An essentially unmetamorphosed sequence of Upper Proterozoic Adelaide System rocks has been folded into a parallel series of north and north-east trending folds on the ORROROO mapsheet area. Rocks of the Adelaide System are up to 50,000 ft thick, and include sediments of Willouran to Marinoan age. Marked facies changes occur from west to east in the Lower Marinoan. Lower Cambrian rocks occur in the keels of tightly folded synclines in the north-west of the survey area, and Triassic beds are preserved in two small basins south-west of Cradock. Lacustrine Tertiary sediments occur at depth in the Willochra and Walloway Basins. Willouran rocks have been forcefully emplaced into upper Adelaidean sediments in several places in the survey area. Some of these piercement events were contemporaneous with sedimentation, whilst others occurred after the main Lower Palaeozoic orogeny. Copper mineralisation in the vicinity of these structures is probably associated with basic igneous rocks which have intruded the piercement breccias. Mineral production from the area has been small and mainly limited to iron flux, copper and gold, all mined towards the end of the 19th Century.