This image shows the western wall of a small pit that is located along the floor of a larger trough in Coprates Catena. Dark layers are exposed along the bottom of the pit wall while light-toned layers are near the top of the pit and the adjacent trough floor.
Based upon where the layers are exposed, we can tell that the dark layers formed first followed by the light layers. The light layers could have been deposited when water filled part of the trough while the dark layers could be older lava flows. [More at link]