302 THE EARTH'S SURFACE AND PLANT LIFE 



their silt into the still waters of transient ice-dammed 

 lakes. Whatever the original surface of the region, at 

 present it is an irregularly filled plain due to the ancient 

 ice sheet. As the soil is composed of pulverized rock not 

 previously exhausted by vegetable growth, it is strong and 



ALFALFA CUTTING ON THE FERTILE PRAIRIES 



enduring, so that this country has, since its settlement, 

 been noted for its productivity. 



Soils Produced by Decay. All the agencies we have 

 discussed and still others have contributed to breaking down 

 the rocky crust of the earth into soil, thus preparing the way 

 for plant life. The very plants themselves and the animal 

 life which they support must die and return to the soil from 

 which they came. If it were not for this the earth would 

 eventually be encumbered with the dead forms of plants and 

 animals; and the substances of which these bodies are 

 composed would eventually be exhausted from the soil. 



