Ismenius Lacus is located in the northern mid-latitudes of the eastern hemisphere of Mars. It includes sections of both the southern highlands and northern plains. The topographic transition is defined by gently sloping surfaces, steep scarps, linear to sinuous channels, and isolated knobs and plateaus. The southern part of the quadrangle is defined by the northernmost extent of the high-standing, ancient cratered highlands of Arabia Terra and Terra Sabaea, at elevations near datum to –3,000 m. The regional highlands in Ismenius Lacus contain large, ancient channel systems – Okavango and Mamers Valles (Figure 5.A; Mangold and Howard, 2013) – as well as networks of linear depressions – Ismeniae and Coloe Fossae. These physiographic features all record complex geologic processes that are associated with the long-term break-up and marginal erosion of the cratered highlands. From the north to the south, the highland–lowland transition in Ismenius Lacus is marked by the high-standing plateaus of Deuteronilus and Protonilus Mensae as well as the irregularly-shaped depressions of Deuteronilus Colles and Colles Nili. The lowlands in the north typically lie at –4,000 m or lower. The 236-km-diameter Lyot crater and its radial and lobate ejecta blanket dominate the center of the quadrangle.