THE WORK OF GLACIERS 



*57 



Erosion by Mountain Glaciers 



Plucking and Abrasion. — Glaciers accomplish their work of erosion 

 in two ways, (i) The ice secures a hold on the material of its bed 

 either by freezing about projecting points of rock or by being pressed 

 into the joints and other cracks by its great weight. As the glacier 

 moves on rock fragments are pulled out and carried along. This 

 process is called plucking, and by it a glacier may remove a great 

 quantity of material in much-jointed rock. The process, however, 

 is of little effect on 

 rocks which have few 

 joints ; and conse- 

 quently one some- 

 times finds that a 

 glacier has been able 

 to deepen its valley 

 more easily in hard 

 granite and gneiss 

 than in the softer 

 limestone, because 

 the former were 

 much fractured, per- 

 mitting the plucking 

 out of blocks, while 

 the latter being less 

 broken was little 

 affected. (2) Glaciers 

 also deepen and widen their valleys by abrasion. The tools which 

 accomplish the work of abrasion are the rocks which have been 

 torn from the bed by plucking and those which have reached 

 the base of the ice from the surface through crevasses. Hold- 

 ing these rock fragments in a firm grasp and pressing with great 

 force, estimated to be 48,600 pounds to the square yard in portions 

 of the Aar Glacier, a glacier acts as a gigantic file, cutting down pro- 

 jecting points, and deepening and smoothing its bed. The hard 

 pebbles scratch and the bowlders groove the bedrock ; while the clay 

 and rock, ground fine by the process, polish the surface, producing the 

 smoothed and striated appearance so characteristic of glaciated rocks. 



It will readily be seen that a thick glacier, because of its great 

 weight, will be able to erode its bed more rapidly than a thin one, 



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Fig. 142. — Roche moutonnee, Bronx Park, City of 

 New York. (U. S. Geol. Surv.) 



