By Colin Reader, January 2011
Introduction
The landscape of Egypt is the product of a long and often tortured geological evolution which reaches back almost to the origins of the planet. In some of Egypt’s more remote areas, such as the high mountains of the Eastern Desert and the Gebel Uweinat area in the south west, rocks possibly as old as two thousand million years are exposed at the surface. By contrast, the honey-coloured limestones that were used to build the pyramids are relatively young: perhaps as little as forty million years old. Over such huge timescales, both climate and the associated ecosystems have changed drastically. 500 million years ago, Africa was part of a single huge southern continent, with the area of Cairo located at about 70 degrees south and was buried deeply beneath glacial ice. The current arid conditions in Egypt have only developed in the last five or six thousand years.
Fig 1 – Simplified geological map of Egypt (after Sampsell)
Geological Timescales
In archaeology, as in most walks of life, the word ‘ancient’ is generally used to describe events within, perhaps, the last ten thousand years: hence the term ‘Ancient Egypt’ nicely encompasses the period from the death of Cleopatra through the pharaonic period and as far back as the earliest identifiable settled communities of the Pre-Dynastic Period. To a geologist timescales such as these are of little relevance: to adequately discuss geological time it is necessary to consider tens, hundreds and even thousands of millions of years, with the words ‘recent’ and ‘ancient’ taking on meanings that to the non-geologist can be difficult to comprehend.
As shown on Fig 2, the geological history of the Earth is divided into two major periods, the Pre-Cambrian and Phanerozoic eons, with the Phanerozoic having three principle subdivisions: the Palaeozoic (meaning ancient animal life), Mesozoic and Cenozoic eras. The Pre-Cambrian Eon accounts for most of the earth’s history, from the planet’s formation some 4,600 million years ago to a point about 550 million years ago when significant life can first be identified in the fossil record To help picture these vast periods of time and to place them in context, a useful approach is to reduce the 4,600 million year geological history of the earth to a single twenty-four hour day, which is then divided into segments to represent the various geological periods (the times of the day that correspond with the various geological periods are given in the right-hand column of Fig 2).
The Geology of Egypt and the Evolution of the Egyptian Landscape
Most of Egypt is underlain at depth by Pre-Cambrian rocks, referred to as the Pre-Cambrian Basement. These ancient rocks form a foundation on which all the younger strata sit. Across most of Egypt, the presence of the Pre-Cambrian Basement has only been demonstrated by deep boreholes usually associated with oil and gas exploration. As shown by the pink areas on the geological map, however, at a number of locations such as Gebel Uweinat, Aswan and the Red Sea Hills, geologically ancient rocks are exposed at the surface.
As we have seen, the oldest Pre-Cambrian rocks in Egypt have been tentatively dated to about 2000 million years ago (about 10:30 am on our 24 hour clock) and are thought to be the remains of an ancient continent. Given their great age, all these strata have undergone tremendous earth-movements over immense periods of time and as a result, these rocks have developed an immense natural mineral wealth. The rocks of the Red Sea Hills, for example, were intensively mined and quarried by the ancient Egyptians and later cultures. As well as precious and semi-precious stones and metal deposits, including significant gold mineralisation, the Red Sea Hills have yielded such unique stones as the purple Imperial Porphyry quarried in Roman times at Mons Porphyrites.
Fig 2 – The Geological Timescale
There is a huge gap in the Egyptian geological record, with very few exposures of Palaeozoic strata found within Egypt at the surface or at depth (the Palaeozoic strata are shown on Fig 1). For the geological history of Egypt, therefore, the Palaeozoic is something of a ‘Dark Age’. The Palaeozoic is a significant period in the Earth’s geological history which on our 24 hour clock extends from just after 9 pm (555 Mya) to about 10:45 pm (240 Mya). The huge gap in the geological record can probably be attributed to the landmass that we now know as Egypt having been raised above sea level throughout this immense period of time. Sedimentary rocks such as sandstones and limestones, are generally laid down under marine conditions: that is under water. By contrast, under terrestrial conditions, when strata are lifted above sea level and become dry land, largely erosive conditions are encountered in which the action of rivers or wind ‘abrade’ the landscape and erode the rocks.
During the long reign of the dinosaurs, throughout the Mesozoic Era (from 240 to 65 Mya or 10:45 to 11:40 pm, see Fig 2), Egypt was progressively inundated from the north by the Tethys Sea. By the start of the Cretaceous (150 million years ago – 11:15 pm) most of the country was under fully marine conditions. The rocks formed from the sediments laid down in the Tethys Sea at this time are known as the Nubian Sandstones which as the geological map shows (Fig 1) are exposed across large parts of Egypt, south of the latitude of Asyut. In the north of the country, a number of more limited Nubian Sandstone exposures occur such as in the Bahariya Oasis from which a number of hitherto unknown dinosaur species, including some of the largest dinosaurs yet identified, have been found in recent years.
The Cenozoic period represents the last twenty minutes or so of our 24 hour day. In terms of the evolution of the Egyptian landscape the Cenozoic is perhaps the most significant period as it is the time during which the limestones that cover much of Egypt (see Fig 1) were laid down under extensive tropical seas. In order to more fully appreciate the sequence of events that unfolded in Egypt during this time, we need to break the Cenozoic into a series of smaller sub-divisions, or Epochs, as shown on Fig 2.
In the Eocene (from about 20 minutes to midnight or 57 million years ago) the Egyptian landscape was very different from that at present. Neither the Red Sea or the Nile existed and the what will become the Mediterranean was a much greater expanse of water – the Tethys sea – which extended a considerable distance to the south and had, by this time, laid down the sands of the Nubian Sandstones. As the Eocene progressed and the shores of the Tethys retreated to the north substantial thicknesses of Eocene limestones were laid down over the Egyptian landmass – although much of this has subsequently been eroded away and the thick deposits of limestones in northern Egypt represent only a fraction of what must have been originally laid down.
As we enter the Oligocene (36 Mya or 11:49 pm), things start to get more interesting. The continued northward retreat of the Tethys that had been underway since the Eocene, is thought to have been the result of earth movements which were lifting the North African landmass. As this uplift continued, run off from the more elevated areas in the east of the landmass led to the formation of rivers in the east and north of Egypt, which drained to the north into the Tethys. This regional uplift also led to a build-up of tension in the crust, along an approximate SE-NW axis through the African landmass and into eastern Asia. This tension was shortly to result in the formation of a series of rifts and faults – tears in the crust – which in turn led to the formation of the African Rift Valley and the Red Sea. These newly former highlands were capped with Nubian Sandstones and Caenozoic limestones just like the rest of Egypt, however, river systems eroded the highlands, eventually exposing the underlying ancient Pre-Cambrian strata with their rich variety of rock types and associated mineral wealth. Continued volcanic activity associated with the opening of the Red Sea also led to the intrusion of younger granites into the ancient strata of the Red Sea Hills. By the mid- to late Miocene (about 5 million years ago), the run-off from the Red Sea Hills had developed to such an extent that it formed an enormous southward flowing river system, referred to as the Qena River.
As a consequence of the continued up lift of the African landmass, but also possibly due to a global reduction in sea level, the Tethys had shrunk so much by the late Miocene (about 5 Mya or three minutes to midnight) that it appears to have dried out across large areas of what is now the Mediterranean basin. This reduction in sea level caused increased flow in many of the rivers that drained into the sea along Egypt’s northern coast and these rivers then began to aggressively erode their channels into deep canyons which also began to cut inland towards the south. This more aggressive phase of river erosion led to possibly one of the most significant episodes in the history of the Nile region.
The headwaters of one of these aggressive rivers advanced southward towards the much older Qena River system. At the point we know today as the Qena Bend – and, geologically speaking this must have happened almost ‘overnight’ – the channel of this northern river broke through into the channel of the Qena River in what must have been a catastrophic event. Within a very short period, the long-established southward flow of the Qena River reversed, to flow to the north along the very young, over-deepened canyon of the northern river and out towards the North African coast ultimately forming a new deeply cut river channel – the Eonile – which extended as far south as Aswan.
The Eonile channel was enormous, longer and deeper than the Grand Canyon in the US. Although previous rivers had been largely free to meander across the landscape, given its huge size the Eonile canyon for once fixed the course of all subsequent phases of the Nile. Although the Nile system was still to undergo a great deal of evolution before settling into the modern system the course of the river was forever to be confined by the towering walls of the Eonile canyon. The high cliffs at places such as Dier el Bahri represent perhaps only the upper 10% of the wall of the canyon, most of which is now buried in sediment associated with the evolving Nile.
Nile Evolution
At some point in the Pliocene, for reasons that are not yet fully understood, the waters of the Atlantic Ocean breached the Straits of Gibraltar and the dry Mediterranean basin was deluged. This deluge flooded the over-deepened Eonile canyon and the Eonile valley was transformed into an arm of the sea. It has been estimated that by the time rivers once more took possession of the valley (some 2 million years ago) marine sediments had accumulated to fill approximately half of the Eonile canyon.
The Paleonile was the earliest true river that flowed within the Eonile Canyon and existed from the Pliocene and into the Quaternary (from about 3.5 Mya to 1.8Mya. The end of the Paleonile phase, as with all subsequent phases of the Nile’s evolution was marked by an arid period during which flow along the Nile appears to have ceased for a period of time. By the end of the Paleonile phase, however, the deep Eonile canyon had become almost completely filled with marine, estuary and then river sediment. The arid event that brought an end to the Paleonile phase of Nile evolution was superseded by a wet phase which introduced the subsequent Protonile phase.
Deposits laid down during the next phase of Nile development, the Prenile (800,000 to 400,000 years ago) included for the first time material eroded from the Ethiopian highlands. Although this can perhaps be seen as the onset of the modern Nile system, the Prenile stage ended in yet another period of relative aridity in which flow from Ethiopia was interrupted to be replaced by flow once more from the Red Sea Hills and through the system of wadis that flow through the Eastern Desert. It was only in the final (and current) stage of Nile evolution (the Neonile – from approximately 12,000 years ago) that the connections with drainage from Ethiopia were re-established though on a much smaller scale than for any of the earlier river stages.