HANGING VALLEYS 
1 r S 
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¬ 
matical ly illustrated in 
figures 56 and 57, is also il¬ 
lustrated in the actual topo¬ 
graphy of many regions 
sculptured by Pleistocene 
glaciers. 
The hanging valley is es¬ 
pecially significant in two 
lines of physiographic in¬ 
terpretation. 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. 
FIGS. 56 AND 57. DIAGRAMS ILLUSTRA¬ 
TING ORIGIN OF HANGING VALLEYS. 
56. A terrestrial block containing a trunk 
glacier and tributary with well adjusted chan¬ 
nels. 
57. The same block without the ice, show¬ 
ing the adjusted glacier channels. The trunk 
channel is deeper than the tributary; the trib¬ 
utary channel is a hanging valley on the side 
of the trunk channel. 
