The Elements of Geology
by
William Harmon Norton
Part 1 out of 7
This etext was produced by Robert Rowe, Charles Franks
and the Online Distributed Proofreading Team
THE ELEMENTS OF GEOLOGY
BY WILLIAM HARMON NORTON
PROFESSOR OF GEOLOGY IN CORNELL COLLEGE
PREFACE
Geology is a science of such rapid growth that no apology is
expected when from time to time a new text-book is added to those
already in the field. The present work, however, is the outcome of
the need of a text-book of very simple outline, in which causes
and their consequences should be knit together as closely as
possible,--a need long felt by the author in his teaching, and
perhaps by other teachers also. The author has ventured,
therefore, to depart from the common usage which subdivides
geology into a number of departments,--dynamical, structural,
physiographic, and historical,--and to treat in immediate
connection with each geological process the land forms and the
rock structures which it has produced.
It is hoped that the facts of geology and the inferences drawn
from them have been so presented as to afford an efficient
discipline in inductive reasoning. Typical examples have been used
to introduce many topics, and it has been the author's aim to give
due proportion to both the wide generalizations of our science and
to the concrete facts on which they rest.
There have been added a number of practical exercises such as the
author has used for several years in the class room. These are not
made so numerous as to displace the problems which no doubt many
teachers prefer to have their pupils solve impromptu during the
recitation, but may, it is hoped, suggest their use.
In historical geology a broad view is given of the development of
the North American continent and the evolution of life upon the
planet. Only the leading types of plants and animals are
mentioned, and special attention is given to those which mark the
lines of descent of forms now living.
By omitting much technical detail of a mineralogical and
paleontological nature, and by confining the field of view almost
wholly to our own continent, space has been obtained to give to
what are deemed for beginners the essentials of the science a
fuller treatment than perhaps is common.
It is assumed that field work will be introduced with the
commencement of the study. The common rocks are therefore briefly
described in the opening chapters. The drift also receives early
mention, and teachers in the northern states who begin geology in
the fall may prefer to take up the chapter on the Pleistocene
immediately after the chapter on glaciers.
Simple diagrams have been used freely, not only because they are
often clearer than any verbal statement, but also because they
readily lend themselves to reproduction on the blackboard by the
pupil. The text will suggest others which the pupil may invent. It
is hoped that the photographic views may also be used for
exercises in the class room.
The generous aid of many friends is recognized with special
pleasure. To Professor W. M. Davis of Harvard University there is
owing a large obligation for the broad conceptions and luminous
statements of geologic facts and principles with which he has
enriched the literature of our science, and for his stimulating
influence in education. It is hoped that both in subject-matter
and in method the book itself makes evident this debt. But besides
a general obligation shared by geologists everywhere, and in
varying degrees by perhaps all authors of recent American text-
books in earth science, there is owing a debt direct and personal.
The plan of the book, with its use of problems and treatment of
land forms and rock structures in immediate connection with the
processes which produce them, was submitted to Professor Davis,
and, receiving his approval, was carried into effect, although
without the sanction of precedent at the time. Professor Davis
also kindly consented to read the manuscript throughout, and his
many helpful criticisms and suggestions are acknowledged with
sincere gratitude.
Parts of the manuscript have been reviewed by Dr. Samuel Calvin
and Dr. Frank M. Wilder of the State University of Iowa; Dr. S. W.
Beyer of the Iowa College of Agriculture and Mechanic Arts; Dr. U.
S. Grant of Northwestern University; Professor J. A. Udden of
Augustana College, Illinois; Dr. C. H. Gordon of the New Mexico
State School of Mines; Principal Maurice Ricker of the High
School, Burlington, Iowa; and the following former students of the
author who are engaged in the earth sciences: Dr. W. C. Alden of
the United States Geological Survey and the University of Chicago;
Mr. Joseph Sniffen, instructor in the Academy of the University of
Chicago, Morgan Park; Professor Martin Iorns, Fort Worth
University, Texas; Professor A. M. Jayne, Dakota University;
Professor G. H. Bretnall, Monmouth College, Illinois; Professor
Howard E. Simpson, Colby College, Maine; Mr. E. J. Cable,
instructor in the Iowa State Normal College; Principal C. C. Gray
of the High School, Fargo, North Dakota; and Mr. Charles Persons
of the High School, Hannibal, Missouri. A large number of the
diagrams of the book were drawn by Mr. W. W. White of the Art
School of Cornell College. To all these friends, and to the many
who have kindly supplied the illustrations of the text, whose
names are mentioned in an appended list, the writer returns his
heartfelt thanks.
WILLIAM HARMON NORTON
CORNELL COLLEGE, MOUNT VERNON, IOWA
JULY, 1905
INTRODUCTORY NOTE
During the preparation of this book Professor Norton has
frequently discussed its plan with me by correspondence, and we
have considered together the matters of scope, arrangement, and
presentation.
As to scope, the needs of the young student and not of the expert
have been our guide; the book is therefore a text-book, not a
reference volume.
In arrangement, the twofold division of the subject was chosen
because of its simplicity and effectiveness. The principles of
physical geology come first; the several chapters are arranged in
what is believed to be a natural order, appropriate to the
greatest part of our country, so that from a simple beginning a
logical sequence of topics leads through the whole subject. The
historical view of the science comes second, with many specific
illustrations of the physical processes previously studied, but
now set forth as part of the story of the earth, with its many
changes of aspect and its succession of inhabitants. Special
attention is here given to North America, and care is taken to
avoid overloading with details.
With respect to method of presentation, it must not be forgotten
that the text-book is only one factor in good teaching, and that
in geology, as in other sciences, the teacher, the laboratory, and
the local field are other factors, each of which should play an
appropriate part. The text suggests observational methods, but it
cannot replace observation in field or laboratory; it offers
certain exercises, but space cannot be taken to make it a
laboratory manual as well as a book for study; it explains many
problems, but its statements are necessarily more terse than the
illustrative descriptions that a good and experienced teacher
should supply. Frequent use is made of induction and inference in
order that the student may come to see how reasonable a science is
geology, and that he may avoid the too common error of thinking
that the opinions of "authorities" are reached by a private road
that is closed to him. The further extension of this method of
presentation is urged upon the teacher, so that the young
geologist may always learn the evidence that leads to a
conclusion, and not only the conclusion itself.
W. M. DAVIS
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
JULY, 1905
CONTENTS
INTRODUCTION.--THE SCOPE AND AIM OF GEOLOGY
PART I
EXTERNAL GEOLOGICAL AGENCIES
I. THE WORK OF THE WEATHER
II. THE WORK OF GROUND WATER
III. RIVERS AND VALLEYS
IV. RIVER DEPOSITS
V. THE WORK OF GLACIERS
VI. THE WORK OF THE WIND
VII. THE SEA AND ITS SHORES
VIII. OFFSHORE AND DEEP-SEA DEPOSITS
PART II
INTERNAL GEOLOGICAL AGENCIES
IX. MOVEMENTS OF THE EARTH'S CRUST
X. EARTHQUAKES
XI. VOLCANOES
XII. UNDERGROUND STRUCTURES OF IGNEOUS ORIGIN
XIII. METAMORPHISM AND MINERAL VEINS
PART III
HISTORICAL GEOLOGY
XIV. THE GEOLOGICAL RECORD
XV. THE PRE-CAMBRIAN SYSTEMS
XVI. THE CAMBRIAN
XVII. THE ORDOVICIAN AND SILURIAN
XVIII. THE DEVONIAN
XIX. THE CARBONIFEROUS
XX. THE MESOZOIC
XXI. THE TERTIARY
XXII. THE QUATERNARY
INDEX
THE ELEMENTS OF GEOLOGY
INTRODUCTION
THE SCOPE AND AIM OF GEOLOGY
Geology deals with the rocks of the earth's crust. It learns from
their composition and structure how the rocks were made and how
they have been modified. It ascertains how they have been brought
to their present places and wrought to their various topographic
forms, such as hills and valleys, plains and mountains. It studies
the vestiges which the rocks preserve of ancient organisms which
once inhabited our planet. Geology is the history of the earth and
its inhabitants, as read in the rocks of the earth's crust.
To obtain a general idea of the nature and method of our science
before beginning its study in detail, we may visit some valley,
such as that illustrated in the frontispiece, on whose sides are
rocky ledges. Here the rocks lie in horizontal layers. Although
only their edges are exposed, we may infer that these layers run
into the upland on either side and underlie the entire district;
they are part of the foundation of solid rock which everywhere is
found beneath the loose materials of the surface.
The ledges of the valley of our illustration are of sandstone.
Looking closely at the rock we see that it is composed of myriads
of grains of sand cemented together. These grains have been worn
and rounded. They are sorted also, those of each layer being about
of a size. By some means they have been brought hither from some
more ancient source. Surely these grains have had a history before
they here found a resting place,--a history which we are to learn
to read.
The successive layers of the rock suggest that they were built one
after another from the bottom upward. We may be as sure that each
layer was formed before those above it as that the bottom courses
of stone in a wall were laid before the courses which rest upon
them.
We have no reason to believe that the lowest layers which we see
here were the earliest ever formed. Indeed, some deep boring in
the vicinity may prove that the ledges rest upon other layers of
rock which extend downward for many hundreds of feet below the
valley floor. Nor may we conclude that the highest layers here
were the latest ever laid; for elsewhere we may find still later
layers lying upon them.
A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as
these are of kinds whose nearest living relatives now have their
home in the sea, we infer that it was on the flat sea floor that
the sandstone was laid. Its present position hundreds of feet
above sea level proves that it has since emerged to form part of
the land; while the flatness of the beds shows that the movement
was so uniform and gentle as not to break or strongly bend them
from their original attitude.
The surface of some of these layers is ripple-marked. Hence the
sand must once have been as loose as that of shallow sea bottoms
and sea beaches to-day, which is thrown into similar ripples by
movements of the water. In some way the grains have since become
cemented into firm rock.
Note that the layers on one side of the valley agree with those on
the other, each matching the one opposite at the same level. Once
they were continuous across the valley. Where the valley now is
was once a continuous upland built of horizontal layers; the
layers now show their edges, or OUTCROP, on the valley sides
because they have been cut by the valley trench.
The rock of the ledges is crumbling away. At the foot of each step
of rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in
the future.
Through the valley runs a stream. The waters of rains which have
fallen on the upper parts of the stream's basin are now on their
way to the river and the sea. Rock fragments and grains of sand
creeping down the valley slopes come within reach of the stream
and are washed along by the running water. Here and there they
lodge for a time in banks of sand and gravel, but sooner or later
they are taken up again and carried on. The grains of sand which
were brought from some ancient source to form these rocks are on
their way to some new goal. As they are washed along the rocky bed
of the stream they slowly rasp and wear it deeper. The valley will
be deeper in the future; it has been less deep in the past.
In this little valley we see slow changes now in progress. We find
also in the composition, the structure, and the attitude of the
rocks, and the land forms to which they have been sculptured, the
record of a long succession of past changes involving the origin
of sand grains and their gathering and deposit upon the bottom of
some ancient sea, the cementation of their layers into solid rock,
the uplift of the rocks to form a land surface, and, last of all,
the carving of a valley in the upland. Everywhere, in the fields,
along the river, among the mountains, by the seashore, and in the
desert, we may discover slow changes now in progress and the
record of similar changes in the past. Everywhere we may catch
glimpses of a process of gradual change, which stretches backward
into the past and forward into the future, by which the forms and
structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long
process is geology. Geology treats of the natural changes now
taking place upon the earth and within it, the agencies which
produce them, and the land forms and rock structures which result.
It studies the changes of the present in order to be able to read
the history of the earth's changes in the past.
The various agencies which have fashioned the face of the earth
may. be divided into two general classes. In Part I we shall
consider those which work upon the earth from without, such as the
weather, running water, glaciers, the wind, and the sea. In Part
II we shall treat of those agencies whose sources are within the
earth, and among whose manifestations are volcanoes and
earthquakes and the various movements of the earth's crust. As we
study each agency we shall notice not only how it does its work,
but also the records which it leaves in the rock structures and
the land forms which it produces. With this preparation we shall
be able in Part III to read in the records of the rocks the
history of our planet and the successive forms of life which have
dwelt upon it.
PART I
EXTERNAL GEOLOGICAL AGENCIES
CHAPTER I
THE WORK OF THE WEATHER
In our excursion to the valley with sandstone ledges we witnessed
a process which is going forward in all lands. Everywhere the
rocks are crumbling away; their fragments are creeping down
hillsides to the stream ways and are carried by the streams to the
sea, where they are rebuilt into rocky layers. When again the
rocks are lifted to form land the process will begin anew; again
they will crumble and creep down slopes and be washed by streams
to the sea. Let us begin our study of this long cycle of change at
the point where rocks disintegrate and decay under the action of
the weather. In studying now a few outcrops and quarries we shall
learn a little of some common rocks and how they weather away.
STRATIFICATION AND JOINTING. At the sandstone ledges we saw that
the rock was divided into parallel layers. The thicker layers are
known as STRATA, and the thin leaves into which each stratum may
sometimes be split are termed LAMINAE. To a greater or less degree
these layers differ from each other in fineness of grain, showing
that the material has been sorted. The planes which divide them
are called BEDDING PLANES.
Besides the bedding planes there are other division planes, which
cut across the strata from top to bottom. These are found in all
rocks and are known as joints. Two sets of joints,
running at about right angles to each other, together with the
bedding planes, divide the sandstone into quadrangular blocks.
SANDSTONE. Examining a piece of sandstone we find it composed of
grains quite like those of river sand or of sea beaches. Most of
the grains are of a clear glassy mineral called quartz. These
quartz grains are very hard and will scratch the steel of a knife
blade. They are not affected by acid, and their broken surfaces
are irregular like those of broken glass.
The grains of sandstone are held together by some cement. This may
be calcareous, consisting of soluble carbonate of lime. In brown
sandstones the cement is commonly ferruginous,--hydrated iron
oxide, or iron rust, forming the bond, somewhat as in the case of
iron nails which have rusted together. The strongest and most
lasting cement is siliceous, and sand rocks whose grains are
closely cemented by silica, the chemical substance of which quartz
is made, are known as quartzites.
We are now prepared to understand how sandstone is affected by the
action of the weather. On ledges where the rock is exposed to view
its surface is more or less discolored and the grains are loose
and may be rubbed off with the finger. On gentle slopes the rock
is covered with a soil composed of sand, which evidently is
crumbled sandstone, and dark carbonaceous matter derived from the
decay of vegetation. Clearly it is by the dissolving of the cement
that the rock thus breaks down to loose sand. A piece of sandstone
with calcareous cement, or a bit of old mortar, which is really an
artificial stone also made of sand cemented by lime, may be
treated in a test tube with hydrochloric acid to illustrate the
process.
A LIMESTONE QUARRY. Here also we find the rock stratified and
jointed (Fig. 2). On the quarry face the rock is distinctly seen
to be altered for some distance from its upper surface. Below the
altered zone the rock is sound and is quarried for building; but
the altered upper layers are too soft and broken to be used for
this purpose. If the limestone is laminated, the laminae here have
split apart, although below they hold fast together. Near the
surface the stone has become rotten and crumbles at the touch,
while on the top it has completely broken down to a thin layer of
limestone meal, on which rests a fine reddish clay.
Limestone is made of minute grains of carbonate of lime all firmly
held together by a calcareous cement. A piece of the stone placed
in a test tube with hydrochloric acid dissolves with brisk
effervescence, leaving the insoluble impurities, which were
disseminated through it, at the bottom of the tube as a little
clay.
We can now understand the changes in the upper layers of the
quarry. At the surface of the rock the limestone has completely
dissolved, leaving the insoluble residue as a layer of reddish
clay. Immediately below the clay the rock has disintegrated into
meal where the cement between the limestone grains has been
removed, while beneath this the laminae are split apart where the
cement has been dissolved only along the planes of lamination
where the stone is more porous. As these changes in the rock are
greatest at the surface and diminish downward, we infer that they
have been caused by agents working downward from the surface.
At certain points these agencies have been more effective than
elsewhere. The upper rock surface is pitted. Joints are widened as
they approach the surface, and along these seams we may find that
the rock is altered even down to the quarry floor.
A SHALE PIT. Let us now visit some pit where shale--a laminated
and somewhat hardened clay--is quarried for the manufacture of
brick. The laminae of this fine-grained rock may be as thin as
cardboard in places, and close joints may break the rock into
small rhombic blocks. On the upper surface we note that the shale
has weathered to a clayey soil in which all traces of structure
have been destroyed. The clay and the upper layers of the shale
beneath it are reddish or yellow, while in many cases the color of
the unaltered rock beneath is blue.
THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are
the leading types of the great group of stratified, or
sedimentary, rocks. This group includes all rocks made of
sediments, their materials having settled either in water upon the
bottoms of rivers, lakes, or seas, or on dry land, as in the case
of deposits made by the wind and by glaciers. Sedimentary rocks
are divided into the fragmental rocks--which are made of
fragments, either coarse or fine--and the far less common rocks
which are constituted of chemical precipitates.
The sedimentary rocks are divided according to their composition
into the following classes:
1. The arenaceous, or quartz rocks, including beds of loose sand
and gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).
2. The calcareous, or lime rocks, including limestone and a soft
white rock formed of calcareous powder known as chalk.
3. The argillaceous, or clay rocks, including muds, clays, and
shales. These three classes pass by mixture into one another. Thus
there are limy and clayey sandstones, sandy and clayey limestones,
and sandy and limy shales.
GRANITE. This familiar rock may be studied as an example of the
second great group of rocks,--the unstratified, or igneous rocks.
These are not made of cemented sedimentary grains, but of
interlocking crystals which have crystallized from a molten mass.
Examining a piece of granite, the most conspicuous crystals which
meet the eye are those of feldspar. They are commonly pink, white,
or yellow, and break along smooth cleavage planes which reflect
the light like tiny panes of glass. Mica may be recognized by its
glittering plates, which split into thin elastic scales. A third
mineral, harder than steel, breaking along irregular surfaces like
broken glass, we identify as quartz.
How granite alters under the action of the weather may be seen in
outcrops where it forms the bed rock, or country rock, underlying
the loose formations of the surface, and in many parts of the
northern states where granite bowlders and pebbles more or less
decayed may be found in a surface sheet of stony clay called the
drift. Of the different minerals composing granite, quartz alone
remains unaltered. Mica weathers to detached flakes which have
lost their elasticity. The feldspar crystals have lost their
luster and hardness, and even have decayed to clay. Where long-
weathered granite forms the country rock, it often may be cut with
spade or trowel for several feet from the surface, so rotten is
the feldspar, and here the rock is seen to break down to a clayey
soil containing grains of quartz and flakes of mica.
These are a few simple illustrations of the surface changes which
some of the common kinds of rocks undergo. The agencies by which
these changes are brought about we will now take up under two
divisions,--CHEMICAL AGENCIES producing rock decay and MECHANICAL
AGENCIES producing rock disintegration.
THE CHEMICAL WORK OF WATER
As water falls on the earth in rain it has already absorbed from
the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks
into the ground and becomes what is termed ground water, it takes
into solution from the soil humus acids and carbon dioxide, both
of which are constantly being generated there by the decay of
organic matter. So both rain and ground water are charged with
active chemical agents, by the help of which they corrode and rust
and decompose all rocks to a greater or less degree. We notice now
three of the chief chemical processes concerned in weathering,--
solution, the formation of carbonates, and oxidation.
SOLUTION. Limestone, although so little affected by pure water
that five thousand gallons would be needed to dissolve a single
pound, is easily dissolved in water charged with carbon dioxide.
In limestone regions well water is therefore "hard." On boiling
the water for some time the carbon dioxide gas is expelled, the
whole of the lime carbonate can no longer be held in solution, and
much of it is thrown down to form a crust or "scale" in the kettle
or in the tubes of the steam boiler. All waters which flow over
limestone rocks or soak through them are constantly engaged in
dissolving them away, and in the course of time destroy beds of
vast extent and great thickness.
The upper surface of limestone rocks becomes deeply pitted, as we
saw in the limestone quarry, and where the mantle of waste has
been removed it may be found so intricately furrowed that it is
difficult to traverse.
Beds of rock salt buried among the strata are dissolved by seeping
water, which issues in salt springs. Gypsum, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched
with the finger nail, is readily taken up by water, giving to the
water of wells and springs a peculiar hardness difficult to
remove.
The dissolving action of moisture may be noted on marble
tombstones of some age, marble being a limestone altered by heat
and pressure and composed of crystalline grains. By assuming that
the date on each monument marks the year of its erection, one may
estimate how many years on the average it has taken for weathering
to loosen fine grains on the polished surface, so that they may be
rubbed off with the finger, to destroy the polish, to round the
sharp edges of tool marks in the lettering, and at last to open
cracks and seams and break down the stone. We may notice also
whether the gravestones weather more rapidly on the sunny or the
shady side, and on the sides or on the top.
The weathered surface of granular limestone containing shells
shows them standing in relief. As the shells are made of
crystalline carbonate of lime, we may infer whether the carbonate
of lime is less soluble in its granular or in its crystalline
condition.
THE FORMATION OF CARBONATES. In attacking minerals water does more
than merely take them into solution. It decomposes them, forming
new chemical compounds of which the carbonates are among the most
important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken
up by the action of water containing carbon dioxide, forming
alkaline carbonates. These carbonates are freely soluble and
contribute potash and soda to soils and river waters. By the
removal of the soluble ingredients of feldspar there is left the
silicate of alumina, united with water or hydrated, in the
condition of a fine plastic clay which, when white and pure, is
known as KAOLIN and is used in the manufacture of porcelain.
Feldspathic rocks which contain no iron compounds thus weather to
whitish crusts, and even apparently sound crystals of feldspar,
when ground to thin slices and placed under the microscope, may be
seen to be milky in color throughout because an internal change to
kaolin has begun.
OXIDATION. Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty
films. Oxygen and water have here united with the iron, forming
hydrated iron oxide. The effects of oxidation may be seen in the
alteration of many kinds of rocks and in red and yellow colors of
soils and subsoils.
Pyrite is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and
sulphur (iron sulphide). Under the attack of the weather it takes
up oxygen, forming iron sulphate (green vitriol), a soluble
compound, and insoluble hydrated iron oxide, which as a mineral is
known as limonite. Several large masses of iron sulphide were
placed some years ago on the lawn in front of the National Museum
at Washington. The mineral changed so rapidly to green vitriol
that enough of this poisonous compound was washed into the ground
to kill the roots of the surrounding grass.
AGENTS OF MECHANICAL DISINTEGRATION
HEAT AND COLD. Rocks exposed to the direct rays of the sun become
strongly heated by day and expand. After sunset they rapidly cool
and contract. When the difference in temperature between day and
night is considerable, the repeated strains of sudden expansion
and contraction at last become greater than the rocks can bear,
and they break, for the same reason that a glass cracks when
plunged into boiling water (Fig. 5).
Rocks are poor conductors of heat, and hence their surfaces may
become painfully hot under the full blaze of the sun, while the
interior remains comparatively cool. By day the surface shell
expands and tends to break loose from the mass of the stone. In
cooling in the evening the surface shell suddenly contracts on the
unyielding interior and in time is forced off in scales.
Many rocks, such as granite, are made up of grains of various
minerals which differ in color and in their capacity to absorb
heat, and which therefore contract and expand in different ratios.
In heating and cooling these grains crowd against their neighbors
and tear loose from them, so that finally the rock disintegrates
into sand.
The conditions for the destructive action of heat and cold are
most fully met in arid regions when vegetation is wanting for lack
of sufficient rain. The soil not being held together by the roots
of plants is blown away over large areas, leaving the rocks bare
to the blazing sun in a cloudless sky. The air is dry, and the
heat received by the earth by day is therefore rapidly radiated at
night into space. There is a sharp and sudden fall of temperature
after sunset, and the rocks, strongly heated by day, are now
chilled perhaps even to the freezing point.
In the Sahara the thermometer has been known to fall 131 degrees
F. within a few hours. In the light air of the Pamir plateau in
central Asia a rise of 90 degrees F. has been recorded from seven
o'clock in the morning to one o'clock in the afternoon. On the
mountains of southwestern Texas there are frequently heard
crackling noises as the rocks of that arid region throw off scales
from a fraction of an inch to four inches in thickness, and loud
reports are made as huge bowlders split apart. Desert pebbles
weakened by long exposure to heat and cold have been shivered to
fine sharp-pointed fragments on being placed in sand heated to 180
degrees F. Beds half a foot thick, forming the floor of limestone
quarries in Wisconsin, have been known to buckle and arch and
break to fragments under the heat of the summer sun.
FROST. By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more
or less porous and all contain more or less water in their pores.
Workers in stone call this "quarry water," and speak of a stone as
"green" before the quarry water has dried out. Water also seeps
along joints and bedding planes and gathers in all seams and
crevices. Water expands in freezing, ten cubic inches of water
freezing to about eleven cubic inches of ice. As water freezes in
the rifts and pores of rocks it expands with the irresistible
force illustrated in the freezing and breaking of water pipes in
winter. The first rift in the rock, perhaps too narrow to be seen,
is widened little by little by the wedges of successive frosts,
and finally the rock is broken into detached blocks, and these
into angular chip-stone by the same process.
It is on mountain tops and in high latitudes that the effects of
frost are most plainly seen. "Every summit" says Whymper, "amongst
the rock summits upon which I have stood has been nothing but a
piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen,
in Kamchatka, and in other frigid lands large areas are thickly
strewn with sharp-edged fragments into which the rock has been
shattered by frost.
ORGANIC AGENTS
We must reckon the roots of plants and trees among the agents
which break rocks into pieces. The tiny rootlet in its search for
food and moisture inserts itself into some minute rift, and as it
grows slowly wedges the rock apart. Moreover, the acids of the
root corrode the rocks with which they are in contact. One may
sometimes find in the soil a block of limestone wrapped in a mesh
of roots, each of which lies in a little furrow where it has eaten
into the stone.
Rootless plants called lichens often cover and corrode rocks as
yet bare of soil; but where lichens are destroying the rock less
rapidly than does the weather, they serve in a way as a
protection.
CONDITIONS FAVORING DISINTEGRATION AND DECAY. The
disintegration of rocks under frost and temperature changes
goes on most rapidly in cold and arid climates, and where
vegetation is scant or absent. On the contrary, the decay of rocks
under the chemical action of water is favored by a warm, moist
climate and abundant vegetation. Frost and heat and cold can only
act within the few feet from the surface to which the necessary
temperature changes are limited, while water penetrates and alters
the rocks to great depths.
The pupil may explain.
In what ways the presence of joints and bedding planes assists in
the breaking up and decay of rocks under the action of the
weather.
Why it is a good rule of stone masons never to lay stones on edge,
but always on their natural bedding planes.
Why stones fresh from the quarry sometimes go to pieces in early
winter, when stones which have been quarried for some months
remain uninjured.
Why quarrymen in the northern states often keep their quarry
floors flooded during winter.
Why laminated limestone should not be used for curbstone.
Why rocks composed of layers differing in fineness of grain and in
ratios of expansion do not make good building stone.
Fine-grained rocks with pores so small that capillary attraction
keeps the water which they contain from readily draining away are
more apt to hold their pores ten elevenths full of water than are
rocks whose pores are larger. Which, therefore, are more likely to
be injured by frost?
Which is subject to greater temperature changes, a dark rock or
one of a light color? the north side or the south side of a
valley?
THE MANTLE OF ROCK WASTE
We have seen that rocks are everywhere slowly wasting away. They
are broken in pieces by frost, by tree roots, and by heat and
cold. They dissolve and decompose under the chemical action of
water and the various corrosive substances which it contains,
leaving their insoluble residues as residual clays and sands upon
the surface. As a result there is everywhere forming a mantle of
rock waste which covers the land. It is well to imagine how the
country would appear were this mantle with its soil and vegetation
all scraped away or had it never been formed. The surface of the
land would then be everywhere of bare rock as unbroken as a quarry
floor.
THE THICKNESS OF THE MANTLE. In any locality the thickness of the
mantle of rock waste depends as much on the rate at which it is
constantly being removed as on the rate at which it is forming. On
the face of cliffs it is absent, for here waste is removed as fast
as it is made. Where waste is carried away more slowly than it is
produced, it accumulates in time to great depth.
The granite of Pikes Peak is disintegrated to a depth of twenty
feet. In the city of Washington granite rock is so softened to a
depth of eighty feet that it can be removed with pick and shovel.
About Atlanta, Georgia, the rocks are completely rotted for one
hundred feet from the surface, while the beginnings of decay may
be noticed at thrice that depth. In places in southern Brazil the
rock is decomposed to a depth of four hundred feet.
In southwestern Wisconsin a reddish residual clay has an average
depth of thirteen feet on broad uplands, where it has been removed
to the least extent. The country rock on which it rests is a
limestone with about ten per cent of insoluble impurities. At
least how thick, then, was that portion of the limestone which has
rotted down to the clay?
DISTINGUISHING CHARACTERISTICS OF RESIDUAL WASTE. We must learn to
distinguish waste formed in place by the action of the weather
from the products of other geological agencies. Residual waste is
unstratified. It contains no substances which have not been
derived from the weathering of the parent rock. There is a gradual
transition from residual waste into the unweathered rock beneath.
Waste resting on sound rock evidently has been shifted and was not
formed in place.
In certain regions of southern Missouri the land is covered with a
layer of broken flints and red clay, while the country rock is
limestone. The limestone contains nodules of flint, and we may
infer that it has been by the decay and removal of thick masses of
limestone that the residual layer of clay and flints has been left
upon the surface. Flint is a form of quartz, dull-lustered,
usually gray or blackish in color, and opaque except on thinnest
edges, where it is translucent.
Over much of the northern states there is spread an unstratified
stony clay called the drift. It often rests on sound rocks. It
contains grains of sand, pebbles, and bowlders composed of many
different minerals and rocks that the country rock cannot furnish.
Hence the drift cannot have been formed by the decay of the rock
of the region. A shale or limestone, for example, cannot waste to
a clay containing granite pebbles. The origin of the drift will be
explained in subsequent chapters.
The differences in rocks are due more to their soluble than to
their insoluble constituents. The latter are few in number and are
much the same in rocks of widely different nature, being chiefly
quartz, silicate of alumina, and iron oxide. By the removal of
their soluble parts very many and widely different rocks rot down
to a residual clay gritty with particles of quartz and colored red
or yellow with iron oxide.
In a broad way the changes which rocks undergo in weathering are
an adaptation to the environment in which they find themselves at
the earth's surface,--an environment different from that in which
they were formed under sea or under ground. In open air, where
they are attacked by various destructive agents, few of the rock-
making minerals are stable compounds except quartz, the iron
oxides, and the silicate of alumina; and so it is to one or more
of these comparatively insoluble substances that most rocks are
reduced by long decay.
Which produces a mantle of finer waste, frost or chemical decay?
which a thicker mantle? In what respects would you expect that the
mantle of waste would differ in warm humid lands like India, in
frozen countries like Alaska, and in deserts such as the Sahara?
THE SOIL. The same agencies which produce the mantle of waste are
continually at work upon it, breaking it up into finer and finer
particles and causing its more complete decay. Thus on the
surface, where the waste has weathered longest, it is gradually
made fine enough to support the growth of plants, and is then
known as soil. The coarser waste beneath is sometimes spoken of as
subsoil. Soil usually contains more or less dark, carbonaceous,
decaying organic matter, called humus, and is then often termed
the humus layer. Soil forms not only on waste produced in place
from the rock beneath, but also on materials which have been
transported, such as sheets of glacial drift and river deposits.
Until rocks are reduced to residual clays the work of the weather
is more rapid and effective on the fragments of the mantle of
waste than on the rocks from which waste is being formed. Why?
Any fresh excavation of cellar or cistern, or cut for road or
railway, will show the characteristics of the humus layer. It may
form only a gray film on the surface, or we may find it a layer a
foot or more thick, dark, or even black, above, and growing
gradually lighter in color as it passes by insensible gradations
into the subsoil. In some way the decaying vegetable matter
continually forming on the surface has become mingled with the
material beneath it.
HOW HUMUS AND THE SUBSOIL ARE MINGLED. The mingling of humus and
the subsoil is brought about by several means. The roots of plants
penetrate the waste, and when they die leave their decaying
substance to fertilize it. Leaves and stems falling on the surface
are turned under by several agents. Earthworms and other animals
whose home is in the waste drag them into their burrows either for
food or to line their nests. Trees overthrown by the wind, roots
and all, turn over the soil and subsoil and mingle them together.
Bacteria also work in the waste and contribute to its enrichment.
The animals living in the mantle do much in other ways toward the
making of soil. They bring the coarser fragments from beneath to
the surface, where the waste weathers more rapidly. Their burrows
allow air and water to penetrate the waste more freely and to
affect it to greater depths.
ANTS. In the tropics the mantle of waste is worked over chiefly by
ants. They excavate underground galleries and chambers, extending
sometimes as much as fourteen feet below the surface, and build
mounds which may reach as high above it. In some parts of Paraguay
and southern Brazil these mounds, like gigantic potato hills,
cover tracts of considerable area.
In search for its food--the dead wood of trees--the so-called
white ant constructs runways of earth about the size of gas pipes,
reaching from the base of the tree to the topmost branches. On the
plateaus of central Africa explorers have walked for miles through
forests every tree of which was plastered with these galleries of
mud. Each grain of earth used in their construction is moistened
and cemented by slime as it is laid in place by the ant, and is
thus acted on by organic chemical agents. Sooner or later these
galleries are beaten down by heavy rains, and their fertilizing
substances are scattered widely by the winds.
EARTHWORMS. In temperate regions the waste is worked over largely
by earthworms. In making their burrows worms swallow earth in
order to extract from it any nutritive organic matter which it may
contain. They treat it with their digestive acids, grind it in
their stony gizzards, and void it in castings on the surface of
the ground. It was estimated by Darwin that in many parts of
England each year, on every acre, more than ten tons of earth pass
through the bodies of earthworms and are brought to the surface,
and that every few years the entire soil layer is thus worked over
by them.
In all these ways the waste is made fine and stirred and enriched.
Grain by grain the subsoil with its fresh mineral ingredients is
brought to the surface, and the rich organic matter which plants
and animals have taken from the atmosphere is plowed under. Thus
Nature plows and harrows on "the great world's farm" to make ready
and ever to renew a soil fit for the endless succession of her
crops.
The world processes by which rocks are continually wasting away
are thus indispensable to the life of plants and animals. The
organic world is built on the ruins of the inorganic, and because
the solid rocks have been broken down into soil men are able to
live upon the earth.
SOLAR ENERGY. The source of the energy which accomplishes all this
necessary work is the sun. It is the radiant energy of the sun
which causes the disintegration of rocks, which lifts vapor into
the atmosphere to fall as rain, which gives life to plants and
animals. Considering the earth in a broad way, we may view it as a
globe of solid rock,--the lithosphere,--surrounded by two mobile
envelopes: the envelope of air,--THE ATMOSPHERE, and the envelope
of water,--THE HYDROSPHERE. Under the action of solar energy these
envelopes are in constant motion. Water from the hydrosphere is
continually rising in vapor into the atmosphere, the air of the
atmosphere penetrates the hydrosphere,--for its gases are
dissolved in all waters,--and both air and water enter and work
upon the solid earth. By their action upon the lithosphere they
have produced a third envelope,--the mantle of rock waste.
This envelope also is in movement, not indeed as a whole, but
particle by particle. The causes which set its particles in
motion, and the different forms which the mantle comes to assume,
we will now proceed to study.
MOVEMENTS OF THE MANTLE OF ROCK WASTE
At the sandstone ledges which we first visited we saw not only
that the rocks were crumbling away, but also that grains and
fragments of them were creeping down the slopes of the valley to
the stream and were carried by it onward toward the sea. This
process is going on everywhere. Slowly it may be, and with many
interruptions, but surely, the waste of the land moves downward to
the sea. We may divide its course into two parts,--the path to the
stream, which we will now consider, and its carriage onward by the
stream, which we will defer to a later chapter.
GRAVITY. The chief agent concerned in the movement of waste is
gravity. Each particle of waste feels the unceasing downward pull
of the earth's mass and follows it when free to do so. All
agencies which produce waste tend to set its particles free and in
motion, and therefore cooperate with gravity. On cliffs, rocks
fall when wedged off by frost or by roots of trees, and when
detached by any other agency. On slopes of waste, water freezes in
chinks between stones, and in pores between particles of soil, and
wedges them apart. Animals and plants stir the waste, heat expands
it, cold contracts it, the strokes of the raindrops drive loose
particles down the slope and the wind lifts and lets them fall. Of
all these movements, gravity assists those which are downhill and
retards those which are uphill. On the whole, therefore, the
downhill movements prevail, and the mantle of waste, block by
block and grain by grain, creeps along the downhill path.
A slab of sandstone laid on another of the same kind at an angle
of 17 degrees and left in the open air was found to creep down the
slope at the rate of a little more than a millimeter a month.
Explain why it did so.
RAIN. The most efficient agent in the carriage of waste to the
streams is the rain. It moves particles of soil by the force of
the blows of the falling drops, and washes them down all slopes to
within reach of permanent streams. On surfaces unprotected by
vegetation, as on plowed fields and in arid regions, the rain
wears furrows and gullies both in the mantle of waste and in
exposures of unaltered rock (Fig. 17).
At the foot of a hill we may find that the soil has accumulated by
creep and wash to the depth of several feet; while where the
hillside is steepest the soil may be exceedingly thin, or quite
absent, because removed about as fast as formed. Against the walls
of an abbey built on a slope in Wales seven hundred years ago, the
creeping waste has gathered on the uphill side to a depth of seven
feet. The slow-flowing sheet of waste is often dammed by fences
and walls, whose uphill side gathers waste in a few years so as to
show a distinctly higher surface than the downhill side,
especially in plowed fields where the movement is least checked by
vegetation.
TALUS. At the foot of cliffs there is usually to be found a slope
of rock fragments which clearly have fallen from above. Such a
heap of waste is known as talus. The amount of talus in any place
depends both on the rate of its formation and the rate of its
removal. Talus forms rapidly in climates where mechanical
disintegration is most effective, where rocks are readily broken
into blocks because closely jointed and thinly bedded rather than
massive, and where they are firm enough to be detached in
fragments of some size instead of in fine grains. Talus is removed
slowly where it decays slowly, either because of the climate or
the resistance of the rock. It may be rapidly removed by a stream
flowing along its base.
In a moist climate a soluble rock, such as massive limestone, may
form talus little if any faster than the talus weathers away. A
loose-textured sandstone breaks down into incoherent sand grains,
which in dry climates, where unprotected by vegetation, may be
blown away as fast as they fall, leaving the cliff bare to the
base. Cliffs of such slow-decaying rocks as quartzite and granite
when closely jointed accumulate talus in large amounts.
Talus slopes may be so steep as to reach THE ANGLE OF REPOSE, i.e.
the steepest angle at which the material will lie. This angle
varies with different materials, being greater with coarse and
angular fragments than with fine rounded grains. Sooner or later a
talus reaches that equilibrium where the amount removed from its
surface just equals that supplied from the cliff above. As the
talus is removed and weathers away its slope retreats together
with the retreat of the cliff, as seen in Figure 9.
GRADED SLOPES. Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges.
On the steeper slopes it is coarser and in more rapid movement
than on slopes more gentle, but mountain sides and hills and
plains alike come to be mantled with sheets of waste which
everywhere is creeping toward the streams. Such unbroken slopes,
worn or built to the least inclination at which the waste supplied
by weathering can be urged onward, are known as GRADED SLOPES.
Of far less importance than the silent, gradual creep of waste,
which is going on at all times everywhere about us, are the
startling local and spasmodic movements which we are now to
describe.
AVALANCHES. On steep mountain sides the accumulated snows of
winter often slip and slide in avalanches to the valleys below.
These rushing torrents of snow sweep their tracks clean of waste
and are one of Nature's normal methods of moving it along the
downhill path.
LANDSLIDES. Another common and abrupt method of delivering waste
to streams is by slips of the waste mantle in large masses. After
long rains and after winter frosts the cohesion between the waste
and the sound rock beneath is loosened by seeping water
underground. The waste slips on the rock surface thus lubricated
and plunges down the mountain side in a swift roaring torrent of
mud and stones.
We may conveniently mention here a second type of landslide, where
masses of solid rock as well as the mantle of waste are involved
in the sudden movement. Such slips occur when valleys have been
rapidly deepened by streams or glaciers and their sides have not
yet been graded. A favorable condition is where the strata dip
(i.e. incline downwards) towards the valley (Fig. 11), or are
broken by joint planes dipping in the same direction. The upper
layers, including perhaps the entire mountain side, have been cut
across by the valley trench and are left supported only on the
inclined surface of the underlying rocks. Water may percolate
underground along this surface and loosen the cohesion between the
upper and the underlying strata by converting the upper surface of
a shale to soft wet clay, by dissolving layers of a limestone, or
by removing the cement of a sandstone and converting it into loose
sand. When the inclined surface is thus lubricated the overlying
masses may be launched into the valley below. The solid rocks are
broken and crushed in sliding and converted into waste consisting,
like that of talus, of angular unsorted fragments, blocks of all
sizes being mingled pellmell with rock meal and dust. The
principal effects of landslides may be gathered from the following
examples.
At Gohna, India, in 1893, the face of a spur four thousand feet
high, of the lower ranges of the Himalayas, slipped into the gorge
of the headwaters of the Ganges River in successive rock falls
which lasted for three days. Blocks of stone were projected for a
mile, and clouds of limestone dust were spread over the
surrounding country. The debris formed a dam one thousand feet
high, extending for two miles along the valley. A lake gathered
behind this barrier, gradually rising until it overtopped it in a
little less than a year. The upper portion of the dam then broke,
and a terrific rush of water swept down the valley in a wave
which, twenty miles away, rose one hundred and sixty feet in
height. A narrow lake is still held by the strong base of the dam.
In 1896, after forty days of incessant rain, a cliff of sandstone
slipped into the Yangtse River in China, reducing the width of the
channel to eighty yards and causing formidable rapids.
At Flims, in Switzerland, a prehistoric landslip flung a dam
eighteen hundred feet high across the headwaters of the Rhine. If
spread evenly over a surface of twenty-eight square miles, the
material would cover it to a depth of six hundred and sixty feet.
The barrier is not yet entirely cut away, and several lakes are
held in shallow basins on its hummocky surface.
A slide from the precipitous river front of the citadel hill of
Quebec, in 1889, dashed across Champlain Street, wrecking a number
of houses and causing the death of forty-five persons. The strata
here are composed of steeply dipping slate.
In lofty mountain ranges there may not be a single valley without
its traces of landslides, so common there is this method of the
movement of waste, and of building to grade over-steepened slopes.
ROCK SCULPTURE BY WEATHERING
We are now to consider a few of the forms into which rock masses
are carved by the weather.
BOWLDERS OF WEATHERING. In many quarries and outcrops we may see
that the blocks into which one or more of the uppermost layers
have been broken along their joints and bedding planes are no
longer angular, as are those of the layers below. The edges and
corners of these blocks have been worn away by the weather. Such
rounded cores, known as bowlders of weathering, are often left to
strew the surface.
DIFFERENTIAL WEATHERING. This term covers all cases in which a
rock mass weathers differently in different portions. Any weaker
spots or layers are etched out on the surface, leaving the more
resistant in relief. Thus massive limestones become pitted where
the weather drills out the weaker portions. In these pits, when
once they are formed, moisture gathers, a little soil collects,
vegetation takes root, and thus they are further enlarged until
the limestone may be deeply honeycombed.
On the sides of canyons, and elsewhere where the edges of strata
are exposed, the harder layers project as cliffs, while the softer
weather back to slopes covered with the talus of the harder layers
above them. It is convenient to call the former cliff makers and
the latter slope makers.
Differential weathering plays a large part in the sculpture of the
land. Areas of weak rock are wasted to plains, while areas of hard
rock adjacent are still left as hills and mountain ridges, as in
the valleys and mountains of eastern Pennsylvania. But in such
instances the lowering of the surface of the weaker rock is also
due to the wear of streams, and especially to the removal by them
from the land of the waste which covers and protects the rocks
beneath.
Rocks owe their weakness to several different causes. Some, such
as beds of loose sand, are soft and easily worn by rains; some, as
limestone and gypsum for example, are soluble. Even hard insoluble
rocks are weak under the attack of the weather when they are
closely divided by joints and bedding planes and are thus readily
broken up into blocks by mechanical agencies.
OUTLIERS AND MONUMENTS. As cliffs retreat under the attack of the
weather, portions are left behind where the rock is more resistant
or where the attack for any reason is less severe. Such remnant
masses, if large, are known as outliers. When
Note the rain furrows on the slope at the foot of the monuments.
In the foreground are seen fragments of petrified trunks of trees,
composed of silica and extremely resistant to the weather. On the
removal of the rock layers in which these fragments were imbedded
they are left to strew the surface in the same way as are the
residual flints of southern Missouri. flat-topped, because of the
protection of a resistant horizontal capping layer, they are
termed mesas,--a term applied also to the flat-topped portions of
dissected plateaus (Fig. 129). Retreating cliffs may fall back a
number of miles behind their outliers before the latter are
finally consumed.
Monuments are smaller masses and may be but partially detached
from the cliff face. In the breaking down of sheets of horizontal
strata, outliers grow smaller and smaller and are reduced to
massive rectangular monuments resembling castles (Fig. 17). The
rock castle falls into ruin, leaving here and there an isolated
tower; the tower crumbles to a lonely pillar, soon to be
overthrown. The various and often picturesque shapes of monuments
depend on the kind of rock, the attitude of the strata, and the
agent by which they are chiefly carved. Thus pillars may have a
capital formed of a resistant stratum. Monuments may be undercut
and come to rest on narrow pedestals, wherever they weather more
rapidly near the ground, either because of the greater moisture
there, or--in arid climates--because worn at their base by
drifting sands.
Stony clays disintegrating under the rain often contain bowlders
which protect the softer material beneath from the vertical blows
of raindrops, and thus come to stand on pedestals of some height.
One may sometimes see on the ground beneath dripping eaves pebbles
left in the same way, protecting tiny pedestals of sand.
MOUNTAIN PEAKS AND RIDGES. Most mountains have been carved out of
great broadly uplifted folds and blocks of the earth's crust.
Running water and glacier ice have cut these folds and blocks into
masses divided by deep valleys; but it is by the weather, for the
most part, that the masses thus separated have been sculptured to
the present forms of the individual peaks and ridges.
Frost and heat and cold sculpture high mountains to sharp,
tusklike peaks and ragged, serrate crests, where their waste is
readily removed.
The Matterhorn of the Alps is a famous example of a mountain peak
whose carving by the frost and other agents is in active progress.
On its face "scarcely a rock anywhere is firmly attached," and the
fall of loosened stones is incessant. Mountain climbers who have
camped at its base tell how huge rocks from time to time come
leaping down its precipices, followed by trains of dislodged
smaller fragments and rock dust; and how at night one may trace
the course of the bowlders by the sparks which they strike from
the mountain walls. Mount Assiniboine, Canada (Fig. 20), resembles
the Matterhorn in form and has been carved by the same agencies.
"The Needles" of Arizona are examples of sharp mountain peaks in a
warm arid region sculptured chiefly by temperature changes.
Chemical decay, especially when carried on beneath a cover of
waste and vegetation, favors the production of rounded knobs and
dome-shaped mountains.
THE WEATHER CURVE. We have seen that weathering reduces the
angular block quarried by the frost to a rounded bowlder by
chipping off its corners and smoothing away its edges. In much the
same way weathering at last reduces to rounded hills the earth
blocks cut by streams or formed in any other way. High mountains
may at first be sculptured by the weather to savage peaks (Fig.
181), but toward the end of their life history they wear down to
rounded hills (Fig. 182). The weather curve, which may be seen on
the summits of low hills (Fig. 21), is convex upward.
In Figure 22, representing a cubic block of stone whose faces are
a yard square, how many square feet of surface are exposed to the
weather by a cubic foot at a corner a; by one situated in the
middle of an edge b; by one in the center of a side c? How much
faster will a and b weather than c, and what will be the effect on
the shape of the block?
THE COOPERATION OF VARIOUS AGENCIES IN ROCK SCULPTURE. For the
sake of clearness it is necessary to describe the work of each
geological agent separately. We must not forget, however, that in
Nature no agent works independently and alone; that every result
is the outcome of a long chain of causes. Thus, in order that the
mountain peak may be carved by the agents of disintegration, the
waste must be rapidly removed,--a work done by many agents,
including some which we are yet to study; and in order that the
waste may be removed as fast as formed, the region must first have
been raised well above the level of the sea, so that the agents of
transportation could do their work effectively. The sculpture of
the rocks is accomplished only by the cooperation of many forces.
The constant removal of waste from the surface by creep and wash
and carriage by streams is of the highest importance, because it
allows the destruction of the land by means of weathering to go on
as long as any land remains above sea level. If waste were not
removed, it would grow to be so thick as to protect the rock
beneath from further weathering, and the processes of destruction
which we have studied would be brought to an end. The very
presence of the mantle of waste over the land proves that on the
whole rocks weather more rapidly than their waste is removed. The
destruction of the land is going on as fast as the waste can be
carried away.
We have now learned to see in the mantle of waste the record of
the destructive action of the agencies of weathering on the rocks
of the land surface. Similar records we shall find buried deeply
among the rocks of the crust in old soils and in rocks pitted and
decayed, telling of old land surfaces long wasted by the weather.
Ever since the dry land appeared these agencies have been as now
quietly and unceasingly at work upon it, and have ever been the
chief means of the destruction of its rocks. The vast bulk of the
stratified rocks of the earth's crust is made up almost wholly of
the waste thus worn from ancient lands.
In studying the various geological agencies we must remember the
almost inconceivable times in which they work. The slowest process
when multiplied by the immense time in which it is carried on
produces great results. The geologist looks upon the land forms of
the earth's surface as monuments which record the slow action of
weathering and other agents during the ages of the past. The
mountain peak, the rounded hill, the wide plain which lies where
hills and mountains once stood, tell clearly of the great results
which slow processes will reach when given long time in which to
do their work. We should accustom ourselves also to think of the
results which weathering will sooner or later bring to pass. The
tombstone and the bowlder of the field, which each year lose from
their surfaces a few crystalline grains, must in time be wholly
destroyed. The hill whose rocks are slowly rotting underneath a
cover of waste must become lower and lower as the centuries and
millenniums come and go, and will finally disappear. Even the
mountains are crumbling away continually, and therefore are but
fleeting features of the landscape.
CHAPTER II
THE WORK OF GROUND WATER
LAND WATERS. We have seen how large is the part that water plays
at and near the surface of the land in the processes of weathering
and in the slow movement of waste down all slopes to the stream
ways. We now take up the work of water as it descends beneath the
ground,--a corrosive agent still, and carrying in solution as its
load the invisible waste of rocks derived from their soluble
parts.
Land waters have their immediate source in the rainfall. By the
heat of the sun water is evaporated from the reservoir of the
ocean and from moist surfaces everywhere. Mingled as vapor with
the air, it is carried by the winds over sea and land, and
condensed it returns to the earth as rain or snow. That part of
the rainfall which descends on the ocean does not concern us, but
that which falls on the land accomplishes, as it returns to the
sea, the most important work of all surface geological agencies.
The rainfall may be divided into three parts: the first DRIES UP,
being discharged into the air by evaporation either directly from
the soil or through vegetation; the second RUNS OFF over the
surface to flood the streams; the third SOAKS IN the ground and is
henceforth known as GROUND or UNDERGROUND WATER.
THE DESCENT OF GROUND WATER. Seeping through the mantle of waste,
ground water soaks into the pores and crevices of the underlying
rock. All rocks of the upper crust of the earth are more or less
porous, and all drink in water. IMPERVIOUS ROCKS, such as granite,
clay, and shale, have pores so minute that the water which they
take in is held fast within them by capillary attraction, and none
drains through. PERVIOUS ROCKS, on the other hand, such as many
sandstones, have pore spaces so large that water filters through
them more or less freely. Besides its seepage through the pores of
pervious rocks, water passes to lower levels through the joints
and cracks by which all rocks, near the surface are broken.
Even the closest-grained granite has a pore space of 1 in 400,
while sandstone may have a pore space of 1 in 4. Sand is so porous
that it may absorb a third of its volume of water, and a loose
loam even as much as one half.
THE GROUND-WATER SURFACE is the name given the upper surface of
ground water, the level below which all rocks are saturated. In
dry seasons the ground-water surface sinks. For ground water is
constantly seeping downward under gravity, it is evaporated in the
waste and its moisture is carried upward by capillarity and the
roots of plants to the surface to be evaporated in the air. In wet
seasons these constant losses are more than made good by fresh
supplies from that part of the rainfall which soaks into the
ground, and the ground-water surface rises.
In moist climates the ground-water surface (Fig. 24) lies, as a
rule, within a few feet of the land surface and conforms to it in
a general way, although with slopes of less inclination than those
of the hills and valleys. In dry climates permanent ground water
may be found only at depths of hundreds of feet. Ground water is
held at its height by the fact that its circulation is constantly
impeded by capillarity and friction. If it were as free to drain
away as are surface streams, it would sink soon after a rain to
the level of the deepest valleys of the region.
WELLS AND SPRINGS. Excavations made in permeable rocks below the
ground-water surface fill to its level and are known as wells.
Where valleys cut this surface permanent streams are formed, the
water either oozing forth along ill-defined areas or issuing at
definite points called springs, where it is concentrated by the
structure of the rocks. A level tract where the ground-water
surface coincides with the surface of the ground is a swamp or
marsh.
By studying a spring one may learn much of the ways and work of
ground water. Spring water differs from that of the stream into
which it flows in several respects. If we test the spring with a
thermometer during successive months, we shall find that its
temperature remains much the same the year round. In summer it is
markedly cooler than the stream; in winter it is warmer and
remains unfrozen while the latter perhaps is locked in ice. This
means that its underground path must lie at such a distance from
the surface that it is little affected by summer's heat and
winter's cold.
While the stream is often turbid with surface waste washed into it
by rains, the spring remains clear; its water has been filtered
during its slow movement through many small underground passages
and the pores of rocks. Commonly the spring differs from the
stream in that it carries a far larger load of dissolved rock.
Chemical analysis proves that streams contain various minerals in
solution, but these are usually in quantities so small that they
are not perceptible to the taste or feel. But the water of springs
is often well charged with soluble minerals; in its slow, long
journey underground it has searched out the soluble parts of the
rocks through which it seeps and has dissolved as much of them as
it could. When spring water is boiled away, the invisible load
which it has carried is left behind, and in composition is found
to be practically identical with that of the soluble ingredients
of the country rock. Although to some extent the soluble waste of
rocks is washed down surface slopes by the rain, by far the larger
part is carried downward by ground water and is delivered to
streams by springs.
In limestone regions springs are charged with calcium carbonate
(the carbonate of lime), and where the limestone is magnesian they
contain magnesium carbonate also. Such waters are "hard"; when
used in washing, the minerals which they contain combine with the
fatty acids of soap to form insoluble curdy compounds. When
springs rise from rocks containing gypsum they are hard with
calcium sulphate. In granite regions they contain more or less
soda and potash from the decay of feldspar.
The flow of springs varies much less during the different seasons
of the year than does that of surface streams. So slow is the
movement of ground water through the rocks that even during long
droughts large amounts remain stored above the levels of surface
drainage.
MOVEMENTS OF GROUND WATER. Ground water is in constant movement
toward its outlets. Its rate varies according to many conditions,
but always is extremely slow. Even through loose sands beneath the
beds of rivers it sometimes does not exceed a fifth of a mile a
year.
In any region two zones of flow may be distinguished. The UPPER
ZONE OF FLOW extends from the ground-water surface downward
through the waste mantle and any permeable rocks on which the
mantle rests, as far as the first impermeable layer, where the
descending movement of the water is stopped. The DEEP ZONES OF
FLOW occupy any pervious rocks which may be found below the
impervious layer which lies nearest to the surface. The upper zone
is a vast sheet of water saturating the soil and rocks and slowly
seeping downward through their pores and interstices along the
slopes to the valleys, where in part it discharges in springs and
often unites also in a wide underflowing stream which supports and
feeds the river (Fig. 24).
A city in a region of copious rains, built on the narrow flood
plain of a river, overlooked by hills, depends for its water
supply on driven wells, within the city limits, sunk in the sand a
few yards from the edge of the stream. Are these wells fed by
water from the river percolating through the sand, or by ground
water on its way to the stream and possibly contaminated with the
sewage of the town?
At what height does underground water stand in the wells of your
region? Does it vary with the season? Have you ever known wells to
go dry? It may be possible to get data from different wells and to
draw a diagram showing the ground-water surface as compared with
the surface of the ground.
FISSURE SPRINGS AND ARTESIAN WELLS. The DEEPER ZONES OF FLOW lie in
pervious strata which are overlain by some impervious stratum.
Such layers are often carried by their dip to great depths, and
water may circulate in them to far below the level of the surface
streams and even of the sea. When a fissure crosses a water-
bearing stratum, or AQUIFIER, water is forced upward by the
pressure of the weight of the water contained in the higher parts
of the stratum, and may reach the surface as a fissure spring. A
boring which taps such an aquifer is known as an artesian well, a
name derived from a province in France where wells of this kind
have been long in use. The rise of the water in artesian wells,
and in fissure springs also, depends on the following conditions
illustrated in Figure 29. The aquifer dips toward the region of
the wells from higher ground, where it outcrops and receives its
water. It is inclosed between an impervious layer above and water-
tight or water-logged layers beneath. The weight of the column of
water thus inclosed in the aquifer causes water to rise in the
well, precisely as the weight of the water in a standpipe forces
it in connected pipes to the upper stories of buildings.
Which will supply the larger region with artesian wells, an
aquifer whose dip is steep or one whose dip is gentle? Which of
the two aquifers, their thickness being equal, will have the
larger outcrop and therefore be able to draw upon the larger
amount of water from the rainfall? Illustrate with diagrams.
THE ZONE OF SOLUTION. Near the surface, where the circulation of
ground water is most active, it oxidizes, corrodes, and dissolves
the rocks through which it passes. It leaches soils and subsoils
of their lime and other soluble minerals upon which plants depend
for their food. It takes away the soluble cements of rocks; it
widens fissures and joints and opens winding passages along the
bedding planes; it may even remove whole beds of soluble rocks,
such as rock salt, limestone, or gypsum. The work of ground water
in producing landslides has already been noticed. The zone in
which the work of ground water is thus for the most part
destructive we may call the zone of solution.
CAVES. In massive limestone rocks, ground water dissolves channels
which sometimes form large caves (Fig. 30). The necessary
conditions for the excavation of caves of great size are well
shown in central Kentucky, where an upland is built throughout of
thick horizontal beds of limestone. The absence of layers of
insoluble or impervious rock in its structure allows a free
circulation of ground water within it by the way of all natural
openings in the rock. These water ways have been gradually
enlarged by solution and wear until the upland is honeycombed with
caves. Five hundred open caverns are known in one county.
Mammoth Cave, the largest of these caverns, consists of a
labyrinth of chambers and winding galleries whose total length is
said to be as much as thirty miles. One passage four miles long
has an average width of about sixty feet and an average height of
forty feet. One of the great halls is three hundred feet in width
and is overhung by a solid arch of limestone one hundred feet
above the floor. Galleries at different levels are connected by
well-like pits, some of which measure two hundred and twenty-five
feet from top to bottom. Through some of the lowest of these
tunnels flows Echo River, still at work dissolving and wearing
away the rock while on its dark way to appear at the surface as a
great spring.
NATURAL BRIDGES. As a cavern enlarges and the surface of the land
above it is lowered by weathering, the roof at last breaks down
and the cave becomes an open ravine. A portion of the roof may for
a while remain, forming a "natural bridge."
SINK HOLES. In limestone regions channels under ground may become
so well developed that the water of rains rapidly drains away
through them. Ground water stands low and wells must be sunk deep
to find it. Little or no surface water is left to form brooks.
Thus across the limestone upland of central Kentucky one meets but
three surface streams in a hundred miles. Between their valleys
surface water finds its way underground by means of sink holes.
These are pits, commonly funnel shaped, formed by the enlargement
of crevice or joint by percolating water, or by the breakdown of
some portion of the roof of a cave. By clogging of the outlet a
sink hole may come to be filled by a pond.
Central Florida is a limestone region with its drainage largely
subterranean and in part below the level even of the sea. Sink
holes are common, and many of them are occupied by lakelets. Great
springs mark the point of issue of underground streams, while some
rise from beneath the sea. Silver Spring, one of the largest,
discharges from a basin eight hundred feet wide and thirty feet
deep a little river navigable for small steamers to its source.
About the spring there are no surface streams for sixty miles.
THE KARST. Along the eastern coast of the Adriatic, as far south
as Montenegro, lies a belt of limestone mountains singularly worn
and honeycombed by the solvent action of water. Where forests have
been cut from the mountain sides and the red soil has washed away,
the surface of the white limestone forms a pathless desert of rock
where each square rod has been corroded into an intricate branch
work of shallow furrows and sharp ridges. Great sink holes, some
of them six hundred feet deep and more, pockmark the surface of
the land. The drainage is chiefly subterranean. Surface streams
are rare and a portion of their courses is often under ground.
Fragmentary valleys come suddenly to an end at walls of rock where
the rivers which occupy the valleys plunge into dark tunnels to
reappear some miles away. Ground water stands so far below the
surface that it cannot be reached by wells, and the inhabitants
depend on rain water stored for household uses. The finest cavern
of Europe, the Adelsberg Grotto, is in this region. Karst, the
name of a part of this country, is now used to designate any
region or landscape thus sculptured by the chemical action of
surface and ground water. We must remember that Karst regions are
rare, and striking as is the work of their subterranean streams,
it is far less important than the work done by the sheets of
underground water slowly seeping through all subsoils and porous
rocks in other regions.
Even when gathered into definite channels, ground water does not
have the erosive power of surface streams, since it carries with
it little or no rock waste. Regions whose underground drainage is
so perfect that the development of surface streams has been
retarded or prevented escape to a large extent the leveling action
of surface running waters, and may therefore stand higher than the
surrounding country. The hill honeycombed by Luray Cavern,
Virginia, has been attributed to this cause.
CAVERN DEPOSITS. Even in the zone of solution water may under
certain circumstances deposit as well as erode. As it trickles
from the roof of caverns, the lime carbonate which it has taken
into solution from the layers of limestone above is deposited by
evaporation in the air in icicle-like pendants called STALACTITES.
As the drops splash on the floor there are built up in the same
way thicker masses called STALAGMITES, which may grow to join the
stalactites above, forming pillars. A stalagmitic crust often
seals with rock the earth which accumulates in caverns, together
with whatever relics of cave dwellers, either animals or men, it
may contain.
Can you explain why slender stalactites formed by the drip of
single drops are often hollow pipes?
THE ZONE OF CEMENTATION. With increasing depth subterranean water
becomes more and more sluggish in its movements and more and more
highly charged with minerals dissolved from the rocks above. At
such depths it deposits these minerals in the pores of rocks,
cementing their grains together, and in crevices and fissures,
forming mineral veins. Thus below the zone of solution where the
work of water is to dissolve, lies the zone of cementation where
its work is chemical deposit. A part of the invisible load of
waste is thus transferred from rocks near the surface to those at
greater depths.
As the land surface is gradually lowered by weathering and the
work of rain and streams, rocks which have lain deep within the
zone of cementation are brought within the zone of solution. Thus
there are exposed to view limestones, whose cracks were filled
with calcite (crystallized carbonate of lime), with quartz or
other minerals, and sandstones whose grains were well cemented
many feet below the surface.
CAVITY FILLING. Small cavities in the rocks are often found more
or less completely filled with minerals deposited from solution by
water in its constant circulation underground. The process may be
illustrated by the deposit of salt crystals in a cup of
evaporating brine, but in the latter instance the solution is not
renewed as in the case of cavities in the rocks. A cavity thus
lined with inward-pointing crystals is called a GEODE.
CONCRETIONS. Ground water seeping through the pores of rocks may
gather minerals disseminated throughout them into nodular masses
called concretions. Thus silica disseminated through limestone is
gathered into nodules of flint. While geodes grow from the outside
inwards, concretions grow outwards from the center. Nor are they
formed in already existing cavities as are geodes. In soft clays
concretions may, as they grow, press the clay aside. In many other
rocks concretions are made by the process of REPLACEMENT. Molecule
by molecule the rock is removed and the mineral of the concretion
substituted in its place. The concretion may in this way preserve
intact the lamination lines or other structures of the rock. Clays
and shales often contain concretions of lime carbonate, of iron
carbonate, or of iron sulphide. Some fossil, such as a leaf or
shell, frequently forms the nucleus around which the concretion
grows.
Why are building stones more easily worked when "green" than after
their quarry water has dried out?
DEPOSITS OF GROUND WATER IN ARID REGIONS. In arid lands where
ground water is drawn by capillarity to the surface and there
evaporates, it leaves as surface incrustations the minerals held
in solution. White limy incrustations of this nature cover
considerable tracts in northern Mexico. Evaporating beneath the
surface, ground water may deposit a limy cement in beds of loose
sand and gravel. Such firmly cemented layers are not uncommon in
western Kansas and Nebraska, where they are known as "mortar
beds."
THERMAL SPRINGS. While the lower limit of surface drainage is sea
level, subterranean water circulates much below that depth, and is
brought again to the surface by hydrostatic pressure. In many
instances springs have a higher temperature than the average
annual temperature of the region, and are then known as thermal
springs. In regions of present or recent volcanic activity, such
as the Yellowstone National Park, we may believe that the heat of
thermal springs is derived from uncooled lavas, perhaps not far
below the surface. But when hot springs occur at a distance of
hundreds of miles from any volcano, as in the case of the hot
springs of Bath, England, it is probable that their waters have
risen from the heated rocks of the earth's interior. The springs
of Bath have a temperature of 120 degrees F., 70 degrees above the
average annual temperature of the place. If we assume that the
rate of increase in the earth's internal heat is here the average
rate, 1 degree F. to every sixty feet of descent, we may conclude
that the springs of Bath rise from at least a depth of forty-two
hundred feet.
Water may descend to depths from which it can never be brought
back by hydrostatic pressure. It is absorbed by highly heated
rocks deep below the surface. From time to time some of this deep-
seated water may be returned to open air in the steam of volcanic
eruptions.
SURFACE DEPOSITS OF SPRINGS. Where subterranean water returns to
the surface highly charged with minerals in solution, on exposure
to the air it is commonly compelled to lay down much of its
invisible load in chemical deposits about the spring. These are
thrown down from solution either because of cooling, evaporation,
the loss of carbon dioxide, or the work of algae.
Many springs have been charged under pressure with carbon dioxide
from subterranean sources and are able therefore to take up large
quantities of lime carbonate from the limestone rocks through
which they pass. On reaching the surface the pressure is relieved,
the gas escapes, and the lime carbonate is thrown down in deposits
called TRAVERTINE. The gas is sometimes withdrawn and the deposit
produced in large part by the action of algae and other humble
forms of plant life.
At the Mammoth Hot Springs in the valley of the Gardiner River,
Yellowstone National Park, beautiful terraces and basins of
travertine are now building, chiefly by means of algae which cover
the bottoms, rims, and sides of the basins and deposit lime
carbonate upon them in successive sheets. The rock, snow-white
where dry, is coated with red and orange gelatinous mats where the
algae thrive in the over-flowing waters.
Similar terraces of travertine are found to a height of fourteen
hundred feet up the valley side. We may infer that the springs
which formed these ancient deposits discharged near what was then
the bottom of the valley, and that as the valley has been deepened
by the river the ground water of the region has found lower and
lower points of issue.
In many parts of the country calcareous springs occur which coat
with lime carbonate mosses, twigs, and other objects over which
their waters flow. Such are popularly known as petrifying springs,
although they merely incrust the objects and do not convert them
into stone.
Silica is soluble in alkaline waters, especially when these are
hot. Hot springs rising through alkaline siliceous rocks, such as
lavas, often deposit silica in a white spongy formation known as
SILICEOUS SINTER, both by evaporation and by the action of algae
which secrete silica from the waters. It is in this way that the
cones and mounds of the geysers in the Yellowstone National Park
and in Iceland have been formed.
Where water oozes from the earth one may sometimes see a rusty
deposit on the ground, and perhaps an iridescent scum upon the
water. The scum is often mistaken for oil, but at a touch it
cracks and breaks, as oil would not do. It is a film of hydrated
iron oxide, or LIMONITE, and the spring is an iron, or chalybeate,
spring. Compounds of iron have been taken into solution by ground
water from soil and rocks, and are now changed to the insoluble
oxide on exposure to the oxygen of the air.
In wet ground iron compounds leached by ground water from the soil
often collect in reddish deposits a few feet below the surface,
where their downward progress is arrested by some impervious clay.
At the bottom of bogs and shallow lakes iron ores sometimes
accumulate to a depth of several feet.
Decaying organic matter plays a large part in these changes. In
its presence the insoluble iron oxides which give color to most
red and yellow rocks are decomposed, leaving the rocks of a gray
or bluish color, and the soluble iron compounds which result are
readily leached out,--effects seen where red or yellow clays have
been bleached about some decaying tree root.
The iron thus dissolved is laid down as limonite when oxidized, as
about a chalybeate spring; but out of contact with the air and in
the presence of carbon dioxide supplied by decaying vegetation, as
in a peat bog, it may be deposited as iron carbonate, or SIDERITE.
TOTAL AMOUNT OF UNDERGROUND WATERS. In order to realize the vast
work in solution and cementation which underground waters are now
doing and have done in all geological ages, we must gain some
conception of their amount. At a certain depth, estimated at about
six miles, the weight of the crust becomes greater than the rocks
can bear, and all cavities and pores in them must be completely
closed by the enormous pressure which they sustain. Below a depth
of even three or four miles it is believed that ground water
cannot circulate. Estimating the average pore spaces of the
different rocks of the earth's crust above this depth, and the
average per cents of their pore spaces occupied by water, it has
been recently computed that the total amount of ground water is
equal to a sheet of water one hundred feet deep, covering the
entire surface of the earth.
CHAPTER III
RIVERS AND VALLEYS
THE RUN-OFF. We have traced the history of that portion of the
rainfall which soaks into the ground; let us now return to that
part which washes along the surface and is known as the RUN-OFF.
Fed by rains and melting snows, the run-off gathers into courses,
perhaps but faintly marked at first, which join more definite and
deeply cut channels, as twigs their stems. In a humid climate the
larger ravines through which the run-off flows soon descend below
the ground-water surface. Here springs discharge along the sides
of the little valleys and permanent streams begin. The water
supplied by the run-off here joins that part of the rainfall which
had soaked into the soil, and both now proceed together by way of
the stream to the sea.
RIVER FLOODS. Streams vary greatly in volume during the year. At
stages of flood they fill their immediate banks, or overrun them
and inundate any low lands adjacent to the channel; at stages of
low water they diminish to but a fraction of their volume when at
flood.
At times of flood, rivers are fed chiefly by the run-off; at times
of low water, largely or even wholly by springs.
How, then, will the water of streams differ at these times in
turbidity and in the relative amount of solids carried in
solution?
In parts of England streams have been known to continue flowing
after eighteen months of local drought, so great is the volume of
water which in humid climates is stored in the rocks above the
drainage level, and so slowly is it given off in springs.
In Illinois and the states adjacent, rivers remain low in winter
and a "spring freshet" follows the melting of the winter's snows.
A "June rise" is produced by the heavy rains of early summer. Low
water follows in July and August, and streams are again swollen to
a moderate degree under the rains of autumn.
THE DISCHARGE OF STREAMS. The per cent of rainfall discharged by
rivers varies with the amount of rainfall, the slope of the
drainage area, the texture of the rocks, and other factors. With
an annual rainfall of fifty inches in an open country, about fifty
per cent is discharged; while with a rainfall of twenty inches
only fifteen per cent is discharged, part of the remainder being
evaporated and part passing underground beyond the drainage area.
Thus the Ohio discharges thirty per cent of the rainfall of its
basin, while the Missouri carries away but fifteen per cent. A
number of the streams of the semi-arid lands of the West do not
discharge more than five per cent of the rainfall.
Other things being equal, which will afford the larger proportion
of run-off, a region underlain with granite rock or with coarse
sandstone? grass land or forest? steep slopes or level land? a
well-drained region or one abounding in marshes and ponds? frozen
or unfrozen ground? Will there be a larger proportion of run-off
after long rains or after a season of drought? after long and
gentle rains, or after the same amount of precipitation in a
violent rain? during the months of growing vegetation, from June
to August, or during the autumn months?
DESERT STREAMS. In arid regions the ground-water surface lies so
low that for the most part stream ways do not intersect it.
Streams therefore are not fed by springs, but instead lose volume
as their waters soak into the thirsty rocks over which they flow.
They contribute to the ground water of the region instead of being
increased by it. Being supplied chiefly by the run-off, they
wither at times of drought to a mere trickle of water, to a chain
of pools, or go wholly dry, while at long intervals rains fill
their dusty beds with sudden raging torrents. Desert rivers
therefore periodically shorten and lengthen their courses,
withering back at times of drought for scores of miles, or even
for a hundred miles from the point reached by their waters during
seasons of rain.
THE GEOLOGICAL WORK OF STREAMS. The work of streams is of three
kinds,--transportation, erosion, and deposition. Streams TRANSPORT
the waste of the land; they wear, or ERODE, their channels both on
bed and banks; and they DEPOSIT portions of their load from time
to time along their courses, finally laying it down in the sea.
Most of the work of streams is done at times of flood.
TRANSPORTATION
THE INVISIBLE LOAD OF STREAMS. Of the waste which a river
transports we may consider first the invisible load which it
carries in solution, supplied chiefly by springs but also in part
by the run-off and from the solution of the rocks of its bed. More
than half the dissolved solids in the water of the average river
consists of the carbonates of lime and magnesia; other substances
are gypsum, sodium sulphate (Glauber's salts), magnesium sulphate
(Epsom salts), sodium chloride (common salt), and even silica, the
least soluble of the common rock-making minerals. The amount of
this invisible load is surprisingly large. The Mississippi, for
example, transports each year 113,000,000 tons of dissolved rock
to the Gulf.
THE VISIBLE LOAD OF STREAMS. This consists of the silt which the
stream carries in suspension, and the sand and gravel and larger
stones which it pushes along its bed. Especially in times of flood
one may note the muddy water, its silt being kept from settling by
the rolling, eddying currents; and often by placing his ear close
to the bottom of a boat one may hear the clatter of pebbles as
they are hurried along. In mountain torrents the rumble of
bowlders as they clash together may be heard some distance away.
The amount of the load which a stream can transport depends on its
velocity. A current of two thirds of a mile per hour can move fine
sand, while one of four miles per hour sweeps along pebbles as
large as hen's eggs. The transporting power of a stream varies as
the sixth power of its velocity. If its velocity is multiplied by
two, its transporting power is multiplied by the sixth power of
two: it can now move stones sixty-four times as large as it could
before.
Stones weigh from two to three times as much as water, and in
water lose the weight of the volume of water which they displace.
What proportion, then, of their weight in air do stones lose when
submerged?
MEASUREMENT OF STREAM LOADS. To obtain the total amount of waste
transported by a river is an important but difficult matter. The
amount of water discharged must first be found by multiplying the
number of square feet in the average cross section of the stream
by its velocity per second, giving the discharge per second in
cubic feet. The amount of silt to a cubic foot of water is found
by filtering samples of the water taken from different parts of
the stream and at different times in the year, and drying and
weighing the residues. The average amount of silt to the cubic
foot of water, multiplied by the number of cubic feet of water
discharged per year, gives the total load carried in suspension
during that time. Adding to this the estimated amount of sand and
gravel rolled along the bed, which in many swift rivers greatly
exceeds the lighter material held in suspension, and adding also
the total amount of dissolved solids, we reach the exceedingly
important result of the total load of waste discharged by the
river. Dividing the volume of this load by the area of the river
basin gives another result of the greatest geological interest,--
the rate at which the region is being lowered by the combined
action of weathering and erosion, or the rate of denudation.
THE RATE OF DENUDATION OF RIVER BASINS. This rate varies widely.
The Mississippi basin may be taken as a representative land
surface because of the varieties of surface, altitude and slope,
climate, and underlying rocks which are included in its great
extent. Careful measurements show that the Mississippi basin is
now being lowered at a rate of one four-thousandth of a foot a
year, or one foot in four thousand years. Taking this as the
average rate of denudation for the land surfaces of the globe,
estimates have been made of the length of time required at this
rate to wash and wear the continents to the level of the sea. As
the average elevation of the lands of the globe is reckoned at
2411 feet, this result would occur in nine or ten million years,
if the present rate of denudation should remain unchanged. But
even if no movements of the earth's crust should lift or depress
the continents, the rate of wear and the removal of waste from
their surfaces will not remain the same. It must constantly
decrease as the lands are worn nearer to sea level and their
slopes become more gentle. The length of time required to wear
them away is therefore far in excess of that just stated.
The drainage area of the Potomac is 11,000 square miles. The silt
brought down in suspension in a year would cover a square mile to
the depth of four feet. At what rate is the Potomac basin being
lowered from this cause alone?
It is estimated that the Upper Ganges is lowering its basin at the
rate of one foot in 823 years, and the Po one foot in 720 years.
Why so much faster than the Potomac and the Mississippi?
HOW STREAMS GET THEIR LOADS. The load of streams is derived from a
number of sources, the larger part being supplied by the
weathering of valley slopes. We have noticed how the mantle of
waste creeps and washes to the stream ways. Watching the run-off
during a rain, as it hurries muddy with waste along the gutter or
washes down the hillside, we may see the beginning of the route by
which the larger part of their load is delivered to rivers.
Streams also secure some of their load by wearing it from their
beds and banks,--a process called erosion.
EROSION
Streams erode their beds chiefly by means of their bottom load,--
the stones of various sizes and the sand and even the fine mud
which they sweep along. With these tools they smooth, grind, and
rasp the rock of their beds, using them in much the fashion of
sandpaper or a file.
WEATHERING OF RIVER BEDS. The erosion of stream beds is greatly
helped by the work of the weather. Especially at low water more or
less of the bed is exposed to the action of frost and heat and
cold, joints are opened, rocks are pried loose and broken up and
made ready to be swept away by the stream at time of flood.
POTHOLES. In rapids streams also drill out their rocky beds. Where
some slight depression gives rise to an eddy, the pebbles which
gather in it are whirled round and round, and, acting like the bit
of an auger, bore out a cylindrical pit called a pothole. Potholes
sometimes reach a depth of a score of feet. Where they are
numerous they aid materially in deepening the channel, as the
walls between them are worn away and they coalesce.
WATERFALLS. One of the most effective means of erosion which the
river possesses is the waterfall. The plunging water dislodges
stones from the face of the ledge over which it pours, and often
undermines it by excavating a deep pit at its base. Slice after
slice is thus thrown down from the front of the cliff, and the
cataract cuts its way upstream leaving a gorge behind it.
NIAGARA FALLS. The Niagara River flows from Lake Erie at Buffalo in
a broad channel which it has cut but a few feet below the level of
the region. Some thirteen miles from the outlet it plunges over a
ledge one hundred and seventy feet high into the head of a narrow
gorge which extends for seven miles to the escarpment of the
upland in which the gorge is cut. The strata which compose the
upland dip gently upstream and consist at top of a massive
limestone, at the Falls about eighty feet thick, and below of soft
and easily weathered shale. Beneath the Falls the underlying shale
is cut and washed away by the descending water and retreats also
because of weathering, while the overhanging limestone breaks down
in huge blocks from time to time.
Niagara is divided by Goat Island into the Horseshoe Falls and the
American Falls. The former is supplied by the main current of the
river, and from the semicircular sweep of its rim a sheet of water
in places at least fifteen or twenty feet deep plunges into a pool
a little less than two hundred feet in depth. Here the force of
the falling water is sufficient to move about the fallen blocks of
limestone and use them in the excavation of the shale of the bed.
At the American Falls the lesser branch of the river, which flows
along the American side of Goat Island, pours over the side of the
gorge and breaks upon a high talus of limestone blocks which its
smaller volume of water is unable to grind to pieces and remove.
A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while
the American Falls retreated at the rate of 0.64 feet in the same
period. We cannot doubt that the same agency which is now
lengthening the gorge at this rapid rate has cut it back its
entire length of seven miles.
While Niagara Falls have been cutting back a gorge seven miles
long and from two hundred to three hundred feet deep, the river
above the Falls has eroded its bed scarcely below the level of the
upland on which it flows. Like all streams which are the outlets
of lakes, the Niagara flows out of Lake Erie clear of sediment, as
from a settling basin, and carries no tools with which to abrade
its bed. We may infer from this instance how slight is the erosive
power of clear water on hard rock.
Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat
Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST,
how long is it since the Niagara River fell over the edge of the
escarpment where now is the mouth of the present gorge?
Back to Full Books
|