Cooperative National Geologic Map: Precambrian 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 Precambrian Geology layer depicts all Precambrian rocks in the United States, inclusive of map units with minimum ages younger than Precambrian, such as the “Paleozoic to Precambrian” units shown on some older maps. The map was composited from all geologic maps that show Precambrian rocks, including where they are mostly buried beneath Phanerozoic sedimentary rocks and inferred from boreholes and geophysical data. In areas covered by maps of both Precambrian outcrops and buried Precambrian basement, the outcropping Precambrian units are preserved instead of the more generalized buried units. Due to the great disparity in scale and degree of interpretation between outcrop maps and subsurface maps, the Precambrian layer is highly discontinuous along map boundaries. Phanerizoic rocks are included in the map, subdivided into stratified rocks vs. igneous and metamorphic rocks. 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.