268 ROCKS AND SOIL 



frozen into its base it marked the bed rock with grooves, 

 so that the direction of the ice streams can be clearly 

 traced. The time during which the ice lasted is called 

 the Glacial Period, and the material which the ice brought 

 with it is called glacial drift. 



292. Erosion. We are now ready to give a name to 

 the total effect of the weathering process. We can see 

 that the land is everywhere being worn down, and that 

 the material formed in weathering is being carried by 

 rains into the rivers. This weathered material, together 

 with what the rivers themselves wear away, is being car- 

 ried into the sea. We must add to the down-tearing 

 agencies the action of the sea itself, as it wears away the 

 land by the pounding of its waves. This general cutting 

 down of the land level we call erosion, from the Latin 

 rodeo, "I gnaw." The agents of erosion are air and water 

 (including ice) ; they owe their physical energy to gravity, 

 the downward pull of the earth. 



293. Exercises. 



1. Is soft coal a rock? To which class does it belong? How was 

 it formed? Cf. 118. 



2. Remembering that shale is hardened clay, and that clay " holds 

 water," what can you say of the conditions under which a deposit of 

 coal was formed, if we now find a layer of shale underneath it? 



3. Old mortar is a mixture of sand and limestone. Which is the 

 cementing material? How can you remove it easily? 



4. Why is sharp sand used in making mortar? Would sand 

 obtained from a lake beach be sharp? 



5. How do pebbles become rounded? If you find a bank of rounded 

 pebbles some distance from any body of water, what do you con- 

 clude? 



