Page:The Journal of geology (IA journalofgeology21894univers).pdf/375

Recapitulating, it appears that of the several elements involved in glacial erosion, the first tends to deepen the gorge and slightly to develop the U form, the second to develop the U form, and perhaps very slightly to deepen the gorge, while the third and predominant one tends strongly to widen the gorge and protect its bottom, and less strongly to develop the U form. It follows that the general tendency of glaciers must be to widen rather than deepen the valleys they occupy, and to transform V to U cañons. Also, since the typical U gorge is just such as would result from temporary occupancy of a V gorge by a glacier, while the ordinary ratio of width to depth is less than would obtain were the gorge eroded by glacial action exclusively, it follows again that the characteristic glacial cañons must be only modified stream-cañons.

This conclusion explains, and is equally and directly corroborated by, the first and sixth features of glacial cañons. It also fully warrants the assumption, in the following as in the foregoing discussion, of originally V shaped glacier-beds.

As elsewhere shown,^[1] corrasion of a stream is a function of its volume, and, ceteris paribus, varies with, but less rapidly than that element. In a region of rapid corrasion then, the main stream must (unless the declivity be materially unlike) more rapidly corrade its channel than does its minor tributary; and the tributary cañon must accordingly enter its principal over a rapid or at least a convex curve in longitudinal profile.

If now the main cañon become filled with ice and be transformed from the V to the U type by its action, the distal extremity of the tributary will be cut off and the original stream-formed declivity replaced by the precipitous side-wall of the normal glacier valley (fig. 3); and this result will follow whether the tributary be filled with or free from ice, provided corrasion