In order to better assess the oil prospects of the Cambrian rocks of Yorke Peninsula, the Fuel Section of the Geological Survey of South Australia undertook during 1956 a detailed investigation of the Proterozoic - Cambrian stratigraphic succession there, based upon several months of field work and a comprehensive review of all previous geological work, published literature and borehole records. Significant results of this investigation are as follows: 1. A reasonably thick and complete succession of Proterozoic (Adelaide System) sediments were recognised and mapped beneath the fossiliferous Cambrian beds of the Kulpara area. These had previously been described as part of the Cambrian succession. 2. Several important stratigraphic features were recognised within the Adelaidean sequence at Port Clinton and Kulpara, viz. (a) an unconformity at the base of the Sturtian Series; (b) a gradual emergence to sub-continental and continental conditions is clearly revealed by the lithology of the Marinoan Series. Evidence is also presented which indicates that the Pound Quartzite (previously regarded as the basal unit of the Cambrian) is of continental origin and comprises an upper member of the Marinoan Series. 3. A disconformity has been recognised at the base of the Cambrian limestone group at Kulpara. 4. Previously unrecorded outcrops of fossiliferous Cambrian limestone were mapped east of Minlaton. 5. Conglomerates marking the northern and western margins of Proterozoic - Cambrian sedimentation within the Curramulka Basin were mapped. 6. Evidence of Early Palaeozoic and of Upper Tertiary (or later) tectonism is described. 7. The evidence of trough-faulting in St Vincent Gulf is reconsidered, and an alternative theory that the Gulf occupies a major synclinal environment is proposed. Because insufficient outcrop and borehole information was available in two critical areas for making a sound geological interpretation, the Department of Mines chose sites for 2 new stratigraphic wells which are still in the process of being drilled. Results to hand from these boreholes are summarised herein, and geological logs of all previous deep boreholes on Yorke Peninsula are also appended to this report.