Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/454

 4. On the Formation of " Cirques," and their bearing upon Theories attributing the Excavation of Alpine Valleys mainly to the Action of Glaciers. By the Rev. T. G. Bonney, M.A., F.G.S.

The following paper is an attempt to examine how far a theory which during the last few years has obtained more or less support from many very eminent geologists, can be applied to one of the most remarkable features in several mountain-valleys. Although " cirques " are more commonly associated with the Pyrenees, they are by no means infrequent or on a small scale in the Alps ; and as I know the former mountain-chain only at second hand, I shall confine myself to the latter, and to those instances which I have personally examined.

In venturing to treat of a subject so full of difficulty, and to oppose the opinions of persons far more eminent than myself, I may venture to plead as my excuse that I do not write without some experience of mountain-regions, seeing that I have twelve times visited the Alps, have wandered, generally on foot, over almost the whole chain, and have had during many of these journeys this subject of mountain-sculpture especially present before my mind, so as to be constantly on the watch for evidence bearing upon the various theories that have been advanced.

First let me briefly describe a few of the most remarkable cirques, beginning with the Creux de Champs.

This is an approximately semicircular amphitheatre on the northern face of the Diablerets, forming the head of a short glen terminating the valley of La Grande Eau, which joins the Rhone at Aigle. Vast precipices of limestone and shale enclose it, which are crowned with short wide glaciers, and surmounted by the blunt peaks of the Diablerets, the highest of which reaches an elevation of 10,666 feet above the sea. The floor of this glen, though encumbered with moraine stuff, and intruded upon by the great taluses beneath the precipices, is tolerably level, and is probably about 4000 feet above the sea*. Above the slopes an almost unbroken wall of limestone rock rises for at least 2000 feet ; shales, alternating with narrower bands of limestone, then reduce the precipitousness and produce the ledges whereon the glaciers rest ; and, finally, the limestone peaks rise like broken battlements on the summit of the wall. From these snow- laden ledges numerous streams gush down the rock, some mere white threads of spray, others turbid cascades, whose volume varies with the hour of the day and the state of the weather. Speaking in general terms, we may say that for an arc of about 80° this cirque is enclosed by a nearly vertical wall of precipices rising full 5000 feet above its comparatively level floor. The strata exposed, though here and there contorted, and in one place at least folded, lie on the whole pretty evenly, and appear to dip gently a little south of west.

The Fer-a-Cheval. — This cirque differs from that last described in being situated just on one side of a large peak, and in opening into


 * The Hotel des Diablerets, at its mouth, is 3832 feet ; and the fall of the valley-bed is not rapid.