Section Iii.—the Surrounding Desert Region.

With the exception of the lake and the cultivated area the depression is practically entirely desert. The southern and south-western parts include the wadies Rayan and Muêla, where freshwater springs occur, surrounded by areas covered by a good deal of wild scrub. Apart from these, however, no springs occur outside the cultivated land.

The topography of the region is so intimately connected with its geological structure that an adequate description of the former is not possible without constant reference to the latter. Full geological details will, however, be reserved for later consideration.

The part of the Libyan Desert dealt with in this report has, excluding the cultivated land and the lake, an area of some ten thousand square kilometres. While some portions have been examined and mapped in detail, others are still very imperfectly known, especially on the south and south-west sides. The irregular cliff-line forming the southern boundary of Rayan and the adjacent wadis may be taken as our limit in this direction, beyond lying an almost unbroken limestone plateau rising gradually and continually to the south. On the north and north-west the area under description is bounded by the southern limit of the great undulating high-lying gravelly desert-plateau which stretches with little change of character to the Mediterranean. On the east side the Nile Valley forms a convenient though not altogether natural boundary; while to the south-west our limit practically coincides with the boundary of the depression, where the floor of the latter insensibly merges into the general desert plateau.

The rocks forming the area within the above limits are almost entirely of sedimentary origin, the exception being a band of hard basalt intercalated at the very top of the series and exposed only on the extreme northernmost limit of the depression. The total thickness of sediments, from the lowest beds exposed in the bottom of the Wadi Rayan to the summit of the escarpments, a day’s march north of Tamia, is some 700 metres. These beds include every kind of sedimentary deposit—limestones, marls, clays, sandstones, sands and gravels, forming an ever-changing succession of rocks, varying considerably in hardness and capacity for withstanding the agents of denudation. It is not too much to say that the coming into existence of the Fayûm, with its plains, lowlying depressions, precipitous cliffs and escarpments, was largely dependent on the existence of this variable series of deposits.

Apart from the presence of sediments varying greatly in hardness and durability, the fact that the whole of the rocks have an almost constant northerly dip of two or three degrees is a point of prime importance. So small a dip may be scarcely noticeable in any one place, but over the large areas with which we have to deal its influence on the position and level of any individual bed is very marked and the topography of the region would have been essentially different if the strata had been quite horizontal.

The unique character of the Fayûm is alone sufficient to show that special causes have acted in its production. Two main causes stand out:—(1) the presence of thick bands of comparatively soft arenaceous and argillaceous strata breaking up the usually continuous hard limestone of the Middle Eocene; (2) the effect of the Nile Valley fault in lowering the whole of the western desert (north of Assiut) relatively to the eastern. The former took place as the result of changed geographical conditions on the continent to the south at the time in question, with which however we need not deal here. On a homogeneous mass of rock weathering has little power to form depressions of any magnitude, and this is the cause of the continuous unbroken plateau which stretches southwards from the Fayûm, the underlying rocks being one continuous thick mass of hard limestone. Wherever softer intercalations are present differential weathering takes place, and all the great depressions of the Libyan desert owe their origin to the presence of soft easily denuded strata; if the great homogeneous mass of Nile Valley limestone had stretched unchanged westwards, the oases of Farafra and Baharia would never have existed. They owe their origin entirely to the presence of the underlying saddle of softer Cretaceous rocks. Similarly if changed conditions had not led to the deposition of soft beds of clay, marl, and sandstone, the western plateau would have continued unbroken northwards.

A comparison of the two sides of the Nile Valley between Cairo and Assiut shows that the tectonic movements, which largely determined the existence of the valley itself, resulted in a considerable lowering of the rocks forming the western side. This was brought about by differential movements along the north and south line or lines of fault, and by the presence of an east to west monoclinal fold which is especially well marked in the neighbourhood of Heluan. The depressions of the Fayûm would doubtless have existed irrespective of this general lowering of the western desert relative to the east, but denudation would have required an additional period of many thousands of years before the floor of the depression was low enough to allow of its actual connection with the Nile river.

As it has been maintained that the Fayûm is an area let down and enclosed by faults, it may be mentioned here that all available evidence points in an opposite direction; this question of faults will however be dealt with in detail later. The influence of the Nile Valley fault has been explained above and it must be remembered it is one affecting not the Fayûm alone but the northern part of the western desert as a whole.

For purposes of description it will be convenient to divide the whole region into three parts: first, the southern portion, including the wadis Muêla and Rayan; secondly, the central area, comprising the extensive plain forming the floor of the depression as a whole, and including the areas under cultivation and the Birket el Qurûn, as well as the desert separating the Fayûm from the Nile Valley. Thirdly, the northern portion, embracing all the rising ground between the floor and the northern rim of the region. These areas will now be taken in order.