In Pleistocene times drainage down the Nile Valley appears to have become definitely established and probably the river in the lower part of its course eventually washed up against and broke down the separating barrier of gravel between the Fayûm and the Nile Valley, so that part of its waters obtained access to the depression, formed a lake on the lowest part, and gradually rose until the whole basin, up to the level of the channel connecting it with the Nile Valley, became filled. Every year thousands of tons of sediment were carried in by the floods and spread out on the floor in the shape of a fan. Probably later, as the Nile level fell, the valley and the depression again became disconnected, until the more modern river, with its gradually rising bed, again attained the requisite altitude. In early historic times the alluvial deposits had probably silted up the lake in its southern central part, and when in the XIIth dynasty the district was first taken in hand by Amenemhat I this part of it must have had the character of a huge marsh, nearly surrounded by open water, rapidly deepening towards the north. SECTION VII.—THE NORTHERN DESERT REGION. [Sidenote: The Plateau bounding the Fayûm depression to the north.] All along the north-west and north sides the ground rises rapidly from the base of the dip-slope of the plain in a series of escarpments to the summit of the rim of the depression, averaging 340 metres above sea level. Northwards from the summit stretches a rolling pebbly desert, the prevailing character of which is a dark brown, relieved by lighter brown grey and yellow patches, and especially flecked by the light sandy slopes of the undulations. Although the latter seldom rise to any considerable height above the general level of the plain, from the top of the most modest eminence an immense view in every direction can frequently be obtained. The monotony of this desert is only relieved by the occasional belts of sand, which although extremely narrow in width, run for immense distances in almost absolutely straight lines, and in a N.N.W.—S.S.E. direction. Although none of these dunes actually reach the rim of the escarpment we may mention here the beautiful Ghart el Khanashat, an almost straight and apparently unbroken ridge of sand, extremely narrow but of great length. Near its southern extremity the width does not exceed 100 metres; the slopes on both sides are frequently as much as 30°. The commencement of the Ghart el Khanashat was observed on a march from Wadi Natrûn to Mogara; it lay some way to the south of a line joining those two localities but could not be accurately fixed from the line of route. The belt dies out 24 kilometres from the rim of the Fayûm depression, its termination being particularly abrupt, although the height of the ridge diminishes gradually throughout the last kilometre or two. The line of the belt if continued would almost strike the western extremity of the Birket el