Fig. 193—Panta Mountain and its glacier system. The talus-covered mass in the center (B) is a terminal moraine topped by the dirt-stained glacier that descends from the crest. The separate glaciers were formerly united to form a huge ice tongue that truncated the lateral spurs and flattened the valley floor. One of its former stages is shown by the terminal moraine in the middle distance, breached by a stream, and impounding a lake not visible from this point of view.
Fig. 194—Recessed southern slopes of volcanoes whose northern slopes are practically without glacial modifications. Summit of the lava plateau, Maritime Cordillera, western Peru, between Antabamba and Cotahuasi.
Test cases are presented in Figs. 191, 192, and 193, Cordillera Vilcapampa, for the determination of the fact of the movement of the snow long before it has reached the thickness Matthes or Hobbs believes necessary for a movement of translation to begin. [191] shows snow masses occupying pockets on the slope of a ridge that was never covered with ice. Past glacial action with its complicating effects is, therefore, excluded and we have to deal with snow action pure and simple. The pre-glacial surface with smoothly contoured slopes is recessed in a noteworthy way from the ridge crest to the snowline of the glacial period at least a thousand feet lower. The recesses of the figure are peculiar in that not even the largest of them involve the entire surface from top to bottom; they are of small size and are scattered over the entire slope. This is believed to be due to the fact that they represent the limits of variations of the snowline in short cycles. Below them as far as the snowline of the glacial period are larger recesses, some of which are terminated by masses of waste as extensive as the neighboring moraines, but disposed in irregular scallops along the borders of the ridges or mountain slopes in which the recesses have been found.
The material accumulated at the lower limit of the snow cover of the glacial period was derived from two sources: (1) from slopes and cliffs overlooking the snow, (2) from beneath the snow by a process akin to ice plucking and abrasion. The first process is well known and resembles the shedding of waste upon a valley glacier or a névé field from the bordering cliffs and slopes. Material derived in this manner in many places rolls down a long incline of snow and comes to rest at the foot of it as a fringe of talus. The snow is in this case but a substitute for a normal mass of talus. The second process produces its most clearly recognizable effects on slopes exceeding a declivity of 20°; and upon 30° and 40° slopes its action is as well-defined as true glacial action which it imitates. It appears to operate in its simplest form as if independent of the mass of the snow, small and large snow patches showing essentially the same results. This is the reverse of Matthes’ conclusion, since he says that though the minimum thickness “must vary inversely with the percentage of the grade,” “the influence of the grade is inconsiderable,” and that the law of variation must depend upon additional observation.[61]
Let us examine a number of details and the argument based upon them and see if it is not possible to frame a satisfactory law of variation.
In [193] the chief conditions of the problem are set forth. Forward from the right-hand peak are snow masses descending to the head of a talus (A) whose outlines are clearly defined by freshly fallen snow. At (B) is a glacier whose tributaries descend the middle and left slopes of the picture after making a descent from slopes several thousand feet higher and not visible in this view. The line beneath the glacier marks the top of the moraine it has built up. Moraines farther down valley show a former greater extent of the glacier. Clearly the talus material at (A) was accumulated after the ice had retreated to its present position. It will be readily seen from an inspection of the photograph that the total amount of material at (A) is an appreciable fraction of that in the moraine. The ratio appears to be about 1:8 or 1:10. I have estimated that the total area of snow-free surface about the snowfields of the one is to that of the other as 2:3. The gradients are roughly equivalent, but the volume of snow in the one case is but a small fraction of that in the other. It will be seen that the snow masses have recessed the mountain slopes at A and formed deep hollows and that the hollowing action appears to be most effective where the snow is thickest.
Summarizing, we note first, that the roughly equivalent factors are gradient and amount of snow-free surface; second, that the unequal factors are (a) accumulated waste, (b) degree of recessing, and (c) the degree of compacting of snow into ice and a corresponding difference in the character of the glacial agent, and (d) the extent of the snow cover. The direct and important relation of the first two unequal factors to the third scarcely need be pointed out.