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. [Illustration: 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).] [Illustration: 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).