GLACIAL SCULPTURE 205 



The tendency toward curves of large radius is more effect- 

 ive where the material of the bed is easily worn than 

 where it is obdurate. 



A third factor affecting rate of abrasion is the material 

 abraded. Some materials yield more easily than others 

 and are worn more rapidly. Where the material of the 

 glacier bed is heterogeneous, including both yielding and 

 obdurate rocks, there is a tendency to hollow out the 

 yielding rocks and leave the obdurate masses prominent. 

 This tendency is opposed by those arising from the vis- 

 cosity of the ice, and the type of the resulting sculpture 

 in each individual case is a compromise. It is thought 

 that the relative importance of viscosity is greater with 

 swift-moving ice than with slow-moving ice. 



A fourth factor is found in the quality and quantity of 

 the abrasive material, the rock particles set in the base 

 of the ice. The particles picked up from shale would not 

 be effective in grinding quartzite, but particles from 

 quartzite would act vigorously on most other rocks. The 

 abrasive action of pure ice is probably nil. The influence 

 of this factor is not easily formulated, but there can be no 

 question that it qualifies the influence of other factors in 

 important ways. 



The sculpture wrought by plucking differs notably 

 from that due to abrasion. The plucking of a block of 

 rock removes a projection and leaves a hollow. A sur- 

 face which has been reduced chiefly by plucking abounds 

 in salient and reentrant angles, and would be called 

 hackly if its pattern were smaller. Usually its salients, 

 and often its reentrants, are rounded by subsequent abra- 

 sion, producing a topography to which Saussure's title of 

 moutonnee is peculiarly applicable. 



The conditions which locally determine plucking rather 

 than abrasion are not clear to me. Evidence of plucking 

 is seen more frequently on hard rocks than on soft. The 



