The Cultana map area lies about 150 miles north-northeast of Adelaide and straddles the narrow northern arm of Spencer Golf, South Australia. Along the western side of the area, which includes the industrial town of Whyalla, are exposures of steeply folded dark sandstone of the Moonabie Formation (pre- Adelaide System lower Precambrian) overlain by the Pandurra Formation, reddish sandstone of possible Willouran (lower Adelaide System) age. These rocks are exposed again along the western shore of Spencer Gulf, where the Moonabie Formation is intruded by the Cultana Granite. Lavas considered to be equivalent to the Roopena Level (about 1 450 million years old) are here interbedded with the Pandurra Formation. Flat-lying quartzite of the Tent Hill Formation, which forms a prominent mesa west of Spencer Gulf, is probably of Marinoan (upper Adelaidean System) age and overlies the poorly-outcropping Tregolana Shale. These little-disturbed beds of the Stuart Stable Shelf contrast with their equivalent geosynclinals facies which appears as steeply-dipping quartzites and shales of the Wilpena and Umberatana Groups of the Adelaide System in the western edge of the Flinders Ranges, northeast corner of the map area. Much of the map area is covered by outwash, sand and swamp deposits.