THE WORK OF SNOW AND ICE. 



285 



for great erosion. Something depends, too, on the character of the 

 load. Coarse, hard, and angular debris is a more effeclive instrument 

 of erosion than fine, soft, or rounded material. The adverse influence 

 of the overloading of the ice on its motion has been hkened to the sliffen- 



FiG. 255. — Striae on bed rock, Kingston, Des Moines Co., la. (Iowa Geol. Surv.) 



ing of a viscous Hquid by the addition of foreign matter, but it may 

 better perhaps be referred to the destruction of the granular-crystalHne 

 continuity on which glacier motion probably depends. 



From the preceding statement, it is evident that erosion is not equally 

 effective at all points beneath a glacier. So far as concerns the ice itself, 

 erosion is not most effective at the end of a valley glacier, or at the edge 

 of an ice sheet, for here the strength of movement is too shght and the 

 load too great; nor is the most effective erosion at the source or near 

 it, for though the ice may here be thick, the movement is slow and the. 

 load Hkely to be shght. Ice conditions only being considered, erosion 

 is most effective somewhere between the source and the terminus, and 

 probably much nearer the latter than the former. The conditions of 

 the surface over which the ice passes may be such as to vary the place of 

 greatest erosion widely. Thus in an Alpine glacier, erosion may be 



