RIVERS AND THEIR WORK 235 



much of the earth's surface has only a few inches of soil, 

 although in some old valleys the soil is many feet thick. 



255. The Formation of Rivers. The soil filled with roots 

 of living plants and the broken rock beneath it retain for 



FIG. 123. ERODED SOIL 



This is a place where a road is being made. Streams caused by a heavy 

 rain have cut deeply into soft earth. 1. What becomes of material taken 

 out by rain water? 2. Why does not this happen on all slopes every time it 

 rains? 3. What reason can you give for the fact that hillsides are not often 

 planted for crops that require cultivation? , 



a short time some of the water which falls upon the ground. 

 Gravity makes the water sink slowly through cracks and 

 porous rock until it reaches some less porous stratum. If 

 this stratum slants downward, the water follows the rock 

 until it comes to the surface lower down the mountain. 

 There is then a constant discharge of water which, running 

 as a little brook in a valley, may be the beginning of a 

 river. Other brooks join it; springs from the valley slopes 

 feed it here and there; rivers from side valleys bring water 



