A programme of structure drilling was carried out within County Grey, South Australia during April and May, 1966 with the object of mapping the upper surface of the Eocene Knight Group. The programme, as originally envisaged, was to have comprised the drilling of approximately twenty structure holes in the northern part of the Hundred of Blanche and the southwestern corner of the Hundred of Young. Data obtained from wells drilled in the search for oil and gas in this area and from water bores indicated that a structural nose, with southward plunge, might be present on the downthrown block of the Tartwaup Fault. In the first stage of the Tartwaup Structure Drilling Project 16 holes were drilled in an attempt to delineate the outline of the structural nose. As a result of this drilling it was found that the structural pattern of the area was more complex than originally had been thought and it became evident that closed structure, if present, would be only of limited areal extent. For these reasons, and because severe lost circulation problems encountered while drilling through the Gambier Limestone made further drilling in the area to the south of the Tartwaup Fault uneconomic, it became necessary to modify the objectives of the project. Accordingly the second stage of the project was conducted in the region extending northwards from the Tartwaup Fault to the northern boundary of the Hundred of Young. Over much of this region the Knight Group is present at relatively shallow depth. Here 20 holes were drilled in a fresh attempt to locate structural closure at this level. Stratigraphic information obtained by drilling in the latter area has provided evidence that the configuration of the upper surface of the Knight Group is related to tectonism occurring after deposition of at least part of the Glenelg Group, with the region to the north of the Tartwaup Fault emergent during part of Upper Eocene or Early Oligocene time. Utilising all the available drillhole data, a depth structure contour map of the base of the Gambier Limestone has been drawn for this region, showing two areas of closure separated by a shallow saddle. The 36 holes drilled in this project totalled 4 664 feet; of these 24 penetrated the Knight Group at an average depth of 171.7 feet, while the remaining 12 were abandoned before reaching the objective. Drill cuttings were sampled and logged at intervals of every 5 feet or less.