268 



GEOLOGY. 



middle. There may be several medial moraines on one glacier, in which 

 case some of them may be far from the center. In alpine glaciers, the 

 surface terminal moraine is less well-defined; in polar glaciers it often 

 connects two lateral moraines, making a loop roughly concentric with 

 the terminus of the glacier. 



Besides the surface moraines, which represent belted aggregations 

 of debris, there may be scattered bowlders and bits of rock of various 

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

 some dust which has been blown upon the ice. 



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

 raphy by influencing the melting of the subjacent and adjacent ice. 

 The rock debris absorbs heat more readily than the ice. A small and 

 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 on snow 



Fig. 242. — Irregular surface due to uneven bottom. Bowdoin glacier, Inglefield 

 Gulf, North Greenland. The dark patches near the left margin of the glacier are 

 lakelets in basins produced by the upward bending of the ice as it overrides an 

 elevation in its bed. The figure also shows a depressed medial moraine. 



will sink because of the increased melting beneath it. Though a good 

 absorber of heat, rock is a poor conductor, and so the lower surface of 

 a large mass of stone is not notably warmed. The ice beneath it is 

 protected from the direct rays of the sun, and is therefore melted more 

 slowly than that around it. The result is that the bowlder presently 

 stands on a protuberance of ice (Fig. 244). When its pedestal becomes 

 high, the obUque rays of the sun and the warm air surrounding it cause 

 it to waste away, and the capping bowlder falls. In high latitudes, 



