134 



WORK OF SNOW AND ICE 



and retreat at intervals 1 , and the periods of advance follow a suc- 

 cession of years when the snowfall was heavy and the temperature 

 low, while the periods of retreat follow years when the snowfall 



was light and the temperature 

 above normal. The periods of 

 advance and retreat lag behind 

 the periods of heavy and light 

 snowfall, respectively, by some 

 years, and a long glacier re- 

 sponds less promptly than a 

 short one. 



Likenesses and unlike- 

 nesses of glaciers and rivers. 

 Slope, roughness of bed, and 

 volume affect the movement of 

 glaciers somewhat as they af- 

 fect the movement of rivers. 

 The temperature of water, on 

 the other hand, has little effect 

 on its flow, so long as it remains 

 unfrozen; but the effect of 

 temperature on the motion of 

 ice is important. In many 

 cases, indeed, the tempera- 

 ature, together with the water 

 that is incidental to it, seems to 

 be the chief factor in determin- 

 ing its rate of movement. Its 

 Fig. 136. Aletsch Glacier, Switzerland. effects will be discussed later. 



From Fig. 136 it will be seen that a valley glacier is an elongate 

 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 a manner which, in the aggre- 

 gate, is similar to the movement 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 the narrow parts of the valley and slower in the broader. These 



1 Reid. Variations of Glaciers. Occasional articles in Journal of Geology, 

 Vol. Ill and later volumes. 



