LAND WATERS STREAMS 



141 



The lettering of inscriptions on exposed marble becomes fainter 

 land fainter as time goes by, and finally disappears, because the 

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

 pase the weathering is effected partly by the atmosphere and partly 

 by water, and these are the chief, though not the only agents 

 Concerned in weathering. 



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



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



lowly some of the constituents of the 



"ock. This tends to make the rock 



rumble, much as mortar does when 



he lime carbonate which cements the 



and is dissolved. The chemical changes 



effected by ground-water and the gases 



lissolved in it, also help to disintegrate 



;he rock, as we have seen. 



There are other processes of weath- 

 ering not due directly either to the 

 itmosphere or to water. Thus the 

 'oots of trees frequently grow into 

 tracks of rocks (Fig. 105), and, in- 

 creasing in size, act like wedges. 

 i\gain, water freezing in cracks works 

 !n the same way. 



From the faces of steep cliffs 

 lasses of rock are frequently loos- 

 ned, by the wedge-work of roots or ice, or by expansion 

 nd contraction due to changes of temperature. The quantities 

 f debris at the bases of many cliffs, forming slopes of talus (Fig. 

 06), testify to the importance of the weathering of the rock, and 

 o the action of gravity in getting loosened material down. An- 

 ther phase of gravity work is shown in Fig. 107. Here, under the 

 nfluence of gravity and expansion and contraction, due to freezing 

 md thawing and wetting and drying, the surface material is creep- 

 ng down slope. In the process the rock is being broken. The de- 

 cent of rock detritus under the influence of gravity is hardly weath- 

 ering, but it is often weathered material which is brought down. 



Fig. 105. Tree growing in 

 crack in a rock, and by 

 its growth splitting the 

 rock. 



