LAND SCULPTURE BY MOUNTAIN GLACIERS 377 



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 



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.). 



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 Corner near Zermatt are such well- 

 known examples (Fig. 403). 



It is characteristic of rivers that the tributaries cut their val- 

 leys more rapidly than does the main stream within the neighbor- 

 ing section, though they cannot cut lower than their outlets 

 the side streams enter accordantly. This is easily explained be- 

 cause the grades of the tributary streams are the steeper, and, as 

 we well know, the corrasion of a valley is augmented at a most 



