With the gradual invasion of the upland upon which the cirques have made their attack, the area from which winds may gather up the snow is steadily diminished, and hence cirque recession is correspondingly retarded. Cirques which have approached each other from opposite sides of the ridge until they have become tangent at one point may, however, still receive nourishment at the sides and so continue to cut down the intervening rock wall to form a pass or _col_. The theoretical curve which results from this intersection is that known as the hyperbola, of which an illustration is afforded by Fig. 396. An approximation to this form is clearly furnished by most of the mountain passes in glaciated mountain districts, and a particularly good illustration is furnished from the vicinity of Glacier on the line of the Canadian Pacific Railway (Fig. 397). [Illustration: 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). [Illustration: FIG. 399.—The U-shaped Kern valley in the Sierra Nevadas of California (after W. B. Scott).]