The region is transected by numerous northeast-trending grabens that form parts of the Tempe, Mareotis, and Tantalus Fossae; these are fracture systems of varying ages that are oriented radially from the Tharsis bulge. The region west of Enipeus Vallis is characterized by Hesperian-aged volcanic flows from Alba Mons. The area to the east consists of fractured and terraced highland plateaus of Noachian age. At the end of the Noachian period, these highland materials underwent an episode of fluvial erosion and resurfacing. Enipeus Vallis probably formed during this fluvial episode.