This mosaic of the head of Ravi Vallis shows a 300-kilometer-long portion of the channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis originates in a region of collapsed and disrupted (“chaotic”) terrain within the planet's older, cratered highlands. Structures in these channels (slides #24 and #25) indicate that the channels were carved by liquid water moving at high flow rates. The abrupt beginning of the channel in this image, with no apparent tributaries, suggests that the water that carved the channel was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. Three such regions of chaotic collapsed material are seen in this image, connected by a channel whose floor was scoured by the flowing water. The flow in this channel was from west to east (left to right). This channel ultimately links up with a system of channels that flowed northward into Chryse Basin.
From Mars Digital Image Map, image processing by Brian Fessler, Lunar and Planetary Institute