GLACIAL SCULPTURE 
203 
tract of ice heavily loaded with drift, its surface was made 
up of hummocks and hollows, varied here and there by 
cliffs of black ice, down which pebbles and boulders oc¬ 
casionally rolled in shifting their position from hill to 
hollow. These irregularities, involving, as they do, in¬ 
equality in the distribution of the drift on the surface of 
the ice, help to account for the irregularity observed in 
terminal moraines. 
GLACIAL SCULPTURE 
The work of rock sculpture accomplished by the middle 
and lower parts of a glacier is performed chiefly by the pro¬ 
cesses of abrasion and plucking. In abrasion, fragments of 
rock held in the under part of the ice, being dragged over 
the fixed rock of the glacier bed, file away and reduce it. 
In plucking, blocks of bed-rock, being partly surrounded 
by the ice, are forced from their bearings and rolled or 
slidden forward. If the plucked blocks have originally 
stood as projections, they may be broken away, even if 
quite firm and flawless; otherwise it is probable that they 
can be removed only if originally separated by joints or 
other structural partings. The waste resulting from abra¬ 
sion is clay and sand; plucking yields boulders. 
One of the chief factors on which the rate of abrasion 
depends is the velocity of the moving ice. If the bed¬ 
rock surface is uneven, the ice does not flow over all parts 
of it at the same rate, but moves slower in the hollows 
and faster across the prominences. This difference results 
partly from the condition of continuity, which demands 
higher average speed where the cross-section is less, and 
partly from the tendency of parts embayed in hollows to 
lag behind the general mass. The prominences are there¬ 
fore abraded more rapidly than the adjacent hollows, and 
the profile is thus reduced to simple forms. 
Another factor on which rate of abrasion depends is 
