Fig. 392.—The marginal crevasse or Bergschrund on the highest margin of a glacier (after Gilbert).
The excavation of the glacial amphitheater or cirque.—It has been found that the marginal crevasse plays a most important rôle in the sculpture of mountains by glaciers, for the great amphitheater which is everywhere the collecting basin for the nourishment of mountain glaciers is not an inherited feature, but the handiwork of the ice itself. This was the discovery of Mr. W. D. Johnson, an American topographer and geologist, who, in order to solve the problem of the amphitheater allowed himself to be lowered into such a crevasse upon the Mount Lyell glacier of the Sierra Nevadas in California.
Fig. 393.—Niches and cirques in the same vicinity in the Bighorn Mountains of Wyoming. A, A, unmodified valleys; B, B, niches on drift sites; C, C, cirques on small glacier sites (after map by F. E. Mathes, U. S. G. S.).
Let down a distance of a hundred and fifty feet, he reached the bottom of the crack, and in a drizzling rain of thaw water stood upon a floor composed of rock masses in part dislodged from a wall which extended some twenty feet upwards upon the cliff side of the crevasse. It was evident that the warm air of the day produced the thaw water which was constantly dripping and which filled every crack and cranny of the rock surface. With the sinking of the sun below the peaks the sudden chill, so characteristic of the end of the day in high mountains, causes this water to freeze and thus rend the rock along its planes of jointing. Broad and thin plates of ice, loosened by melting at the walls, could be extracted from the crevices of the rock as mute witnesses to the powerful stresses developed by this most vigorous of weathering processes.
Fig. 394.—Subordinate small cirques in the amphitheater on the west face of the Wannehorn above the Great Aletsch Glacier of Switzerland.
In short, the rock wall above the glacier, which in its initial stage was the upper wall of the niche hollowed beneath the snowdrift, is first steepened and later continually both recessed and deepened by an intensive frost rending which is in operation at the base of the marginal crevasse. The same process does not go on as rapidly above the surface of the névé for the reason that the necessary wetting of the rock surface does not there so generally result from the daily summer thaw. At the bottom of the marginal crevasse alone is this condition fully realized. Intensive frost action where the rock is wet with thaw water daily is thus a fundamental cause, both of the hollowing of the early drift site to form the niche, and of the later enlargement of this niche into an amphitheater or cirque when the drift has been transformed into the névé of a glacier. Inasmuch as the crevasse forms where the snow and ice pull away from the rock toward the middle of the depression, the cirque wall in its early stage has the outline of a semicircle. In the Bighorn Mountains of Wyoming, all stages, from the unmodified valley heads to the full-formed cirque, may be seen near one another ([Fig. 393]). It will be noted that wherever a glacier has formed, as indicated by the cirque, there is a series of lakes which have developed in the valley below (see [p. 412]).
