The geologic map in the National Atlas of the United States of America shows the age, distribution, and general character of the rocks that underlie the Nation, including Alaska, Hawaii, Puerto Rico, and the Virgin Islands (but excluding other small island possessions). (The National Atlas of the United States can be accessed at URL http://nationalatlas.gov/natlas/Natlasstart.asp.) The map depicts the bedrock that lies immediately beneath soils or surficial deposits except where these deposits are so thick and extensive that the type of bedrock beneath them can only be inferred by deep drilling or geophysical methods, or both. Thus, it does not show the extensive glacial deposits of the North Central and Northeastern States, the deep residuum of the Southeastern and South Central States, the relatively thin alluvium along many major rivers and basins, and extensive eolian deposits on the high plains. However, it does show, in a general way, the thick alluvial deposits along the lower Mississippi River and on the Atlantic and Gulf Coastal Plains, and in the deep basins of the western cordillera.
The rocks are classified as either sedimentary, volcanic, plutonic, or metamorphic, and their geologic ages are given in terms using a simplified version of the 1999 Geological Society of America geologic time scale. In some places rocks depicted as sedimentary are interlayered with volcanic rocks, including tuff, volcanic breccia, and volcanic flows. Conversely, many of the rocks shown as volcanic include interlayered sedimentary rocks. Plutonic rocks are classified by age and as granitic, intermediate, mafic, or ultramafic, but no similar classification has been attempted for the volcanic rocks in this version of the map. Where sedimentary or volcanic rocks have been metamorphosed but still retain clear evidence of their depositional age and origin, the extent of the metamorphism is shown by a pattern. Where the metamorphism has been so intense that the rocks bear little resemblance to the rocks from which they were derived, they are mapped as gneiss, but the age given is generally the age of the original rocks.
The map in the National Atlas is a generalization of a new geologic map of North America that has recently been published by the Geological Society of America. The original compilation was prepared at a scale of 1:2,500,000 for publication at a scale of 1:5,000,000. This generalized version is intended for viewing at scales between about 1:10,000,000 and 1:7,500,000.
Posted November 2007
Reed, J.C., and Bush, C.A., 2007, About the geologic map in the National Atlas of the United States of America: U.S. Geological Survey Circular 1300, 52 p.
Conterminous United States
Central Interior Region
Appalachian and Ouachita Mountain Systems
Ouachita Mountain System
Coastal Plain Province
Atlantic Coastal Plain
Gulf Coastal Plain
Cordilleran Mountain System
Laramide Rocky Mountains
Rocky Mountain Fold and Thrust Belt
Basin and Range Province
Rio Grande Rift
Paleogene Volcanic Fields of the Eastern Cordillera
Accreted and Subduction-Related Terranes of the Western Cordillera
Lava Plains and Plateaus of the Columbia Intermontane Region
Brooks Range and Arctic Slope
Central Intermontane Plateaus and Basins
Aleutian-Alaska Range Mountain System
Volcanoes of the Aleutian Islands, Alaska Peninsula, and Wrangell Mountains, and the Bering Sea Volcanic Field
Puerto Rico and the U.S. Virgin Islands
Continental Ice Sheets