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ELEMENTS OF GENERAL SCIENCE 



grind or tear away the rocks in its path and carry the frag- 

 ments along with it. Often these rocks are ground over one 

 another, and marks or scars are left as evidence of their 

 contact and the force which acted upon them. When the 

 temperature becomes warmer or when the glacier reaches a 



FIG. 139. The end of a glacier 



The ice is covered with stones and gravel. In the foreground is an accumulation 



of the material which has been brought down by the glacier. Photograph by 



Dr. G. E. Nichols 



place of warmer temperature, the ice may melt, thus leaving 

 the rock and sand that were carried (fig. 139). The melting 

 ice forms streams which, as they flow from the glacier, carry 

 away some of the solid material. 



A large part of North America (fig. 140) was once covered 

 by glaciers, and we still have many evidences of the tremen- 

 dous effect they have had upon the surface and upon soil for- 

 mation. Much of the richest soil of the central United States 



