Imagine to yourselves the whole of St. Paul's churchyard filled
with oyster-shells, built up in a large square till they reached
half as high again as the top of the cathedral, then you will
have some idea of the amount of chalk carried invisibly past
Bonn in the water of the Rhine every year.
Since all this matter, whether brought down as mud or dissolved,
comes from one part of the land to be carried elsewhere or out
to sea, it is clear that some gaps and hollows must be left in
the places from which it is taken. Let us see how these gaps are
made. Have you ever clambered up the mountainside, or even up
one of those small ravines in the hillside, which have generally
a little stream trickling through them? If so, you must have
noticed the number of pebbles, large and small, lying in patches
here and there in the stream, and many pieces of broken rock,
which are often scattered along the sides of the ravine; and
how, as you climb, the path grows steeper, and the rocks become
rugged and stick out in strange shapes.
The history of this ravine will tell us a great deal about the
carving of water. Once it was nothing more than a little furrow
in the hillside down which the rain found its way in a thin
thread-like stream. But by and by, as the stream carried down
some of the earth, and the furrow grew deeper and wider, the sides
began to crumble when the sun dried up the rain which had soaked
in. Then in winter, when the sides of the hill were moist with the
autumn rains, frost came and turned the water to ice, and so made
the cracks still larger, and the swollen steam rushing down,
caught the loose pieces of rock and washed them down into its bed.
Here they were rolled over and over, and grated against each
other, and were ground away till they became rounded pebbles, such
as lie in the foreground of the picture (Fig. 25); while the grit
which was rubbed off them was carried farther down by the stream.
And so in time this became a little valley, and as the stream cut
it deeper and deeper, there was room to clamber along the sides of
it, and ferns and mosses began to cover the naked stone, and small
trees rooted themselves along the banks, and this beautiful little
nook sprang up on the hill-side entirely by the sculpturing of
water.
Shall you not feel a fresh interest in all the little valleys,
ravines, and gorges you meet with in the country, if you can
picture them being formed in this way year by year? There are
many curious differences in them which you can study for
yourselves. Some will be smooth, broad valleys and here the
rocks have been soft and easily worn, and water trickling down
the sides of the first valley has cut other channels so as to
make smaller valleys running across it. In other places there
will be narrow ravines, and here the rocks have been hard, so
that they did not wear away gradually, but broke off and fell in
blocks, leaving high cliffs on each side. In some places you
will come to a beautiful waterfall, where the water has tumbled
over a steep cliff, and then eaten its way back, just like a saw
cutting through a piece of wood.
There are two things in particular to notice in a waterfall like
this. First, how the water and spray dash against the bottom of
the cliff down which it falls, and grind the small pebbles
against the rock. In this way the bottom of the cliff is
undermined, and so great pieces tumble down from time to time,
and keep the fall upright instead of its being sloped away at the
top, and becoming a mere steam. Secondly, you may often see
curious cup-shaped holes, called "pot-holes," in the rocks on the
sides of a waterfall, and these also are concerned in its
formation. In these holes you will generally find two or three
small pebbles, and you have here a beautiful example of how water
uses stones to grind away the face of the earth. These holes are
made entirely by the falling water eddying round and round in a
small hollow of the rock, and grinding the pebbles which it has
brought down, against the bottom and sides of this hollow, just as
you grind round a pestle in a mortar. By degrees the hole grows
deeper and deeper and though the first pebbles are probably ground
down to powder, others fall in, and so in time there is a great
hole perforated right through, helping to make the rock break and
fall away.
In this and other ways the water works its way back in a
surprising manner. The Isle of Wight gives us some good
instances of this; Alum Bay Chine and the celebrated Blackgang
Chine have been entirely cut out by waterfalls. But the best
know and most remarkable example is the Niagara Falls, in
America. Here, the River Niagara first wanders through a flat
country, and then reaches the great Lake Erie in a hollow of the
plain. After that, it flows gently down for about fifteen miles,
and then the slope becomes greater and it rushes on to the Falls
of Niagara. These falls are not nearly so high as many people
imagine, being only 165 feet, or about half the height of St.
Paul's Cathedral, but they are 2700 feet or nearly half-a-mile
wide, and no less than 670,000 tons of water fall
over them every minute, making magnificent clouds of spray.
Sir Charles Lyell, when he was at Niagara, came to the conclusion
that, taking one year with another, these falls eat back the
cliff at the rate of about one foot a year, as you can easily
imagine they would do, when you think with what force the water
must dash against the bottom of the falls. In this way a deep
cleft has been cut right back from Queenstown for a distance of
seven miles, to the place where the falls are now. This helps us
a little to understand how very slowly and gradually
water cuts its way; for if a foot a year is about the average of
the waste of the rock, it will have taken more than thirty-five
thousand years for that channel of seven miles to be made.
But even this chasm cut by the falls of Niagara is nothing
compared with the canyons of Colarado. Canyon is a Spanish word
for a rocky gorge, and these gorges are indeed so grand, that if
we had not seen in other places what water can do, we should
never have been able to believe that it could have cut out these
gigantic chasms. For more than three hundred miles the River
Colorado, coming down from the Rocky Mountains, has eaten its way
through a country made of granite and hard beds of limestone and
sandstone, and it has cut down straight through these rocks,
leaving walls from half-a-mile to a mile high, standing straight
up from it. The cliffs of the Great Canyon, as it is called,
stretch up for more than a mile above the river which flows in
the gorge below! Fancy yourselves for a moment in a boat on this
river, as shown in Figure 27, and looking up at these gigantic
walls of rock towering above you. Even half-way up them, a man,
if he could get there, would be so small you could not see him
without a telescope; while the opening at the top between the
two walls would seem so narrow at such an immense distance that
the sky above would have the appearance of nothing more than a
narrow streak of blue. Yet these huge chasms have not been made
by any violent breaking apart of the rocks or convulsion of an
earthquake. No, they have been gradually, silently, and steadily
cut through by the river which now glides quietly in the wider
chasms, or rushes rapidly through the narrow gorges at their feet.
"No description," says Lieutenant Ives, one of the first
explorers of this river, "can convey the idea of the varied and
majestic grandeur of this peerless waterway. Wherever the river
turns, the entire panorama changes. Stately facades, august
cathedrals, amphitheatres, rotundas, castellated walls, and rows
of time-stained ruins, surmounted by every form of tower,
minaret, dome and spire, have been moulded from the cyclopean
masses of rock that form the mighty defile." Who will say, after
this, that water is not the grandest of all sculptors, as it
cuts through hundreds of miles of rock, forming such magnificent
granite groups, not only unsurpassed but unequalled by any of
the works of man?
But we must not look upon water only as a cutting instrument, for
it does more than merely carve out land in one place, it also
carries it away and lays it down elsewhere; and in this it is
more like a modeller in clay, who smooths off the material from
one part of his figure to put it upon another.
Running water is not only always carrying away mud, but at the
same time laying it down here and there wherever it flows. When
a torrent brings down stones and gravel from the mountains, it
will depend on the size and weight of the pieces how long they
will be in falling through the water. If you take a handful of
gravel and throw it into a glass full of water, you will notice
that the stones in it will fall to the bottom at once, the grit
and coarse sand will take longer in sinking, and lastly, the fine
sand will be an hour or two in settling down, so that the water
becomes clear. Now, suppose that this gravel were sinking in the
water of a river. The stones would be buoyed up as long as the
river was very full and flowed very quickly, but they would drop
through sooner than the coarse sand. The coarse sand in its turn
would begin to sink as the river flowed more slowly, and would
reach the bottom while the fine sand was still borne on. Lastly,
the fine sand would sink through very, very slowly, and only
settle in comparatively still water.
From this it will happen that stones will generally lie near to
the bottom of torrents at the foot of the banks from which they
fall, while the gravel will be carried on by the stream after it
leaves the mountains. This too, however, will be laid down when
the river comes into a more level country and runs more slowly.
Or it may be left together with the finer mud in a lake, as in
the lake of Geneva, into which the Rhone flows laden with mud
and comes out at the other end clear and pure. But if no lake
lies in the way the finer earth will still travel on, and the
river will take up more and more as it flows, till at last it
will leave this too on the plains across which it moves
sluggishly along, or will deposit it at its mouth when it joins
the sea.