Desiccated fluvial valley
Nirgal Vallis is the relict of a stream that once flowed over the surface of Mars. It is a typical example of an amphitheater-headed valley, characterized by steep-sided valley walls, smooth valley floors, and U-shaped cross sections. This kind of valleys has short tributaries with semi-circular head terminations. Examples on Earth include valleys in the Atacama Desert in Chile, on the Colorado Plateau, and on the Hawaiian islands. Apart from Nirgal Valles, Nanedi Valles and Echus Chasma headwater region are prominent examples of amphitheater-headed valleys on Mars.