African river valley once hosted big lake.The valley of the White Nile White Nile, river, one of the chief tributaries of the Nile, E Africa. The name is sometimes used for the 600 mi (970 km) long section of the river known as the Bahr el Abiad that extends upstream from Khartoum to the junction of the Bahr el Jebel and the Bahr el , one of two main tributaries of Africa's longest river, may long ago have held a shallow lake that sprawled 70 kilometers across and stretched more than 500 km along the river. Satellite images clearly show a continuous string of arc-shaped features that extends about 280 km southward south·ward adv. & adj. Toward, to, or in the south. n. A southward direction, point, or region. south from Esh Shawal, Sudan, along the eastern margin of the White Nile valley. Previous ground surveys found that the elevations of those land-forms vary by no more than 2 meters, says Martin A.J. Williams, a geographer at Australia's University of Adelaide Its main campus is located on the cultural boulevard of North Terrace in the city-centre alongside prominent institutions such as the Art Gallery of South Australia, the South Australian Museum and the State Library of South Australia. . West of these contours, the terrain slopes gently toward the river, dropping about 15 centimeters across each westward kilometer. East of the features, the ground rises at a steeper gradient of 3 meters per kilometer. The consistent level of the features, like that of a gargantuan gar·gan·tu·an adj. Of immense size, volume, or capacity; gigantic. See Synonyms at enormous. gargantuan Adjective huge or enormous [after Gargantua, a giant in Rabelais' bathtub ring, marks the wave-scoured shoreline of an ancient lake, says Williams. North of Esh Shawal, traces of the lake's eastern shoreline become vague, because sediments dumped there by the Blue Nile Blue Nile, Arab. Al Bahr al Azraq, river, c.1,000 mi (1,600 km) long, the chief headstream of the Nile, rising in Lake Tana, NW Ethiopia, at an altitude of c.6,000 ft (1,800 m). have covered the area. However, signs of erosion along the opposite shore can be found more than 200 km north of the town. Neither the satellite images nor previous surveys provide any clue about where the ancient lake's southern shore may have been located. Williams and his colleagues describe their findings in the November Geology. Because the southeastern shoreline is well defined, the arc-shaped features probably took a long time to form, says Michael R. Talbot, a geologist at the University of Bergen The University of Bergen (Universitetet i Bergen) is located in Bergen, Norway. Although founded as late as 1946, academic activity had taken place at Bergen Museum as far back as 1825. The university today caters for more than 16,000 students. in Norway. The apparent absence of multiple shorelines at various elevations along the river valley indicates the lake was a permanent and stable feature, not one that rose and fell with the seasons and reached different levels each year. The White Nile's ancient lake couldn't exist today because the river doesn't carry enough water to keep up with evaporation evaporation, change of a liquid into vapor at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity over a lake-size area. At some sites in the region, reservoirs can lose as much as 1 cm of water per day, says Williams. Analyses of clay-rich sediments excavated from a trench near Esh Shawal suggest that layers now 5 m below ground level were deposited on the lake bottom more than 250,000 years ago. Williams and his colleagues suggest that the lake formed and existed between 420,000 and 360,000 years ago, during a wetter-than-normal period between ice ages. |
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