From the faces of steep cliffs masses of rock frequently fall. However dislodged, their descent is effected by gravity. The quantities of débris at the bases of many cliffs, forming slopes of _talus_ (Fig. 94), testify to the importance of the action of gravity in getting material from higher to lower levels. Another phase of gravity-work is shown in Fig. 95. Here, under the influence of gravity and expansion and contraction, due to freezing and thawing and wetting and drying, the surface material is creeping down slope. In the process the rock is being broken. The process illustrated by the figure involves weathering as well as other factors. The foregoing are among the commoner processes of weathering, although they do not exhaust the list. The more active and tangible processes by which surface rocks are broken up, such as wave wear, river wear and glacier wear, are processes of corrasion. The mechanical wear effected by wind-driven sand might be considered either as corrasion or as weathering. It is more likely to be regarded as corrasion if the amount of wear is considerable enough to be obvious. Rock is sometimes decomposed by the chemical action of hot vapors, gases, and waters rising to the surface from considerable depths. This is often seen in volcanic regions. A conspicuous illustration is seen in the canyon of the Yellowstone in the National Park. Decay of this sort is perhaps not properly weathering, but is not always readily distinguished from it. [Illustration: +Fig.+ 95.—Shows the downward creep of soil and slaty rock under the influence of gravity.]