+------------------------+----------------------------+---------+ | | |Thickness| | Geological System. | Stage. | in | | | | Metres. | +------------------------+----------------------------+---------+ | | | | | |{Sand-Dunes }| | |RECENT AND } |{Spring Deposits (modern) }| Very | |PLEISTOCENE} |{Lacustrine Sands and Clays}| variable| | |{Calcareous Tufa }| | | | | | | | | | | | | | |LOWER } Lower Libyan| Plateau Limestone | 115 | |EOCENE} | | | | Passage Beds| Esna Shales and Marls | 55 | | | | | | |{White Chalk }| 70 | | {Danian |{Ash-grey Shales }| | | { |{Exogyra Beds | 30 | |UPPER { | | | |CRETACEOUS { |{Phosphate Beds | 70 | | {Campanian |{Purple Shales | 50 | | {(Nubian |{Surface-water Sandstone | 45 | | { Series) |{Impermeable Grey Shales | 75 | | |{Artesian-water Sandstone | 120 | | +---------+ | Total | 630 | +-----------------------------------------------------+---------+ places the still older underlying grey shales are exposed. The purple or red shales generally form the rising ground towards the escarpments, at the base of which are usually found the phosphatic beds, with hard, pronounced bands made up of fish-remains and phosphatic nodules. Above come the Exogyra Beds, with thick bands of limestone almost entirely composed of large oyster-shells. Rising up above these is the generally well-marked cliff of grey shales, capped by a snow-white chalk of much the same age geologically as the well-known chalk of the South of England. The summit of the chalk frequently forms a separate plateau, subsidiary to the high desert tableland, and separated from it by the cliffs formed of the massive Eocene limestones.