Description: The USGS National Cooperative Geologic Mapping Program (NCGMP) was directed by Congress to "bring together detailed national and continental-resolution 2D and 3D information produced throughout the Survey and by federal and state partners" (House Report 116-100), to create a new geologic map of the United States. To fulfill this goal, the NCGMP created a map synthesis engine (Johnstone and others, 2025) based on the Geologic Map Schema (GeMS) standard and used it to bring together and (to some extent) to harmonize source geologic maps originally published by the USGS and by State Geological Surveys. Currently, this map covers the conterminous U.S.; however, work is underway to expand the coverage.The Pre-Quaternary geology layer depicts geologic units older than Quaternary, except for units whose ages span the Quaternary-Neogene boundary. The map was assembled from State geologic maps showing Pre-Quaternary "bedrock" in the glaciated regions of the Midwest and Northeast, which is mostly buried by Quaternary glacial deposits and, elsewhere, from State geologic maps showing geology at the earth’s surface. Where present, Quaternary and Quaternary-Tertiary units are categorized as a generic cover unit. This map corresponds either to the Earth’s surface (where no Quaternary geology is present) or the subsurface base of Quaternary deposits in the glaciated regions (often called the "top of rock" surface).This layer is one of four created for the new national geologic map. The other layers available are: Earth’s surface (the geology exposed at the Earth’s surface) Quaternary (geologic units of Quaternary age, exposed at the Earth’s surface) Precambrian (emphasizes Precambrian units at Earth's surface and those depicted in "basement" maps).Each layer is published by the USGS as GeMS-compliant Esri geodatabases. Those geodatabases are products of the overall geospatial database, which is published as a geopackage. These products and static images of the layers are available from the National Geologic Map Database, at https://ngmdb.usgs.gov/Prodesc/proddesc_118545.htm. For a more performant viewing and data exploration experience for the four layers, please visit the web application at ngmdb.usgs.gov/nationalgeology.