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Shear zones and bedrock units as thin as 3 to 4 m in width can be detected using radon and equivalent uranium (eU) concentrations in soils overlying the Hylas zone area, Virginia. Soil radon concentrations in the area range from 144 to 12,081 pCi/L, with distinct contrasts in concentrations among many of the rock types. Equivalent uranium concentrations range from 0.4 to 9.8 ppm; they also reflect bedrock geology but less distinctly than soil radon, which is subject to surficial mechanical and chemical processes. The formation of saprolite and subsequent soil processes produce variability in uranium concentrations and consequently contacts and other fine geologic details can be obscured near the surface. Soil radon analyses at a depth of 75 cm, and using small (20 ml) samples, largely circumvent these problems and can be used as an effective geologic mapping tool in covered areas, provided that (1) there is a uranium concentration contrast in the bedrock, and that (2) the soil is derived directly from bedrock. The scale of variability of radon in soil gas indicates that radon potential maps should be constructed on the basis of geology rather than political divisions.

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