The nearly 700 kilometers long Nirgal Vallis with its tributaries dissects old volcanic plains, lava flows, and eolian deposits. The east-west orientation of the main valley follows a regional trend of tectonic features seen also in the nearby Valles Marineris canyon system. The flat valley floor of Nirgal Vallis is widely covered by transverse dunes, which indicate wind directions more or less parallel to the valley walls. The images shown here cover the upstream reach of Nirgal Vallis, which is about 200 meters deep and about 2 kilometers wide. 400 kilometers farther to the East, the valley opens out into the much larger Uzboi Valles that has presumably been occupied by a lake in the past.