This report describes the geology of the central Virgin River basin in southwestern Utah, and characterizes the structure and lithology of the Jurassic Navajo Sandstone and the thickness and stratigraphy of Quaternary-late Tertiary unconsolidated deposits, the two main aquifers in the region. Southwestern Utah has experienced rapid population growth and increased demand on water supplies during the past 15 years, and the purpose of this report is to better define the ground-water resources of the area. Ground water provides approximately half of the public water supply in southwestern Utah, so future decisions regarding water use must be based on careful geologic characterization of the aquifers and their relation to the regional hydrologic system. The results of this study will be applied to ground-water modeling, evaluating regional and local hydrogeologic conditions, and assessing sites for development of ground water. 53 pages + 7 plates