200 



PHYSICAL GEOLOGY 



from the thinner snows of the portion above. This fissure, or 

 zone of fissures, where the glacier proper is sometimes con- 

 sidered as beginning, is called the bergschrund (Fig. 208). The 



Fia. 208. Bergschrund on east side of Fremont Peak, Wind River Range, 

 Wyoming. (Baker.) 



upper walls of crevasses formed in these or other ways, being 

 more exposed to the sun and weather than the lower walls, 

 melt faster, so that the openings often become conspicuously 

 V-shaped, and are separated by a complex of crests and sharp 

 ridges. Were it not for melting follow- 

 ing cracking, most crevasses extending 

 crosswise of a glacier would probably 

 be closed by its onward movement. 

 Rock debris weathered from the slopes 

 above may accumulate in quantity on 



FIG. 209. -Rock-capped the ice - If such fragments are too 

 ice pillars. The rock thick to be heated through in the course 



S'ete'o^h!;"^! of d^. they protect the ice beneath. 



and the melting away The surrounding ice melting mean- 



while > the y COme to stand On C lumilS 

 of ice (Fig. 209). Thin deposits of 



earthy matter such, for example, as wind-deposited dust, have 

 an opposite effect. Dust absorbs heat faster than ice does, 

 and thin deposits, heating through readily in the course of a 



