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More than one-third of the United States is estimated to have high geologic radon potential. A high radon potential area is defined as an area in which the average indoor radon screening measurement is expected to be 4 pCi/L or greater. Geologic terrains of the United States with high radon potential include:1. Uraniferous metamorphosed sediments, volcanics, and granite intrusives that are highly deformed and often sheared. Shear zones in these rocks cause the highest indoor radon problems in the United States.2. Glacial deposits derived from uranium-bearing rocks and sediments and glacial lake deposits. Clay-rich tills and lake clays have high radon emanation because of high specific surface area and high permeability due to desiccation cracking when dry.3. Marine black shales. The majority of black shales are moderately uraniferous and have high emanation coefficients and high fracture permeability. 4. Soils derived from carbonate, especially in karstic terrain. Although most carbonates are low in uranium, the soils derived from them are very high in uranium and radium.5. Uraniferous fluvial, deltaic, marine, and lacustrine deposits. Much of the nation's reserve uranium ores are contained within these sedimentary deposits, which dominate the stratigraphy of the western U.S.