This line shapefile shows the outlines of major calderas and impact structures in the United States. This layer is part of the Geologic Map of the United States, originally published at a scale of 1:2,500,000 (King and Beikman, 1974b). It excludes Alaska and Hawaii. These data depict the geology of the bedrock that lies at or near the land surface, but not the distribution of surficial materials such as soils, alluvium, and glacial deposits. This is a revised version of the April 2004 data set. This map complements the Generalized Geologic Map of Alaska, Hawaii, Puerto Rico, and the U.S. Virgin Islands by Reed and Bush. A geographic information system (GIS) allows combining and overlaying of layers for analysis of spatial relations not readily apparent in the standard paper publication. This layer is part of the 1997-2014 edition National Atlas of the United States. Conversion of the geologic map of the U.S. to a digital format was undertaken to facilitate the presentation and analysis of earth-science data. Digital maps can be displayed at any scale or projection, whereas a paper map has a fixed scale and projection. However, these data are not intended to be used at any scale finer than 1:2,500,000. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.
