Blown Sand, carried along by the prevailing north and north-west winds, is heaped up against all obstacles in its path, such as hills and vegetation; sometimes it is carried over these obstacles, only to fall and accumulate in the wind-shadow on the other side. The cultivated lands of the villages are for the most part surrounded by sand-accumulations, and some low dunes occur in various parts of the oasis; the positions of the latter will be better gathered from the maps than from any mere description. Baharia itself is more free from sand than any of the oases to the south of it, but even here the deposits of this material are very considerable, and as already mentioned extensive sand-dunes occur on the desert to the east, at a short distance from the depression.
Salt-swamps occur in the north of the oasis, having probably been produced by the overflowing of springs forming lagoons, which have taken the salt from the surrounding clays, the water from the springs only containing a small percentage of salt. The salt-swamps round Mandisha are now covered with a thin rough dry white crust of salt mixed with clay and sand; when this is broken through a salty sludge is seen to exist below, sometimes in a quite liquid state; the depth of sludge is probably not very great, but there is enough to immerse a camel up to its neck, as was found by experience during the survey.
Clays.—Sandy clays of recent origin occur at isolated spots on the floor of the oasis, notably near Ain el Haiss, where they are now being rapidly removed by the action of the wind. Many of the springs are also depositing a certain amount of clay on the low ground.
The Tectonics of the oasis.
The rocks of the district have undergone a considerable amount of disturbance along certain lines.
The most important fold is a well-marked syncline beginning at the west scarp in lat. 28° 7′ N. and running across the depression in a direction 30° north of east until it strikes the eastern escarpment in lat. 28° 23′. This synclinal fold is remarkably sharp, the width being generally only a few hundred metres, but the dips so strong as to bring the limestone beds nearly vertical in places. Along the line of fold we find the whole of the series 6 and 7 brought down to the level of the floor of the oasis, but on either side the beds so quickly regain their horizontal position that no trace of the folding is to be seen ([Plate VII]). The White Chalk in these folds is always altered by pressure into a hard crystalline limestone, as are also the limestones of the Cenomanian below. The fold is not actually continuous across the area, but is in reality made up of a number of long narrow ellipsoidal depressions, or cup-shaped hills, in which the end-dips are always less strong than those of the sides. At its most southerly point, where it runs into the scarp 14 kilometres north-west of Ain el Haiss, the two series 6 and 7 are found dipping northwards at angles up to 40° (and at one spot 78°). On the other side of the fold the same beds dip southwards at only 5°, and the appearance of the fold at this point suggests the existence of an accompanying fault (fig. 1).
Fig. 1.—Section across Syncline, 12½ kilometres N.W. of Ain Haiss.
a. Shales and sandstones with Exogyra, Arca, Cucullaea, etc., Neolobites vibrayeanus, etc. b. Hard crystalline limestone with flints. c. Sandstone passing into hard quartzite. d. Variegated sandstones with bands of sandy limestone. e. Hard dark-brown siliceous crystalline limestone. f. Calcareous grit (≡ bone-bed) passing in part into sandstone, and capped by hard crystalline limestone with shells.
Between this point and Jebel Hefhuf the fold is marked by a line of low isolated limestone-capped, cup-shaped hills, the largest and best marked of which is 16 kilometres north of Ain el Haiss. Here the maximum dip amounts to 55° on the western side of the basin-shaped hill.