On the 17th of December we reached the foot of the last ledges, on the western borders of Ennedi, at the altitude of about 1800 feet—that is to say, about that of the depression separating Erdi from the plateaux of Erdebe—and pitched our tents in the valley of Archeï, the most picturesque of the beautiful valleys of the Ennedi. The century-long erosion of wind and water, carving the great sandstone masses that line the valley, lavished throughout the landscape the most admirable effects of natural architecture. The approaches of the great grotto, above all, and of the sheet of water teeming with little fish, were a pure delight for the eyes: the sheer cliffs, fretted into colonnades crowned with turrets and belfries, were burnt to tones of faded ochre that made the blue of the sky seem deeper and more luminous still. [Illustration: MOURDIA WOMEN AND CHILDREN, PLATEAU OF ERDÉBÉ (1000 m.), ENNEDI] [Illustration: THE FORT OF FADA, ENNEDI] [Illustration: CAVES OF ARCHEÏ, ENNEDI] From this exploration it became apparent that Ennedi is, roughly speaking, a triangle covering about 12,000 square miles (30,000 square kilometres). It consists of a succession of sandstone plateaux rising in tiers from the base level of 1600 feet to that of 4300 and possibly even 4800 or 5000 feet in the parts of the country which had to be left out of our investigations (Basso and eastern Erdébé). It falls by steep slopes to the plains of the Libyan desert. The plateaux of Ennedi are ravined by many valleys, most of them very deep, whose waters only flow for a few days or weeks each year after the rains (August and September). These waters hurl themselves from ledge to ledge in waterfalls, hollowing out at the foot of each fall natural cisterns in the rock, where the water remains a longer or shorter time according as it is well or ill sheltered from the torrent beds. The roads usually follow the torrent beds, except when blocked by masses of crumbled rock, in which case a more or less awkward circuit has to be made. At the points where the main valleys converge great muddy ponds are usually formed, but they are shallow and short-lived. In all the valleys splendid grazing-land is found, where not only camels but also thousands of oxen could live if the problem of drinking-troughs did not present itself every year in the height of the dry season. For at that moment the natural cisterns that have still kept some store of water are grown few in number, and are nearly always very hard to get at. Most of the great temporary pools are dry, and subterranean water is no longer found except in the great wadis, where the wells (that have to be dug out afresh every year) go as deep as 20 or 25 yards.