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GLACIAL CAÑONS.
357

influence in any direction must be slight, and its effect may be disregarded).

Combining the several antagonistic factors, it appears uncertain whether the general tendency of the friction element is to widen or deepen the gorge, but certain that it is to develop concavity of the valley sides and the U form of cañon.

Since the third and fourth factors in down-stream impulse (available potential energy and friction) are indeterminate, the problem as to the declivity required to render such impulse equal to the weight at any point in a given glacier, or even as to whether such equality ever obtains in nature, cannot be analytically solved; and very few observations showing the relative value of these components have ever been made. Niles,[1] however, found that in the Great Aletsch glacier the ice usually rides upon projecting rugosities and seldom fills the intervening depressions of its bed, and that a bowlder (itself slowly moving) three feet high had formed an inverted trough thirty feet long in the base of the incumbent ice; whence the down-stream impulse must have exceeded ten times the weight. Bonney,[2] also, in the Glacier des Bois and the Glacier d' Argentiere, found all broad and gentle depressions in the glacier beds filled with impressed ice, the narrower depressions not quite filled, the lee of projecting knobs protected for a distance equal to their height, and bowlders lying in situ beyond the present terminus of the ice glaciated above and below (showing that here also motion took place along the two planes), all of which phenomena indicate that, in these glaciers, the down-stream impulse is in excess of weight, but in a less degree than in the Great Aletsch. The several observations then demonstrate (1), that down-stream impulse may greatly exceed weight, and (2), that the relation is variable. All were in the upper portions of the valleys where the declivity is great (15° to 20° in the examples described by Bonney), and where the office of the glaciers is preëminently one

  1. ↑ Proc. Boston Soc. Nat. Hist., XIX., 1878, 330; Am. Jour. Sci., XVI., 1878, 366.
  2. ↑ Geol. Mag., Dec. II., Vol. III., 1876, 197.
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