Mount Toondina lies near to the centre of the Arckaringa Basin, herein redefined somewhat provisionally, on seismic evidence, as a significantly thicker development of Upper Palaeozoic sediments. The Mount Toondina Beds are herein formally described. Their type section, located at Mount Toondina, consists of a 263 feet thickness of shale and argillaceous siltstone with interbeds of fine sandstone and coal. These sediments were deposited mainly in a lake environment but also in intermittent swampy phases, and their included fossil floral remains indicate an Early Permian age. So far, the Mount Toondina Beds are limited in known exposure to the small area of the type locality, where they are severely deformed and dislocated together with remnants of Upper Jurassic Algebuckina Sandstone. However, the surrounding outcropping Lower Cretaceous strata, and underlying Upper Jurassic and Upper Palaeozoic sequences inferred from seismic surveys, are only mildly deformed. An isolated, small, positive gravity anomaly coincides approximately with the Permian inlier. To explain the structurally anomalous occurrence of the Permian sediments at Mount Toondina, it is suggested, by assimilation of present geological and geophysical findings, that they were elevated by piercement of the overlying strata, probably in Middle or Late Tertiary time. The question of motivation for such a piercement has not been resolved.