38 



PEACTICAL FOKESTBY. 



Water in motion was nature's most powerful tool in shaping the 

 present surface of the earth. In places where the slopes are steep, the 

 structure of the ground loose, and the raiafall abundant, water may- 

 work very rapidly in cutting away the heights and filling the valleys. 

 The destruction of the forest in such a region exposes the surface to 

 the direct action of falling rain and is certain to be followed by the 

 formation of torrents. The danger is greatest when the soil has been 

 laid bare by the browsing and the hoofs of grazing animals, among 

 which sheep and goats are especially destructive, or where the forest 

 floor has been burned a#ay. 



>'J 





Fig. 23.— Rich bottomland washed out by floods. North Carolina. 



When these conditions are both present, as in parts of the Sierra 

 Nevada Mountains of California, of the Cascade Range in Oregon, and 

 in many other parts of the West, the prosperity of the valleys is ia 

 serious danger. Fire and overgrazing on the mountains combine to 

 endanger the future water supply of irrigated or irrigable areas in the 

 valleys below. When rain falls over mountains which have so been 

 deprived of their natural protection it is no longer caught and held 

 back by the trees and the forest floor. The roots, which were once the 

 strongest means of binding the soil together, now are gone and leave it 

 without protection against the rushing water. Heavy rains or sudden 



358 



