=The surface mantle of the lithosphere.=—The surface of the lithosphere is very generally mantled by a layer of loose material composed of soil, clay, sand, gravel, and broken rock. This loose material is sometimes known as _mantle rock_, and sometimes as _rock waste_. On the land, mantle rock is often composed of the disintegrated products of underlying rock formations. It represents the results of the recent action of the atmosphere, of water, of changes of temperature, and of other physical agencies acting on the outer part of the rock sphere. The surface of this mantle is being constantly removed by wind and water, but as constantly renewed by continued decomposition of the rock below. In some areas, especially in the northern part of North America and the northwestern part of Europe, the soil graduates down into an irregular sheet of mixed clay, sand, gravel, and bowlders, known as _drift_. From this and other evidence it is inferred that at a time not greatly antedating our own, ice, chiefly in the form of glaciers, spread extensively over the high latitudes of the northern hemisphere. In some parts of the earth the surface is still covered by fields of snow and ice, comparable to those which formed the drift. In still other places, especially along the flood plains of streams, the mantle rock consists of deposits made by streams which were unable to carry their loads of sediment to the sea. =The crust of the lithosphere.=—Much of the detritus washed down from the land finds its way to bodies of standing water, and beneath lakes and seas the mantle of loose material is made up largely of the gravel, sand, and mud derived from the land. Before deposition these materials are more or less assorted and arranged in layers by waves and currents. When consolidated they constitute rock. The weathering of the rocks of the land, the wearing away of the resulting detritus, and its deposition beneath standing water, are among the most important processes of geologic change. On the land, the mantle of loose material is sometimes absent, and in such places the surface of solid rock of the crust appears. Bare surfaces of rock are most commonly seen where the topography is rough, especially on the slopes of steep-sided valleys and mountains, and on the slopes of cliffs which face seas or lakes. Solid rock, without covering of soil or loose material of any sort, is also frequently seen in the channels of streams, especially where there are falls or rapids.