MATURITY IN ALPINE GLACIAL EROSION 573 



that of abrasion. Abrasion accomplishes deepening vertically and 

 directly. In the case of a "continental" glacier upon a level plain, 

 abrasion would be operative alone. But that process was not to be 

 invoked in explanation of the scalloped, tabular forms of the High 

 Sierra; these pointed only to basal sapping. 



Basal sapping in its details, however, was unintelligible. It was 

 not immediately apparent, at least, how a glacier, originating against 

 a precipitous rock slope, and drawing away from that slope, could 

 undercut and cause it to recede. 



To return to the narrative form, one feature of the small ice-body 

 lying deep in a great amphitheater opening northward from Mount 

 Lyell — one of perhaps a dozen "glacierets" of the High Sierra — 

 seemed to offer the explanation. 



Among the numerous crevasses or schrunds of several diverse 

 systems sharply hning the snowy surface upon which I looked directly 

 down as upon a map, one master opening, the Bergschrund of the 

 Swiss mountaineers, paralleled the amphitheater wall, a little out 

 upon the ice. In detail it was ragged and splintered, but its general 

 effect was that of a symmetrical great arc. I had already in mind, 

 vaguely formulated, the working hypothesis that the glacier makes 

 the amphitheater; that it is not by accident that glaciated mountains, 

 and such mountains only, abound in forms peculiarly favorable to 

 hesLYj snow-drift accumulation. My instant surmise, therefore, 

 was that this curving great schrund penetrated to the foot of the wall, 

 or precipitous rock slope, and that a causal relation determined the 

 coincidence in position of the hne of deep crevassing and the line of 

 the assumed basal undercutting. 



So much of assumption, so plausibly grounded, rendered direct 

 observation at this critical point on the glacier floor compellingly 

 desirable ; and, returning to camp for all hands and the pack ropes, the 

 rather appalling task was in fact very easily accomplished. 



The depth of descent was about one hundred and fifty feet. In 

 the last twenty or thirty feet, rock replaced ice in the up-canyon wall. 

 The schrund opened to the cliff foot. I cannot say that the floor 

 there was of sound rock, or that it was level; but there was a floor to 

 stand upon, and not a steeply incHned talus. It was somewhat cum- 

 bered with blocks, both of ice and of rock; and I was at the disad- 



