The meaning of this feature is represented in [199] , in which three successive stages in cirque development are shown. In A, as displayed in small valleys or mountainside alcoves which were but temporarily occupied by snow and ice, or as in all higher valleys during the earlier stages of the advancing hemicycle of glaciation, snow collects, a short glacier forms, and a bergschrund develops. As a result of the concentrated frost action at the base of the bergschrund a rapid deepening and steepening takes place at a. As long as the depth of snow (or snow and névé) is slight the bergschrund may remain open. But its existence at this particular point is endangered as the cirque grows, since the increasing steepness of the slope results in more rapid snow movement. Greater depth of snow goes hand in hand with increasing steepness and thus favors the formation of névé and even ice at the bottom of the moving mass and a constantly accelerated rate of motion. At the same time the bergschrund should appear higher up for an independent reason, namely, that it tends to form between a mass of slight movement and one of greater movement, which change of function, as already pointed out, would appear to be controlled by change from snow to névé or ice on the part of the bottom material.
The first stages in the upward migration of the bergschrund will not effect a marked change from the original profile, since the converging slopes, the great thickness of névé and ice at this point, and the steep gradient all favor powerful erosion. When, however, stage C is reached, and the bergschrund has retreated to c″, a broader terrace results below the schrundline, the gradient is decreased, the ice and névé (since they represent a constant discharge) are spread over a greater area, hence are thinner, and we have the cirque taking on a compound character with a lower, less steep and an upper, precipitous section.
It is clear that a closely jointed and fragile rock might be quarried by moving ice at c′-c″ and the cirque wall extended unbroken to x; it is equally clear that a homogeneous, unjointed granite would offer no opportunities for glacial plucking and would powerfully resist the much slower process of abrasion. Thus Gilbert[64] observed the schrundline in the granites of the Sierra Nevada, which are “in large part structureless” and my own observations show the schrundline well developed in the open-jointed granites of the Cordillera Vilcapampa and wholly absent in the volcanoes of the Maritime Cordillera, where ashes and cinders, the late products of volcanic action, form the easily eroded walls of the steep cones. Somewhere between these extremes—lack of a variety of observations prevents our saying where—the resistance and the internal structure of the rock will just permit a cirque wall to extend from x to c′ ″ of [199] .
A common feature of cirques that finds an explanation in the proposed hypothesis is the notch that commonly occurs at some point where a convergence of slopes above the main cirque wall concentrates snow discharge. It is proposed to call this type the notched cirque. It is highly significant that these notches are commonly marked by even steeper descents at the point of discharge into the main cirque than the remaining portion of the cirque wall, even when the discharge was from a very small basin and in the form of snow or at the most névé. The excess of discharge at a point on the basin rim ought to produce the form we find there under the conditions of snow motion outlined in earlier paragraphs. It is also noteworthy that it is at such a point of concentrated discharge that crevasses no sooner open than they are closed by the advancing snow masses. To my mind the whole action is eminently representative of the action taking place elsewhere along the cirque wall on a smaller scale.
What seems a good test of the explanation of cirques here proposed was made in those localities in the Maritime Cordillera, where large snowbanks but not glaciers affect the form of the catchment basins. A typical case is shown in [201] . As in many other cases we have here a great lava plateau broken frequently by volcanic cones of variable composition. Some are of lava, others consist of ashes, still others of tuff and lava and ashes. At lower elevations on the east, as at 16,000 feet between Antabamba and Huancarama, evidences of long and powerful glaciers are both numerous and convincing. But as we rise still higher the glaciated topography is buried progressively deeper under the varying products of volcanic action, until finally at the summit of the lava fields all evidences of glaciation disappear in the greater part of the country between Huancarama and the main divide. Nevertheless, the summit forms are in many cases as significantly altered as if they had been molded by ice. Precipitous cirque walls surround a snow-filled amphitheater, and the process of deepening goes forward under one’s eyes. No moraines block the basin outlets, no U-shaped valleys lead forward from them. We have here to do with post-glacial action pure and simple, the volcanoes having been formed since the close of the Pleistocene.
Likewise in the pass on the main divide, the perpetual snow has begun the recessing of the very recent volcanoes bordering the pass. The products of snow action, muds and sands up to very coarse gravel, glaciated in texture with an intermingling of blocks up to six inches in diameter in the steeper places, are collected into considerable masses at the snowline, where they form broad sheets of waste so boggy as to be impassable except by carefully selected routes. No ice action whatever is visible below the snowline and the snow itself, though wet and compact, is not underlain by ice. Yet the process of hollowing goes forward visibly and in time will produce serrate forms. In neither case is there the faintest sign of a bergschrund; the gradients seem so well adjusted to the thickness and rate of movement of the snow from point to point that the marginal crack found in many snowfields is absent.
The absence of bergschrunds is also noteworthy in many localities where formerly glaciation took place. This is notoriously the case in the summit zone of the Cordillera Vilcapampa, where the accumulating snows of the steep cirque walls tumble down hundreds of feet to gather into prodigious snowbanks or to form névé fields or glaciers. From the converging walls the snowfalls keep up an intermittent bombardment of the lower central snow masses. It is safe to say that if by magic a bergschrund could be opened on the instant, it would be closed almost immediately by the impetus supplied by the falling snow masses. The explanation appears to be that the thicker snow and névé concentrated at the bottom of the cirque results in a corresponding concentration of action and effect; and cirque development goes on without reference to a bergschrund. The chief attraction of the bergschrund hypothesis lies in the concentration of action at the foot of the cirque wall. But in the thickening of the snow far beyond the minimum thickness required for motion at the base of the cirque wall and its change of function with transformation into névé, we need invoke no other agent. If a bergschrund forms, its action may take place at the foot of the cirque wall or high up on the wall, and yet sapping at the foot of the wall continue.
