Cooperative National Geologic Map: Pre-Quaternary geology

The USGS National Cooperative Geologic Mapping Program (NCGMP) was directed by Congress to create a new geologic map of the United States” by “bring[ing] 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 fulfill this goal, the NCGMP created a map synthesis engine (Johnstone and others, 2025) based on the Geologic Map Schema (GeMS) standard (NCGMP, 2020) and used it to bring together source geologic maps originally published by the USGS and State Geological Surveys. This thematic map layer was derived from this database using procedures described by Johnstone and others (2025) and is intended to serve as a legible national map of the nation; while it retains full descriptions of geologic units from source materials it does not include the full search functionality (described in Johnstone and others, 2025) that is facilitated by a true relational database.
The pre-Quaternary Geology layer depicts geologic units older than Quaternary, exclusive of 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 from State geologic maps showing geology at the earth’s surface elsewhere. 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). The map is structured as a single GeMS database with three additional tables (also described in Johnstone and others, 2025); a Source_DescriptionOfMapunits table preserves all source inputs, a synthesis_to_source_units table relates aggregated national units to original map units, and a Symbol_Lookup table provides definitions of all FGDC geologic map symbol code values (U.S. Geological Survey, 2006) used for stylizing the map.