The larger southern portion of Tharsis (pictured right) lies on old cratered highland terrain. Its western boundary is roughly defined by the high lava plains of Daedalia Planum, which slope gently to the southwest into the Memnonia and Terra Sirenum regions. To the east, the southern Tharsis bulge consists of the Thaumasia Plateau, an extensive stretch of volcanic plains about 3,000 km wide. The Thaumasia Plateau is bounded to the west by a highly elevated zone of fractures (Claritas Fossae) and mountains (the Thaumasia Highlands) that curves south then east to northeast in a wide arc that has been compared to the shape of a scorpion's tail. The plateau province is bounded to the north by Noctis Labyrinthus and the western three-quarters of Valles Marineris. It is bounded to the east by a north-south oriented ridge called the Coprates rise. These boundaries enclose a broad high plateau and shallow interior basin that include Syria, Sinai, and Solis Plana (see list of plains on Mars). The highest plateau elevations on the Tharsis bulge occur in northern Syria Planum, western Noctis Labyrinthus, and the plains east of Arsia Mons.