Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/460

 Bourg d'Oisans, to the head of the Glacier de la Pilatte. This glacier occupies an extensive glen, guarded on one side by the huge precipices of the Ailefroide (12,877 feet), on the other by a nameless mass, the highest peak on which is probably about 12,000 feet. The lowest point in the loop-like crete connecting these two portals is about 10,800* feet above the sea. The exact height of the foot of the glacier I do not know ; but probably 6800 feet is an estimate rather above than below the truth ; for the ascent from La Berarde (5702 feet), 3-1/2 miles down the valley (on map), is not great. Measuring in the same way, it is just two miles from the Col du Sele to the foot of the glacier (=10,560 feet). Neglecting the odd hundreds, which we are justified in doing, as the lower part of the glacier is flat, we have a fall of 4000 feet in 10,000 feet. From La Berarde to St. Christophe, 6-3/4 miles (=35,640 feet), the fall is 879 feet, nearly 25 in 1000 feet. From the latter place to Venose (3363 feet), a distance of about 3| miles, the fall is 1460 feet ; and from Venose to the opening of the valley, perhaps about 4-1/2 miles, the fall must be 950 feet ; for the descent afterwards to Bourg d'Oisans (2380 feet) is but slight. The fall of the valley between La Berarde and St. Christophe is rather more than is given by the difference of the two stations, as the latter place stands some height above the stream, and this may a little affect the next measurement below.

The result of this is, that the fall of the valley-bed during the first two miles (measured on the map) is 40 in the 100, and after that, for the remaining 17 miles of its course, its fall is always considerably less than 10 in 100. Moreover, in that uppermost portion, the slope very perceptibly increases as the watershed is approached, so that the last few hundred feet, whether of snow or rock, are of formidable steepness, the ascent to the only accessible point (the Col du Sele) being a stiff bit of rock- climbing. The same configuration is exhibited by most of the tributary glens that join the main valley — by one, the Vallon de la Bonne Pierre, leading up to the Col des Ecrins, to an even greater degree. Three of these are of considerable size†, and must in former times have brought down immense tributary glaciers ; they join the main valley at or below La Berarde ; and yet there is not any marked deepening or widening of the valley caused by the great increase of the ice-stream.

Take, again, the glacier on the north side of the crete of the Bateau (fig. 2) ; how could it have excavated that cliff which rises full 2000 feet above it ? while, as if to make the puzzle greater, within a mile to the west, begins the Glacier du Mont de Lans, which covers a vast plateau, about 11,000 feet in height, sloping gently to the north. Here the glacier reclines like a white cloth upon a table, protruding a few tongues of broken ice into two or three comparatively shallow notches that have been worn out in its northern edge.

Mathews) ; and several other parts of the ridge are of much the same height,
 * The Col du Sele is 10,814 feet, mean of B. P. (Tuckett) and Bar. (W.

† The Vallons des Etancons, de la Muande, and de la Selle.