Ma'adim Vallis
A large, ancient river valley, called Ma'adim Vallis, enters at the south rim of Gusev Crater, so Gusev Crater was believed to be an ancient lake bed. However, it seems that a volcanic flow covered up the lakebed sediments. Apollinaris Patera, a large volcano, lies directly north of Gusev Crater.
Recent studies lead scientists to believe that the water that formed Ma'adim Vallis originated in a complex of lakes. When the largest lake spilled over the low point in its boundary, a torrential flood would have moved north, carving the sinuous Ma'adim Vallis. At the north end of Ma'adim Vallis, the flood waters would have run into Gusev Crater.
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Section of Ma'adim Vallis as seen by HiRISE. A more recent flow of water may have formed the smaller, deeper channel to the right.
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Apollinaris Patera
Read more about this topic: Aeolis Quadrangle