up by the thirsty ground. They could scarcely follow it, except by the
groove cut by the rivulet in seasons when the flow was greater. They
followed the runaway brook, through the grass roots, that almost hid it.
As the ground grew steeper, it hurried faster. Soon it gathered the
water of other springs, which hurried toward it in small rivulets,
because its level was lower. Water always seeks the lowest level it can
find. Sometimes marshy spots were reached where water stood in the holes
made by the feet of cattle that came there to drink. The water was
muddy, and seemed to stand still. But it was settling steadily, and at
one side the little river was found, flowing away with the water it drew
from the swampy, springy ground. All the mud was gone, now; the water
was clear. It flowed in a bed with a stony floor, and there were rough
steps where the water fell down in little sheets, forming a waterfall,
the first of many that make this river beautiful in the upper half of
its course. To get from the high level of that hillside spring to the
low level of the sea, the water has to make a fall of twenty-three
hundred feet, but it makes the descent gradually. It could not climb
over anything, but always found a way to get around the rocks and hills
that stood in its way. When the flat marsh land interfered, the water
poured in and overflowed the basin at the lowest margin.
In the rocky ground the two explorers found that the stream had widened
its channel by entering a narrow crevice and wearing away its walls. The
continual washing of the water wears away stone. Rocks are softened by
being wet. Streaks of iron in the hardest granite will rust out and let
the water in. Then the lime in rocks is easily dissolved. Every dead
leaf the river carried along added an acid to the water, and this made
easier the process of dissolving the limestone.
Every crumbling rock gives the river tools that it uses like hammer and
chisel and sandpaper to smooth all the uneven surfaces in its bed, to
move stumbling blocks, and to dig the bed deeper and wider. The steeper
the slope is, the faster the stream flows, and the larger the rocks it
can carry. Rocks loosened from the stream bed are rolled along by the
current. Then bang! against the rocks that are not loose, and often they
are able to break them loose. The fine sand is swept along, and its
sharp points strike like steel needles, and do a great work in polishing
roughness and loosening small particles from the stream bed. The bigger
pebbles of the stream have banged against the rock walls, with the same
effect, smoothing away unevenness and pounding fragments loose, rolling
against one another, and getting their own rough corners worn away.
The makers of stone marbles learned their business from a brook. They
cut the stone into cubical blocks, and throw them into troughs, into
which is poured a stream of running water. The blocks are kept in
motion, and the grinding makes each block help the rest to grind off the
eight corners and the twelve ridges of each one. The water becomes muddy
with the fine particles, just as the drip from a grindstone becomes
unclean when an axe is ground. Pretty soon all the blocks in the trough
are changed into globes--the marbles that children buy at the shops when
marble season comes around.
I suppose if the troughs are not watched and emptied in time, the
marbles would gradually be ground down to the size of peas, then to the
size of small bird shot, and finally they would escape as muddy water
and fine sand grains.
Sure it is that the sandy shores that line most rivers are the remnants
of hard rocks that have been torn out and ground up by the action of the
current.
Not very many miles from its first waterfall the stream had grown so
large that my two friends knew that they would soon find their canoes.
The plan now was to float down the curious, winding river and to learn,
if the river and the banks could tell them, just why the course was so
crooked on the map. They came into a broad, level valley where streams
met them, coming out of deep clefts between the hills they were leaving
behind them. The banks were pebbly, but blackened with slimy mud that
made the water murky. The current swerved from one side to the other,
sometimes quite close to the bank, where the river turned and formed a
deep bend. On this side the bank was steep, the roots of plants and
trees exposed. On the opposite side a muddy bank sloped gently out into
the stream. Here building up was going on, to offset the tearing down.
The sharp bends are made sharper, once the current is deflected from the
middle of the stream to one side. At length the loops bend on each other
and come so near together that the current breaks through, leaving a
semicircular bayou of still water, and the river's course straightened
at that place. It must have been in a spring flood that this cut-off was
made, and, the break once made was easily widened, for the soil is fine
mud which, when soaked, crumbles and dissolves into muddy water.
Stately and slow that river moves down to the bay, into which it empties
its load. The rain that falls on hundreds of square miles of territory
flows into the streams that feed this trunk. The little spring that is
the headwater of the system is but one of many pockets in the hillsides
that hold the water that soaks into the ground and give it out by slow
degrees. Surface water after a rain flows quickly into the streams. It
is the springs that hold back their supply and keep the rivers from
running dry in hot weather.
Do they feel now that they know their river? Are they ready to leave it,
and explore some other? Indeed, no. They are barely introduced to it.
All kinds of rivers are shown by the different parts of this one. It is
a river of the mountains and of the lowland. It flows through woods and
prairies, through rocky passes and reedy flats. It races impetuously in
its youth, and plods sedately in later life. The trees and the other
plants that shadow this stream, and live by its bounty, are very
different in the upland and in the lowland. The scenery along this
stream shows endless variety. Up yonder all is wild. Down here great
bridges span the flood, boats of all kinds carry on the commerce between
two neighbour cities. A great park comes down to the river-bank on one
side. Canoes are thick as they can paddle on late summer afternoons.
No one can ever really know a river well enough to feel that it is an
old story. There is always something new it has to tell its friends. So
my two explorers say, and they know far more about their friendly river
than I do.
THE WAYS OF RIVERS
A canal is an artificial river, built to carry boats from one place to
another. Its course is, as nearly as possible, a straight line between
two points. A river, we all agree, is more beautiful than a canal, for
it winds in graceful curves, in and out among the hills, its waters
seeking the lowest level, always.
No artist could lay out curves more beautiful than the river forms.
These curves change from year to year, some slowly, some more rapidly.
It is not hard to understand just why these changes take place.
Some rivers are dangerous for boating at certain points. The current is
strong, and there are eddies and whirlpools that have to be avoided, or
the boat becomes unmanageable. People are drowned each season by
trusting themselves to rivers the dangerous tricks of which they do not
know. Deep holes are washed out of the bed of the stream by whirling
eddies. The pot-holes of which people talk are deep, rounded cavities,
ground out of the rocky stream-bed by the scouring of sand and loose
stones driven by whirling eddies in shallow basins. Every year deepens
each pot-hole until some change in the stream-bed shifts the eddy to
another place.
No stream finds its channel ready-made; it makes its own, and constantly
changes it. The current swings to one side of the channel, lifting the
loose sediment and grinding deeper the bed of the stream. The water lags
on the opposite side, and sediment falls to the bottom. So the
building-up of one side is going on at the same time that the
tearing-down process is being carried on on the other. With the lowering
of the bed the river swerves toward one bank, and a hollow is worn by
slow degrees. The current swings into this hollow, and in passing out is
thrown across the stream to the opposite bank. Here its force wears away
another hollow; and so it zigzags down-stream. The deeper the hollows,
the more curved becomes the course, if the general fall is but moderate.
It is toward the lower courses of the stream that the winding becomes
more noticeable. The sediment that is carried is deposited at the point
where the current is least strong, so that while the outcurves become
sharper by the tearing away of the stream's bank, the incurves become
sharper by the building up of this bank.
The Mississippi below Memphis is thrown into a wonderful series of
curves by the erosion and the deposit caused by the current zigzagging
back and forth from one bank to the other. Gradually the curves become
loops. The river's current finally jumps across the meeting of the
curves, and abandons the circular bend. It becomes a bayou or lagoon of
still water, while the current flows on in the straightened channel.
All rivers that flow through flat, swampy land show these intricate
winding channels and many lagoons that have once been curves of the
river.
No one would ever mistake a river for a lake or any other body of water,
yet rivers differ greatly in character. One tears its way along down its
steep, rock-encumbered channel between walls that rise as vertical
precipices on both sides. The roaming, angry waters are drawn into
whirlpools in one place. They lie stagnant as if sulking in another,
then leap boisterously over ledges of rock and are churned into creamy
foam at the bottom. Outside the mountainous part of its course this same
river flows broad and calm through a mud-banked channel, cut by
tributary streams that draw in the water of low, sloping hills.
The Missouri is such a wild mountain stream at its headwaters. We who
have seen its muddy waters from Sioux City to St. Louis would hardly
believe that its impetuous and picturesque youth could merge into an old
age so comfortable and placid and commonplace.
This thing is true of all rivers. They flow, gradually or suddenly, from
higher to lower levels. To reach the lowest level as soon as possible is
the end each river is striving toward. If it could, each river would cut
its bed to this depth at the first stage of its course. Its tools are
the rocks it carries, great and small. The force that uses these tools
is the power of falling water, represented by the current of the stream.
The upper part of a river such as the Missouri or Mississippi engages in
a campaign of widening and deepening its channel, and carrying away
quantities of sediment. The lower reaches of the stream flow through
more level country; the current is checked, and a vast burden of
sediment is laid down. Instead of tearing away its banks and bottom, the
river fills up gradually with mud. The current meanders between banks of
sediment over a bottom which becomes shallower year by year. The Rocky
Mountains are being carried to the Gulf of Mexico. The commerce of the
river is impeded by mountain debris deposited as mud-banks along the
river's lower course.
Many rivers are quiet and commonplace throughout their length. They flow
between low, rounded hills, and are joined by quiet streams, that occupy
the separating grooves between the hills. This is the oldest type of
river. It has done its work. Rainfall and stream-flow have brought the
level of the land nearly to the level of the stream. Very little more is
left to be ground down and carried away. The landscape is beautiful, but
it is no longer picturesque. Wind and water have smoothed away
unevennesses. Trees and grass and other vegetation check erosion, and
the river has little to do but to carry away the surface water that
falls as rain.
But suppose our river, flowing gently between its grassy banks, should
feel some mighty power lifting it up, with all its neighbour hills and
valleys, to form a wrinkle in the still unstable crust of the earth.
Away off at the river's mouth the level may not have changed, or that
region may have been depressed instead of elevated by the shrinking
process. Suppose the great upheaval has not severed the upper from the
lower courses of the stream. With tremendous force and speed, the
current flows from the higher levels to the lower. The river in the
highlands strikes hard to reach the level of its mouth. It grinds with
all its might, and all its rocky tools, upon its bed. All the mud is
scoured out, and then the underlying rocks are attacked. If these rocks
are soft and easily worn away, the channel deepens rapidly. One after
another the alternating layers are excavated, and the river flows in a
canyon which deepens more and more. As the level is lowered, the current
of the stream becomes slower and the cutting away of its bed less rapid.
The stream is content to flow gently, for it has almost reached the old
level, on which it flowed before the valley became a ridge or
table-land.
The rivers that flow in canyons have been thousands of years in carving
out their channels, yet they are newer, geologically speaking, than the
streams that drain the level prairie country. The earth has risen, and
the canyons have been carved since the prairies became rolling, level
ground.
[Illustration: This little pond is a basin hollowed by the same glacier
that scattered the stones and rounded the hills]
[Illustration: Every stream is wearing away its banks, while trees and
grass blades are holding on to the soil with all their roots]
The Colorado River flows through a canyon with walls that in places
present sheer vertical faces a mile in depth, and so smooth that no
trail can be found by which to reach from top to bottom. The region has
but slight erosion by wind, and practically none by rain. The local
rainfall is very slight. So the river is the one force that has acted to
cut down the rocks, and its force is all expended in the narrow area of
its own bed. Had frequent rains been the rule on the Colorado plateau,
the angles of the mesas would have been rounded into hills of the
familiar kind so constantly a part of the landscape in the eastern half
of the continent.
The Colorado is an ancient river which has to carry away the store of
moisture that comes from the Pacific Ocean and falls as snow on the high
peaks of the Rocky Mountains. Similar river gorges with similar stories
to tell are the Arkansas, the Platte, and the Yellowstone. All cut their
channels unaided through regions of little rain.
When the earth's crust is thrown up in mountain folds, and between them
valleys are formed, the level of rivers is sometimes lowered and the
rapidity of their flow is checked. A stream which has torn down its
walls at a rapid rate becomes a sluggish water-course, its current
clogged with sediment, which it has no power to carry farther. When such
a river begins to build and obstruct its own waters it bars its progress
and may form a lake as the outlet of its tributary streams. Many ancient
rivers have been utterly changed and some obliterated by general
movements of the earth's crust.
THE STORY OF A POND
Look out of the car window as you cross a flat stretch of new prairie
country, and you see a great many little ponds of water dotting the
green landscape. Forty years ago Iowa was a good place to see ponds of
all shapes and sizes. The copious rainfall of the early spring gathered
in the hollows of the land, and the stiff clay subsoil prevented the
water from soaking quickly into the ground. The ponds might dry away
during the hot, dry summer, leaving a baked clay basin, checked with an
intricate system of cracks. Or if rains were frequent and heavy, they
might keep full to the brim throughout the season.
Tall bulrushes stood around the margins of the largest ponds, and
water-lilies blossomed on the surface during the summer. The bass and
the treble of the spring chorus were made by frogs and toads and little
hylas, all of which resorted to the ponds to lay their eggs, in coiled
ropes or spongy masses, according to their various family traditions. On
many a spring night my zooelogy class and I have visited the squashy
margins of these ponds, and, by the light of a lantern, seen singing
toads and frogs sitting on bare hummocks of grass roots that stood
above the water-line. The throat of each musician was puffed out into a
bag about the size and shape of a small hen's egg; and all were singing
for dear life, and making a din that was almost ear-splitting at close
range. So great was the self-absorption of these singers that we could
approach them, daze them with the light of the lantern, and capture any
number of them with our long-handled nets before they noticed us. But it
was not easy to persuade them to sing in captivity, no matter how many
of the comforts of home we provided in the school aquariums. So, after
some very interesting nature studies, we always carried them back and
liberated them, where they could rejoin their kinsfolk and neighbours.
It was when we were scraping the mud from our rubber boots that we
realized the character of the bottoms of our prairie ponds. The slimy
black deposit was made partly of the clay bottom, but largely of
decaying roots and tops of water plants of various kinds. Whenever it
rained or the wind blew hard, the bottom was stirred enough to make the
water muddy; and on the quietest days a pail of pond water had a tinge
of brown because there were always decaying leaves and other rubbish to
stain its purity.
The farmers drained the ponds as fast as they were able, carrying the
water, by open ditches first, and later by underground tile drains, to
lower levels. Finally these trunk drain pipes discharged the water into
streams or lakes. To-day a large proportion of the pond areas of Iowa
has disappeared; the hollow tile of terra-cotta has been the most
efficient means of converting the waste land, covered by ponds, into
fertile fields.
But the ponds that have not been drained are smaller than they used to
be, and are on the straight road to extinction. This process one can see
at any time by visiting a pond. Every year a crop of reeds and a dozen
other species of vigorous water plants dies at the top and adds the
substance of their summer growth to the dust and other refuse that
gathers in the bottom of the pond. Each spring roots and seeds send up
another crop, if possible more vigorous than the last, and this top
growth in turn dies and lies upon the bottom. The pond level varies with
the rainfall of the years, but it averages a certain depth, from which
something is each year subtracted by the accumulations of rotting
vegetable matter in the bottom. Evaporation lowers the water-level,
especially in hot, dry summers. From year to year the water plants draw
in to form a smaller circle, the grassy meadow land encroaches on all
sides. The end of the story is the filling up of the pond basin with the
rotting substance of its own vegetation. This is what is happening to
ponds and inland marshes by slow degrees. The tile drain pipes
obliterate the pond in a single season. Nature is more deliberate. She
may require a hundred years to fill up a single pond which the farmer
can rid himself of by a few days of work and a few rods of tiling.
THE RIDDLE OF THE LOST ROCKS
Outside of my window two robins are building a nest in the crotch of a
blossoming red maple tree. And just across the hedge, men are digging a
big square hole in the ground--the cellar of our neighbour's new house.
It looks now as if the robins would get their house built first, for
they need but one room, and they do not trouble about a cellar. I shall
watch both houses as they grow through the breezy March days.
The brown sod was first torn up by a plough, which uncovered the red New
Jersey soil. Two men, with a team hitched to a scraper, have carried
load after load of the loose earth to a heap on the back of the lot,
while two other men with pickaxes dug into the hard subsoil, loosening
it, so that the scraper could scoop it up.
This subsoil is heavy, like clay, and it breaks apart into hard clods.
At the surface the men found a network of tree roots, about which the
soil easily crumbled. Often I hear a sharp, metallic stroke, unlike the
dull sound of the picks striking into the earth. The digger has struck a
stone, and he must work around it, pry it up and lift it out of the way.
A row of these stones is seen at one side of the cellar hole, ranged
along the bank. They are all different in size and shape, and red with
clay, so I can't tell what they are made of. But from this distance I
see plainly that they are irregular in form and have no sharp corners.
The soil strewn along the lot by the scraper is full of stones, mostly
irregular, but some rounded; some are as big as your head, others grade
down to the sizes of marbles.
When I went down and examined this red earth, I found pebbles of all
shapes and sizes, gravel in with the clay, and grains of sand. This
rock-sprinkled soil in New Jersey is very much like soil which I know
very well in Iowa; it looks different in colour, but those pebbles and
rock fragments must be explained in the same way here as there.
These are not native stones, the outcrop of near-by hillsides, but
strangers in this region. The stones in Iowa soil are also imported.
The prairie land of Iowa has not many big rocks on the surface, yet
enough of them to make trouble. The man who was ploughing kept a sharp
lookout, and swung his plough point away from a buried rock that showed
above ground, lest it should break the steel blade. One of the farmer's
jobs for the less busy season was to go out with sledge and dynamite
sticks, and blast into fragments the buried boulders too large to move.
Sometimes building a hot fire on the top of it, and throwing on water,
would crack the stubborn "dornick" into pieces small enough to be loaded
on stone-boats.
I remember when the last giant boulder whose buried bulk scarcely showed
at the surface, was fractured by dynamite. Its total weight proved to be
many tons. We hauled the pieces to the great stone pile which furnished
materials for walling the sides of a deep well and for laying the
foundation of the new house. Yet for years stones have been
accumulating, all of them turned out of the same farm, when pastures and
swampy land came under the plough.
Draw a line on the map from New York to St. Louis, and then turn
northward a little and extend it to the Yellowstone Park. The
boulder-strewn states lie north of this line, and are not found south of
it, anywhere. Canada has boulders just like those of our Northern
States. The same power scattered them over all of the vast northern half
of North America and a large part of Europe.
What explanation is there for this extensive distribution of unsorted
debris?
THE QUESTION ANSWERED
The rocks tell their own story, partly, but not wholly. They told just
enough to keep the early geologists guessing; and only very recently has
the guessing come upon the truth.
These things the rocks told:
1. We have come from a distance.
2. We have had our sharp corners worn off.
3. Many of us have deep scratches on our sides.
4. At various places we have been dumped in long ridges, mixed with much
earth.
5. A big boulder is often balanced on another one.
The first thing the geologist noted was the fact that these boulders are
strangers--that is, they are not the native rocks that outcrop on
hillsides and on mountain slopes near where they are found. Far to the
north are beds of rock from which this debris undoubtedly came. Could a
flood have scattered them as they are found? No, for water sorts the
rock debris it deposits, and it rounds and polishes rock fragments,
instead of scratching and grooving them and leaving them angular, as
these are.
Professor Agassiz went to Switzerland and studied the glaciers. He found
unsorted rock fragments where the glacier's nose melted, and let them
fall. They were worn and scratched and grooved, by being frozen into the
ice, and dragged over the rocky bed of the stream. The rocky walls of
the valley were scored by the glacier's tools. Rounded domes of rock
jutted out of the ground, in the paths of the ice streams, just like the
granite outcrop in Central Park in New York, and many others in the
region of scattered boulders.