Generalized configuration of the top of the limestone unit of the lower part of the surficial aquifer, Duval County, Florida

The top of the limestone unit ranges from about 25 feet above sea level in western Duval County to about 75 feet below sea level locally in the eastern part of the county. The limestone, which is part of the surficial aquifer, is the principal source of water to shallow wells in the county. The unit generally consists of a soft, friable, cavernous, sandy limestone, which ranges from about 5 to 40 feet in thickness. Along the coast and in the southern part of the county, the limestone becomes discontinuous.