STRATIFIED ICE DEPOSITS 



157 



of ice. About the margin of the ice-sheet, small lakes are formed, 

 the water being held in place by the ice barrier, but these lakes 

 are subject to great fluctuations, and often their waters escape 

 through tunnels in the ice. In some of these lakes stratified 

 deposits are made by the inflowing streams. Innumerable streams, 

 some of them quite large, rise from under the glacier, and many 

 others flowing from the north pass under the free margin of the ice 

 by means of long tunnels. All of these streams are loaded to their 



Fig. 61. 



•The Chaix Hills,, Alaska. Moraine material stratified by water. 

 (U. S. G. S.) 



utmost capacity with sediment, gravel, and boulders ; by blocking 

 up their own openings from the ice, they likewise cause the deposi- 

 tion of sand, gravel, and boulders within their tunnels, which, when 

 the glacier retreats, will be left standing as the gravel ridges called 

 asars, while conical mounds are built up where the streams burst 

 from under the ice, and sometimes, owing to the great pressure, 

 rise like fountains. This kind of deposition is characteristic of 



