Systematic Asymmetry of Crest-Lines. 283, 



wall of the same trough, but with the east wall of the 

 adjacent trough, so that the rock crest between the two 

 troughs is not symmetric. 



Usually in viewing a cirque it is possible to trace 

 about its wall a somewhat definite line separating a cliff 

 or steeper slope above from a gentler, usually scalable, 

 slope below. This line I conceive to mark the base of 

 the bergschrund at a late stage in the excavation of the 

 cirque basin. I have called it in my notes "the schrund 

 line." It can usually be traced for some little distance 

 beyond the cirque, and sometimes for several miles on 

 one wall or other of the glacier trough. Advancing 

 along the trough wall, it descends gradually with a slope 

 which may be assumed to represent the gradient of the 

 ice surface, that surface having been somewhat higher 

 than the schrund line. Its expression outside a cirque 

 may be seen in the lower figure of Plate XLI and the 

 upper figures of Plates XLII and XLIII. 



At a somewhat lower level than that to which the 

 preceding paragraphs apply, the Pleistocene glaciers 

 occupied a smaller share of the surface, and there are 

 considerable unglaciated areas. The photographs shown 

 in Plate XLIII were made in this region. The upper view 

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

 of this trough is another glacier trough descending west- 

 ward, and the dividing ridge was partly destroyed by the 

 head-erosion of the glaciers, so that the typical amphi- 

 theater is not shown. The south wall of the 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. 



