The earth's surface is continually being changed; the outcropping hard rock is worn away by wind and rain, and is broken up by frost, by solution of some minerals, etc. The loose material formed is blown away or washed away by rain and deposited elsewhere by streams in gravel bars, sand beds, and mud flats. The streams cut away their beds, aided by the sand and pebbles washed along. Thus the hills are being worn down and the valleys deepened and widened, and the materials of the land are slowly being moved toward the sea, again to be deposited in beds.
Wave-cut cliff with beach and spit built by waves and currents
Along the coast the waves, with the pebbles washed about, are wearing away the land and spreading out its materials in new beds elsewhere. The shore is being cut back in some places and built out in others. Rivers bring down sand and mud and build deltas or bars at their mouths.
Volcanoes pour out melted rock on the surface, and much fine material is blown out in eruptions. Swamps are filled {115} by dead vegetable matter and by sand and mud washed in. These materials form new rocks and build up the surface. Thus the two processes, the wearing down in some places and the building up in others, are tending to bring the surface to a uniform level. Another process, so slow that it can be observed only through long periods of time, tends to deform the earth's crust and to make the surface more irregular. In times past, layers of rock once horizontal have been bent and folded into great arches and troughs, and large areas of the earth's surface have been raised high above sea-level.
Rock ledge rounded smooth and scratched by ice
Sand-dune with wind-rippled surface
At almost any rock outcrop the result of {116} the breaking-up process may be seen; the outer portion is softer, more easily broken, and of different color from the fresh rock, as shown by breaking open a large piece. The wearing away of the land surface is well shown in rain gullies, and the carrying along and depositing of sand and gravel may be seen in almost any stream. In the Northern states and Canada, which at one time were covered by a great sheet of ice, moving southward and grinding off the surface over which it passed, most of the rock outcrops are smoothly rounded and many show scratches made by pebbles dragged along by the ice. The hills too have {117} smoother and rounder outlines, as compared with those farther south where the land has been carved only by rain and streams. Along the coast the wearing away of the land by waves is shown at cliffs, found where the coast is high, and by the abundant pebbles on the beaches, which are built of material torn from the land by the waves. Sand bars and tidal flats show the deposition of material brought by streams and spread out by currents. Sand dunes and barrens illustrate the carrying and spreading out of fine material by the wind.