Eos Chaos may be made of layer after layer of lava flows, probably mixed with deposits of volcanic ash that fell out of the air following big eruptions. It is likely the rock strata in the walls preserve a long geological history of Mars. Dark layers may be due to dark lava flows. The dark volcanic rock basalt is common on Mars. However, light-toned deposits may have resulted from rivers, lakes, volcanic ash, or wind blown deposits of sand or dust. The Mars Rovers found light-toned rocks to contain sulfates. Probably having been formed in water, sulfate deposits are of great interest
West Rock Ridge in faults created by the rifting apart of North America from Eurasia and Africa over a period of 20 million years. Erosion occurring between the eruptions deposited deep layers of sediment between the lava flows and around the dike, which formed sedimentary rocks. The resulting "layer cake" of igneous and sedimentary sheets was later faulted and tilted upward. Subsequent erosion wore away the weaker sedimentary layers a faster rate than the lava layers, leaving the abruptly tilted edges of the lava sheets exposed, creating the distinct linear ridge and dramatic cliff faces visible today along the Metacomet Ridge. West Rock