46 PHYSIOGRAPHY 



processes, of which those mentioned are but a part. Plants and 

 animals also assist in rock-breaking and rock-decay. The roots 

 of the former penetrate the soil, loosening it, and thereby make it 

 easier for water to get below the surface. Roots sometimes grow 



Fig. 31. A cliff of limestone with talus blocks which have weathered off. 

 Crazy Woman Hill, Wyoming. (From photo.by Hole.) 



in cracks in the rock, and as they grow they act like wedges (Fig. 33). 

 Large masses of rock are sometimes loosened in this way. When 

 a tree is uprooted, the ground is torn up (Fig. 34), and rock material 

 to the depth of several feet is at times exposed to the action of freez- 

 ing water, air, and rain. Burrowing animals of all sorts loosen the 

 ground, and develop channels for the entrance of water. Even 

 small animals like ants and earthworms do an important work in 

 this connection. In Massachusetts, ants have been estimated to 

 bring one-fourth of an inch of fine soil to the surface each year. 

 This would amount to several tons per acre. 



The importance of rock-weathering is great. Much soil is but 

 weathered rock, and without the weathering of rock, much of the 

 land would be free of soil, and therefore without vegetation. The 

 weathering of the rock also prepares fine material for removal by 

 wind or water. 



