The Phoenicis Lacus quadrangle shows the heart of the Tharsis region that dominates the western hemisphere of Mars. Elevations are high: except for the floor of Valles Marineris, essentially the entire quadrangle lies above datum. Arsia Mons rises over 11 km from the surrounding plain and is more than 400 km across. It marks the southwest end of the northeast-trending Tharsis Montes, which also include Pavonis Mons and Ascraeus Mons (MC-9). The eastern half of the map is dominated by Syria, Sinai, and Solis Plana, high plateaus that are capped by a broad field of dozens of smaller volcanic shields. From Arsia Mons across Syria Planum the elevation is 6,000 m or more, descending to 2,000 m at the southwest corner of the map. Wrapping for 1,000 km around the north and west margin of Syria Planum is the Noctis Labyrinthus, where numerous canyons and depressions intersect in a maze-like pattern (see Figure 2.3 in Chapter 2). A large, rugged promontory, informally referred to as Claritas rise, lies along the southern margin of the quadrangle and east of Claritas Fossae (Dohm et al., 2009a). Valles Marineris extends east of Noctis Labyrinthus for several thousand kilometers (across MC-18).