Our knowledge of the junction of the Cretaceous and Eocene in several parts of the country leaves much to be desired. Where the Eocene is most fully developed its basal member consists of a group of green argillaceous deposits, known as the Esna shales, well seen at the base of the cliffs throughout the Esna-Qena reach of the valley. These beds everywhere pass conformably upwards into the Lower Eocene (Libyan) limestones above, but in the Nile Valley and the Eastern Desert the exact line of demarcation between them and the lithologically similar Cretaceous clays below is still somewhat obscure. In Kharga and Farafra they form a well-marked band between the White Chalk (and associated clays) at the top of the Cretaceous and the Libyan limestone of the Lower Eocene. The Esna shales may in fact be regarded as passage beds, and where they exist appear to bridge over the lapse of time which is represented by a decided unconformity between the Cretaceous and Eocene in the north of the country, as in Baharia Oasis and at Abu Roash.
The thick mass of limestone which forms the plateaux and cliffs on both sides of the valley from lat. 25° N. to Cairo is of Lower Eocene (Libyan stage) and Middle Eocene (Mokattam stage) age. These limestones, frequently nummulitic and typically marine calcareous accumulations, exceed 500 metres in thickness, and over a wide area are unrelieved by a single band of clay or sandstone. Towards the summit of the Middle Eocene, however, terrigenous deposits were laid down, the Upper Mokattam consisting of an alternating series of impure limestones, clays, and sandstones. In the Fayûm the Middle Eocene is followed by a great thickness of fluvio-marine deposits of Upper Eocene age, in which the remains of the animals that inhabited the land to the south and the adjoining seas at the time are abundantly preserved.
50. Oligocene and Miocene.
Throughout Oligocene and Miocene times conditions similar to those which led to the deposition of the Upper Eocene formation in the Fayûm prevailed, accompanied by a continual retreat of the sea to the north. In the littoral area marine beds were intermingled with the sediments brought down by rivers from the land to the south; and throughout these deposits the remains of land animals and great quantities of large silicified trees are common. A considerable part of the deserts east and west of the valley north of lat. 29° 30´ is covered with deposits of this age, and shallow water Miocene beds, unconformably overlying the Eocene, form marked flanking plateaux to some of the igneous ranges of the Red Sea Hills.
51. Pliocene, Pleistocene and Recent.
In Pliocene times the relative areas of land and sea approximated to those of to-day and powerful earth-movements initiated the formation of the lower part of the Nile Valley. The determining faults and the huge blocks of displaced rock are visible along the cliff walls in many parts of the valley, and at Gebelain isolated ridges of highly tilted limestone protrude above the floor of the trough, though as a rule, except near the cliffs, the faulted rocks are invisible, being buried under great thicknesses of lacustrine and fluviatile deposits. A few kilometres south of Jebel Silsila, however, Eocene and Cretaceous limestones are met with at river level in the centre of the valley and point to the Kom Ombo plain being let down by a fault of over 400 metres throw.
The Nile Valley trough or “grab” became a marine fiord in later Pliocene times, sea-beaches being formed up to 70 metres above present sea level. Extensive terraces of gravel, perched up on the surrounding slopes of the Fayûm, prove that the sea, or a great inland lake, stood at 180 metres in latest Pliocene or early Pleistocene times. From this time also dates the Red Sea (in its modern aspect), the highest Older Pleistocene coral reefs being now found at some 200 metres above sea-level; younger reefs associated with later Pleistocene gravels occur at a lower level. In later Pleistocene and early pre-human times, under the very moist climate which preceded the present desert conditions, the Nile Valley north of latitude 24° was occupied by a series of deep freshwater lakes, perhaps co-existent with that in which the Fayûm gravel terraces were accumulated. The denudation of the surrounding country was rapid, and tributary streams from the plateaux on either side brought down fine limestone detritus, which was deposited along the margins of the lakes in the form of compact beds of re-made limestone, interbedded with frequent layers of conglomerate and gravel, washed down by the larger streams and by torrential floods. In the quieter parts of these lakes clays and calcareous tufas were laid down and are visible to-day from Kom Ombo to Heluan. Subsequently, owing to the breaking down of the dividing barriers, or as the result of a general slight elevation, drainage became more pronounced and the river cut its way down through these lacustrine deposits. It was probably at this time that, following the partial removal of the gravel ridge between the Fayûm and the valley, part of the drainage obtained access to the Fayûm depression and a lake, the precursor of the historical Mœris, was formed. Subsequently, under climatic conditions similar to those of to-day, the accumulation of Nile alluvium commenced within the wide trough cut out in the older lake beds. Flood plains were formed on either side of the river, and by successive deposits, at the rate of about twelve centimetres a century, were built up to their present level.
52. Economic products.
Limestones for building and other purposes are abundant in the Lower and Middle Eocene formations, though as a rule of only medium quality. The chief quarries are those of Jebel Mokattam, Tura, Heluan, Abu Foda, Harîdi, and el-Tarif. At Isawia, near Tahta, a fine tough freshwater tufaceous limestone of Pleistocene age occurs, and was largely used in the construction of the Assiut barrage. Clays of good quality are not widely extended, though certain bands of the Esna shales are very largely used near Qena in the manufacture of pottery. For bricks the Nile alluvium is used throughout the country.
Sandstone is quarried for local purposes at J. Ahmar near Cairo and in several localities in the south part of the country; it was formerly extensively quarried at J. Ahmar for the temples of the Delta, at J. Silsila for those of Upper Egypt, and at Girtassa, south of Assuân, for the Nubian monuments. Although a fairly hard and good weather-resisting stone when carefully selected, the frequent presence of soft uncemented and clayey laminae gives it an unreliable character, and a good deal of the decay of many of the ancient Egyptian monuments is attributable to this cause.