CREVASSE CYCLE 



201 



FIG. 98. LEVEL TRACT ON MUIR 



GLACIER. 

 The rock retards the melting of the ice on 



for the features of the new cycle thus instituted at first 

 combine with those of the old and eventually supplant 

 them. 



As my excursions on glaciers were all short, it hap- 

 pened that I never saw a 

 complete illustration of the 

 ablation cycle on one gla- 

 cier, but such examples 

 should be readily discover- 

 able. Where the even bed 

 of a glacier increases its 

 grade, where it is interrupted 

 by a step, producing a cas- 

 cade, or where the overrid- * sts>aE thus preserves a pedestaL 

 ing of a submerged peak produces breaking strains in the 

 upper part of the ice stream, the smooth ice plain above, 



the pr is mo idal 

 blocks, the acute 

 peaks, the gradually 

 subsiding waves, and 

 the final ice plain 

 should appear in reg- 

 ular sequence, sub- 



Illustrating the formation and obliteration of crevasses e fot-ftolli7 oo t-^r^o 

 and seracs ol.ciiiLia.LLy d.a I epic 



sented in figure 99. 



An exceptional condition was observed on the tongue of 

 the Columbia Glacier which flows into the western embay- 

 ment of its valley. A plain surface was there interrupted 

 by an extensive plexus of crevasses, which were filled to 

 the brim with water and snow (fig. 100). The normal 

 cycle appeared to be varied in this case by a lack of 

 drainage, surface ablation progressing only where the air 

 had access, and truncating the seracs down to the water- 

 line. It is conceivable that under such conditions a set 

 of crevasses originating from horizontal stresses may pro- 



FIG. 99. 



IDEAL PROFILE AND SECTION 

 OF A GLACIER. 



