THE WORK OF SNOW AND ICE. 



265 



crevasses may affect both the river-Hke part of a glacier and its deploying 

 end, and are the result of tension developed by movement within the 

 ice itself, to wliich, again, rivers offer no analogy. Somewhat similar 



V 





\ 











^ \ 



~~!j3 



h - ..; 



— v-f -^ . ■ ----^-^ _^^ 



.^^H 



Fig. 239.— Seracs of glacier. (Reid.) 



cracks develop in the outer crust of asphalt, when a mass of it is allowed 

 to stand and spread; but in this case there is evaporation of the vol- 

 atile ingredients, giving to the outer part relative rigidity and brittle- 

 ness, while the inner part remains more fluent. The analogy is therefore 

 not perfect and probably not really illustrative. The crevices may 

 be narrow or wide, and both narrow and wide may be found in the same 

 glacier. The narrow crevices that never open much are the most sig- 

 nificant, as they show that very Httle stretching is needed to satisfy 

 the tension. The opening of a gaping crevice is sometimes the work 

 of weeks, and, in the slow-moving glaciers of high latitudes, somelimes 

 the work of successive seasons. All this shows that the glacier is a 

 very brittle body, incapable of resisting even very moderate strains 

 brought to bear upon it very slowly. Had the ice even moderate due- 



