ElsrLAEGEME:NrTS OF GLACIAL CHANNELS. 317 



tains water warmed above 32° is proved by the overhang at the margin of 

 glacial lakes and by the enlargement at the bottoms of glacial pools and 

 lakelets. On Hagues Peak, Colorado, is an ice field that is sliding, if not 

 flowing, and the walls of the siibglacial outlet of a small lake overhang at 

 an angle of 45° or more in a curve convex downward. 



In case of superficial and englacial channels the bottom as well as the 

 sides is more or less melted and eroded by the glacial waters ; hence the 

 base does not enlarge laterally so much as when the bed is composed of 

 rock, and such streams gen- 

 erally form more canyon- 

 like channels. But if they 

 succeed in melting their beds 



down to the ground the chan- Fig. 25.— Ideal sections across channels of superficial glacial streams. 

 ■,,■,. -, ct, before reaching the base ; 6, after reaching the base. 



nels then begin to enlarge 



at the base, and the walls to overhang, like those of a subglacial stream. 

 Gravel dejDOsited in such a channel would be a ridge with arched cross 

 section, like that found in a subglacial tunnel. 



The accompanying cut (fig. 25) was drawn in 1888, and can be com- 

 pared by the critical reader with the more recently published photographs 

 of the Malespiua glacier by Russell. 



EXTRAORDINARY ENLARGEMENTS OF THE GLACIAL RIVER CHANNELS. 



When w^e follow one of the ordinary osars for 50 miles, we become 

 greatly impressed by the narrowness and steepness of the ridge. Some 

 of the hillside and smaller osars are, toward their northern ends, only 5 to 

 15 feet wide at the base. Their material here is very little worn and 

 rounded, and the streams that deposited them were brooks. The height of 

 the osar proper usually exceeds one-eighth, and sometimes locally reaches 

 to one-foui'th or one-third, of its base. That rivers capable of transporting 

 so great a quantity of sediment should occupy so narrow channels is truly 

 wonderful when we consider the softness of ice as comjjared with the 

 hardness of the debris transported and its consequent liability to mechan- 

 ical erosion, also that it was liable to melting, a process which has its 

 analogue in the action of subterranean waters or calcareous rocks, and 

 might be expected to result in the formation of subglacial and englacial 

 channels comparable to the great limestone caves. That the glacial rivers 

 do not ordinarily succeed in doing this is due, I conceive, chiefly to the 



