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Page:Popular Science Monthly Volume 70.djvu/121

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117
GLACIAL EROSION IN ALASKA

do scour their beds at all, as every one admits, and that there is plenty of time available, as is well known to be the case.

Accepting ice erosion as a doctrine now established, as it seems to me we must, we will briefly examine some of the consequences of such erosion. Hanging valleys, U-shaped valleys, aligned spurs, and steepened valley slopes are among the more prominent of these consequences. From their existence we must of necessity infer enormous vertical as well as lateral erosion, such erosion occurring in places where actively moving streams of ice were concentrated in valleys along relatively narrow lines. Along the Inside Passage, and in Yakutat Bay, the two sections immediately under consideration in this paper, the amount of erosion which must be deduced from the evidence is in places not less than two thousand feet vertically; and erosion of this magnitude has occurred along hundreds of miles of fiords.

In discussions of the significance of hanging valleys, it has been rather common to speak as if the main valleys were eroded while the tributaries were left undeepened. This has been done here, as doubtless in other writings, in order not to introduce an unnecessary complexity into the discussion. It would, however, be entirely erroneous to suppose that the lateral valleys were not eroded also. It requires only an examination of the photographs accompanying this paper to see that the normal cross-section of the hanging tributary valleys has the same curve as that of the main valleys; that is, the curve which glacial erosion produces.

From the statement just made, it follows that the level at which a lateral valley now hangs above the main trough is not to be taken as the full measure of vertical erosion along the main valley. That this is true is indicated by the fact that of several valleys tributary to a main trough, no two usually hang at exactly the same level. There may be, and in many cases are, wide differences in the hanging levels of neighboring valleys (Fig. 10); some being perched far up on the mountain side, others so far lowered that the sea water enters and drowns their mouths (compare Figs. 2 and 5), which, however, are still hanging above the bottom of the fiord. Such differences in the hanging level are, in the main, a measure of the difference in amount of erosive work performed by glaciers in the several hanging laterals.

In general, those valleys occupied by the largest glaciers have been lowered most; and it may be stated as a law that, other conditions being equal, the height of a hanging valley above the bottom of the main trough varies inversely with the size of the glacier. The operation of this law is, of course, modified by the influence of varying rock texture, slope and other causes which tend to modify the rate of ice erosion. We are not yet in full enough possession of the facts relating to the process of glacial erosion to warrant an attempt at a full statement of the nature and result of the various influences which tend to modify