302 



GEOLOGY. 



ice and all deposited from the base of the ice during its dissolution con- 

 stitutes the ground moraine. The thickness of the ground moraine is 

 notably unequal. In general, it is thicker toward the terminus of the 

 glacier and thinner toward its source, but considerable portions of a 

 glacier's bed are often left without debris when the ice melts. In general, 

 the terminal moraine is not only thicker, but more irregularly disposed 

 than the ground moraine. 



The lateral moraines. — The surface lateral moraines of valley glaciers 

 are let down on the surface beneath when the ice melts out from under 

 them; but the lateral moraines in a valley from which the ice has melted 

 are not merely the lateral moraines which were on the glacier at a given 

 lime. They are often far more massive than any which ever existed 

 on the ice itself at any one time (Fig. 278). As a glacier retreats, its 



Fig. 278. — A lateral moraine from which the ice has retreated. Bighorn Mountains, 



Wyo. (Blackwelder.) 



lateral moraine material is more or less bunched. Thus if the ice 

 advances 200 feet while its end is being melted back 300 feet, the lateral 

 moraines on the 300 feet melted are concentrated into 100 feet, as they 

 are delivered on to the land by the melting of the ice from beneath. If 

 the retreat of the end of a glacier be very slow, the bunching may 

 be great. But even this cannot explain the massiveness of som^e lateral 

 moraines. Furthermore, the materials of which many lateral mioraines 

 are composed are nearly as well worn as those of the ground moraine. 

 The massive lateral moraines of which this is true are often made up 



