CREVASSE CYCLE 
201 
FIG. 98. 
LEVEL TRACT ON MUIR 
GLACIER. 
The rock retards the melting of the ice on 
which it rests, and thus preserves a pedestal. 
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¬ 
ing of a submerged peak produces breaking strains in the 
upper part of the ice stream, the smooth ice plain above, 
the prismoidal 
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 stantiallv US reDre- 
andseracs. J C J JA 
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. 
