The highland–lowland boundary transects the central part of the Aeolis quadrangle. Elevations range from 3 km above datum in the highlands to 3 km below datum in the lowlands at the north edge of the quadrangle. Aeolis Mensae and de Vaucouleurs display the rugged, gradational nature of the boundary in this region. Along the boundary is Gusev crater, the landing site of the Mars Exploration Rover (MER-A), Spirit (Squyres et al., 2004; Arvidson et al., 2006), while near the west edge of the map is Gale crater, location of the Mars Science Laboratory (MSL) rover, Curiosity (Golombek et al., 2012; Grotzinger et al., 2012). Ma’adim Vallis originates from a complex of proposed paleolake basins and enters Gusev crater from the south (MC-29; Irwin et al., 2002). About 200 km directly north of Gusev is a prominent, 200-km-wide and 5-km-high shield volcano, Apollinaris Mons.