CHAPTER XXVIII 



THE GLACIER'S SURFACE FEATURES AND THE 

 DEPOSITS UPON ITS BED 



The glacier flow. The downward flow of the ice within a 

 mountain glacier has been the subject of many investigations and 

 the topic of many heated discussions since the time when Louis 

 Agassiz and his companions set a line of stakes across the Aar 

 glacier and numbered the surface bowlders in 

 L preparation for repeated observations. Their 



first observation was that the line of stakes, 

 which had run straight across the glacier, was 

 distorted into a curve which was convex down- 

 stream (Fig. 461, A'), thus showing that the 

 surface layers have more rapid motion in pro- 

 portion as they are distant from the side mar- 

 gins. Summarizing these and later studies, it 

 may be stated that the glacier increases its rate 

 of motion from its side margin towards its cen- 

 ter line, from its bed upwards towards its sur- 

 face, and below the neve the velocity is greatest 

 where the fall is greatest and also wherever the 

 cross section diminishes. In all these particu- 

 lars, then, the ice of the glacier behaves like a 

 stream of water. The average rate of flow of 

 Alpine glaciers varies from a few inches to a few 

 feet per day, and is greater during the warm 

 summer season. The Muir glacier of Alaska 

 has been shown to move at the rate of about 

 seven feet per day. 



In traveling from the neVe* downward to the 

 glacier foot, the snow not only changes into 

 ice, but it undergoes a granulating process with continued increase 

 in the size of the nodules until at the foot of the glacier these may 



390 



FIG. 416. Diagram 

 to illustrate the mi- 

 grations of lines of 

 stakes crossing a 

 glacier, due to its 

 surface movement, 

 A, original position 

 of lines ; A', later 

 positions ; a and a', 

 original and dis- 

 torted forms of a 

 square section of 

 the glacier surface 

 near its margin ; r, 

 r', diagonal ere- 



