Fig. 401.—View of the Vale of Chamonix from the séracs of the Glacier des Bossons. The alb of the opposite side is well brought out.
If uplift of the land occurs while glaciers occupy the valleys of mountains, an increased capacity for deepening the valley is imparted to these ice streams, and we find, as a result, a deep central valley of U cross section excavated within a relatively broad trough visible above the shoulder on either side of the later furrow. Save only for its characteristic curves, such a valley bears close resemblance to a mature stream valley which has been rejuvenated (see [p. 173]). The remnants of the earlier glacier-carved valley are, as already stated, gently curving high terraces so common in Switzerland, where they are known as albs or high mountain meadows. These albs may be seen to special advantage on the sides of the Chamonix valley ([Fig. 401]), the Lauterbrunnen valley, or in fact almost any of the larger Alpine valleys.
The cascade stairway in glacier-carved valleys.—If now, instead of giving our attention to the cross section, we follow the course of the valley that has been occupied by a glacier, we find that it descends by a series of steps or terraces having many backwardly directed treads ([plate 19]), whereas a normal and well-established river valley has only forward grades. Because of these backward grades the stream waters are impounded, and so lakes are found strung along the valley in chains as the larger beads are found in a rosary, and these are the characteristic rock basin lakes sometimes referred to as “Paternoster Lakes” (see [p. 412] and [Fig. 402]).
Plate 20.
Map of the surface modeled by mountain glaciers in the Sierra Nevadas of California (after I. C. Russell).
When the backward grades upon the valley floor are especially steep, the rock step becomes a rock bar, or Riegel, of which nearly every Alpine valley has its examples. In a walk from the Grimsel to Meiringen many such bars are passed. Carrying in suspension the sharp rock sand from the glacier deposits along its bed, the stream which succeeds to the glacier as it vacates its valley saws its way through these obstructions with a rapidity that is amazing, thus producing narrow defiles, of which the Gorge of the Aar near Meiringen and that of the Gorner near Zermatt are such well-known examples ([Fig. 403]).
Fig. 402.—Map of an area near the continental divide in Colorado, showing an unglaciated surface to the west of the divide, where the westerly winds have cleared the ground of snow, and the glacier-carved country to the eastward. Note the regular forms of the youthful cirque, the glacier stairway, and the rock basin lakes (U. S. G. S.).