James L. Bischoff, Roy J. Shlemon, T. L. Ku, Ruth D. Simpson, Robert J. Rosenbauer, Fred E. Budinger, Jr.; Uranium-series and soil-geomorphic dating of the Calico archaeological site, California. Geology ; 9 (12): 576–582. doi: https://doi.org/10.1130/0091-7613(1981)9<576:UASDOT>2.0.CO;2
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Lithic specimens identified as artifacts have been recovered from near the base of the Yermo fan deposits at Calico, California. The soil on the fan surface is a strongly developed relict paleosol. Comparison of this soil with dated paleosols elsewhere in the southwestern United States suggests that the surface is about 80,000 to 125,000 yr old. Clasts near the base of the deposit are well cemented by laminated CaCO3 that probably formed from groundwater action while the fan was still active. Uranium-thorium assays on the CaCO3 indicate an age of 200,000 yr.