In the summer of 1869, whilst walking up the Valley of the Durance from Mont Dauphin to Briançon, I noticed, when about five kilometres from the latter place, some pinnacles on the mountain-slopes to the west of the road. I scrambled up, and found the remarkable natural pillars which are represented in the annexed engraving.439 They were formed out of an unstratified conglomerate of gritty earth, boulders, and stones. Some of them were more thickly studded with stones than a plum-pudding usually is with plums, whilst from others the stones projected like the spines from an echinoderm. The earth (or mud) was extremely hard and tenacious, and the stones, embedded in it, were extricated with considerable difficulty. The mud adhered very firmly to the stones that were got out, but it was readily washed away in a little stream near at hand. In a few minutes I extracted fragments of syenite, micaschist, several kinds of limestone and conglomerates, and some fossil plants characteristic of carboniferous strata. Most of the fragments were covered with scratches, which told that they had travelled underneath a glacier. The mud had all the character of glacier-mud, and the hillside was covered with drift. From these indications, and from the situation of the pinnacles, I concluded that they had been formed out of an old moraine. The greatest of them were 60 to 70 feet high, and the moraine had therefore been at least that height. I judged from appearances that the moraine was a frontal-terminal one of a glacier which had been an affluent of the great glacier that formerly occupied the Valley of the Durance, and which, during retrogression, had made a stand upon this hillside near Sachas. This lateral glacier had flowed down a nameless vallon which descends towards the E.S.E. from the mountain called upon the French Government map Sommet de l’Eychouda (8,740).

Only one of all the pinnacles that I saw was capped by a stone (a small one), and I did not notice any boulders lying in their immediate vicinity of a size sufficient to account for their production in the manner of the well-known pillars near Botzen. The readers of Sir Charles Lyell’s Principles (10th ed. vol. I p. 338) will remember that he attributes the formation of the Botzen pillars chiefly to the protection which boulders have afforded to the underlying matter from the direct action of rain. This is no doubt correct⁠—the Botzen pinnacles are mostly capped by boulders of considerable dimensions. In the present instance this does not appear to have been exactly the case. Running water has cut the moraine into ridges (shown upon the right hand of the engraving), and has evidently assisted in the work of denudation. The group of pinnacles here figured, belonged, in all probability, to a ridge which had been formed in this way, whose crest, in course of time, became sharp, perhaps attenuated. In such a condition, very small stones upon the crest of the ridge would originate little pinnacles; whether these would develop into larger ones, would depend upon the quantity of stones embedded in the surrounding moraine-matter. I imagine that the largest of the Sachas pinnacles owe their existence to the portions of the moraine out of which they are formed having been studded with a greater quantity of stones and small boulders than the portions of the moraine which formerly filled the gaps between them; and, of course, primarily, to the facts that glacier-mud is extremely tenacious when dry, and is readily washed away. Thus, the present form of the pinnacles is chiefly due to the direct action of rain, but their production was assisted, in the first instance, by the action of running water.