Plate 19.
A. Contour map of a grooved upland, Bighorn Mountains, Wyoming (U. S. Geol. Survey).
B. Contour map of a fretted upland, Philipsburg Quadrangle, Montana (U. S. Geol. Survey).
The features carved above the glacier.—The ranges of pinnacles carved out by mountain glaciers have become known by various names of foreign derivation, such as arête, grat, aiguille mountains, “files of gendarmes”, etc. They may, perhaps, be best referred to as comb ridges, and according to their position they are differentiated into main and lateral comb ridges, as will be clear from the second map of [plate 19].
Fig. 397.—A col shaped like a hyperbola between Mount Sir Donald and Yogo Peak in the Selkirks (after a plate by the Keystone Plate Co.).
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]).