The Margaritifer Sinus quadrangle comprises the distal eastern portions of the Valles Marineris canyon floor, including Eos, Ganges, and Capri Chasmata. Those features transition into the Iani, Margaritifer, Aurorae, and Aureum Chaoses that dominate much of the remaining northwestern and north–central region of the quadrangle. The heavily cratered, high-standing plains of Noachis Terra, at 1 km above datum, in the southern portion of the map, transition into the moderately cratered, middle-relief plains of Arabia Terra to the northeast. The elevation descends to 3,000 m below datum in parts of Xanthe and Margaritifer Terrae and drops another 1,000 m or more into eastern Valles Marineris. Located in a portion of Arabia Terra, Meridiani Planum was the hematite-rich landing site of the Mars Exploration Rover-B, Opportunity (Figure 19.A; Christensen and Ruff, 2004; Christensen et al., 2005). A portion of the Xanthe Terra highlands in the northwest shows how the formation of Valles Marineris isolated some of the highland terrains. The large channel and lake system of the Uzboi–Ladon–Morava (ULM) Valles runs diagonally from the south–southwest to the north–northeast, through several large ancient crater basins, thought to have held lakes, including Holden crater.