200 
ALASKA GLACIERS 
gether, leaving a plain surface over which progress is 
absolutely unimpeded (fig. 98). 
FIG. 96. PINNACLES ON COLUMBIA GLACIER. 
Thus the cycle of ice degradation by ablation is strictly 
analogous to 
the cycle of 
land degrada¬ 
tion by erosion; 
an original 
plain is rapidly 
converted into 
a region of 
highly acci- 
dented topog- 
FIG. 97. BACK OF COLUMBIA GLACIER. rapllV Which 
Illustrating gradual smoothing of surface. ? 
slowly returns 
to the plain condition through the removal of its promi¬ 
nences. If the cycle is interrupted by the formation 
of a new system of crevasses, the analogy still holds, 
