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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.
