The geological constitution of Egypt is very simple. In the Etbāi chain, whose northern offshoots go beyond the 28 ° lat. Between the Nile and the Red Sea, the most ancient precambric soils emerge, forming the backbone of the region: gneiss and crystalline schists, with intrusions of normal granites and amphibolic red granites, diorites, syenites – which derive their name from Siene, the current Aswan (Aswān) – and large concentrations of sometimes auriferous quartz. There were also reported ores of copper, lead and zinc and gems.
According to allpubliclibraries, this formation also extends into the Sinai (v.), Of which it forms the main massif, and widens towards the south; there it is engraved by the Nile at Kalābshah and then at Aswan, where it determines the formation of a rapid: the first cataract (see aswan). The Paleozoic deposits are represented only by sandstones and marls with an intercalation of fossiliferous limestones of the Carbonic, which emerge only in the lower part of the W. el-‛Arabah, but continue on the opposite side of the Gulf of Suez, into Sinai. On the Paleozoic, or on the most ancient rocks, a very monotonous arenaceous-marly formation, known as Nubian sandstone, rests directly, the lower part of which has intercalations of limestone and marl with marine fossils from the Liassic (already reported by the Italian Figari in 1864) and the Oolithic, recently recognized by Sadek in the G. el-Qallālah el-Baḥarīyah (valley el- ‛ Arabah; the exact spelling of the name would be el-Baḥariyyah, and so analogously for names with the same ending; however for practical considerations a simplified transcription has been adopted in this treatment).
Further south, however, the variegated sandstones, red, gray, etc., interspersed with marl, sometimes with hardly decipherable vegetal footprints, rest directly on the archaic substrate and represent the whole series up to the Middle Cretaceous period. These Nubian sandstones emerge largely on the Nilotic side of Etbāi and in the Nile valley itself as far as es-Sebā‛īyan, and extend into the Libyan Desert over a vast area south of the el-Khārǵah and ed-Dākhlah oases (v.), reappearing below the Cretaceous period at the oasis el-Baḥarīyah (v.): the groundwater of these oases circulates in the porous layers of the sandstone. An invasion of the sea produced from the north during the Middle Cretaceous, and extending up to around 23 ° lat., Determined the formation of limestone and fossiliferous marl not very widespread in Egypt del Nilo, where however they appear, in strips, from Wādī el-‛Arabah to Edfū; rather extensive in the Libyan Desert, especially on the edge of the depression of el-Khārǵah, ed-Dākhlah, el-Farāfrah and el-Baḥarīyah and in the vicinity of Cairo itself, in Abū Ra‛āsh. All the middle and upper Cretaceous levels are represented by fossiliferous strata: the Vraconnian, the Cenomanian, the Turonian, the Senonian, the Danian.
The Cretaceous fossils of Egypt are perhaps the first known fossils: the ammone horn (ammonite or as others claim a gastropod shell) is cited by Pliny as a sacred symbol of Egypt, which in the oasis of Sīwah (v.) revered Jupiter Ammon.
The middle Senonian (Campanian) often contains banks rich in phosphates as well in the oases of ed-Dākhlah and el-Khārǵah, as in the Western Desert and in the region of el-Qoṣeir, where the field is successfully exploited by an Italian company (see. el – qoṣeir). From the Cretaceous to the Eocene there is a gradual transition, represented by the schists of Isnā (v.) Which in the surroundings of that city provide the “ṭafla” used as fertilizer. The lower Tertiary with its various levels very rich in fossils, forms a large part of the soil of Egypt: due to its calcareous nature, more resistant than the schists and sandstones below, it remains in relief on them, and its horizontal banks form plateaus which can be seen flanking the Nilotic valley on both sides. In the Western Desert it goes as far as the border, from here extending continuously to the NW, obliquely, up to Sīwah, to N. up to the immediate surroundings of Cairo: the platform where the pyramids of el-Gīzah stand is full of large nummulites (Nummulites gizehensis). In the Eastern Desert the Eocene extends with isolated edges up to Luxor and forms the great plateau of the el-Ma‛āzah which from Qinā extends north to the heights of Jebel Atāqa near Suez and those el-Muqaṭṭam overlooking Cairo. The highest levels of Nummulitic (Oligocene) are represented in the Libyan Desert by deposits at least largely brackish and continental, arenaceous, which supplied the Geb. el-Qaṭrānī (Northern Fayyūm) leftovers of silicate wood (Nicolia), of fish, tortoises and crocodiles, monkeys (Propliopithecus), anthracoterae, protocarnivores (Hyaenodon) and very interesting leftovers of primitive pachyderms (Moeritherium, Palaeomastodon). At the same age, most fossil woods are attributed, which in large numbers lie on the ground in the hills west of Cairo: the so-called “petrified forests”. In the Miocene, the sea returned to invade Egypt, pushing beyond the oasis of Sīwah to el-Qaṭṭārah, el-Maghārah, Wādī en-Naṭrūn, el-Qallālah el-Baḥarīyah south of Suez, and leaving behind limestone deposits rich in shells ; but the upper Miocene is again continental, with freshwater faunas in el-Qallālah. A new but more restricted marine invasion took place in the Pliocene, and at this age we must ascribe certain arenaceous-calcareous deposits, which also emerge near Cairo, pushing as far as el-Fashn between Benī Suéf and el-Minyā, and from which the bei Clypeaster aegyptiacus, which the Bedouins offer to tourists who visit the pyramids of el-Gīzah. The Wādī en-Naṭrūn had emerged and populated with hippos and mastodons.
During the Tertiary, and more especially (it is believed) in the Oligocene, a notable volcanic activity occurred in various points of Egypt, leaving basaltic flows in the oasis of el-Baḥarīyah, al Geb. el-Qaṭrānī, al W. Natash in Etbāi and also in the region between Cairo and Suez, where extinct volcanism still manifests itself with thermal springs (Heliopolis); and the formation of the quartzites of Jebel el-Aḥmar is also due to thermal waters. After the end of the Pliocene there was a period of greater rainfall (Pluvial), traces of which are found especially in the Arabian Desert and in the Libyan oases: the Red Sea then communicated for a short time with the Mediterranean. Subsequently new movements determined the emergence of
From these recent coralligenous deposits, some think may have come the petroleum which gushes out at some point of the coast, especially at Jebel ez-Zeit, Rās Gimsah and a little north of Safāgiah; however, most believe them to be connected to more ancient formations. As for the continental formations, the most ancient deposits of the Nile would be the pebbles and sands of the Oligocene, which cover the limestone platforms for large stretches. The recent alluvial deposits, which form a long and narrow strip on the two banks of the river and fan out into the Delta, consist of repeatedly alternating layers of sand and clay; and of these the former correspond to some aquifer levels, while the superficial clays represent the substrate of a very fertile vegetable soil.
Along the sea the Delta is bordered by sandstone-limestone dunes, partly cemented and oolithic, as in el-Maks near Alessandria, partly still mobile; in the Arabian Desert, shifting dunes of not great extension abound; the Libyan Desert, on the other hand, is encumbered on vast areas by dunes, which are partly intended to be reconnected with the ancient floods of the primitive Nile, now taken over by the wind.
The architecture of the region is of the simplest. Only the precambric rocks of Etbāi are intensely folded and raised; the more recent deposits generally have, as in almost all of the rest of Africa, a tabular structure: they have been raised together more or less noticeably, but not folded; the only tectonic accidents observed there (apart from three broad folds in the northern region, the latest repercussions of the Syriac retreat) are more or less extensive and deep fractures.
The trench of the Red Sea, that of the Gulf of el-‛Aqabah and of Palestine, would be due to the most grandiose, perhaps, of the phenomena of this kind and similar phenomena would also have influenced the formation of the depressions of the oases (see Africa: Geology).