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.