254 GEOLOGY 



with one another (Fig. 205). It does not follow, however, that 

 erosion is greatest when there is most material in the bottom of the 

 ice; for with increase of debris there may be decrease of motion, 1 

 and decrease of motion interferes seriously with the efficiency of 

 erosion. When any considerable thickness of ice at the bottom 

 of a glacier is full of debris, the loaded portion may approach 

 stagnancy, while the cleaner ice above shears over it. A moderate 

 but not excessive load of debris is, therefore, favorable for great 

 erosion. Something depends, too, on the character of the load. 

 Coarse, hard, and angular debris is a more effective instrument 

 of abrasion than fine, soft, or rounded material. So far as plucking 

 is concerned, rate of motion is probably more important than load. 

 Other things being equal, ice which is unyielding, as when full of 

 debris, would be more effective in plucking than ice which is more 

 yielding. , 



In connection with the resistance of the rock, it should be noted 

 that resistance is not a matter of hardness and softness simply. 

 Rock which is affected by cleavage planes, whether joints or bed- 

 ding planes or both, is subject to erosion by plucking, as well as 

 by the abrasion effected by the load in the bottom of the ice. 

 On steep slopes, plucking is probably more important, on the 

 whole, than wear by the debris carried. 



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 the most effective erosion at the source or near it, for though the 

 ice here may be thick, the movement is slow and the load likely to 

 be slight. Ice alone considered, erosion is most effective some- 

 where between the source and the terminus of a glacier, and prob- 

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

 the surface over which the ice passes may be such as to make the 

 place of greatest erosion vary widely. 



Summary. In summary it may be said that rapidly moving 

 ice of sufficient thickness to be working under <i<><><lly pressure. 

 shod with a sufficient but not excessive quantity of hard-rock 

 material, passing over non-resistant formations possessing a topog- 



1 Russell. Jour, of Geol., Vol. Ill, p. 823. 



