This geodatabase includes spatial datasets that represent the Cambrian-Ordovician aquifer system in the States of Wisconsin, Michigan, Minnesota, Iowa, and Illinois. Included are: (1) a polygon dataset that represents the aquifer system extent, (2) raster datasets for the altitude of each aquifer subunit, (3) altitude, or if applicable, thickness contours used to generate the surface rasters, (4) georeferenced images of the figures that were digitized to create the altitude or thickness contours, (5) the line datasets representing the altitude contours that were digitized from the images, and (6) an ArcGIS Toolbox used to develop the surface raster datasets. The images and digitized contours are supplied for reference. The extent of the Cambrian-Ordovician aquifer system is from the linework of the Cambrian-Ordovician aquifer system extent maps in U.S. Geological Survey Professional Paper 1405-B (PP1405B), and a digital version of the aquifer extent presented in the Groundwater Atlas of the United States (the U.S. Geological Survey Hydrologic Atlas 730-J, (USGS HA 730-J) and 730-K (USGS HA 730-K)), available at http://water.usgs.gov/ogw/NatlAqCode-reflist.html . The Cambrian-Ordovician Aquifer System has four aquifer subunits, in order from the most surficial to the deepest: (A1) St. Peter Sandstone Aquifer, (A2) Prairie du Chien Aquifer, (A3) Ironton and Galesville Sandstone Aquifer, and (A4) Mt. Simon or Lamotte Sandstone Aquifer. The altitude contour line datasets for each subunit available were digitized from georeferenced figures of altitude contours in PP1405B, and the altitude values were interpolated into surface rasters within a GIS using tools that create hydrologically correct surfaces from contour data, derives the altitude from the thickness (depth from the land surface), and merges the subareas into a single surface. The primary tool was "Topo to Raster" used in ArcGIS, ArcMap, Esri 2014. (ArcGIS Desktop: Release 10.2 Redlands, CA: Environmental Systems Research Institute). The surface rasters were corrected for the areas where the altitude of the top of the aquifer exceeded the land surface, and where the bottom of an aquifer exceeded the altitude of the corrected top of the aquifer.