(1.) I dismiss at once any theory of " craters of upheaval," by which some distinguished geologists have attempted to account for valleys which approach the cirque form, such as that of La Berarde*, because, after examination of this and many others, I can conceive of no theory of upheaval or fracture which could leave them in a state at all resembling their present one. That a valley like the one at La Berarde, to say nothing of these cirques, should be a " crater of elevation ' appears to me a physical impossibility.
(2.) Can these cirques be explained by any theory of marine erosion, and compared with the coves which we not unfrequently find on the sea-coast ? Though possibly this explanation might be applied to some cases, I do not think it will fit all or nearly all that exist. After seeing how little effect the sea has in the Fjords of Norway, I cannot attribute the Fer-a-Cheval or the Creux de Champs in these remote and sheltered valleys to marine erosion, whatever effect the waves may once have had in blocking out the main features of the Alpine peaks†.
(3.) Can we call in the intervention of glaciers, now so much in favour as nature's carving-tools among geologists ? and may these cirques be regarded as results of what Mr. Buskin would call " minor fury of ice-foam "‡? We shall perhaps best answer the question by considering first this particular case : — whether, assuming that glaciers have been principal agents in the excavation of Alpine valleys, we can suppose the cirques to have been formed by them ; and second, whether the assumption just made, that glaciers have been principal agents in the excavation of Alpine valleys, is a correct one. Now, if these cirques are the result of glacial erosion, they were either fashioned (a) by a glacier which took its rise in them, or (6), like the concavities in the course of a river, by the action of a passing ice- stream. The first of these suppositions appears to me physically impossible. Granting, as we must do, to begin with, some kind of hollow or slight combe on the mountain-side in which the snows could collect and form a neve, the erosive action on this part would always be very slight (for the ice here is less compact than that below) ; the friction would be that due to the weight alone of the super-incumbent ice ||, and, what is most important, there is but little grit between it and the subjacent rock ; for comparatively few stones are engulfed in the neve of a glacier. I can conceive it possible that if a glacier, after wearing away a stratum of hard rock, reached one in which erosion proceeded more rapidly, it might deepen its angle of descent to some degree in the upper stratum also ; but I cannot conceive that precipices more than a thousand feet high could be thus
- In Dauphine. See Forbes's ' Norway and its Glaciers,' p. 259.
† My own examination of mountains has led me to conclusions very different from those advocated by Mr. Mackintosh in his ' Scenery and Geology of England and Wales,' although I think it possible that some peaks and ranges may have originated in insular fragments.
‡ Geol. Mag. vol. ii. p. 50.
|| Friction __ normal pressure ; in many of the lower parts of a glacier, normal pressure at any point = weight of column of ice above that point + a pressure derived from ice behind.
