202 



ALASKA GLACIERS 



duce no crests and pinnacles, but remain as mere inter- 

 ruptions of a general plain surface until obliterated by 

 progressive ablation. 



This exceptional condition draws attention to the fact 

 that crevasse systems are normally underdrained. Look- 

 ing into crevasses on a warm day, 

 one may sometimes see the water 

 of ablation in slender rills disap- 

 pearing down tunnels or shafts in 

 FIG. ico. IDEAL SECTION OF the compact blue ice below to 

 WATER-FILLED CREVASSES AND be gathered doubtless in englacial 



TRUNCATED SERACS. . , - 



or subglacial streams, and eventu- 

 ally escape at the end of the glacier. 



Where the ice carries a heavy back-load of drift the 

 normal crevasse cycle is greatly modified (fig. 101) and 

 its completion indefinitely postponed. 

 As already mentioned in connection 

 with the Hidden Glacier, the drift 

 falls into the crevasses and gathers at 

 their bottoms. As the intervening 

 ice blocks acquire acute crests the 

 greater part of the drift rolls and 

 slides from them, and they retain only 

 enough to darken the surface. Under 

 the familiar law that a sprinkling or 

 thin cover of drift promotes melting, c 

 while a heavy mantle retards it, the 

 pinnacles are rapidly reduced, and 

 their sites are eventually depressed 

 below the drilt masses accumulated 

 in the crevasses. The original asperi- 

 ties made by the crevasses have now 

 been destroyed, but a secondary system has been evolved, 

 which tends in similar manner to produce a tertiary sys- 

 tem, and so on indefinitely. Wherever we found a broad 



FIG. 101. INFLUENCE 

 OF DRIFT ON SURFACE 

 CHARACTER OF GLACIER. 

 Crevasses formed in drift- 

 covered ice (a) accumulate the 

 drift (), and eventually be- 

 come the sites of hills (c). 



