Volcanic rocks from the Wirrulla region include dacites, rhyodacites and rhyolites. They comprise both magmatic and pyroclastic types, and are similar to the Gawler Range Volcanics. Sedimentary rocks from the same region include quartz sandstones and mudstones, but almost all have been modified by pedogenic processes to form silcrete, calcrete and laterite. Dominantly sandy lithologies with zircon may be Garford Formation, while shaley siltstones and mudstones with microfossils may be Pidinga Formation.