This digital data release contains previously published contours on the tops of the Mowry Shale, Morrison Formation, and Minnelusa Formation. The maps are from a series of U.S. Geological Survey Miscellaneous Field Studies (MF) showing computer-generated structure contours, isopachs, and cross sections of selected formations in the Powder River basin, Wyoming and Montana (Crysdale 1990, 1991a, 1991b). The maps were constructed from information stored in a U.S. Geological Survey Evolution of Sedimentary Basins data base. This data base contains picks of geologic formation and (or) unit tops and bases determined from electric resistivity and gamma-ray logs of 8,592 wells penetrating Tertiary and older rocks in the Powder River basin. Well completion cards (scout tickets) were reviewed and compared with copies of all logs, and formation or unit contacts determined by N. M. Denson, D.L. Macke, R. R. Schumann and others. These maps are based on information from 2,429 of these wells that penetrate the Minnelusa Formation and equivalents.