The Acheron Fossae region is named after Acheron Fossae, an approximately 800 km long tectonic graben system about 1200 km north of Olympus Mons on the outskirts of its aureole. This region appears as a block of ancient terrain surrounded by younger plains in form of a reversed crescent moon on the Martian surface. The HRSC image frames the eastern part of the Acheron Fossae region where the edge of the graben system leads into the lower plains.