hole is artificial.
(_Photo by Darton, U. S. Geological Survey._)]
Geysers are periodically eruptive hot springs found only in a few of
the volcanic regions of the world. They are most wonderfully displayed
in the Yellowstone National Park, where they send columns of hot
water to all heights up to 250 feet at various intervals of time.
Almost incredible amounts of hot water are sent into the air every
day in the geyser basins of Yellowstone Park. The single geyser "Old
Faithful," which erupts at inte
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