20O 



ALASKA GLACIERS 



gather, 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- 

 raphy, which 

 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, 



FIG. 97. BACK OF COLUMBIA GLACIER. 

 Illustrating gradual smoothing of surface. 



