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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 Fayum 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 Fayum 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° 3d 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-move- 
ments initiated the formation of the lower part of the Nile Yalley. The determin- 
ing 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 Yalley trough or “grab 5 ’ 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 Fayum, 
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 
Yalley north of latitude 24° was occupied by a series of deep freshwater lakes, 
perhaps co-existent with that in which the Fayum gravel terraces were accumu- 
lated. 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 
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