Arising from: T. M. McCollom & B. M. Hynek Nature 438, 1129–1131 (2005); McCollom & Hynek reply. The Mars Exploration Rover Opportunity discovered sulphate-rich sedimentary rocks at Meridiani Planum on Mars, which are interpreted by McCollom and Hynek1 as altered volcanic rocks. However, their conclusions are derived from an incorrect representation of our depositional model2, 3, which is upheld by more recent Rover data4, 5, 6, 7. We contend that all the available data still support an aeolian and aqueous sedimentary origin for Meridiani bedrock.