Search geologic maps of the US

Missouri Spatial Data Information Service. Click on the data tab for downloads.

Free maps and templates that can be opened directly in ArcMap. Good source of base maps.

Large source of data for US government GIS layers.

Links to the US Geological Survey data distribution sites.

This portal is a catalog of geospatial information containing thousands of metadata records (information about the data) and links to live maps, features, and catalog services, downloadable data sets, images, clearinghouses, map files, and more.

Guide to US GIS data resources listed by State, from U of Oregon Libraries