We have then an inequality in amount of accumulated material to be explained by either an inequality in the extent of the snow and therefore an inequality of snow action, or an inequality due to the presence of ice in one valley and not in the other, or by both. It is at once clear that if ice is absent above (A) and the mountain slopes are recessed that snow action is responsible for it. It is also recognized that whatever rate of denudation be assigned to the snow-free surfaces this rate must be exceeded by the rate of snow action, else the inequalities of slope would be decreased rather than increased. The accumulated material at (A) is, therefore, partly but not chiefly due to denudation of snow-free surfaces. It is due chiefly to erosion beneath the snow. Nor can it be argued that the hollows now occupied by snow were formed at some past time when ice not snow lay in them. They are not ice-made hollows for they are on a steep spur above the limits of ice action even in the glacial period. Any past action is, therefore, represented here in kind by present action, though there would be differences in degree because the heavier snows of the past were displaced by the lighter snows of today.
While it appears that the case presents clear proof of degradation by snow it is not so clear how these results were accomplished. Real abrasion on a large scale as in bowlder-shod glaciers is ruled out, since glacial striæ are wholly absent from nivated surfaces according to both Matthes’ observations and my own. Yet all nivated surfaces have very distinctive qualities, delicately organized slopes which show a marked change from any original condition related to water-carving. In the absence of striæ, the general absence of all but a thin coating of waste even in rock hollows, and the accumulation of waste up to bowlders in size at the lower edge of the nivated zone, I conclude that compacted snow or névé of sufficient thickness and gradient may actually pluck rock outcrops in the same manner though not at the rate which ice exhibits. That the products of nivation may be bowlders as well as fine mud would seem clearly to follow increase in effectiveness, due to increase in amount of the accumulated snow; that bowlders are actually transported by snow is also shown by their presence on the lower margins of nivated tracts.
Our argument may be made clearer by reference to the observed action of snow in a particular valley. Snow is shed from the higher, steeper slopes to the lower slopes and eventually accumulates to a marked degree on the bottoms of the depressions, whence it is avalanched down valley over a series of irregular steps on the valley floor. An avalanche takes place through the breaking of a section of snow just as an iceberg breaks off the end of a tide-water glacier. Evidently there must be pressure from behind which crowds the snow forward and precipitates it to a lower level.
As a snow mass falls it not only becomes more consolidated, beginning at the plane of impact, but also gives a shock to the mass upon which it falls that either starts it in motion or accelerates its rate of motion. The action must therefore be accompanied by a drag upon the floor and if the rock be close-jointed and the blocks, defined by the joint planes, small enough, they will be transported. Since snow is not so compact as ice and permits included blocks easily to adjust themselves to new resistances, we should expect the detached blocks included in the snow to change their position constantly and to form irregular scratches, but not parallel striæ of the sort confidently attributed to stone-shod ice.
It is to the plasticity of snow that we may look for an explanation of the smooth-contoured appearance of the landscape in the foreground of Fig. 135. The smoothly curved lines are best developed where the entire surface was covered with snow, as in mid-elevations in the larger snowfields. At higher elevations, where the relief is sharper, the snow is shed from the steeper declivities and collected in the minor basins and valley heads, where its action tends to smooth a floor of limited area, while snow-free surfaces retain all their original irregularities of form or are actually sharpened.
The degree of effectiveness of snow and névé action may be estimated from the reversed slopes now marked by ponds or small marshy tracts scattered throughout the former névé fields, and the many niched hollows. They are developed above Pampaconas in an admirable manner, though their most perfect and general development is in the summit belt of the Cordillera Vilcapampa between Arma and Choquetira, [135] . It is notable in all cases where nivation was associated with the work of valley glaciers that the rounded nivated slopes break rather sharply with the steep slopes that define an inner valley, whose form takes on the flat floor and under-cut marginal walls normal to valley glaciation.
A classification of numerous observations in the Cordillera Vilcapampa and in the Maritime Cordillera between Lambrama and Antabamba may now be presented as the basis for a tentative expression of the law of variation respecting snow motion. The statement of the law should be prefaced by the remark that thorough checking is required under a wider range of conditions before we accept the law as final. Near the lower border of the snow where rain and hail and alternate freezing and thawing take place, the snow is compacted even though but fifteen to twenty feet thick, and appears to have a down-grade movement and to exercise a slight drag upon its floor when the gradient does not fall below 20°. Distinct evidences of nivation were observed on slopes with a declivity of 5° near summit areas of past glacial action, where the snow did not have an opportunity to be alternately frozen and thawed.
The thickness of the former snow cover could, however, not be accurately determined, but was estimated from the topographic surroundings to have been at least several hundred feet. Upon a 40° slope a snow mass 50 feet thick was observed to be breaking off at a cliff-face along the entire cross-section as if impelled forward by thrust, and to be carrying a small amount of waste—enough distinctly to discolor the lowermost layers—which was shed upon the snowy masses below. With increase in the degree of compactness of the snow at successively lower elevations along a line of snow discharge, gradients down to 25° were still observed to carry strongly crevassed, waste-laden snow down to the melting border. It appeared from the clear evidences of vigorous action—the accumulation of waste, the strong crevassing, the stream-like character of the discharging snow, and the pronounced topographic depression in which it lay—that much flatter gradients would serve, possibly not more than 15°, for a snow mass 150 feet wide, 30 to 40 feet thick, and serving as the outlet for a set of tributary slopes about a square mile in area and with declivities ranging from small precipices to slopes of 30°.
We may say, therefore, that the factors affecting the rate of motion are (1) thickness, (2) degree of compactness, (3) diurnal temperature changes, and (4) gradient. Among these, diurnal temperature changes operate indirectly by making the snow more compact and also by inducing motion directly. At higher elevations above the snowline, temperature changes play a decreasingly important part. The thickness required varies inversely as the gradient, and upon a 20° slope is 20 feet for wet and compact snow subjected to alternate freezing and thawing. For dry snow masses above the zone of effective diurnal temperature changes, an increasing gradient is required. With a gradient of 40°, less than 50 feet of snow will move en masse if moderately compacted under its own weight; if further compacted by impact of falling masses from above, the required thickness may diminish to 40 feet and the required declivity to 15°. The gradient may decrease to 0° or actually be reversed and motion still continue provided the compacting snow approach true névé or even glacier ice as a limit.
From the sharp topographic break between the truly glaciated portions of the valley in regions subjected to temporary glaciation, it is concluded that the eroding power of the moving mass is suddenly increased at the point where névé is finally transformed into true ice. This transformation must be assumed to take place suddenly to account for so sudden a change of function as the topographic break requires. Below the point at which the transformation occurs the motion takes place under a new set of conditions whose laws have already been formulated by students of glaciology.