240 



GEOLOGY 



water, on the other hand, has little effect on the flow of a river so 

 long as it remains unfrozen; but the effect of temperature on the 

 motion of ice is important. In many cases ,indeed, the temperature, 



together with the water that is 

 incidental to it, seems to be 

 the chief factor in determining 

 the rate of movement. The 

 way in which its effects are felt 

 will be discussed later. 



From Fig. 196 it will be seen 

 that a valley glacier is an elon- 

 gate body of ice, following the 

 curves of the valley in stream- 

 like fashion. It has its origin 

 in the snows collected on the 

 mountain heights, and it works 

 its way down the valley in u 

 manner which, in the aggre- 

 gate, is similar to the move- 

 ment of a stiff liquid. The 

 likeness to a river extends to 

 many details. Not only does 

 the center move faster than 

 the sides, and the upper part 

 faster than the bottom, as in 

 the case of streams, but the 

 movement is more rapid in 

 narrow parts of the valley and 

 slower in the broader parts. These and other likenesses, some of 

 which are apparent rather than real, have given rise to the view 

 that glacier ice moves like a stiff viscous liquid. 



But while the points of likeness between glaciers and rivers arc 

 several, their differences are at least equally numerous and signif- 

 icant. Though the trains of debris on the surface (the dark bands 

 in Fig. 196) pass nearer the projecting points of the valley walls 

 and farther from the receding bends like the central currents of 

 streams, they do not conform in detail to the course of the windin<i 



Fig. 196. 



Aletsch Glacier, Switzer- 

 land. 



