Flowing Rivers on Ancient Mars

This image of ridges in Aeolis Planum tells a story of ancient rivers and a Mars very different to that of today. River beds often get filled with gravel and the surrounding terrain is often built up of fine-grained mud from river overflows. The gravely river bottom and the fine-grained surroundings can lead to a strange phenomenon that geologists call inverted channels. After the river disappears, the fine-grained surroundings can be easily eroded away leaving the gravely river bed as a high-standing ridge. These ridges show the location of the old river beds in Mars' distant past. The angle at which the ridges join together indicate that these rivers flowed from top-right to bottom-left (i.e. southwest). https://photojournal.jpl.nasa.gov/catalog/PIA22822