156 



PHYSICAL GEOLOGY 



Fig. 140. — Aar Glacier showing lateral and medial 

 moraines, and cirques. (Photo. L. E. Westgate.) 



All bowlders transported and deposited by glaciers are given the 

 general name erratics. Figure 141 shows a balanced bowlder. 



Subglacial Material. 



— The bottom por- 

 tions of glaciers con- 

 tain stones and ground- 

 up rock. This ma- 

 terial is derived, either 

 directly from the rock 

 bed or from the super- 

 glacial material which 

 reaches the bottom 

 through the crevasses. 

 The subglacial mate- 

 rial is usually much 

 worn. 



Englacial Material. 



— Between the sur- 

 face and the bottom of a glacier some debris may be carried. This 

 is derived in part from the superglacial material which has not 

 reached the bottom 

 through the crevasses, 

 in part from that 

 which gathered on the 

 surface of the snow or 

 neve and was subse- 

 quently covered, and 

 in part from that 

 which was scraped ofF 

 an elevation in the 

 bed. The englacial 

 material may become 

 superglacial by abla- 

 tion, and subglacial 

 by gradually settling 

 or by the melting of 

 the lower ice. All wi 



be deposited when the ' K; - H 1 - — Balanced bowlder, Hoosac Mountain, 



, , w i , r fc r 1 • Massachusetts. The bowlder is so nicely balanced that 



end 01 the glacier is , , f • , , , J •, . , 



although of great weight it can be made to vibrate with 



reached. little effort. 



