In the latest edition of Prof. Zittel’s map, the depression was shown as consisting of Cretaceous beds surrounded by an Eocene plateau, which was also supposed to separate it from the oasis of Farafra to the south.
Geological Survey, Oct.-Dec. 1897.A glance at [Plate I] will be sufficient to show that the work of the Geological Survey has resulted in large and important additions to our knowledge of this oasis. The Eocene is seen to form the surrounding plateau only on the north; while the Cretaceous, consisting of several well-marked divisions, occupies not only the depression itself, but forms part of the surrounding plateaux on the east and west sides, and is continuous with that of Farafra to the south. Moreover, the relations of the two great formations of Cretaceous and Eocene are now shown to be those of unconformability and overlap. The actual extent of volcanic rocks and post-Eocene lacustrine deposits, the latter a newly-discovered and important series, is now shown for the first time.[40] Important folding has been detected and numerous organic remains have been discovered.
The deposits met with in the district admit of classification as follows:—
| RECENT. | |||
| 1. | Sand-dunes, salines, and superficialdeposits. | ||
| POST EOCENE (Oligocene?). | |||
| 2. | Basalt and Dolerite intrusions. | ||
| 3. | Ferruginous sandstones and quartzites,with limonite and pisolitic iron-ore, probably lacustrine inorigin. | ||
| EOCENE (Middle-Lower.) | |||
| LowerMokattam-Upper Libyan. | ⎰ ⎱ | 4. | Limestones with Nummulites,Operculina, Echinids, Lucina, etc. |
| UPPER CRETACEOUS. | |||
| Danian | | | 5. | Thick-bedded White Chalk and greycrystalline limestone. |
| Cenomanian (Some Turonian or Senonian). | ⎱ ⎰ | 6. | Limestones and variegated sandstones. |
| Cenomanian | | | 7. | Sandstones, clays and marls. |
These groups will now be dealt with in succession, commencing with the oldest.
UPPER CRETACEOUS.
Cenomanian.
7.—Sandstones, Clays and Marls—These, the lowest and oldest beds, form the floor and parts of the walls of the depression. They are best developed and exposed in the north end, where their maximum thickness is about 170 metres; the base of the series is not seen, and thus their total thickness cannot be estimated. In the north part of the oasis they are capped, with apparent conformability, by Eocene limestones, although an intervening band of limestone-grit may occasionally occur. In the south, the series is followed by the higher Cretaceous divisions, while in the isolated hills within the depression these beds are capped either by Eocene limestones, by basalt or dolerite, or by Post-Eocene ferruginous sandstones and quartzites.
They consist of friable false-bedded variegated sands and sandstones, with harder dark-brown ferruginous bands, alternating with sandy shales and clays, passing through every gradation. Some of the sandstones are micaceous.
The clays are frequently saliferous, and bands of fibrous gypsum are occasionally seen.