Although the Nile seems like an ancient river - after all, it was there long before one of the earliest civilizations began to develop on its banks - it is really a very young river and has gone through many changes over the recent (in geologic terms) past. The Nile consists of a series of steeper and flatter segments, and this is thought to indicate that several independent drainage systems existed in the region now drained by the Nile. There is much that we do not know about how the Nile formed, and much work remains for future scientists to discover. What follows is an outline of what we think we know now.
The oldest parts of the Nile drainage are probably those associated with the Sudd. These follow the axes of sediment-filled rifts that formed over 65 million years ago, and which have continued to slowly sink and fill with sediments since that time. This part of what is now the Nile only became part of the great transcontinental river in the past 1 or 2 million years. The best record of the great river is recorded where the sedimentary sequence is best preserved, in Egypt.
As a result of studying sedimentary deposits in the delta and Egyptian Nile, we know that great river systems carried sediments - preserved today as the Nubian Sandstone - north from central Africa as far back as Cretaceous time, about 100 million years ago, but the course of these rivers is poorly known. The vast amount of sediments shed northward at this time was derived from uplift , some of which was responsible for formation of the rift basins that lie beneath the Sudd. No link can be established between the Cretaceous rivers and the course of the present Nile, because this system of rivers was obliterated when the sea invaded Africa from the north towards the end of the Cretaceous. Much of NE Africa was a shallow sea during early Tertiary time, about 70 to 40 million years ago. Sealevel slowly dropped throughout the early Tertiary, allowing new rivers to extend north after the retreating sea. River deposits of late Eocene-early Oligocene age (about 35 million years ago) are known from west of the present Nile, but these sediments did not travel far, indicating that this was a relatively small river. This may have been the precursor of the stream responsible for carving the great canyon following evaporation of the Mediterranean Sea about 6 million years ago. This is the so-called 'Messinian Salinity crisis' and is a critical event in the formation of the Nile.
The Nile system is traced back in time to the evaporation of the Mediterranean Sea. From this time on, five main episodes in the evolution of the Nile have been deduced. These are, from oldest to youngest: Eonile, Paleonile, and three Pleistocene Niles: Protonile, Prenile, and Neonile. The deposits of the Neonile are indistinguishable from those of the present river.
The Eonile formed in response to the Messinian Salinity crisis. More water evaporates from the Mediterranean Sea than is supplied by the rivers that flow into it, and this deficit is compensated by sea water flows into the Mediterranean from the Atlantic. When collision between African and Europe shut the Straits of Gibraltar, the flow of Atlantic seawater stopped and the Mediterranean slowly dried up. This was a huge depression, almost certainly a tremendous and sterile desert like the region around Dead Sea of Israel and Jordan. A major difference between the present Dead Sea region and the Mediterranean desert is that the former at about 400 m below sea level is presently the deepest spot on the continents, but the Mediterranean seafloor, lying as much as 3000m below sealevel, was probably a mile deeper and a thousand times more vast.
Evaporation of the Mediterranean profoundly affected the streams that flowed into it. As the level of the Mediterranean got lower and lower, streams that once flowed placidly into it began to cut down into the underlying rocks, becoming steeper and with more erosive power as sealevel dropped and the stream cut down into relatively soft limestones. The enhanced erosive power allowed its upper tributaries to extend into the headwaters and 'capture' upstream drainages. The increased water from the captured streams further increased the streams erosive power, further stimulating the expansion of the drainage system upstream. This led to the development of the so-called ïEonileÍ, which carved a huge canyon that was deeper than the Grand Canyon of Arizona and many times longer. This canyon is buried beneath all of the Egyptian Nile, but it cannot be traced south of Aswan.
In time, the barrier at Gibraltar ruptured and a tremendous waterfall brought Atlantic seawater to refill the Mediterranean basin. The "Grand Canyon of Egypt" became a drowned river valley or estuary, similar to the fjords of Norway but very different in origin. Slowly, this estuary filled with sediments brought in by the Paleonile flowing from the south, and a landscape not too different from the present was established by 3 or 4 million years ago. Sediments deposited by the Paleonile were not derived from Ethiopia; the distinctive mineral pyroxene - common in Ethiopian basalts - is not found. The Paleonile was probably fed by a drainage basin that was much more limited in size than the present Nile, probably restricted to SE Egypt. Paleonile sediments are very fine-grained, suggesting that it drained a moist and vegetated area more very different from the desert now found in NE Africa. The Paleonile flowed through Egypt from about 4 to 1.8 million years ago.
The interval between the Paleonile and the Protonile was marked by a dramatic change in climate. This was the beginning of Pleistocene time, a period of widespread glaciation in northern Europe and North America, but a time when a harsh desert was first established in North Africa. The Nile stopped flowing north during this transition, and sand dunes drifted into the abandoned river channel. Torrential winter rains occasionally filled the channel, but no water reached the Egyptian Nile from south of the Nubian Swell, until the flow regime of the Protonile was established about 1.5 million years ago. Coarse sediments characterize the deposits of this time, including conglomerate, gravel and coarse sand. There is no hint of material eroded from the Ethiopian highlands. It is not clear to what extent the Nubian Swell and the Bayuda Uplift blocked flow from the south at this time, or whether there was simply too little water flowing northward to cross the Sahara.
Part of our uncertainty comes from a lack of information about whether or not the Blue Nile existed at this time, and if so, how much water flowed through it. The Ethiopian highlands began to form about 40 or 50 million years ago as a result of tremendous volcanic activity as a mantle plume punctured the crust, but the distinctive basaltic sediments derived from these highlands is not recognized in Egypt until deposits of the Prenile were laid down, about 700,000 years ago. Increased strength of northward flow from the Ethiopian highlands may be due in part to development and intensification of monsoonal circulation in the recent past. Monsoonal circulation is due to the change position of atmospheric low-pressure cells, which lie over the equatorial Indian Ocean during northern winter and over south-central Asia during northern summer. The result is that cold, dry winds blow south from Asia during northern winter, but warm, moist winds blow from the sea towards Asia in northern summer. The westward deflection of summer winds due to the Coriolis Effect brings part of the moisture-laden air currents over Ethiopia, where the air cools as it rises. Cool air can hold less moisture than warm air, so clouds and then rain forms as the monsoon rises over the Ethiopian plateau. This brings the long, drenching rains in Ethiopia that cause the annual Nile flood. The monsoonal circulation has intensified over the last few millions of years due to continued uplift of the Tibetan Plateau. It would be a wonderful thing to be able to go back in time and tell the ancient Egyptians that the mystery of the Nile Flood could best be understood by knowing about mountain-building events that were occurring thousands of miles away! Water from the Ethiopian highlands may not have reached Egypt because the Nubian Swell acted as a barrier, perhaps deflecting the water to the west. It may be that only with the additional water provided as a result of the intensifying monsoon that the upstream Nile was able to erode its way through the Nubian Swell and continue north to the Mediterranean Sea.
The Prenile flowed from perhaps 700,000 until about 200,000 years ago, when a desert occupied N. Africa.. It can be safely said that the Prenile was the largest and most vigorous of the Nile precursors, with a wide floodplain. Its sources lay south of Egypt, and the presence of abundant pyroxene in these sediments indicates that the Ethiopian highlands were imprint sediment sources for the first time. A large proportion of sediments in the Nile Delta were deposited by this phase of the river, as much as 1000m thick. The Prenile marks the dawn of the present transcontinental river system, establishing flow from Ethiopia to the Mediterranean Sea and probably also from the Sudd.
The Neonile began about 120,000 years ago and was established at a time when North Africa was well-watered, with numerous lakes. Crude stone fashioned by humans are found in these sediments. The Neonile was significantly less vigorous than the Prenile. Contributions from the White Nile have grown slowly with time, and probably were important for the development of the Neonile. Lake Victoria did not exist prior to about 12,000 years ago. Before this time, the streams of the Ugandan highlands flowed west to join the Congo, which drains into the equatorial Atlantic. Very recent uplift tilted the region to make the lake and direct its excess to flow north. This was important because the waters of the White Nile provide most of the Nile's water during the Ethiopian dry season. Several episodes when the N. Africa was wetter or drier can be identified. It was after the last wet period, sometime after 10,000 years ago, that hungry nomads migrated to the Nile Valley and Delta and took up farming. This led in turn to the establishment of civilization in Egypt, about 5,000 years ago.
Seguo nei prossimi post con la traduzione.