This preliminary map is the first product of the ground-water resources study of Logan County, which has an area of 1,015 mi2 (2,629 km2) in south-central North Dakota (see location map). This study is part of a statewide program to determine the location, extent, chemical quality, and hydrologic characteristics of the ground-water reservoirs (aquifers) in North Dakota. The study was made by the U.S. Geological Survey in cooperation with the North Dakota State Water Commission, North Dakota Geological Survey, and Logan County Water Management District.