1979. United States. Soil Conservation Service. Base source: USGS county base dated 1976; interpretations derived from soil map constructed 1961.; "Projection and 10,000-meter grid ticks, zone 18... Dept. of Agriculture, Soil Conservation Service (Lanham, Md.).
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1980. United States. Soil Conservation Service. Base source: USGS 1:100,000 county base dated 1977; interpretations derived from soil map constructed 1967.; "Projection and 10,000-meter grid tick... Dept. of Agriculture, Soil Conservation Service (Lanham, Md.).