574 



WILLARD D. JOHNSON 



vantage, for close observation, of having to clamber over these, with 

 a candle, in a dripping rain, but there seemed to be definitely pre- 

 sented a line of glacier base, removed from five to ten feet from the 

 foot of what was here a hterally vertical cliff. 



The glacier side of the crevasse presented the more clearly defined 

 wall. The rock face, though hard and undecayed, was much riven, 

 its fracture planes outhning sharply angular masses in all ^stages of 

 displacement and dislodgment. Several blocks were tipped forward 

 and rested against the opposite wall of ice; others, quite removed 

 across the gap, were incorporated in the glacier mass at its base. 

 Icicles of great size, and stalagmitic masses, were abundant; the 

 fallen blocks in large part were ice- sheeted; and open seams in the 

 cliff face held films of this clear ice. Melting was everywhere in 

 progress, and the films or thin plates in the seams were easily remov- 

 able. 



These thinning plates, especially, were demonstrative of alternate 

 freezings and thawings, in short-time intervals, probably diurnal. 

 Without, upon the cirque or amphitheater wall, above the glacier, 

 such intervals would be seasonal. Thus, apparently, to generahze 

 from observation at a single point, the arc of the bergschrund foot, 

 and the coincident arc of the cirque-wall foot, is a narrow zone of 

 relatively vigorous frost-weathering. The glacier is a cover, pro- 

 tective of the rock surface beneath it against changes of temperature. 

 Probably the bed temperature does not fall below that of melting ice. 

 Hence, if (in summer) the bed at the wall foot is exposed, through 

 the open bergschrund, to daily temperature changes across the freezing- 

 point, frost-weathering must be sharply localized. The glacier will 

 be efficient as the agent for debris removal; the result, therefore, 

 must be quarrying and excavation, and basal sapping. 



The amphitheater floor has been described as characteristically 

 reversed in grade, though at a slight angle, and ponded backward 

 against its head wall. It may be assumed that the disrupting action 

 of frost at the bergschrund foot is directed against the floor, as well 

 as against the cliff. Likely enough, also, the glacier is a highly 

 efficient agent for removal, supplementing, by "plucking," the 

 initial rupturing work of frost. Its plucking action may be directed 

 downward as well as backward; but downward action, at an early 



