As a B.Sc. Honours project with the Geology Department of the University of Adelaide, which was sponsored by the South Australian Department of Mines and Energy, the author during 1978 carried out geological mapping of a nearly 400 square km area surrounding Charlie Swamp. This locality lies 20 km north-east of the Stuart Creek Opal Field, which in turn is 60 km north of Andamooka. In addition, an investigation of the nature of the Marree Formation, in particular its coarse clastic content, was undertaken in the diggings at Stuart Creek. 87 backhoe trenches were opened and examined, while several hundred samples were collected from the trenches and adjacent outcrop for later laboratory study, to supplement field notes. In the Charlie swamp area, Mesozoic sedimentation commenced with the "Stuart Creek Beds", a transgressional unit of fine sand infilling depressions in the pre-Mesozoic topography. The overlying Marree Formation, of Early Cretaceous age, comprises a marine sequence of mud with sandy conglomeratic interbeds and "erratics". Much of the coarse material has been transported by shore-ice rafting. A major conglomerate bed, closely parallelled by the base of a bleached profile, extends over a distance of 80 km. Following a period of deep weathering, silicification and erosion, the ?Middle Tertiary Mount Sarah Sandstone, a fluviatile-lacustrine unit of sand and sandy limestone was deposited overlying the Marree Formation. ?Late Tertiary ferruginisation and silicification have produced varicoloured stains and silcrete within the Mount Sarah Sandstone and the Marree Formation. Late Tertiary or Quaternary events include a further bleaching and the formation of red-brown colluvial sediment and development of silcrete and gypcrete.