222 



PHYSICAL GEOLOGY 



FIG. 235. Glacial trough with hanging valleys. Upper canon of Green 

 River. (Baker.) 



of the widening of the bottoms of the main valleys, the 

 floors of the tributary valleys at their mouths are left stand- 

 ing higher (sometimes 1000 feet or more) than the opposite 



bottoms of the main val- 

 leys. After the disap- 

 pearance of the ice, the 

 streams of the tributary 

 valleys descend in rapids 

 or falls to the main 

 streams. Such elevated 

 tributary valleys are 

 known as hanging valleys 

 (Figs. 24 land 235). The 

 same condition is of 

 course brought about 

 where a main valley is 



FIG. 236. View in the Bighorn Moun- 

 tains, Wyo. The cirque in the back- 

 ground contains Cloud Peak Glacier, 

 which has a length of nearly a mile. 

 The cirque walls are in places about 

 1500 feet in height. (Trowbridge.) 



glaciated, while its tribu- 

 taries remain free from 

 ice. 



The topographic iVa- 



