The National Geologic Map Database (NGMDB) is the product of a partnership between the United States Geological Survey (USGS) and the Association of American State Geologists (AASG). NGMDB makes widely available a national archive of standardized maps. The project was authorized by the Geologic Mapping Act of 1992. The collection is comprised of >100,000 maps from >600 publishers.
There are 4 main avenues: the catalog, the lexicon, the map interface and the historical topographic map collection. The lexicon has stratigraphic information – geologic names, charts, time scales, and guidelines. “Significant publications” are linked to the geologic terms. The lexicon is also referred to as Geolex, the geological time scale, which is still being adjusted to accommodate all the additional geological scholarship and knowledge. Epochs are not yet searchable until the migration of the originally-reported (in the literature) time scales can be reassigned to the present scheme.
The homepage prominently displays keyword searching -- by title, author, or map number. Theme limits to select from include: Geology, Geophysics, Marine, Resources (Natural), or Hazards. The “Other” category includes Geochemistry, Geochronology and Paleontology.
One may search by State, Territory or County, by map scale, or by latitude and longitude. Publication date(s) are limitable and formats, too: in paper, available online, with map preview, GeoTiff/ Google Earth KMZ or GIS vector data. One may search by publisher, which besides USGS and AASG includes other government agencies, societies, universities, and the private sector. Mousing/browsing with the cursor over maps will produce the reference from which the map is taken.