A geological reconnaissance has almost completed the mapping of the remainder of the MARREE 4-mile mapsheet area, which straddles the edge of the Great Artesian Basin. Earlier mapping, carried out under supervision of Bruce Webb, had finished three of its 1-mile sheets. Precambrian basement rocks include part of the Mount Painter granitic complex and strata of the Adelaide System extending from Willouran to Marinoan Series. Within the Mesozoic basin are exposed sandstones equivalent to the Blythesdale Group and a predominantly shaley Aptian to Cenomanian succession. The base of the Cenomanian has not been recognised, but Albian limestones should provide a useful marker. A post-Winton succession has been noted. Tertiary sandstones and duricrust are extensive, and reach a thickness of about 200 feet near Blanchewater. A possible dolomitic equivalent of the Etadunna Formation occurs near the Clayton Embankment. Quaternary deposits include clays, ancient outwash gravels and more recent gypsum-covered pediments and sand ridges. Folding within the basin has been mostly very gentle, and has produced broad structures such as the Coryanna Dome. This appears explicable by gentle lasoment folding. Localised, more intense folding, such as that which produced near-vertical dips at Blanchewater, has possibly been caused by minor gravitational gliding.