Safsaf Oasis Egypt

Visions of the Sahara today do not invoke images of major rivers and connecting multiple tributaries. Today the landscape looks like vast, open areas of nothing but sand for miles in all directions. As in the previous examples, even though the surface may not show any signs of water, this does not mean that water did not flow and cut tributaries at one time, only to later dry up and become covered with sand. At one time, abundant rivers in the Safsaf, Egypt, carved channels through canyons and formed lakes, and today these "fossil rivers" are buried under the sand.

In the photograph, the image on the left was obtained from a LANDSAT satellite. The surface looks hard and smooth, but upon close inspection one can see a very faint river channel that runs across the image. The image on the right of the same area, however, is what the subsurface of the ground looks like. This image was obtained from the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR), which uses radar to penetrate the thin sand cover on the Earth's surface, and was taken aboard the NASA space shuttle Endeavour. This image shows that the area was once very different—the oasis was once a very productive, lush river valley. The sinuous dark channels (especially on the lower left) were cut by a meandering river and its tributaries. Such evidence helps climatologists reconstruct a region's climate in order to better understand the past, present, and fUture.

central sahara, Africa

NASA acquired a moderate-resolution imaging spectroradiometer (MODIS) image of the Sahara in Africa just north of Algeria and Libya. They studied three major rock formations among the reddish sand dunes: the Tassili, Tadrart-Acacus, and Amsak. Remote sensing specialists were able to identify several ancient riverbed structures in the Acacus and Amsak regions that followed a dendritic (treelike) pattern. Paleoclimatologists have interpreted this image and determined that the area was wet during the last glacial era, covered with forests, and probably inhabited by several species of animals. In addition, several renditions of ancient rock art have been found in the area, which indicates that the area provided a home for an ancient civilization. Experts have determined that the area became extremely arid about 3000 b.c.e.

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