CREVASSE CYCLE 
I 99 
only moderate depth, the ice being welded into a practi¬ 
cally continuous mass beneath. Wherever a crevassed 
tract was exhibited in section in a tidal cliff, the crevasses 
were seen to terminate uniformly along a definite zone, 
which was usually nearer the top of the cliff than its base. 
The depth of this zone was estimated in different instances 
at from 50 to 125 feet, but this did not represent the full 
original depth of the crevasses, as something had in every 
case been lost by ablation (fig. 94). 
As soon as the cracks are opened, melting begins on 
FIG. 95. CREVASSES AND SERACS, MUIR GLACIER. 
their faces, the rate being greatest above, and the flat 
tops of the ice blocks, the seracs of alpinists (fig. 95), are 
converted into roof-like crests and pinnacles. In this 
condition the surface is nearly or quite impassable (fig. 
96). With continuance of ablation the height of the 
seracs is reduced, their slopes become less steep, and 
many connecting cross ridges become available to the 
wayfarer, so that with the exercise of care and patience 
one can make his way safely in any direction. Figure 
97 shows a characteristic field of this sort, crossed by 
some of our party on their way to the great nunatak of the 
Columbia Glacier. A continuance of the same reduction 
by ablation eventually obliterates the ice waves alto- 
