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.
Section IV.—WADI RAYAN AND NEIGHBOURHOOD.
This part of the Fayûm is of special interest in consequence of its possible future as a reservoir. Although the area has not yet been examined in detail by the Geological Survey it will be useful to bring together all the information that is at present available.
Colonel Western’s Survey.In 1882, as a counter-project to other irrigation schemes, Cope Whitehouse suggested[17] utilising as a reservoir the Wadi Rayan, a depression which had been referred to by Linant de Bellefonds.[18] At the request of Sir Colin Scott Moncrieff the Government deputed Colonel Western to make plans of the Wadi Rayan and surrounding country and to ascertain the capacity of the depression and its capability of being used as a reservoir. Liernur Bey under his direction prepared a contoured map, and Colonel Western’s report, plans, and estimates were published.[19] Some general details of the wadi and surrounding hills are given and the detailed survey showed that the 30 metre contour line (above sea-level) enclosed an area of 706 square kilometres (170,000 feddans). The lowest points of the depression were found at 42 metres below sea-level. The sand, scrub and springs are briefly referred to and the discharge of the latter is given as equal to that of a very slow-going four inch hand pipe, the water running out at about + 20 m. and disappearing in the sand. Wadi Muêla was found to be separated from the Rayan depression by sandhills and rock at a mean level of + 50 metres, the lowest point in Muêla being at + 25 metres. A line of levels was run from Rayan through Muêla to the Nile Valley, the highest point crossed being at + 105 metres; for fifteen kilometres the level was not below + 75 metres. In order to find the most suitable passage for a canal to connect the Nile with the Wadi Rayan two lines of level were made after a reconnaissance of the hills near Sidmant el Jebel: the southern, from Ezba Menesi Ali, near the Gharaq canal, to Mazana on the Bahr Yusef, being considered the best. Along this line the highest point was only at + 44·7 metres and the average + 35 metres along four kilometres. Borings were not made here but judging from the surface excavation would be mostly in soft limestone, sand, and conglomerate. A much shorter route is from Deshasleh on the Bahr Yusef over the hills about 5 kilometres to the south of Mazana or Sidmant into the Wadi Gharaq, a distance of 30 kilometres. This route was not however levelled but is fairly straight and apparently not much higher than the Mazana passage.
The survey of the + 30 metre contour line of the Wadi Rayan proved that there were only two outlets into the Fayûm, both on the northern side: these two openings are only from 400-500 metres wide and their lowest points are not below + 25 or + 26 metres.
Later Government Publications by Scott Moncrieff and Willcocks.In 1889 Sir C. Scott Moncrieff published[20] a further note, in which he briefly discussed the probable cost and benefits to be derived from the suggested reservoir, concluding that at least the project was one worthy of being thoroughly examined.
In 1894 the plans and designs in connection with the Wadi Rayan were published[21] and the possibility of utilizing the Wadi Rayan was examined by Sir William Willcocks, then Director General of Reservoirs, from an engineering point of view, and the questions of its probable cost and future utility were discussed. In this report it is stated that the routes proposed by Colonel Western in 1888 pass through salty marls and clays unsuitable for holding canals. Another route is suggested, which after leaving the Nile Valley crosses the high desert ridge in a straight line, passing through the so-called Wadi Liernur (Wadi Lulu of Cope Whitehouse); this depression is 12 kilometres long and has its bed some 24 metres below the general level of the desert. Plate 15 of the report shows the Wadi Rayan, the deserts between it and the Nile Valley and the cultivated land. The map was begun by Col. Western and completed by Willcocks. The lowest point of Wadi Rayan is shown as − 42 metres and the depression is separated from the Fayûm by a limestone ridge generally from + 34 to + 60 metres, except at two places where it falls to + 26 metres above sea level on a length of 600 metres. Within the + 27 metre contour line the wadi has an area of 673 square kilometres and a capacity of 18,743,000,000 cubic metres. Between it and the Nile Valley lie 30 kilometres of desert, of which 11 are occupied by a marked depression discovered by Liernur Bey in 1887. At the extreme western edge of the Nile Valley (here 20 kilometres wide) runs the Bahr Yusef. Comparing the proposed Wadi Rayan reservoir and the ancient Mœris and allowing for a difference of 4·5 metres between the levels of the Nile Valley in B.C. 2,000 and to-day, Willcocks assumes that the high water mark of Lake Mœris was at + 22·5 metres and its area 2,500 square kilometres, against 673 square kilometres of the Wadi Rayan at + 27 metres. It is pointed out that the ancient lake had the great advantage that in those days the Bahr Yusef was an important branch of the Nile, if not the main river itself, and the reservoir was connected with the Nile by a natural ravine of great length and short breadth, across which a massive embankment was thrown and destroyed annually, the surplus water of high floods being stored for the deficiency of low floods.
The published sections along the lines of borings put down show the different strata cut through by the proposed canal. The Nile Valley, along the line of the inlet canal, consists of hard clay 6 to 10 metres thick, lying on coarse sand. Along the outlet canal sandy clays and clays alternate to a depth of 10 metres. On entering the desert sands and sandy conglomerate, with gypsum and salt, are met with below the surface, then a yellow marl with salts, and finally a plastic black clay overlying the Parisian limestone. These beds are most extensive in the narrow neck of land between the Nile Valley and the Fayûm and to some 10 kilometres to the south of it. They rise to + 70 metres. There are some other marls inside the Wadi Rayan or in the adjacent depressions and as they have to be traversed by the canals form a serious factor, being easily dissolved in water; in consequence Willcocks chose the alignment of the inlet canal along the Bahr Belama where the extent of these beds would only be 2·5 kilometres against 9 kilometres on the alternative route marked on the plan. A narrow neck of land, some 15 kilometres in length, runs between the Fayûm and the depressions traversed by the proposed Wadi Rayan canal; this neck is the continuation of the salty marls and clays, but the limestone is near the surface and is overlain by a thin deposit of sand and pebbles, with freshwater shells on its northern slope at + 22·50 metres; the southern slope is entirely devoid of them. Willcocks points out that it is evident the ancient Mœris rose to + 22·50 metres but its water never penetrated into the Wadi Rayan. The report goes into details of inlet and outlet canals, discharge, necessary masonry works, cost, and compares the different reservoir schemes.