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Page:Popular Science Monthly Volume 70.djvu/117

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113
GLACIAL EROSION IN ALASKA

the grade of this valley out into the main trough of Disenchantment Bay, where the nearest soundings show a depth of from 600 to 1,000 feet, the profile falls far short of reaching the bed of the bay. It is assumed, therefore, to be a hanging valley with the lip at or just below the surface of the fiord water. If we grant that this particular hanging valley may be due to faulting, which can not be disproved, we are still left with the necessity of assuming block faulting along the axis of the Russell Valley to account for the hanging condition of its own tributaries whose lips lie fully a thousand feet above the Russell Valley bottom (Fig. 12). In some cases even a third series of laterals have been seen hanging above a tributary, which itself hangs above another, which is hanging above a main trough.

To propose faulting as an explanation for such a complex system of hanging valleys does not seem rational without definite evidence of the faulting, and without some explanation of why the results of such recent faulting are so common in glaciated regions and so rare in unglaciated areas. Moreover, in some of the cases mentioned, for example, the Russell Valley itself, if there had been such faulting, it would be easily detected in the sedimentary rocks which form the walls of the valley. Since a search for evidence of recent faulting in this valley failed to find it, I feel warranted in asserting that there has been no such faulting as the theory demands. A glance at the photographs (Figs. 10 and 11) is sufficient to show that the form of this valley could not be accounted for on the basis of block faulting. Its flaring, curving, U-shaped sides are not the forms characteristic of cliffs due to faulting. Should it be stated that block faulting occurred at a date sufficiently remote to permit the weathering back of the valley walls to the present curve, it is sufficient to answer that in all the time required for this, the lateral streams must of necessity have trenched the bottoms of the hanging valleys and reduced them to an accordant grade with the Russell Valley stream. As Fig. 12 clearly shows, this is far from being the case.

From the above statement of hypotheses it will be seen that it is generally admitted that hanging valleys are a peculiar phenomenon calling for special explanation. It is also true that this phenomenon is practically confined to regions of former glaciation. Together with the U-shaped valley, truncated spurs, and steepened main valley slopes, the condition of hanging valleys is reported not only from a wide area in Alaska and British Columbia, but in such other regions of former glaciation as the Sierra Nevada, the Rocky Mountains, the Finger Lake Valleys of central New York, the coast of Norway, the Alps, the Himalayas and New Zealand. While exceptional instances of hanging valleys, which are readily explained in other ways, have been reported from unglaciated regions, these are so few and scattered, and