GEOLOGIC MAP OF THE CONTERMINOUS UNITED STATES: EARTH’S SURFACE 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 we have created a database engine for synthesizing geologic maps (described in a companion poster) based on the Geologic Map Schema (GeMS) and used it to bring together geologic maps published by the USGS and State Geological Surveys and create derivative thematic geologic maps.

This Earth’s Surface geology layer depicts geologic units exposed at the earth’s surface over the entire conterminous United States, inclusive of everything from Quaternary glacial deposits to Precambrian crystalline bedrock. In the West and Southeast, it is a composite of 29 state geologic maps depicting geology at the earth’s surface. In the glaciated region of the Midwest and Northeast, it is a composite of 21 state geologic maps depicting pre-Quaternary rocks (“bedrock”), 8 state geologic maps depicting Quaternary deposits, and 18 USGS Quaternary Atlas Series maps depicting Quaternary deposits; the Quaternary Atlas maps were used where modern state geologic maps were not available. Quaternary and pre-Quaternary maps were composited by replacing bedrock, residuum, and colluvium on Quaternary maps with the corresponding content from the underlying pre-Quaternary maps (including line and point features).

About 5200 source map units from 76 maps are assigned to 106 synthesis map units, broadly grouped by age and lithology, from the International Commission on Stratigraphy (ICS) timescale and GeMS Geomaterials fields, respectively. Synthesis units are primarily divided into sedimentary and igneous rocks and grouped by age, with metamorphic rocks more often grouped into separate units in the Precambrian and parts of the Lower Paleozoic. Synthesis map units are colored according to USGS guidelines such that they are readable as a traditional geologic map. The locations of contacts and faults are unchanged from the original sources, but only those that bound synthesis units are solid; those within them are symbolized as “internal.” Additional line features such as dikes, key beds, and fold axes are included.