136 



WORK OF SNOW AND ICE 



again, rivers offer no analogy. All cracks show that the glacier is 

 a very brittle body, incapable of resisting even very moderate strains 

 brought to bear upon it very slowly. In its behavior under tension, 

 therefore, a glacier is notably unlike a river. 



Surface moraines. The surfaces of many glaciers are affected 

 by rock debris, some of which is disposed in the form of belts or 



moraines (Fig. 138). The 

 surface moraines may be 

 lateral, medial, or ter- 

 minal. A lateral moraine 

 is any considerable accu- 

 mulation of debris in a 

 belt on the side of a gla- 

 cier. A medial moraine 

 is a similar accumula- 

 tion at some distance 

 from the margins, but 

 not necessarily in or even 

 near the middle. There 

 may be several medial 

 moraines on one glacier. 

 In valley glaciers, the 

 surface terminal moraine 

 may connect two lateral 

 moraines, making a loop 

 roughly concentric with 

 the end of the glacier. 

 Besides the surface mo- 

 raines, there may be 

 scattered bowlders and 

 bits of rock of various 

 sizes on the ice, and, in addition to the coarse material, there is in 

 many cases some dust which has been blown upon the ice. 



Relief due to surface debris. The debris on the ice affects its 

 topography by influencing the melting of the ice beneath and about 

 it. Rock debris absorbs heat more readily than the ice. A thin 

 piece of stone lying on the ice is warmed through by the sun's rays, 

 and, melting the ice beneath, sinks, just as a piece of black cloth 

 would. Though a good absorber of heat, rock is a poor conductor, 

 and so the lower surface of a thick mass of stone is not warmed 



Fig. 138. Lateral and medial moraines, 

 the latter formed by the union of glaciers. 



