112 GEOLOGY. 



other factors less susceptible of brief statement. The weathering ef- 

 fected by ground water has a wider range both in area and depth than 

 that due to changes of temperature, for while the latter is effective 

 only where temperature changes are considerable, and where coherent 

 material lies at the surface (p. 45), the former is operative to all depths 

 to which water sinks. 



Fig. 94. — Talus accumulation at the base of a steep bluff. Weber Canyon, Uinta 

 Mountains, Utah. The talus has accumulated since the last glaciation of the val- 

 ley and is therefore of very recent origin. (Church.) 



There are other processes of weathering not due directly either to 

 the atmosphere or to water. The roots of trees and smaller plants 

 frequently grow into cracks of rocks, and, increasing in size, act much 

 like freezing water (p. 45) in similar situations. This wedge-work 

 of roots is a phase of weathering. 



From the faces of steep cliffs masses of rock frequently fall. How- 

 ever dislodged, their descent is effected by gravity. The quantities 

 of debris 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 



