Page:Popular Science Monthly Volume 70.djvu/108

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104
THE POPULAR SCIENCE MONTHLY.
 

PSM V70 D108 Hanging valley grenville channel inside passage.jpg

Fig. 5. Hanging Valley, Grenville Channel, Inside Passage. The forest-covered lip is solid rock, but it looks like a dam. Doubtless if one went to the crest of this lip he would find the broad valley extending, with moderate grade up the distant mountain. Note the waterfall near center of picture. Photograph by Lawrence Martin.

 

of the main valley to which they are tributary, instead of entering at grade, as is normal.

Where these Alaskan hanging valleys are most typically developed, the appearance is quite remarkable. The valley wall of the long, straight 'Beach' or 'Canal' is broken by a broad U-shaped tributary valley, whose cross section, if explained by ordinary methods of valley formation, would require a long period of time for its formation. The stream occupying the hanging valley flows with moderate grade up to the point where the tributary valley is intersected by the straight wall of the main 'Reach.' Then, instead of continuing into the main valley with the same grade, it tumbles over the lip of the hanging valley and descends to the fiord in a succession of leaps, sometimes on the very face of the main valley wall (Fig. 4), sometimes in a shallow gorge (Figs. 3 and 5).

In these most typical cases, there is such an absolute discordance of conditions as to cause comment from even the most casual observers, as I had occasion to observe in many instances in sailing through the Inside Passage. The first feature to attract attention was the waterfall. It was then noticed that the stream emerged from a broad valley, far up which one could look, though without seeing its bottom (Figs. 3-5). This produced the impression that the lip of the hanging valley was really a dam across the mouth of a broad tributary valley.