Fig. 195—Curve of snow motion. Based on many observations of snow motion to show minimum thickness of snow required to move on a given gradient. Figures on the left represent thickness of snow in feet. The degrees represent the gradient of the surface. The gradients have been run in sequence down to 0° for the sake of completing the accompanying discussion. Obviously no glacially unmodified valley in a region of mountainous relief would start with so low a gradient, though glacial action would soon bring it into existence. Between +5° and -5° the curve is based on the gradients of nivated surfaces.
The foregoing readings of gradient and depth of snow are typical of a large number which were made in the Peruvian Andes and which have served as the basis of [195] . It will be observed that between 15° and 20° there is a marked change of function and again between +5° and -5° declivity, giving a double reversed curve. The meaning of the change between 15° and 20° is inferred to be that, with gradients over 20°, snow cannot wholly resist gravity in the presence of diurnal temperature changes across the freezing point and occasional snow or hail storms. With increase of thickness compacting appears to progress so rapidly as to permit the transfer of thrust for short distances before absorption of thrust takes place in the displaced snow. At 250 feet thorough compacting appears to take place, enabling the snow to move out under its own weight on even the faintest slopes; while, with a thickness still greater, the resulting névé may actually be forced up slight inclines whose declivity appears to approach 5° as a limit. I have nowhere been able to find in truly nivated areas reversed curves exceeding 5°, though it should be added that depressions whose leeward slopes were reversed to 2° and 3° are fairly common. If the curve were continued we should undoubtedly find it again turning to the left at the point where the thickness of the snow results in the transformation of snow to ice. From the sharp topographic break observed to occur in a narrow belt between the névé and the ice, it is inferred that the erosive power of the névé is to that of the ice as 2:4 or 1:5 for equal areas; and that reversed slopes of a declivity of 10° to 15° may be formed by glaciers is well known. Precisely what thickness of snow or névé is necessary and what physical conditions effect its transformation into ice are problems not included in the main theme of this chapter.
It is important that the proposed curve of snow motion under minimum conditions be tested under a large variety of circumstances. It may possibly be found that each climatic region requires its special modifications. In tropical mountains the sudden alternations of freezing and thawing may effect such a high degree of compactness in the snow that lower minimum gradients are required than in the case of mid-latitude mountains where the perpetual snow of the high and cold situations is compacted through its own weight. Observations of the character introduced here are still unattainable, however. It is hoped that they will rapidly increase as their significance becomes apparent; and that they have high significance the striking nature of the curve of motion seems clearly to establish.
BERGSCHRUNDS AND CIRQUES
The facts brought out by the curve of snow-motion ([Fig. 195]) have an immediate bearing on the development of cirques, whose precise mode of origin and development have long been in doubt. Without reviewing the arguments upon which the various hypotheses rest, we shall begin at once with the strongest explanation—W. D. Johnson’s famous bergschrund hypothesis. The critical condition of this hypothesis is the diurnal migration across the freezing point of the air temperature at the bottom of the schrund. Alternate freezing and thawing of the water in the joints of the rock to which the schrund leads, exercise a quarrying effect upon the rock and, since this effect is assumed to take place at the foot of the cirque, the result is a steady retreat of the steep cirque wall through basal sapping.
While Johnson’s hypothesis has gained wide acceptance and is by many regarded as the final solution of the cirque problem it has several weaknesses in its present form. In fact, I believe it is but one of two factors of equal importance. In the first place, as A. C. Andrews[62] has pointed out, it is extremely improbable that the bergschrund of glacial times under the conditions of a greater volume of snow could have penetrated to bedrock at the base of the cirque where the present change of slope takes place. In the second place, the assumption is untenable that the bergschrund in all cases reaches to or anywhere near the foot of the cirque wall. A third condition outside the hypothesis and contradictory to it is the absence of a bergschrund in snowfields at many valleys heads where cirques are well developed!