HANGING VALLEYS 
117 
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 necessar}/, 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, 
