Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Though the same processes act in much the same manner beneath mountain glaciers, though here upon all parts of the bed, they are, in the earlier stages at least, subordinated to a third process more important than the two acting together. Sculpture by mountain glaciers, instead of reducing surface irregularities and softening outlines, increases the accent of the relief and produces the most sharply rugged topography that is known. In nearly all places where Alpinists resort for difficult rock climbing, mountain glaciers are to be seen, or the evidence for their former presence may be read in unmistakable characters.

Wind distribution of the snow which falls in mountains.—Until quite recently students of glaciation have concerned themselves but little with the work of the wind in lifting and redistributing the snow after it has fallen. We have already seen that, for the continental glaciers, wind appears to be the chief transporting agent, if we except the marginal lobes where glacier flow assumes large importance. In the case of mountain glaciers, also, we are to find that for the earlier stages particularly wind is of the first importance as a redistributing agent. In the higher levels snow is swept up from the ground by all high winds, and does not find a resting place until it is dropped beneath an eddy in some irregularity of the surface; and if the inherited surface be relatively smooth, this will be found in most cases upon the lee of the mountain crest.

In normal cases at least the inherited irregularities of the higher zones of mountain upland are the gentle depressions which develop at the heads of streams. These become, then, the sites of snowdrifts that are augmented in size from year to year, though at first they melt away in the late summer.

Fig. 390.—Snowdrift hollowing its bed by nivation and building a delta (at the left). Quadrant Mountain, Yellowstone National Park.

The niches which form on snowdrift sites.—Wherever a drift is formed, a process is set in operation, the effect of which is to hollow out and lower the ground beneath it, a process which has been called nivation. The drift shown in [Fig. 390] was photographed in late summer at an elevation of some 9000 feet in the Yellowstone National Park. The very gently sloping surface surrounding the drift is covered with grass, but within a zone a few feet in width on the borders of the drift no grass is growing, and in its place is found a fine brown soil which is fast becoming the prey of the moving water derived by melting of the drift. This is explained by the water permeating the crevices of the rock and being rent by the nightly freezing. Farther from the drift the ground is dry, and no such action is possible. With each succeeding spring the augmented drift as it melts carries all finely comminuted rock material down slopes beneath the snow to emerge at the lowest margin and be there deposited in the form of a delta. By the operation of this process of nivation the higher parts of the drift site are lowered as deposition goes on upon the lower. The combined effect is thus to produce a niche or faintly etched amphitheater upon the slope of the mountain ([Fig. 391]).

Fig. 391.—Amphitheater formed on a drift site in northern Lapland (after a photograph by G. von Zahn).

The augmented snowdrift moves down the valley—birth of the glacier.—In still lower air temperatures the drifts enlarge with each succeeding year until they endure throughout the summer season. From this stage on, an increment of snow is left from each succeeding season. No longer entirely wasted by melting, the time soon comes when the upper snow layers will by their weight compress the lower into ice, and the mass will begin to creep down the slope along the course of the inherited valley. The enlarged snowdrift which feeds this ice stream is called the névé or firn.

Against the sloping cliff which had been shaped by nivation at the upper margin of the snowdrift, that snow which is not of sufficient depth to begin a movement towards the valley separates from the moving portion, opening as it does so a cleft or crevasse parallel to the wall. This crack in the snow is called by its German name Bergschrund or Randspalte, and may perhaps be referred to as the marginal crevasse ([Fig. 392]).