Nile Valley, Geological Evolution
Nile River (6670km) stretches across 34 degrees of latitude, from two degrees south of the equator to 32 degrees north, at its delta’s tip. Lake Tana, located on the Ethiopian Plateau at 1830 meters, and Lake Victoria, located in the equatorial region of East Africa at 1134 meters, supply the system with water. Even so, its drainage basin is only moderately sized (2.87 million km2), about half of which contributes no runoff, and its volume is about half that of the Danube.
Several tectonic provinces intersect at the irregular Nile watershed, leading to a complex geological history that is poorly understood. At least 40 million years ago, a river flowed northward near the western Egyptian Nile course, but it did not yet reach the sub-Saharan basins of the Blue and White Niles.
Basalt flows occurred 24 million years ago, which began Ethiopia’s updoming 30 million years ago. As a result, much of the Ethiopian drainage would have been directed to the older sedimentary basin in southern Sudan. The Saharan Nile axis was defined by the uplift and initial erosion of the Red Sea Hills 20 to 17 million years ago. A 5 million-year-old drainage system connected to the Blue Nile drainage system. Egypt’s landscape consists of three main components:
(1) The Eastern Desert
An ancient spine of igneous and metamorphic rocks uplifted from the African Shield, forms the eastern perimeter. Red Sea Hills are formed by these rocks. There are several small basins and drainage lines that facilitate travel from the Nile Valley to the Red Sea. These low ranges are rough and jagged in profile.
(2) The Western Desert
Western Desert plateaus and plains stretch westward from the valley and are level and tabular in shape. These sedimentary rocks have been exposed to erosion for more than 100 million years in the south (Cretaceous: Nubia Sandstone) and 20 million years in the north (Miocene limestones). There are steep escarpments or shallow depressions, partly excavated by wind, that intersect aquifers at very wide intervals. There are springs in these depressions that support oases such as Kharga and Dakhla.
(3) The Nile Valley
There is an incised valley in the eastern Sahara resulting from erosion. On its northward course to the Mediterranean Sea, it runs roughly parallel to the Red Sea Hills axis. A shallow valley cuts into Nubia Sandstone through Nubia. Between Khartoum and Aswan, it has six cataracts formed by dense, igneous rocks. Tectonic basins intersect the valley at Kom Ombo. Eocene limestone cliffs (some 50 million years old) close in on the valley near Esna, remaining prominent downstream to Minya.
The valley’s margins open up into sand-swept plains to the west, and open hill country to the east. Overland and subsurface links connect the Fayum Depression to the Nile, which has a bedrock floor 50m below sea level.
The Wadi Natrun has a shallower counterpart. When the Mediterranean Sea dried up six to four million years ago, the deep entrenchment of the Nile Valley and its delta can be traced back to the Messenian, when river action, facilitated by crustal movements, cut a remarkable canyon to 2000m below modern sea level near Cairo, and 175m below sea level upstream to Aswan.
This over-deepened canyon was once filled with marine, estuarine, and fluvial beds, whose remnants are visible along the valley margins. Delta sediment depressed the underlying crust 4km beneath the surface, despite the weight of the accumulated sediment. A progressively lower level of desert cliffs was formed when river gravel was washed together as river terraces during the last 1–2 million years.
At an elevation of 60 to 15m above the modern Nile floodplain, these terraces were fragmentarily preserved. There is a small percentage of sand derived from the Upper Nile Basin in these gravel “terraces” Evidence of early Paleolithic occupation, such as Acheulian handaxes, can be found in the younger units.
First appearing within Acheulian terrace gravels, Ethiopian flood silts are distinctive. Around 75,000 years ago, the Nile ceased to accumulate gravel and switched to its modern regime of silts deposited during summer floods. Rather than the Upper Nile changing behavior, it was the tributary wadis in the Egyptian deserts that resulted in this change; their channels have become almost inactive, with only sporadic activity in recent years.
The Egyptian Nile Valley has been inhabited continuously since roughly 20,000 years ago, as evidenced by Late Paleolithic industries. A hunter-fisher-gatherer economy of the early Holocene evolved from these industries.
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