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 
