The Wirrealpa and Aroona Creek Limestones are early Middle Cambrian formations which crop out in the Flinders Ranges of eastern South Australia. They are up to 140 m thick and are underlain and overlain conformably by thick red bed sequences. This report concentrates on the formations in their outcrops in the Flinders Ranges. Temporally-equivalent carbonate formations also crop out near Mt. Arrowsmith in northwestern New South Wales and on Yorke Peninsula in South Australia. These outcrops have not been dealt with fully in this study. The petrography and field relationships of the rocks are used to interpret the depositional history and palaeogeography. Twenty four sections were measured, 14 of them in the main outcrop area near Wirrealpa Homestead, and more than 450 samples were studied by stained acetate peels. Nine major rock types, including cryptalgal deposits, were recognised and their respective depositional environments interpreted