=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). [Illustration: 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). [Illustration: FIG. 392.—The marginal crevasse or Bergschrund on the highest margin of a glacier (after Gilbert).]