ii 6 
ALASKA GLACIERS 
Its value as an earmark depends on the principle of 
exclusion; glaciation is the only physiographic process 
known to produce such forms. It is true that discordance 
of level between trunk and tributary valleys is not by itself 
diagnostic of glaciation, for it often occurs as a temporary 
condition in systems of stream-made valleys, especially 
when fresh uplift stimulates down-cutting by trunk 
streams; but in such cases discordance is associated with 
the narrow trenches of youthful or rejuvenated topog¬ 
raphy. It is true also that the glacial U-trough is some¬ 
times (though rarely) simulated by products of stream 
erosion, and that a hollow closely resembling the glacial 
cirque is occasionally produced by aqueous process; but 
these imitative forms belong to the middle life of a stream, 
when down-cutting has so slackened as to permit valleys 
to broaden, and they imply a harmonious grading of stream 
beds, inconsistent with discordance of level at the junc¬ 
tion of tributary and trunk. But the combination of dis¬ 
cordance of level with U-shaped cross-profiles constitutes 
a physiographic type peculiar to the work of glaciers. 
The significance of the hanging valley for the valuation 
of glacial erosion depends largely on the assumption that 
the discordance of level was 
produced by the glacial exca¬ 
vation of the main trough, and 
this assumption requires qual¬ 
ification. If ABC in the dia- 
fig. 58. diagram illustrating gram (fig. 58) be the cross- 
r\Tcnr4T» rv % ur k *vr^ T^mir k t t t7Vc» ^ 
a main glacial trough, 
DE the longitudinal profile of a tributary trough, and EF 
the produced floor of the tributary, the ‘discordance’ is rep¬ 
resented by the height of F above B. If the preglacial 
stream valleys were accordant in grade, their junction 
was at F\ or some point above Fj and FB is the meas¬ 
ure either of the deepening of the main trough by the 
