272 



THE ICE AGE IN NORTH AMERICA. 



One of the most striking features in the sculpturing of the 

 high Sierra is furnished by the grand amphitheatres or cirques, 

 occurring about the more elevated peaks and crests. These are 

 deep semicircular excavations, bounded on all sides, except that 

 through which the drainage escapes, by bold cliffs or by per- 

 pendicular walls from a few hundred to more than a thousand 



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Fig. 83.— Glacial furrows on Gibraltar Island, one half mile from preceding, but running 

 nearly north and south. 



feet in height. The bottoms of these excavations are often 

 depressed below that portion of the rim through which the 

 drainage escapes, and form, rock-basins ; at other times the 

 basins are partially inclosed by debris, and in some instances 

 they have well-formed terminal moraines across their outlets. 

 In these hollows there are transparent lakes of azure blue, 

 which reflect the grandeur of the sheltering walls with wonder- 

 ful distinctness from their unruffled surfaces. A horizontal 

 cross-section of a cirque is semicircular or horseshoe-shaped, 

 and in certain portions of the range these are so numerous that 

 they give a scalloped contour to the faces of the cliffs. The 



