GLACIAL EROSION IN ALASKA 



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explained in this way. In consequence of this inability to accept the 

 conclusion that glaciers are powerful agents of erosion, a number of 

 alternate hypotheses have been suggested, of which the following are 

 some of the most prominent. 



One of these special explanations is based upon the conception that 

 glaciers act to protect rather than to erode. This explanation assumes 

 that glaciers occupied and protected the tributary valleys while the 

 main valleys were free from ice, and that, while this condition lasted, 

 the main valleys were so deepened that, when the ice finally melted from 

 the protected tributary valleys, they were hanging well above the over- 

 deepened main troughs. When it is considered that thousands of 

 hanging valleys are already known, and that in each case it was neces- 

 sary for a small glacier to linger with its terminus at the very lip 



Fig. 8. Looking into Hanging Valley (Fig. 7) from Rock Lip at Elevation of 7°0 

 Feet. The stream flows in a small gorge at the right. The elevation in the middle background 

 of the valley is the moraine-covered terminus of a dwindling glacier. The valley floor is all 

 rock, and rock extends continuously across its mouth. Photograph by O. von Engeln. 



of the hanging valley throughout the long period of time required 

 to deepen the main channel, this explanation seems almost too 

 absurd to consider. It furthermore fails to account for the aligned 

 spurs, and, above all, for the great breadth and U-shape of fche 

 main troughs. While one might admit this as a possible cause for 

 individual cases, it fails utterly as a general explanation. 



A second hypothesis proposed, is that glacial erosion is lateral 

 rather than vertical, and that the hanging valleys are due to the wear- 

 ing back of the tributary mouths so that they are left hanging. That 



