Again, you have learnt that the restless agitation of the sea is due to movements of the air, and that the destruction which the sea can effect on the land is due chiefly to the action of the waves caused by wind. But this action must be merely a surface one. The influence of the waves can not reach to the bottom of the deep sea. Consequently that bottom lies beyond the reach of the various kinds of destruction which so alter the face of the land. The materials which are derived from the waste of the land can lie on the sea floor without further disturbance than they may suffer from the quiet flow of such ocean currents as touch the bottom.
In what way, then, are the gravel, sand and mud disposed of when they reach the sea?
As these materials are all brought from the land, they accumulate on those parts of the sea floor which border the land, rather than at a distance. We may expect to find banks of sand and gravel in shallow seas and near land, but not in the middle of the ocean.
You may form some notion, on a small scale, as to how the materials are arranged on the sea bottom by examining the channel of a river in a season of drought. At one place, where the current has been strong, there may be a bank of gravel; at another place, where the currents of the river have met, you will find, perhaps, a ridge of sand which they have heaped up; while in those places where the flow of the stream has been more gentle, the channel may be covered with a layer of fine silt or mud. You remember that a muddy river may be made to deposit its mud if it overflows its banks so far as to spread over flat land which checks its flow.
The more powerful a current of water, the larger will be the stones it can move along. Hence coarse gravel is not likely to be found over the bottom of the sea, except near the land, where the waves can sweep it out into the path of strong sea currents. Sand will be carried further out, and laid down in great sheets, or in banks. The finer mud and silt may be borne by currents for hundreds of miles before at last settling down upon the sea bottom.
In this way, according to the nearness of the land, and the strength of the ocean currents, the sand, mud, and gravel worn from the land are spread out in vast sheets and banks over the bottom of the sea.