304 SUMMARY 



SUMMARY 



The surface of the earth is constantly changing; in fact, 

 change is the fundamental law of life. There are forces 

 constantly building up, and other forces just as steadily 

 tearing down. Among those forces which produce change 

 are running water, with its power to erode and dissolve; 

 freezing water, with its tremendous expansion ; the moisture 

 of the air; the gases of the atmosphere; heat; and the 

 winds. 



But the agent that has had most to do with preparing the 

 soils of the great grain-bearing regions of the northern 

 hemisphere is ice in the form of glaciers. Glaciers have their 

 origin in upper latitudes or altitudes, where the snow accumu- 

 lates from season to season and is gradually transformed by 

 pressure into ice. This may spread out and creep down the 

 valleys like slow flowing rivers. As glaciers creep down the 

 valleys, the dirt and rocks fall upon their edges from the 

 upper valley sides and are borne along upon the ice. These 

 are called lateral moraines. If two glaciers unite to form a 

 larger one, the debris upon the two sides which come to- 

 gether forms a layer of dirt and rocks which is called a medial 

 moraine. The pressure of the glacier on its bed also wears 

 away the rocks and pulverizes them into soil. When 

 the end of a glacier melts, the debris that is deposited 

 is known as a terminal moraine. 



Almost the whole of the island of Greenland is covered 

 with a deep sheet of ice. The depth of this ice sheet is not 

 known, but probably in some places it is at least several 

 thousand feet. In the antarctic region an area vastly 

 greater than Greenland is covered with ice, probably of a 

 greater thickness. When a glacier extends out into deep 



