GLACIERS 



199 



The exact nature of the movement of glacier ice is a mooted 

 question. The effect, so far as the form of the glacier is con- 

 cerned, is much the same as in the movement of a thick mass of 

 tar or wax. It is doubt- 

 ful, however, if the mo- 

 tion is flowage. 



Lower limits of gla- 

 ciers. Glaciers descend 

 from their parent snow 

 fields to a level so low 

 and so warm that the 

 wastage of the ice bal- 

 ances its forward move- 



FIG. 206. Muir Glacier, Alaska. 



ment. Many large gla- 

 ciers reach far below the 

 snow line ; some of those in Switzerland end near grain fields 

 and orchards. In high latitudes glaciers may reach the sea 

 (Fig. 204). Turbid streams, fed by the melting ice, flow 

 from the lower ends of many valley glaciers (Fig. 202). 



Character of the surface of valley glaciers. The surfaces 

 of valley glaciers are in many cases notably irregular (Figs. 

 205 and 206). Varying in compactness, the surface ice 

 melts unevenly. Changes in the slope of the surface down 

 which the glacier moves cause the ice to crack open (Fig. 207). 



Where steep or precipi- 

 tous descents occur in the 

 bed, icefalls correspond- 

 ing to waterfalls in rivers 

 form, and the ice is often 

 shattered by a multitude 

 of cracks. Great cracks 

 (crevasses') may be formed 

 also by the more rapid 

 motion of the center of the glacier, as compared with the sides. 

 One or more crevasses, often large, sometimes form where the 

 neve of the lower part of the parent snow field moves away 



FIG. 207. Portion of a glacier, showing 

 crevasses in the ice due to changes in the 

 slope of the bed. 



