RECORDS LEFT BY RIVERS
When we come to examine more closely the work which rivers do in
removing mineral substances from the land by washing particles
of them from the surface, we find that the records they leave in
geological history must be plainly marked. Every stream, large or
small, is always busy carrying mud, sand, or gravel. Rivers are the
"navvies" of geology. When they are swollen by rain they sweep large
stones away with them. If we look at the bed of a mountain torrent
we shall often see huge blocks of stone that have fallen from the
cliffs on either side blocking the pathway of the stream. To all
appearance the stream is quite powerless to remove these blocks,
and has to circumnavigate them. But visit such a torrent when the
snows are melting, or heavy rain has fallen, and you will hear the
stones knocking against each other or on the rocky bottom as they are
driven downwards by the flood. It is not easy to estimate the driving
power of water. M. Gustave le Bon has furnished an illustration of
its power which is very curious. In the south of France a stream
is led downwards from the mountains to drive the turbine of some
machinery at a manufactory. It comes down several thousand feet. In
the manufactory there is a vent-hole, out of which the water can be
allowed to shoot. The vent-hole is about an inch in diameter; and the
water rushes out with such swiftness and force that the water-jet
becomes as rigid as steel. It is impossible to cut through this
water-jet; and if any one were to try to do so with a sword, the
sword might break but it could never pierce or pass through those
swiftly moving particles of water. A more commonplace illustration is
the use that is sometimes made of water-jets to break up the surfaces
of rock in quarries; nor must it be forgotten that horse-power of
great value and extent for electric lighting and other purposes is
always being drawn from waterfalls. Thus as a mechanical force merely
the river can be immensely powerful; and must leave marks of its
power on the rocks.
The aspect of its force with which we are, however, most concerned is
that which is directed to lowering gradually the surface of the land.
In the last chapter we showed how much mineral might be dissolved
in the waters of rivers. If we are to include also the amount of
mud, sand, and other things classed altogether as silt which a river
carries down, the figures become much more imposing. Sir Archibald
Geikie says that, taking the Mississippi as a typical river (it is
as good an example as would be found, because in its great length
it passes through many different kinds of land, soil, and climate),
we may assume that the average amount of sediment carried down by a
river is one part of sediment to every 1500 parts of water.
If now, says he, we assume that all over the world this is the amount
carried down, we can see how seriously the level of the land is
lowered by rivers. The Mississippi carries from the land it drains
every year the 1/6000th part of a foot of rock. If we take the
general height of the land of the whole globe to be 2100 feet, and
suppose it to be continuously wasted at this rate, then the whole
dry land would be carried into the sea in 12,600,000 years. Or if
we assume the average height of the continent of Europe to be 940
feet, and to be lowered by its rivers at the same rate, then the last
vestige of Europe would have disappeared in 8,640,000 years. Such
figures are of course not exact; and it must always be remembered
that the rivers are merely robbing Peter to pay Paul, and whatever
they take away are always putting somewhere else, but we may learn
from the foregoing considerations that the lowering of the land is
much more rapid than is sometimes supposed. Another thing about the
excavating work of rivers has to be remembered. The torrents carry
sand, shingle, and rock with them, and these very materials act as
agents of destruction on the beds of the water-courses. If we want
to polish brass or steel we mix emery powder (or something finer or
coarser) with the polishing liquid. The torrent or river uses sand
or shingle as its polishing powder. It then wears out the rock over
which it travels, and sometimes carves it into holes or caverns,
gorges or ravines. Sometimes the process is varied, as when a stream
finds its way over a hard rock which overlies a softer rock. If the
arrangement is like that of a series of steps (there may be only one
or two steps) it is possible for the river as it foams in a waterfall
over the hard step at the top to eat its way into the lower softer
step. The lower softer step will gradually disappear, and then the
waterfall, still eating its way in, will begin to undermine the hard
top step, and when that has gone on long enough the hard top step
will fall down and the waterfall will have to begin a little farther
up the stream. In this way a waterfall, gorge, or ravine can be
constructed by a river.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
The Grand Cañon of Arizona
The Colorado River at this point is nearly 200 feet wide. The man
is seated about 1200 feet above the river's level. This whole cañon
up to the top of the mountains in the distance has been worn away
by prehistoric current; and the river has gradually cut its bed
deeper.]
The Falls of Niagara are an illustration of this method. The river
flows from Lake Erie through a level country for a few miles, then
begins to go faster as the path becomes steeper, and finally plunges
over a hard limestone precipice. Beneath the hard limestone (the top
step) are softer beds of shale and sandstone. As the water eats into
them and removes them, large portions of the face of the limestone
precipice on the top fall into the stream below. Thus gradually the
Falls of Niagara are eating their way back to Lake Erie, and have
been doing so for hundreds of thousands of years. In the process of
doing so the Niagara River has cut out below the Falls a gorge which
is not less than seven miles long, from two hundred to four hundred
yards wide, and from two hundred to three hundred feet deep. There
is no reason to doubt that the Niagara gorge has been entirely cut
out in this way, and that at first the river fell over cliffs seven
miles farther down its course at Queenstown. The amount of rock thus
tunnelled would make a rampart about twelve feet high and six feet
thick going round the world at the Equator. Still more gigantic are
the gorges or caverns of the Colorado and its tributaries in Western
America. The Grand Cañon of the Colorado is three hundred miles long,
and in some places more than six thousand feet deep. The country
traversed by it is a network of deep ravines, at the bottom of which
flow the streams that have dug themselves down from the top of the
Colorado tableland.
Now suppose that the river has dug itself in as far as it can go.
There must be a limit, and the limit is reached when the slope of
the bed has been made so slight that the current can only go on
languidly. In that case it cannot sweep along stones, or shingle,
or even coarse gravel; and then the river so far from deepening
its channel begins to raise it by allowing more of the transported
sediment to settle down. If a fast stream meets a slower one
deposition of material will take place; and the same thing will
occur when the rivers meet a lake or a sea. Whatever checks the
swiftness of a current weakens its carrying power and causes it to
drop some of its sediment to the bottom. Therefore accumulations of
sediment occur at the foot of torrent slopes along the lower and
more level ground. These deposits we call alluvium, and sometimes
when the mountain torrent ends abruptly in the plain they may stand
up in cones of silt. They are sometimes called _alluvium cones_ or
_fans_. Quitting the steep descents, and reinforced by tributaries on
either side, the stream ceases to be a torrent and becomes a river.
It goes fast enough at first to carry still coarse gravel; but the
big angular blocks of rock have been dropped, and the stones it now
leaves in its bed are smaller, and become rounded and smoothed as it
goes farther and farther across the plain. At many places it deposits
gravel or sand, more especially at the inner side of the curves which
the stream makes as it winds down the valley. When the stream runs
low in summer, strips of bare sand and shingle are seen at each of
these bends; and the stones are always well smoothed and lie on the
whole regularly. Those that are oblong are so placed that the greater
length of the stone points across the stream; those that are flat
usually slope upstream. These facts, though apparently insignificant,
are really of importance, because they point to us a method by which
geology can determine, after a river has disappeared, the slope of
the bed and the direction of the curves which once it had. If we
examine the steep banks or cliffs by the side of a river the layers
of gravel or shingle in the strata may be found to lie not flat on
one another but in sloping planes. That at once will furnish a clue
to the direction of the river. Another thing of great importance
are the terraces which a river forms by the side of itself. When it
overflows in floods it deposits mud on either side, and when after
the flood it subsides the mud is left. If the reader will imagine
the river in the course of ages sinking lower into its bed he will
see that successive eras of flood-levels will leave their mark in a
series of steps, or _river terraces_ as they are called, on either
side of the channel.
But besides the stones and gravel and mud carried down by a river, we
must also consider the fate of the remains of plants and animals that
are swept along by it, especially in flood-time. In any ordinary
flood trees and shrubs, and the smaller animals like mice and moles
and rabbits, are drowned by the flood. In greater floods birds and
even large animals are drowned, and their remains are buried in
the sediment. If they are quite covered over they may perhaps be
preserved, and their bones may last for an indefinite period. If,
further, the mud deposit hardens, these remains may be preserved so
well and so long that they become the fossil records of creatures
which lived before man emerged to dwell in the world and to become
the arbiter of many of its destinies.
What we have said of rivers is true also of lakes. Rivers pour
into lakes, bringing with them, especially in flood-time, enormous
freights of gravel, sand, and mud, and mingled with them the remains
of vegetation and of animal life. Hundreds of thousands of tons may
be swept down by one storm. To the Lake of Lucerne, for example, the
River Reuss, which comes down from the St. Gothard, brings seven
million cubic feet of sediment every year with it. Since the time of
the Romans the Rhone has so filled up part of the Lake of Geneva that
the Roman harbour, Port Valais, is now nearly two miles from the edge
of the lake. The ground between it and the Lake first became marsh.
It is now farm land. And though these accumulations are most marked
where the rivers drain into the lake, there are deposits always
taking place from the hills all round the lake. Thus lake bottoms
become most interesting and valuable receptacles of the life that has
for ages lived by or near their shores. These deposits are in many
ways peculiar. The snails that live in lake waters are distinct from
the land snails of the adjoining shores. Their dead shells gather at
the bottom of some lakes in such numbers as to form there a deposit
of white crumbling _marl_, sometimes many yards in thickness. On the
sites of lakes that have been gradually filled up, or artificially
drained, this marl shows at once where the lake borders were, and,
roughly, the period of the lake. In some lakes also are found
concretions of iron-oxide, which are formed by the chemical action
of the water on some of the rocks by the lake-side; and in several
Swedish lakes this ironstone forms so fast that the lakes are
regularly dredged for it.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
Cleopatra Terrace, with its Mirror-like Pools, Yellowstone Park,
U.S.A.
These beautiful basins are formed of incrustations of volcanic
limestone. They are of all colours: pink, orange-yellow, green,
and blue. The water in them is of a brilliant blue, caused by the
growth of water plants (algæ), which live in water with as high a
temperature as 150 F.]
Thus among the rocks which form the dry lands of the globe there
occur masses of limestone, sand, marl, and other materials which we
know were deposited in lakes, because they contain types of plants
and animals like those found in the lakes of our own time. From this
kind of evidence we can mark out the places of great lakes that have
long ago vanished from the face of Europe and North America.
There are also the so-called Salt Lakes to consider. These are
generally the lakes that have no outlet and into which a small
amount of water now flows, but never enough to cause the lake now to
overflow, whatever it may have done in past times. The water that
now runs in escapes merely by evaporation. But just as the bottom of
a kettle in which hard water is constantly boiled gradually becomes
furred, so a lake bottom into which water is continually pouring,
bringing dissolved in it all sorts of mineral salts, becomes coated
with sediment. The mineral salts are not evaporated, consequently
the lakes become gradually more mineral--or, for convenience, let us
say, become salter. Among the mineral salts common salt and gypsum
are most important; but some bitter lakes contain sodium carbonate
or magnesium chloride. The Dead Sea and the Great Salt Lake of Utah
show by the deposits round them how they have changed their shape
and depth. In the upper terraces of the Great Salt Lake, 1000 feet
above the present level of the water, fresh-water shells occur,
showing that the basin was at first fresh. The valley bottoms around
salt lakes are now crusted with gypsum, salt, or other deposits,
and their waters are without sign of life. Such conditions help us
to understand how great deposits of salt or gypsum were once laid
down in England, Poland, and Germany, and in many other places where
now the climate would not permit of the necessary evaporation and
condensation of the water.
[Illustration:
_Stereo Copyright, Underwood & U._ _London and New York_
A Petrified Tree
This magnificent fossil is in the Petrified Forest of Arizona; and it
affords one of the most striking examples known of the solidification
and petrifaction of material by the infiltration of mineral salts.
The trunk is now not merely encrusted with stone: it is permeated by
silica, and is, in fact, itself a stone as hard as flint.]