150 WORK OF SNOW AND ICE 



more effective for abrasion than fine, soft, or rounded material. 

 In plucking, rate of motion is probably more important than load. 



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 slight and the load too great; nor 

 is it most effective at the source or near it, for the ice here moves 

 slowly and its load is likely to be slight. Ice alone considered, 

 erosion is most effective somewhere between the source and the 

 terminus of a glacier, and probably much nearer the latter than the 

 former. 



In summary it may be said that rapidly moving ice of sufficient 

 thickness to be working under goodly pressure, shod with a sufficient 

 but not excessive quantity of hard-rock material, passing over non- 

 resistant formations possessing a topography of sufficient relief to 

 offer some resistance, and yet too little to retard the progress of the 

 ice seriously, will erode most effectively. 



Varied nature of glacial debris. From its mode of erosion it will 

 be seen that a glacier may carry various sorts of material. At 

 its bottom there may be (i) bowlders which the ice has picked up 

 from the surface, or which it has broken off from projecting points 

 of rock over which it has passed; (2) smaller pieces of rock of the 

 size of cobbles, pebbles, etc., either picked up by the ice from its 

 bed or broken off from larger masses; (3) the fine products (rock- 

 flour) produced by the grinding of the debris in the ice on the rock- 



Fig. 148. A mountain valley in the Wasatch Mountains, not glaciated. 

 (Photo, by Church.) 



