Il6 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 AB C in the dia- 



FIG. 58. DIAGRAM ILLUSTRATING gram (fig. 58) be the cross- 



DISCORDANCE OF HANGING VALLEYS. ~. r . , . , . , 



profile of 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 J3. If the preglacial 

 stream valleys were accordant in grade, their junction 

 was at F) or some point above F; and FB is the meas- 

 ure either of the deepening of the main trough by the 



