74 



WORK OF RUNNING WATER 



natural processes which tend to loosen or change the exposed sur- 

 faces of rock. The inscriptions on exposed marble become fainter 

 and fainter as time goes by, and finally disappear, because the 

 rock in which the letters were cut has weathered away. In this 

 case the weathering is effected partly by air and partly by water, 

 two important agents of W3athering. 



The rain which falls upon the surface of exposed rock, and that 

 which sinks through the soil to the solid rock below, dissolves 



slowly some of the constituents of the 

 rock. This tends to make the rock 

 crumble, much as mortar does when 

 the lime carbonate which cements the 

 sand is dissolved. The chemical 

 changes effected by ground-water and 

 the gases dissolved in it, also help to 

 disintegrate the rock, as we have seen 



(p- 38). 



There are processes of weathering 

 not due directly either to the atmos- 

 phere or to water. Thus the roots of 

 trees frequently grow in cracks of rocks 

 (Fig. 62), and, increasing in size, act 

 like wedges. Water freezing in cracks 

 works in the same way. From the 

 faces of steep cliffs masses of rock are 

 loosened frequently by the wedge-work 

 of roots or ice, or by expansion and contraction due to changes of 

 temperature. The quantities of debris at the bases of many cliffs, 

 forming slopes of talus (Fig. 63), testify not only to the importance 

 of weathering, but also to the effectiveness of gravity in getting 

 loosened material down. 



The importance of weathering in erosion is shown in many ways. 

 Where the mantle rock is the product of the decay of the solid rock 

 beneath, and this is the case over a large part of the earth's surface, 

 the soil and subsoil represent the excess of weathering over trans- 

 portation. Since most of the earth's surface is covered with soil 

 and subsoil, it is clear that, on the whole, weathering keeps ahead of 

 transportation. The loosening of rock by weathering greatly in- 

 creases erosion, not only by running water, but by all other agents 

 of erosion. Though weathering is the first step in most erosion, it 



Fig. 62. 

 crack in a 



Tree growing in 

 rock, and by its 



growth splitting the rock. 



