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- 



