This Mars Global Surveyor image, acquired in March and released last week, shows dozens of repeated layers of sedimentary rock in a western Arabia Terra crater. Wind has sculpted the layered forms into hills somewhat elongated toward the lower left (southwest). The dark patches at the bottom (south) end of the image are drifts of windblown sand. These sedimentary rocks might indicate that the crater was once the site of a lake — or they may result from deposition by wind in a completely dry, desert environment, said scientist at Malin Space Science Systems, which operates the Mars Orbiter Camera aboard the orbiting spacecraft.