Simud Vallis (on the left) and Ares Vallis on the right can be seen on this mosaic of Viking images. They are members of the group of the largest outflows on Mars. These floods were generated catastrophically from the collapse of the ground (see chaotic terrain) and the discharge of underpressure water from the subsurface aquifer. These floods occurred episodically on Mars through time, and as late as during the Amazonian.

Source: NASA