Fig. 395.—“Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of Wyoming (after Atwood).
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]).
Grooved and fretted uplands.—The partial investment by cirques of a mountain upland yields a type of topography quite unlike that produced by any other geological process. The irregularly connected remnants of the inherited upland resemble nothing so much as a layer of dough from which biscuits have been cut ([Fig. 395]). The surface as a whole, furrowed as it is below the cirques, may be described as a grooved upland ([plate 19 A]). A further continuation of the process removes all traces of the earlier upland, for the cirques intersect from opposite sides and thus yield palisades of sharp rock pinnacles which rise on precipitous walls from a terraced floor. This ultimate product of cirque sculpture by glaciers is called a fretted upland ([plate 18 A] and [19 B]).
Plate 18.
A. Fretted upland of the Alps seen from the summit of Mount Blanc.
B. Model of the Malaspina Glacier and the fretted upland above it (after model by L. Martin).