(3) An extensive beach or terrace of gravel on the south-east, east, and north sides of the Fayûm, attaining a maximum level of about 170 to 180 metres above sea-level.
All the known facts seem to be satisfied if we imagine that in the Middle Pliocene the sea occupied the area, depression probably continuing until the 112 metre level with the highest borings was reached. Perhaps the lowest part of the terraces was formed during this time. In the later Pliocene times we may infer that the area was occupied, up to 180 metres above present sea-level, by a vast inland lake, perhaps of brackish water, connected with the sea on the one hand and the fiord or lakes of the Nile Valley on the other, the deposits of which have been described elsewhere[88].
Along the margins of this gigantic lake, these great accumulations of gravel might well have been formed, chiefly of material derived from the immediate shores, augmented perhaps by a certain amount of sediment brought by river-currents from the south.
Finally, from the way in which the gypsum and gypseous deposits are laid on the terrace gravels, and from their extension and thickness, we may presume that they were deposited on the bottom of just such a lake on evaporation of its water, when the sulphate of lime in solution, becoming more and more concentrated, may have been finally precipitated.[89]
Section XIII.—PLEISTOCENE.
The course of events in Pleistocene times is at present obscure. As far as can be judged it was during this period that a freshwater lake, the precursor of the great Mœris, came into existence. It might be thought that the early Pleistocene prehistoric lake was a relic of the still older body of water of Pliocene times, in which the gravel terraces and gypseous deposits were laid down. But such a remnant would have been of a high degree of salinity and could not have given rise to the fresh water Mœris. Most probably at the close of the Pliocene period, after the formation of the gypseous deposits, the area became elevated and cut off from the sea and from the Nile Valley marine fiord; probably an extensive body of water remained as an isolated lake, but this, cut off from external supply, would have gradually evaporated, its salt being left as a superficial deposit on the dried up bed. In early Pleistocene times we may presume the area became dry and was gradually eroded to its present shape and depth. The superficial deposits of salt and gypsum were for the most part removed as the depression was deepened, while the continuous terrace of gravel laid round the greater part of the rim was broken through, except where protected in favourable localities, the constituents being washed down and spread out over the lower ground. In course of time the region was moulded to its present form and dimensions, or rather to what it would be if the local alluvial deposits were stripped off and the water of the lake baled out. The area was an inland depression, probably sparsely vegetated like the rest of the higher country and separated from the Nile Valley by a low rocky ridge surmounted by a more of less continuous terrace of gravel of considerable height and thickness. In our opinion desert conditions had already set in before the early Nile broke down the ridge and formed a lake in the Fayûm; the date when this important event first took place is a matter of considerable doubt, as has already been mentioned (pp. [24,] [25]). We know that at the close of the Pliocene period the Nile Valley was a marine fiord (connected with the Fayûm and the Mediterranean) which was replaced in Pleistocene times, probably in consequence of slight elevation, by a series of fresh water lakes throughout the valley. These lakes were probably two or three in number and drained one into the other; the exact position of the barriers is as yet a matter of conjecture. Within these lakes thick lacustrine deposits were accumulated, so that the basins eventually became to a great extent silted up. In later Pleistocene times drainage down the Nile Valley appears to have become more pronounced, the barriers between the lakes were broken down and the river cut for itself a channel through the lacustrine beds, filling up the old lake basins. From this time onwards to early prehistoric times the bed of the river would appear to have steadily fallen, as it eroded its channel deeper and deeper. That this early Nile was a river of considerable size is evident from the amount of erosion it accomplished in the trough of the valley, whence the older lacustrine beds have been almost completely removed. Probably in the lower part of its course it swept against the base of the dividing ridge between the Nile Valley and the Fayûm depression. If, as we imagine, the river was at that time flowing some 20 metres higher than at present its currents would have met with little resistance from the loosely cemented terrace of gravel which formed the upper part of the dividing ridge. Once this was broken down the waters must have poured into the depression behind, until a lake of considerable size was formed. The sediment spread out over the floor in the shape of a fan, while at the same time deposits of sands and fine clays, blown and washed into the waters from the surrounding shores, were being slowly accumulated in the quieter and more remote parts of the lake.
For some time subsequent to the first connection between the Fayûm depression and the Nile, the latter continued to fall in level owing to continued erosion along its course, possibly the Fayûm again became completely isolated for a time. Subsequently in the earliest historic times under changed conditions the river commenced to carry and lay down the modern alluvial deposit of “Nile mud”, and from this time to the present day its bed has gradually risen. This is shown by the high Nile flood-readings on the early gauges of the Nile Valley; the nilometer at Roda shows a difference of 1·22 metres in 1026 years, or an annual rise of 0·12 centimetre, which is equivalent to 12 metres in 10,000 years[90]. Whether there was ever complete disconnection between the Nile and the originally formed lake in Pleistocene times is uncertain, but even so it was probably only during a comparatively short period: in early historic times the rise of the Nile bed must have brought about a reconnection.
The geological evidence for the existence of a great freshwater lake in Pleistocene and prehistoric times is afforded by the well-marked lacustrine clays and sands which are found over such a large area of the northern and western deserts of the Fayûm; the great extent of this lake will be seen by an examination of the accompanying maps. Its area must have been about 2250 square kilometres or about ten times the size of the modern Birket el Qurûn. The western limit may even have been further west than shown on [fig. 10.,][91] as some of the desert in the neighbourhood of Gar el Gehannem is very low-lying; or there may have existed subsidiary lakes in that direction. The upper limit of these ancient lacustrine clays is between 22 and 23 metres above sea-level, which exactly agrees with the figure adduced by Sir Hanbury Brown as the height of the more modern Lake Moeris from the evidence of levels. That Lake Moeris was simply the older prehistoric lake placed under artificial control admits of no doubt; the difficult question being as to when the lake first came into existence in prehistoric times.
The clays abound in freshwater shells and semi-fossil fish-bones of exactly the same species of fish (siluroid, etc) as still inhabit the Birket el Qurûn; probably some are even hardly different specifically from the Middle Eocene forms of the district. In addition remains of large animals are common, and include Hippopotamus, Elephas, Bubalis, sheep or goat, and Canis, with crocodiles and turtles, etc.[92]
Plate XV.