The Medusae Fossae Formation is a large geological unit of probable volcanic origin on the planet Mars. It is named for the Medusa of Greek mythology. Fossae is Latin for trenches. Located roughly at , it straddles the highland - lowland boundary near the Tharsis and Elysium volcanic areas. The Medusae Fossae Formation lies partly in five quadrangles: the Amazonis quadrangle, the Tharsis quadrangle, the Memnonia quadrangle, the Elysium quadrangle, and the Aeolis quadrangle.