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). [Illustration: FIG. 395.—“Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of Wyoming (after Atwood).] [Illustration: FIG. 396.—Two intersecting inverted cones representing glacial cirques of different sizes, to show that their intersection is the arc of a hyperbola, the curve to which the col approximates.] =Life history of the cirque.=—In its earliest stage the cirque is more or less uniformly supplied with snow from all sides, and so it enlarges by recession in a manner to retain its early semicircular outline. In a later stage a larger proportion of the snow reaches the cirque at its sides so that its further enlargement causes it to broaden and to flatten somewhat that part of its outline which represents the head of the valley (Fig. 389, p. 364). As the territory of the upland is still further invested by the cirques, their nourishment becomes still more irregular, and the circular outline gives place to a scalloped border, as the amphitheater becomes differentiated into subordinate smaller cirques, each of which corresponds to a scallop of the outline (Fig. 398 and Fig. 394).