| Madison Group, Williston Basin
The final potentiometric map by Downey (1986) and used in this project was the result of a detailed analysis also using the previous potentiometric maps contoured for the Madison Group by Miller and Strausz (1980, in Downey, 1986).

Downey, J. S., 1986, Geohydrology of bedrock aquifers in the northern Great Plains in parts of Montana, North Dakota, South Dakota and Wyoming: U.S. Geological Survey Professional Paper 1402-E, 87 p.
Downey, J. S., 1984, Geohydrology of the Madison and associated aquifers in parts of Montana, North Dakota, South Dakota and Wyoming: U.S. Geological Survey Professional Paper 1273-G, 47 p.
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