The results of in-situ measurements, seismic studies of the Robertson Earthquake of 1961 and the Burragorang Earthquake of 1973, geological observations made during construction of Warragamba Dam and the Pipe Head-Potts Hill Tunnel, and foundation investigations into dams in other catchment areas, do not indicate agreement as to the direction of maximum horizontal stress in the southern part of the Sydney Basin. The geological observations include rock movements recorded as a consequence of excavation affecting the stress pattern. These movements were displacement of strata along bedding planes in calyx and diamond drill holes, and cracking of rock in excavations and tunnels. Results of drilling and water testing of the foundations of the older dams, constructed between 1904 and 1941, indicate that stress relief could have taken place beneath the river beds to at least the depth of drilling, which, in one case, was some 90 m. The major structures of the Sydney Basin probably formed as a result of the main horizontal compression being in an E'-W direction; this E-W stress field would have been active during the cratonisation of the Permo-Triassic sediments to the older Palaeozoic rocks. Subsequent to this cratonisation, the maximum direction of horizontal stress has changed to a general N-S direction.
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