Rocks that make up the Arbuckle-Simpson aquifer crop out over ~500 mi2 in the Arbuckle Mountains province in south-central Oklahoma. The aquifer consists of limestone, dolomite, and sandstone of the Arbuckle and Simpson Groups of Late Cambrian to Middle Ordovician age and is about 5,000-9,000 ft thick. The rocks were subjected to intensive folding and faulting associated with major uplift of the area during Early to Late Pennsylvanian time.