The Duperow Formation is the most widespread Devonian rock unit in the subsurface of North Dakota. It thickens northwestward from its erosional limit in south-central South Dakota to 700 feet in Saskatchewan. The Formation is dominantly carbonate with some anhydrite, shale, and siltstone. Thin sections of cores indicate six major rock types: fine-grained limestone, calcarenitic limestone, calcarenite, biolithite, anhydrite and associated fine-grained rock, and crystalline dolomite and dolomitized limestone. All transitions occur between these rock types. Limestone pellets and oolites are present in parts of the Formation. Recrystallization and dolomitization of the limestone is widespread.