82 I. C. RUSSELL — HANGING VALLEYS 



and in alignment with the adjacent portions of the side of the main 

 valley. Very commonly, too, the mouth of the tributary hanging valley 

 is about on a level with the highest lateral moraine, or other evidence 

 of glaciation, on the sides of the receiving valley, but this is not always 

 the case. 



DISCUSSION AS TO ORIGIN 



It is chiefly in reference to the mode of origin of hanging valleys of 

 the type just described that differences of opinion have arisen among 

 students of glacial topography. 



Having in mind the several processes by which glaciers may produce 

 hanging valleys, as enumerated above, it seems evident that those of the 

 type just referred to must have been produced in one of two ways or by 

 a combination of the two methods : Either they have resulted from the 

 differential ice erosion of the main valley and of its tributary, or a pre- 

 Glacial drainage s}^stem has been modified by glaciers so as to have the 

 characteristics referred to, or these two processes have been active at the 

 same locality. 



If the first of these explanations is accepted, it follows that the rock 

 removed to form the portion of a main valley lying at a lower level than 

 the bottom of a tributary hanging valley is a minimum measure of the 

 amount of ice erosion in the main valley. What the maximum amount 

 of ice erosion in such valley may be is not apparent and, as it seems, has 

 not received serious consideration. Unless pre-Glacial erosion is admitted, 

 however, in the case of such mountains as the Sierra Nevada and Cas- 

 cades, the hypothesis of differential ice erosion to account for hanging 

 valleys implies that far more rock has been removed by ice than is ac- 

 counted for by the differences in elevation between a main valley and 

 its tributary hanging valleys. 



The hypothesis which essays to account for hanging valleys by the 

 enlargement and modification of pre-Glacial stream valleys rests mainly 

 for its support on the principle that a glacier developed in a main 

 stream-eroded valley would not only tend to straighten and broaden it, 

 and thus truncate the ends of tributary vallej^s, but establish a baselevel 

 for lateral glaciers, and by determining the depth to which they would be 

 able to erode lead to the origin of hanging valleys at the level of its 

 surface, minus the thickness of the tributaries. Under this hypothesis 

 also some differential ice erosion must be admitted, since other condi- 

 tions being the same a thick glacier must be accredited with greater 

 erosive power than a thinner one ; but several qualifications would have 

 to be considered in this connection in attempting a complete analysis. 



One of the hypotheses before us, then, demands a vast amount of 



