Layered outcrops of the West Candor Chasma region of Mars. These images from Mariner 9 spacecraft in 1972 reveled that some of the mesas and mounds found within the chasms of the Martian Valles Marineris are layered. Speculations about the origin of these layers included in volcanic ash deposits and sediments laid down in takes that could have filled the Vallis Marineris troughs, much as the lakes of the rift valleys of eastern Africa. The proposal that the Valles Marineris once hosted deep martian lakes led to speculation about the prospects of finding fossil evidence of martian life. Recent Mars Orbiter Camera (MOC) images have made other areas of layered rock visible, showing that these layers might not have formed in the Valles Marineris, but were deposited in craters that were buried before the chasms opened up. Layered rock is now visible because of faulting and erosion.