Central Ultimum Chasma (THEMIS visible image mosaic, 18 m/pixel). a. Inset. b, c. Close-ups showing unconformable contacts between the Planum Australe 1a and 1b units. Npl2 = subdued unit of the plateau sequence, Hdc = chasma member of the Dorsa Argentea Formation, Aa1a = Planum Australe 1a unit, Aa1b = Planum Australe 1b unit, Aa2 = Planum Australe 2 unit, white lines = contacts, black lines = 250-m MOLA-based elevation contours. THEMIS VIS images used in mosaic: V08471002, V08516007, V08628007, V08653006, V08678007, V08703009, V08728013, V08733011, V08753002, V08778005, V08828007.