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 



