The undissolved products of rock weathering, consisting largely of intermingled sand, clay, and un-decomposed rock fragments, are subjected to the vicissitudes of erosion and transportation and are ultimately deposited in the sea, in lakes, or in valleys on the land surface. During their transportation the fragmental materials go through a natural process of separation or classification, due to differences in size and shape of grain and in density, so that there is a tendency for clay to be deposited in one place, sand in another, and still coarser material elsewhere.