LAND SCULPTURE BY MOUNTAIN GLACIERS 375 



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). 



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 

 striae which point down the val- 

 ley. Above the level of the gla- 

 cier's upper surface, on the other 

 hand, erosion is accomplished 

 through undermining or sapping, 

 a process which always leaves 

 precipitous slopes of ragged sur- 

 face 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 



FIG. 399. The U-shaped Kern valley 

 in the Sierra Nevadas of California 

 (after W. B. Scott). 



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). 



