TOPOGRAPHY AND STRUCTURAL GEOLOGY.
The Fayûm, a large circular depression in the Libyan Desert, is situated immediately west of that part of the Nile Valley lying between Kafr el Ayat and Feshn ([Plate XVII.])
The depression, which has an area, roughly speaking, of 12,000 square kilometres, is primarily divisible into three distinct parts—cultivated land, lake, and desert.
Section I.—CULTIVATED LAND.
The cultivated land has an area of about 1,800 square kilometres and, with the exception of the lake and part of the Wadi Rayan, occupies the lowest part of the depression. Cultivation is necessarily strictly limited to the area covered with alluvial soil. The latter, for the most part identical in origin and composition with the river-alluvium of the Nile Valley, covers a leaf-shaped tract between the bounding desert on the east side and the lake (the Birket el Qurûn) on the north-west. The easterly and central part of the cultivated area forms a more or less level table-land, from which the ground slopes gently away, especially on the north side, where the slope is towards the lake and very marked. The cultivated land of the Fayûm is directly connected with that of the Nile Valley by a narrow strip of low ground, a natural passage through the desert separating the Nile Valley and the depression of the Fayûm. Through this gap runs the natural canal known as the Bahr Yusef, which is practically the sole source of water in the Fayûm and irrigates the entire district.
The canal leaves the Nile Valley at Lahûn ([Plate II]), and follows a somewhat serpentine course through the desert for about 5 kilometres, irrigating a narrow strip of land on either side, which at Hawara rapidly broadens out into the wide cultivated area of the Fayûm. Once within the latter, the Bahr Yusef gives off numerous subsidiary canals which traverse the country in all directions, constantly splitting up into smaller branches until the water-supply is divided throughout the whole area. With the exception of the self-contained basin of Gharaq, on the south side of the Fayûm, the entire district drains into the Birket el Qurûn, which occupies the lowest part of the depression, to the north of the cultivation. The basin of Gharaq is irrigated by the Bahr el Gharaq, a canal which takes off from the Bahr Yusef soon after the latter enters the Fayûm[6].
The cultivated land of the Fayûm is traversed by two main ravines, cut down in many places to the Eocene limestone below the alluvium (Plates [III] and [V.]) At the present time these ravines carry canals for irrigating the lower parts of the district, and also act largely as drains to the higher lands. They were probably initiated by the escape of water through breaches in the Bahr Yusef during flood time, and have since been deepened to their present dimensions.
In addition to the main central cultivated area, the soil of which, as mentioned above, is essentially identical with that of the Nile Valley, large tracts of the surrounding country, more especially on the north, north-west, and west sides, are also covered with alluvial deposits. These latter, which include sands, sandy clays, and clays of a quite distinct type, represent the slowly formed accumulations of the quieter and more remote parts of the ancient Lake Moeris (and the earlier prehistoric lake). The material was mostly derived from the Eocene strata which formed the shores of the lake, augmented no doubt by a certain amount of very fine sediment drifted from the Bahr Yusef, and by sand blown in by wind.
It is noticeable that the thickest and most sandy deposits occur near the borders of the lake site; when close under the Eocene cliffs, as along the north side above the Birket el Qurûn, the deposits closely resemble those of the latter. The finer more calcareous beds occur further out and the true marls were accumulated only at some distance from the shores of the lake.