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Page:Popular Science Monthly Volume 70.djvu/115

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111
GLACIAL EROSION IN ALASKA

surface. This valley wall extends completely across the mouth of the hanging valley, forming a rock lip seven hundred feet high (Fig. 7). Climbing to the crest of this lip, one is able to look up the hanging valley to its mountain-walled head (Fig 8). It is found to be a broad, U-shaped valley with a flat floor and moderate grade.

The ice-born stream which flows along the bottom of this valley has cut only a shallow trench in the rock floor, through which it flows with moderate grade until the lip of the hanging valley is reached, when its grade abruptly increases and it tumbles down the main valley wall, as a succession of waterfalls, in the bottom of a gorge so shallow that the entire series of cascades, from the crest of the lip to its bottom, is plainly visible from the fiord. The stream has begun to lower its grade to harmonize with the main valley; but it has not had time yet to carry the process very far. That there is no possibility of the presence of a drift-filled valley of earlier date is proved by the fact that bed rock outcrops across the entire lip.

On any assumption of stream rejuvenation, it is utterly incredible that all the time required to deepen the main trough of Nunatak Fiord, and to broaden it into the form of maturity which it possesses (Figs. 9 and 14), should have been too short to have permitted the stream in the hanging valley to cut a more profound gorge, on such a steep slope, and to attain a better approximation to that accordance of grades toward which all tributaries tend in their relation to the main streams. Wherever one critically examines a hanging valley in its relation to the main trough, the same conclusion is necessitated.

A fifth explanation that has been proposed is faulting. It is of course admitted that a block fault, by dropping down the bottom of a main valley, would leave the tributary valleys hanging. Although admitted as a possibility for individual cases, the application of such an explanation to Alaskan conditions in general, fails utterly to account for the facts. It would not explain the truncated spurs on both sides, nor the U-shape of the main and lateral valleys. Furthermore, without the introduction of complicated secondary faulting, it would not account for the difference in level at which the valleys hang above the main trough to which they are tributary (Fig. 10). Another fact which ordinary block faulting would fail to explain is the frequent presence of a condition of double hanging valleys,—a lateral hanging above the main valley, and a tributary of this lateral hanging above it.

Such a condition of double hanging valleys may be illustrated by the case of Russell Valley (Fig. 10) which enters the lower end of Disenchantment Bay, a part of the Yakutat Bay inlet. This valley has a moderate slope and a remarkably well-developed U-shape (Fig. 11). Where it joins the fiord it has-built a gravel delta, so that there the actual rock bottom is not visible; but about a mile back from the fiord, bed rock occurs in the valley bottom near its center. Extending