HANGING VALLEYS 



from the unequal erosion of valleys by glaciers of un- 

 equal size. 



Where a trunk glacier of alpine type receives a rela- 

 tively small lateral tributary the main ice stream gives 

 base-level to the tributary. Their relation in this respect 

 is homologous with the relation of main and tributary 

 streams of water. If the magnitude of trunk and tribu- 

 tary have remained constant long enough for erosion to 

 bring about an adjustment of grades, the surface of the 

 tributary at the point of junction has the same level as the 

 adjacent surface of the trunk. But the bottoms of the 

 two channels do not have the same level. The trunk 

 stream is normally deeper than the tributary, and at their 

 junction its bottom lies at 

 a lower level. If the 

 streams (of ice or water) 

 be removed, the bottom of 

 the tributary channel is 

 found to end high up on 

 the side of the main 

 channel. 



The ideal case, diagram- 

 matically illustrated in 

 figures 56 and 57, is also il- 

 lustrated in the actual topo- 

 graphy of many regions FIGS . 56 AND 57 . DIAGRAMS ILLUSTRA . 



sculptured by Pleistocene TING ORIGIN OF HANGING VALLEYS. 



i 56. A terrestrial block containing a trunk 



giaciei b. glacier and tributary with well adjusted chan- 



The hanging valley is es- nels - 



. . . 57. The same block without the ice, show- 



Significant in tWO ing the adjusted glacier channels. The trunk 

 /!_ i_* channel is deeper than the tributary; the trib- 



OI pnySlOgrapniC in- utary chan nel is & hanging valley on the side 



of the trunk channel. 



It is a COn- 



spicuous earmark of the former presence of glaciers ; and 

 it helps to a conception of the magnitude of Pleistocene 

 glacial erosion. 



