1 54 



PHYSICAL GEOLOGY 



however, show a simultaneous forward movement. The reason for 

 this is to be found in the size of the glaciers. It was discovered that 

 the smaller glaciers advanced more quickly and had, indeed, in 1909 

 already passed completely through the period of advance, while the 

 long glaciers were at that date (1909) either still advancing or merely 

 beginning to advance. 1 



Transportation of Mountain Glaciers 



Surface Moraines. — We have seen (p. 29) that in regions where 

 frost is an active agent of the weather, talus, composed of angular rock 



^ fragments of various sizes, 

 rests at the bases of cliffs. 

 If the bottom of a valley 

 with high and steep sides is 

 occupied by a glacier, these 

 fragments will fall upon its 

 surface ; and as the ice moves 

 on all parts of the glacier's 

 side, will pass under the cliffs 

 which supply the debris. In 

 the process of time, a regular 

 ridge of angular rocks and 

 soil will rest upon the edge of 

 the ice. Such a deposit is 

 called a lateral moraine (Figs. 



138, 139, 140). The ice of 

 glaciers does not commingle 

 at their confluence, but the 

 masses move on side by side, 

 the two adjacent lateral 

 moraines uniting to form a 

 medial moraine (Figs. 138, 



139, 140). In this way, by 

 the union of several branches, a glacier may be covered with several 

 medial moraines. A medial moraine may also be formed when a 

 glacier passes over an elevation in its bed from which it scrapes off 

 rock fragments. After the glacier has passed this point the debris 



1 Physiography and Glacial Geology of the Yakulal Bay Region, Professional Paper 

 No. 64, U. S. Geol. Surv., 1909. 



Fig. 138. — Map showing the formation of 

 the Mer de Glace by the union of several gla- 

 ciers. The development of lateral and medial 

 moraines is illustrated also. 



