GLACIER MOTION 107 



A glacier moves in much the same way as a river, but at a very 

 much slower rate. The centre moves faster than the sides, because 

 the latter are retarded by the friction of the banks, and, for the 

 same reason, the top moves faster than the bottom. While be- 

 having like a plastic substance under pressure, ice yields readily 

 to strain, and even a slight change in the slope of the bed will 

 cause a great transverse crack, or ci'evasse, to form, which, like an 



FIG. 34. — Crevasse in a glacier, partly concealed by a snow-bridge. 



eddy in a stream, seems to be stationary, because always formed 

 again at the same spot. Other systems of cracks, the marginal 

 crevasses, are formed along the sides of the glacier, and are due 

 to the more swiftly moving centre pulling away from the retarded 

 sides. 



The rate of glacier movement depends upon the snow supply, 

 upon the slope of the ground, and the temperature of the season. 

 The comparatively small glaciers of the Alps move at rates varying 

 from two to fifty inches per day in summer and at about half that 



