The contour map is a compilation of data from the S.A. Department of Mines and oil exploration companies working in the south-west part of the Great Artesian Basin. The datum for the plan is the pre-Permian angular unconformity, and is largely non-stratigraphic. Information was obtained from seismic surveys with control from oil exploration, wells. Over the area, the unconformity lies at various positions in the stratigraphic sequence; but generally there are two cases. In areas of Permian sedimentation, the unconformity occurs at the base of the Permian where Permian rocks generally overlie Devonian or older rocks. Where there are no Permian rocks, Mesozoic sediments unconformity overlie lower Palaeozoic rocks, the unconformity lying at the base of the Triassic, or the Jurassic. There are a number of wells in which the depth to the unconformity does not agree with the seismic results, generally the result of using an incorrect velocity function. Because of the near or complete absence of Permian in many of these wells, the average velocity to the unconformity is considerably lower than the value used in the seismic interpretations.