Page:Popular Science Monthly Volume 70.djvu/111

Rh 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 overdeepened main troughs. When it is considered that thousands of hanging valleys are already known, and that in each case it was necessary for a small glacier to linger with its terminus at the very lip

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