266 ROCKS AND SOIL 



so shifting the whole bank (Fig. 228). A strong wind carrying sand 

 uses the sand as a cutting tool, just as a sand blast etches glass (cf. 43). 



Water can carry along much larger objects than air, because it is 

 denser. Mountain torrents often roll down great boulders ; but usually 

 a river carries small gravel, sand, and the still finer particles that make 

 up mud and clay (cf. 287) . And just as wind carrying sand sculptures 

 rocks, so a river carrying sediment scours out its bed. 



The expansion of water in freezing (cf. 70) is one of the most im- 

 portant causes of weathering. Most rocks are porous, and all have 

 cracks. When water freezes in the rock, the rock is forced apart. 

 It is a common experience that fragments fall from a cliff in the spring, 

 when we have frequent freezes and thaws, and when the frozen water 

 in the rock crevices melts, and releases the portions broken from the 

 cliff. 



Plants have already been named as causes of weathering. The 

 decay of plants gives acids, and these attack rock (cf. 132 and 217). 

 Plant roots also act mechanically, tearing rocks apart, much as a tree 

 root breaks a sidewalk. 



291. Drift. What is the origin of mantle rock and 

 soil? And why are soils so different? We have answered 

 these questions partly in our study of weathering. Mantle 

 rock and soil are decayed bed rock. But in this country 

 we find the mantle related to its bed rock in two entirely 

 different ways. In our southern states (except in the 

 Gulf Region) the mantle rock and the bed rock it covers 

 are of the same material. As we dig down we find the 

 rock less and less decayed, until we come to bed rock. If 

 there are boulders, they are simply harder parts of the bed 

 rock, which have not decayed as rapidly as the rest. In 

 such a case we must believe that the mantle rock was 

 formed, where it now lies, by the weathering of the bed 

 rock. Mantle rock of this sort is called residual mantle 

 rock or soil. 



