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Please note: This section was written to accompany a paper on the Geology of the Faiyum. It was never intended to be a geology of Egypt as a whole. However, in the course of explaining some of the processes that lead to the development of the Faiyum Depression, it was necessary to describe other aspects of the Egyptian geological background. Please be aware that this section is a narrative leading towards the Faiyum, and does not attempt to be a comprehensive description of Egypt’s geology. For the full text, including the Faiyum Depression, please see my sister website www.faiyum.historians.co.uk and for other geology links, please see www.egypt.cd2.com. For an excellent introductory book on the subject, please refer to Bonnie Sampsell’s “Traveller’s Geology of Egypt” (AUC). Anyone wishing for more detailed references should email me: andie@easynet.co.,uk. It is not obvious that there should be any such thing as a “Geology of Egypt.” Egypt is a state, a social construct. Political boundaries can be drawn anywhere. What they enclose may bear little relation to physical realities. In fact, Egypt not only has a coherent geology, but one which is coherently related to human occupation. First, throughout the historic period, the Nile, the desert and the Red Sea mountains have defined a territory, and constrained human activities. Second, a small set of physical parameters have worked together to produce the landscapes of Egypt, now and over geological time. The detail is incredibly complex, but the outline is surprisingly clear. We approach the geology and geomorphology of the Faiyum by sketching out the processes which have shaped Egypt as a whole. Tectonics, Transgressions and Deflation The geological history of Egypt is a drama played out by a few geological agents, on a moving platform. The platform is the African Tectonic Plate. The main actors are:
The remainder of this Section introduces the key components one by one. It then sketches out the plot. The Plate is the stage on which the geological history of Egypt and the Faiyum has been played out. Under the Faiyum and Western Desert it is immensely stable, but it has active edges in the east, along the Red Sea, and in the South at Gebel Uwaynat and Gilf Kebir. The active margins of the plate have been the engines driving the geomorphology of Egypt. Egypt is situated on the north east portion of the African Plate. In Egypt the “basement level” of the Plate, below which no sediments occur, is buried deeply below marine sediments, the results of successive transgressions, at paces 9km deep (Hantar 1990). In the north of the Western Desert these deep sediments are covered by a thin layer of marine, fluvio marine, or aeolian deposits. They give little indication of the plateau’s long, complex geological history, but have been central to the formation of the Faiyum, and its history of human occupations. To the north of the Plate is a continental shelf 15 to 50 km wide, bounded by a tectonically active fault line. Under the Western Desert, the Plate has not been subject to the rigorous mountain building and folding found to the north in Europe. It has not experienced the tectonic upheavals which have produced the Great Rift Valley and the Equatorial Lakes. The movement at the margins has produced many smaller faults, mainly in a band running westwards between the coast and Beni Suef (Hantar 1990). They result in minor seismic activity, like the earthquake which severely damaged the pyramids of Abusir, in 1914. They are the origin of the sill which caps Gebel Qatrani in the Faiyum. But “there are few lines of major faults and the few folds noted are minor rolls with gentle dips and large amplitude” (Hantar 1990). The African Plate as a whole is shaped into a number of basins with rims or rolls at their edges. North of the equator, they include Chad and the Sudan and, to the south, the Congo. Each forms a major drainage system (Said 1993) with its own river systems. Map 3, below, illustrates schematically the tectonics of Egypt, the Plate, the active edges and the basin rims. The Nile appears as an obvious anomaly, draining the Sudan basin, the Ethiopian Highlands and the Equatorial Lakes, across the basin rim. The human occupation of Egypt is the gift of a geological anomaly. See map 3 above, and map 5 later in the text
Map 3: Sketch Map of Egyptian Tectonics Stable Northern Desert Plate Active Plate Margins and Rifts Basin Boundaries (after Said 1992) Uplifted Highlands Shorelines and Streams Egyptian Border
The sea has flowed across the Plate from the beginning of the Palaeozoic, leaving sediments kilometres deep. It has then receded and erosion has set in. The sea then returns. Each transgression, and the erosions which followed, are described in the section on the Western Desert. Here it is necessary only to observe the sea advancing and retreating, building up the geological map of Egypt. There were three transgressions, one minor one, and one local advance.
In addition to this sequence of marine sediment, much of northern Egypt is covered with Oligocene fluvial deposits left by rivers draining the newly rising Red Sea Mountains. See Map 4, below:
Depression Escarpments Dotted lines mark the boundaries of Said’s Geomporhpoligical zones: Sampsell’s Simplified Groupings of Sediments (2003)
Deflation, Deposition and Landforms When the sea withdraws, wind and water begin to remove the sediments, returning them to the sea, or redistributing them across the land. In the process, they create the landforms and the topography which we find in place today. The period of erosion which most concerns the Faiyum and the topography of modern Egypt is the stripping of the limestone of the Eocene transgression and the erosion of the Red Sea Mountains. Both processes took place from the end of the Eocene, throughout the Oligocene, and into the Pliocene. It is described in some detail below, in the history of the Western Desert, the Nile and the Faiyum. In outline, it is simple:
At this point in geological time, the rivers have stripped the Eocene limestone off much of south and central Egypt, expulsing older sediments, in bands which run roughly east to west, with the oldest exposed sediments in the south. Large areas of the north are covered with river gravels transported from the Red Sea Mountains. The geological and topographical maps of Egypt are almost in place. See map 6, below:
The Nile, in its present form, does not come on stage until the Holocene, about 10,000 years ago, but it has had many precursors. They are described in the section on “The Making of the Nile.” The history of the making of the Nile at first follows the growth and evolution of the river systems which we have just described, the Eonile carving a deep canyon when the Mediterranean dries up and capturing the headwaters of the south-flowing Quena. With the Atlantic connection established, the canyon fills with sediment. The succeeding “Palaeonile” dries up, in desert conditions. The second part of the story beings during the Pleistocene, when the “Prenile” and “Neonile” make African connections. The modern Nile is established when the connection becomes permanent, in a period of high rainfall, towards the end of the Pleistocene. Then, the Nile has to cut through the Nubian Massif and established a permanent route across the Sudan. The Equatorial lakes cut channels to each other. A perennial river, fed all year from the Equatorial lakes, and in summer from the Ethiopian monsoon, begins to flow. This outline geological history of Egypt is filled out in the following sections. The marine transgressions, the tilting plate, the river systems and the Nile appear at every stage. They come together in the making of the Faiyum. See Map 5, below:
The Making of the Western Desert The Faiyum is a part pf the Western Desert, but a very distinctive part. It has been shaped by the processes which shaped the Desert and the other great depressions. However, from its inception it has been influenced from the north and east – from the north by the Mediterranean, from the east by the Red Sea Mountains, and by the precursors to the Nile. The deep sediments which overlaid the Basement Complex were laid down in four marine transgressions, flowing from north to south. The Palaeozoic Transgression laid down sediment kilometres deep. The sea flowed in and out many times, each advance marking on of the major Periods of the Palaeozoic Era. The only period visible today is the Carboniferous, exposed by the erosion of the Gilf Kebir Plateau in the far south west of the country. The Cretaceous Transgression overlaid the Palaeozoic Transgression, it was a planetary event. The Tethys Sea covered Egypt, depositing muds and sands on its advance, laying down deep layers of limestone once the land was covered. About 40% of the present land surface of Egypt is covered by exposed Cretaceous sediment. To the north of Esna it is mainly limestone and, to the south, mainly sandstone. The Eocene Transgression was not a major event on a world scale, but, in Egypt, it reached to Aswan. Once again, the sea advanced and retreated many times, covering the Cretaceous with shales and limestones. Eocene limestone is the bedrock out of which the Faiyum and the other desert depressions is carved. The Oligocene, which followed, did not see a marine transgression, but rivers draining the north of the Red Sea Mountains left huge swathes of sand, conglomerate and pebbles in the latitude of the Faiyum. The Miocene Transgression was a minor affair on a word scale, but it reached to about 60km south of Cairo, and its sediments make up the surface of the desert to the north of the Faiyum. The fact that the Cretaceous sediments are exposed across 40% of Egypt’s surface means that strata laid down in the Eocene have been stripped off to expose them. If the stripping had been uniform, by a set of rivers flowing northwards towards the Mediterranean, Map 4 would be a set of somewhat wavy bands going from east to west, with the oldest sediments exposed in the south. To the east of Farafra, the strata more or less follow the script. But Map 4 shows the Cretaceous band narrowing to a point at Kharga and, to the west of Kharga, both the Eocene and Cretaceous throw great lobes southwards. Map 4 also shows that all the depressions are located at the boundaries of the Cretaceous and Palaeocene, or the Eocene and the Miocene. And, to the northwest of the Faiyum, there is an extensive area of Oligocene sediments which have no place in the history of marine transgressions. To explain why these things should be, we need to follow the erosion of the plateau surface, step by step, from the end of the Eocene. New Knowledge, Radar Rivers and an Empty Sea A widely accepted account of the erosion of the Western Desert, the Oligocene sediments, and the formation of the depressions, was proposed by Issawi and McCauley in 1992. Issawi, Osman and Meibed extend that account to provide a step by step narrative of the formation of the Faiyum (Issawi et al 2001). Barring a few questions on the Faiyum, we follow Issawi and McCauley. Even to the layman theirs is a brilliant and comprehensive account. Two recent discoveries are central to the theory:
Issawi and McCauley’s account begins towards the end of the Eocene. It then moves forward in stages.
The Present Geomorphology of the Western Desert The outcome of the marine transgressions and the Issawi and McCauley river systems, is the modern Western Desert. Only 3% of the area of Egypt is agricultural land. There are small mountainous areas in the south west and along the Eastern border but most of Egypt is desert with the Western Desert occupying about two thirds of its surface area. The average rainfall for Egypt as a whole is about 1cm per year. Even on the Mediterranean littoral it is less than 20cm. There have been wet periods when active wadis and carbonic acid solution of the sediments have played a major role, but the current geomorphology of the Western Desert is dominated by the effects of wind and sun. Said (1990) divides the Western Desert into the three geomorphological zones, indicated in map 4.
Although the Faiyum is not a spring-fed oasis, all accounts of the Western Desert speculate upon the aquifer, which provided the water essential to human habitation in the other great depressions. The aquifer is held in the Nubian and other Cretaceous sandstones, which are exposed in the south, but lie between the Palaeozoic and Eocene under the desert to the north. The amount of water is incredible. Sampsell expresses it as the equivalent of 600 years of Nile discharge. The water originates in the wet areas to the south of Egypt, and migrates northwards through the permeable strata. Popular accounts talk about the aquifer as if it were a single underground river, looping its way around the oases, on its way to the sea. There is no rationale for this pleasant conceit. Said proposes that the water, while widely spread, is held in the series of high rimmed basins revealed by oil exploration. Unfortunately, tests show the water to be of ancient origin, and susceptible to depletion.
The Faiyum is the offspring of the Western Desert and the Nile. Being at the north of the plateau, it has been subject to influences which have not affected Bahariya and the depressions to the south:
The modern Nile travels, all year, through the 2700 kilometers between Atbara and the sea, without receiving water from tributaries. It floods every August. Said describes it as “a geological freak” and “the most reliable river in the world” (Said 1993). It is easy enough to explain why the Nile is perennial, floods on time, and needs no new water on its journey through Egypt. The Nile receives its waters from two sources:
Explaining how this happy state has come to pass is far from simple. The following account follows Said 1992. It follows Issawi and McCauley’s account of the capture of the Quena river’s headwaters by Said’s Eonile. Said and Issawi trace the making of the Nile through a number of precursor rivers:
At the beginning of the Holocene, the Nile becomes a perennial river and a connection is cut to the Faiyum. At this point the geological histories of the Nile and the Western Desert converge. A modern Faiyum takes shape. The barrier to a permanent African connection were overcome by three coupled sets of events
About 6,500 years ago, the sea level rose as the ice melted. The river ceased to incise its bed. The historic Nile regime was in place. With some repetition this complicated genealogy can be summarised in terms of Said’s Nile precursors. The Sediments of the Nile Valley A complex geomorphological history has left a complex river bed, valley and flood plain. An adequate account is beyond our present scope. Below the bed of the modern river are sediments laid down by each of the Nile precursors, from the Pliocene transgression to the historical Nile silts. Their composition reflects the origins of the waters feeding the river in each of its different stages. Many years of excavation, in the river bed and valley, have revealed river bottom and flood plain sediments, deep river channels, natural levees, alluvial fans, and runs of river terraces. The most influential periods have been:
For those with a more specific interest in the valley there are, in addition to Said, the classical studies of Butzer (Butzer 1976) and Wendorf and Schild (Wendorf and Schild 1976), and the work of Adamson et al 1980. Butzer 1966 is a superb short introduction both to the Nile Valley and alluvial archaeolog
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All text and images on this site are the copyright of Andrea Byrnes andie@easynet.co.uk 2003, 2004, 2005 unless otherwise stated |
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Map 4: Western Desert. Schematic Geology and Geomorphology
