CREST LINES IN THE HIGH SIERRAS 



585 



graphs apply, the Pleistocene glaciers occupied a smaller share of 

 the surface, and there are considerable unglaciated areas. The 

 photograph shown in Fig. 6 was made in this region. The view is 

 westward up the trough of a glacier. Beyond the head of this trough 

 is another glacier trough descending westward, and the dividing 

 ridge was partly destroyed by the head-erosion of the glaciers, so 

 that the typical amphitheater is not shown. The south wall of the 



Fig. 5. — Spur between two glacial amphitheaters near Mount McClure, Sierra 

 Nevada, showing schrund line. 



trough is steep, and shows distinct evidence of sapping. The cliff at 

 top, being composed of thoroughly jointed granite, does not stand 

 vertical, and the fragments recently fallen from it have built a talus 

 which conceals the schrund line. But it is evident that here the 

 sapping action at the schrund line was more active than the glacial 

 erosion lower down on the slope, so as to create a sort of shoulder 

 or terrace near that level. Stating the interpretation in another 

 way, the excessive alimentation along the south wall of the glacier 

 was here developing a branch glacier and a tributary cirque. This 

 cirque was eating its way back into the ridge bounding the glacier. 



