The main geographic features of the Eastern Sahara are the Great Sand Sea and several oases, the main ones being, from north to south, Siwa, Farafra, Baharia, Dakhleh, and Kharga. The Fayum Oasis, which was formerly a lake, has been another very important water reservoir. Unlike in Lower (northern) Egypt, in Upper (southern) Egypt, the flood plain of the Nile is only a few kilometers wide and the desert extends near the river. The desert is not as flat as one may think: mountains and deep depressions exist in several parts, including the Gilf Kebir plateau, south of the Great Sand Sea, which has an altitude of about 2000 m. In spite of the elevated areas, no rivers or streams fed by precipitation, drain into or out of the Western Desert. Nevertheless, the oases live, thanks to freshwater provided by the Nile or local groundwater.
Important environmental changes affected the Eastern Sahara during the shifts corresponding to the glacial and interglacial stages in continental Europe. Although there were no glaciations in most of North Africa, the environment dramatically changed as the sea level of the Mediterranean withdrew by up to
70 m and precipitation totally disappeared. Consequently, the Sahara became a hyper-arid desert, drier than at present. During the Penultimate Glacial, corresponding to oxygen isotope stage (OIS) 6, dated between about 186 000 and 127 000 years ago, a series of sand sheets, formed by winds, covered the Eastern Sahara. Later, lacustrine and fluvial sediments were recorded at Bir Tarfawi, indicating the occurrence of lake basins and a moist climate during the Last Interglacial of OIS 5, dated to 127 00071 000 BP. Furthermore, a significant drainage network was active during various periods of OIS 5 in southwestern Egypt and northwestern Sudan. The following Last Glacial period of OIS 4, dated to
71 000-57 000 BP, witnessed abrupt climatic changes with the formation of sand sheets again, as clearly shown in the Selima region, across the Egyptian-Sudanese border. The remaining part of the Upper Pleistocene featured a harsh environment and dry climate. Although the climate during OIS 3 (57 00024 000 BP) improved, it was not as temperate as OIS 5, and OIS 2 (24 000-11 000 BP) was much more arid than at present. Considerable climatic ameliorations occurred only in the Eastern Sahara at the beginning of the Holocene. Groundwater levels started to rise about 9300 years BP and the aquifers rose to levels as much as 25 m higher than today. Freshwater lakes and swamps formed in the lowlands and a fluvial system developed in the uplands, attracting animals and favoring human occupation in many areas of the Eastern Sahara. Rainfall decreased from south to north, forming lacustrine sediments in the south, below 22° N, and playa sediments between 10° and 22° N. Such favorable conditions lasted until about 6000 years ago, when the landscape started to dry up again and, by 3000 BP, the Sahara became the desert we know today.
World History
