At the wall tops in Gangis Chasma, and throughout the whole Valles Marineris, are layers of resistant rock that form cliffs or steeper slopes Three layers can be seen in the wall top cliffs, alternating dark, light, and dark from the top down. Together, these layers are about 400 meters thick, and are present at the wall tops over almost the full 4000 km length and 800 km width of Valles Marineris. The position and thickness of the layers are essentially identical, independent of position in the Valles, elevation, or age of the surrounding plateaus. These features suggest that the layers formed in place, and are not sediments or lavas deposited on the land. The layers may represent a vast hardpan or solidified soil layer, formed by percolation and evaporation of groundwater.