58 SOILS AND FERTILIZERS 



than at present. 20 Changes of climate caused a recession 

 of these, and resulted in the movement of large bodies 

 of ice, carrying along rocks and frozen soil. The move- 

 ment and pressure of the ice pulverized the rock and 

 produced soil. This action is well illustrated at the 

 present time where mountains rise above the snow line, 

 and the ice and snow melting at the base are replaced by 

 ice and snow from farther up, moving down the side of 

 the mountain and carrying along crushed stones and 

 soil. King estimates that an ice sheet 10 feet in depth 

 exerts a pressure of 570 pounds to the square foot. The 

 frozen mass contains boulders, gravel, and sand which 

 act as a grinding plate upon the rocky surfaces with 

 which it comes in contact. 15 The rubbing of these two 

 surfaces against each other under pressure for cen- 

 turies has resulted in the production of vast areas of 

 drift soil. 



When the glacier receded, stranded ice masses were 

 distributed over the land. These melted slowly and by 

 their grinding action hollowed out places some of which 

 finally became lakes. The numerous lakes at the source 

 of the Mississippi River are supposed to have been 

 formed by glacial action. The terminal of a glacier is 

 called a moraine and is covered with large boulders 

 which have not been disintegrated. The course of a 

 glacier is frequently traced by the markings or scratches 

 of the mass on fock ledges. In glacial soils, the rocks 

 are never angular, but are smooth because of the grind- 



