roches nivelées. They indicate a comparatively early and coarse stage of glacier-action.
§ 10. More or less water is always found flowing underneath glaciers. It is produced by ablation of the surface of the glacier, and by other causes. In the earlier stages of glacier action (§§ 2-7) it finds a free course among the depressions beneath the ice; but as the rocks become smoother and flatter it has more difficulty in discovering outlets, and must materially assist in reducing the friction of the ice upon the rocks, and in the production of highly-polished surfaces, by causing less violent and more uniform abrasion.
Such, it appears to me, are the ways in which glaciers work upon rocks, and produce surfaces moutonnées or nivelées. Before I quit this subject, I wish to make one or two remarks upon the facts which have been stated, and to draw one or two conclusions which they seem to warrant.
1. The production of the peculiar rounded rock-forms which are termed roches moutonnées, is to be attributed to the extremely slow rate at which the bottoms of glaciers move, not less than to the plasticity of the ice. That the rate is very slow may be inferred from the fact, that the smallest fractures on rocks upon which glacier has worked for any length of time, have their weather and their lee sides. That is to say, before the ice is able to move in
always rectilinear, tending in the direction in which the glacier is moving, and most distinct on that side of the surface-inequalities facing the direction of the moving mass, while the lee-side remains mostly untouched. . . .
"Here and there on the sides of the glacier it is possible to penetrate between the walls and the ice to a great depth, and even to follow such a gap to the very bottom of the valley; and everywhere do we find the surface of the ice fretted as I have described it, with stones of every size, from the "pebble to the boulder, and also with sand and gravel of all sorts, from the coarsest grain to the finest, and these materials, more or less firmly set in the ice, form the grating surface with which, on its onward movement down the Alpine valleys, it leaves everywhere unmistak cable traces of its passage." — Agassiz, in The Atlantic Monthly.
