Page:Whymper - Scrambles amongst the Alps.djvu/185

Rh § 4. But whilst the motion of the glacier is urging it onwards and over depressions, the weight of its ice is pressing it into the depressions, and hence the ice strikes the next projection at a lower level than it left the last one. For example, after passing the hollow D, the ice strikes the eminence B at a lower level than it left A (Fig. 1).

§ 5. The immediate effect is, that the minor asperities of the rock suffer, and chiefly those which are opposed to the direction of motion of the glacier. They may be actually crushed, or fragments which are already loose may be brushed or scraped away; in any case they disappear (Fig. 2).

§ 6. In consequence of this, the glacier becomes supported upon a larger area, and its power is exerted over a greater surface. It follows, also, that the amount, in depth, of the matter which is removed constantly diminishes, if the power that is employed continues to be the same.

§ 7. A long continuance of abrasion, from the friction of the ice and by the rasping of foreign matter contained in it, lowers the level of the rock eminences; but surfaces of fractures or depressions in the rock which are not opposed to the direction of the motion of the glacier remain unabraded, if they are perpendicular to the direction of the motion, or anything like perpendicular to it; and they will continue to exist (although becoming less and less) until the entire bed of the glacier (that is, the surface of the rocks) has been reduced, over large areas, nearly to a plane surface.

Rocks which have been rounded by glacier action (such as in Figs. 2, 3) are termed roches moutonnées, and unabraded surfaces of roches moutonnées (such as D, F, Figs. 2, 3) are termed lee-sides. The lee-sides often afford useful indications of the directions in which extinct glaciers have moved.

§ 8. If glaciers still continue to work upon roches moutonnées, the effects which are produced are only an extension of those