HANGING VALLEYS 



glacier, or of the difference between that deepening and 

 the deepening of the tributary. If the preglacial val- 

 leys were discordant, the main valley must have been 

 young, and was probably so narrow that its conversion 

 into a U-trough involved much excavation at the sides. 

 In either case, therefore, the occurrence of high hanging 

 valleys is indicative of great erosion by ice. 



Yet another qualification is necessary, for there is a 

 special condition under which a relatively young stream 

 valley may have the width of a glacial U-valley. When 

 the activity of streams is revived by uplift, the general 

 degradation may be outstripped not only by trunk streams 

 but by strike streams following outcrops of weak rocks; 

 and if the contrasts of rock resistance are great, a broad 

 belt of weak rock may induce the development of an open 

 valley or wide trough, while neighboring uplands of re- 

 sistant rock are still little modified. Thus may arise a 

 topographic condition susceptible of conversion by only a 

 moderate amount of subsequent ice work into a typical 

 glacier trough with hanging valleys. 



Theoretically, then, there are at least three cases to be 

 borne in mind in inferring the quantity of glacial erosion 

 from the existence of hanging valleys. First, the grades 

 of preglacial streams may have been accordant, in which 

 case the discordance of hanging valleys with trunk valleys 

 yields a rough measure of the depth and amount of glacial 

 erosion. Second, the grades of preglacial streams may 

 have been discordant, and without dominant control of 

 contrasted rock texture, in which case the erosive work 

 of ice may have consisted chiefly in the enlargement of 

 V-gorges to U-troughs. Third, the discordance of pre- 

 glacial grades may have been associated with the rapid 

 opening of valleys in weak rocks, in which case the erosive 

 work of ice may have been small. The criteria for dis- 

 criminating the three cases have not been worked out, 



