198 ALASKA GLACIERS 



ice cliff and rock wall constituting the sides of a narrow 

 valley or fosse, usually 50 to 100 feet deep. A stream of 

 water sometimes followed the valley. This feature, fa- 

 miliar in all glacier districts, has been explained as due to 

 the heat acquired by the rock through insolation and then 

 conveyed by radiation to the adjacent ice; and the stream 

 of water, when present, would help to account for the 

 valley, for so much of its volume as came from the sun- 

 heated rock would be warmer than the water of ablation 

 and have some power to melt ice. 



Crevasse Cycle. Wherever the work of the sun is not 

 complicated by the presence of rock debris, the inequali- 

 ties initiated by crevassing are carried by ablation through 

 a regular cycle of change, ending in their complete re- 



FIG. 94. DOWNWARD LIMIT OF CREVASSES IN MUIR GLACIER. 



The lower part of the ice is undivided ; the upper is split into slabs and columns. The 

 dark hill in foreground is of ice with a cover of gravel, a remnant of the retreating glacier. 



moval. In the crevassing which begins abruptly at the 

 head of a cascade, the cracks divide the ice into flat- 

 topped, elongated blocks, usually tapering toward the ends 

 and more or less connected at the surface by slender 

 masses analogous to the slivers of half-broken timber. 

 Whatever the distance downward to which the cracks may 

 originally extend, the resulting permanent crevasses have 



