The present Nile is at least the fifth river that has flowed north from the Ethiopian Highlands. Satellite imagery was used to identify dry watercourses in the desert to the west of the Nile. An Eonile canyon, now filled by surface drift, represents an ancestral Nile called the Eonile that flowed during the later Miocene (23–5.3 million years before present). The Eonile transported clastic sediments to the Mediterranean; several natural gas fields have been discovered within these sediments.
During the late-Miocene Messinian salinity crisis, when the Mediterranean Sea was a closed basin and evaporated to the point of being empty or nearly so, the Nile cut its course down to the new base level until it was several hundred feet below world ocean level at Aswan and 8,000 feet (2,400 m) below Cairo. This created a very long and deep canyon which was filled with sediment when the Mediterranean was recreated. At some point the sediments raised the riverbed sufficiently for the river to overflow westward into a depression to create Lake Moeris.
Lake Tanganyika drained northwards into the Nile until the Virunga Volcanoes blocked its course in Rwanda. The Nile was much longer at that time, with its furthest headwaters in northern Zambia.
0 comments:
Post a Comment