Nirgal Vallis seen by MOC. The valley illustrates the typical morphology of flows generated by groundwater sapping. In the background, a large and short theater-head tributary has now its floor covered with dunes. This tributary was formed when water seeped through the ground, starting a process of heaward erosion. Small other tributaries are visible towards the right of the image. On the bottom left, a Viking image for context. The resolution of the MOC image is about 11m per pxl.
Source: NASA, JPL, Malin Space Science Systems