The most significant feature of the area, covered by the Naracoorte 1:250 000 sheet is the Padthaway Ridge which underlies all but the southernmost part of the area, where Mesozoic sediments, of the Otway Basin lap onto the ridge (Padthaway Archipeligo on the Sheet's tectonic sketch).The Padthaway Ridge consists of early Palaeozoic low grade metamorphics and igneous rocks and appears to have been a "basement high" since its inception of Ordovician time. It separates the Mesozoic developments of the Murray Basin, situated to the north of it, from those of the Otway Basin to the south, and is of considerable importance to the understanding of the Mesozoic palaeogeography of these two basins. In the Lower Cretaceous for instance, fresh-water conditions prevailed in the Otway Basin whereas marine sediments were deposited in the Murray Basin; the situation was reversed in the Upper Cretaceous. For these reasons, the mapping project was carried out by the Petroleum Exploration Division although the major portion of the map area is underlain by "non-prospective" rocks. The mapping project was initiated during the early stages of the Otway Basin Study, a joint project of the Geological Surveys of South Australia and Victoria, and the results of the mapping of the NARACOORTE 1:250 000 Sheet area have been incorporated in the "Geological Map of the Otway Basin", 1:500 000 (Wopfner, et at., 1971). Field work commenced in 1964 and was virtually completed by 1967. Due to extensive inundation of the area during winter months, field mapping had to be restricted to the dry summer period. In addition to the mapping systematic sampling of the crystalline basement was carried out for petrologic analyses and where suitable , for radiometric age dating. As most of the Sheet area is covered by Pleistocene and Recent deposits, considerable attention was given to the compilation of sub-surface data, in order to obtain a better understanding of the distribution of Mesozoic and Tertiary strata. The project was carried out by Mr. K.A. Rochow, Assistant Senior Geologist of the Petroleum Geology Section. Mr. B.E. Cornish, Geologist, also of the Petroleum Geology Section assisted during the early stages of the field programme. The project was supervised by Dr. H. Wopfner, Supervising Geologist of the Petroleum Exploration Division.