Fig. 398.—Diagrams to illustrate the progressive investment of an upland by cirques with the formation of comb ridges, cols, and horns. I, early stage, youth; II, intermediate stage; III, late stage, maturity.
Upon either side of the col the land mass is left in high relief, rising from a more or less triangular base ([Fig. 398], III) into a sharp horn or tooth. An illustration of such a horn is furnished by the Matterhorn in the Swiss Alps, or by Mount Sir Donald in the Selkirks, though less noteworthy examples may be found in every maturely glaciated mountain district.
The features shaped beneath the glacier.—Those features which are carved above the glacier—the comb ridge, the col, and the horn—are all shaped as a result of intensive weathering upon the cirque wall. The shaping at lower levels is accomplished by processes in operation below the glacier surface, where weathering is excluded and where plucking and abrasion work together to tear away and grind off the rock surface. By their joint action the valley is both deepened and widened, directly to the height of the glacier surface, and indirectly through undermining as far up as rock extends. Thus the valley is transformed into one of broad and flat bed and precipitous side walls—the U-shaped section illustrated by valleys of the Swiss Alps and in fact in all districts which have been strongly glaciated by mountain glaciers ([Fig. 399]).
Fig. 399.—The U-shaped Kern valley in the Sierra Nevadas of California (after W. B. Scott).
As high up in the valley as it has been occupied by the glacier, the bed is rounded, smoothed, and polished, and marked by the characteristic glacial scorings or striæ which point down the valley. Above the level of the glacier’s upper surface, on the other hand, erosion is accomplished through undermining or sapping, a process which always leaves precipitous slopes of ragged surface made up of the joint planes on which the fallen blocks have separated from the cliff. Thus there is found a sharp line which separates the smoothly rounded rock surface below from the jagged and precipitous one above ([Fig. 400]). Inasmuch as this boundary usually separates the scalable from the inaccessible slopes above, snow is apt to lodge at this level and make it strikingly apparent.
Fig. 400.—Glaciated valley wall in the Sierra Nevadas of California, showing the sharp line which separates the abraded from the undermined rock surface (after a photograph by Fairbanks).
