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 
rnoutonnee 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 
