Fig. 403.—Gorge of the Albula River near Berkum in the Engadine, cut through a rock bar by the river which has succeeded to the earlier glacier.
Fig. 404.—Idealistic sketch showing both glaciated and nonglaciated side valleys tributary to a glaciated main valley (after Davis).
It is characteristic of rivers that the tributaries cut their valleys more rapidly than does the main stream within the neighboring section, though they cannot cut lower than their outlets—the side streams enter accordantly. This is easily explained because the grades of the tributary streams are the steeper, and, as we well know, the corrosion of a valley is augmented at a most amazing rate for each increase of its grade. No such law controls the processes of plucking and abrasion by which the glacier lowers its floor, for these processes appear to depend for their efficiency upon the depth of the ice, and the supply of cutting tools, quite as much as upon the grade of the bed. To apply a homely illustration, the hollowing of flagstones upon our walks is dependent more upon the number of persons that pass over them, and upon their size and the number of protruding nails in their boot heels, than upon the grades upon which they are placed. At all events we find that the main glacier valleys are cut deeper than the side valleys, so that the latter become hanging valleys—they enter the main valley, not upon its bed, but some distance above it ([Fig. 404]).
The U-shaped hanging valleys, like the main valley, are much too large for the streams which now fill them, and these diminutive side streams plunge over the steep wall of the main valley in ribbon-like falls so thin that the wind turns them aside and disperses the water in the spray of a “bridal veil.” Such falls are found by the hundred in every glaciated mountain district, imparting to it one of the greatest of its scenic charms.
Fig. 405.—Character profiles in landscapes sculptured by mountain glaciers.
Fig. 406.—Flat dome shaped under the margin of a Norwegian ice cap with projecting rock knobs and moraines in foreground.