Mount Willoughby 1 was drilled in the northern part of the Arckaringa Basin as a continuation of the SA Department of Mines stratigraphic study of this area. The well was drilled on the western plateau of a positive gravity high on the Wintinna 1 mile sheet, principally in an attempt to identify the high speed seismic refractor, with an estimated velocity of 21,000 feet per second, which was known to occur in the area at a depth of approximately 2000 feet. The well intersected a very hard, dense dolomite sequence, tentatively dated as Devonian, at 2046 feet KB. The physical properties of drill core samples, particularly a specific gravity of 2.77 g/cc, leave no doubt that this rock unit is the high speed refractor. 52.8 feet of the ?Devonian dolomite (Unit 3?) were drilled before the well reached total depth at 2098.8 feet KB. The dolomite interval contains very interesting microcrystalline silica and dolomite concretions and/or pebbles of unknown origin, together with algal stromatolites. An unconformity separates Unit 3 from the overlying Lower Permian glacigene Unit 2 sediments, which are 1086 feet thick. Unit 1 is absent and 305 feet of Lower Permian Mount Toondina Beds overlie Unit 2. It is believed that the lower part of the Mount Toondina Beds sedimentation occurred during the time that Unit 1 was being deposited in the more southerly parts of the basin. 655 feet of normal Mesozoic western Great Artesian Basin sediments were intersected. From well velocity survey data obtained at the cessation of drilling in this well, seismic interval and average velocities have been determined down to a depth of 2084 feet below surface. The dense crystalline dolomite intersected at 2042 feet below surface exhibits a measured interval velocity of about 18000 feet per second. This information will now enable a more certain interpretation of seismic and other geophysical data in this region.