This GIS dataset presents contours and maps from previously published interpretations of hydrogeologic units of the northern Great Plains by Downey (1986). The original source publication was completed as a component of the United States Geological Survey’s (USGS) Regional Aquifer System Analysis (RASA) professional paper series. Isopach contour maps in this study delineate the upper surfaces of the Bakken Formation, Big Snowy Group, Deadwood Formation, Interlake Formation, Madison Group, Red River Formation as well as the depositional boundary extents of the Charles Salt, Prairie Salt, Opeche Salt, Pine Salt, Middle Madison Salt(s), and Dunham Salt. Geologic Period isopach contours included are Cambrian, Ordovician, Devonian, Pennsylvanian, Permian, Triassic, Jurassic, and Lower Cretaceous. The Northern Great Plains regional bedrock aquifer system is broadly composed of five major bedrock aquifer systems described in the source report, extending beneath large portions of Montana, North Dakota, South Dakota, Wyoming, and into Nebraska.
