The Cockfield Formation of the Claiborne Group of Tertiary age underlies approximately 4,000 sq mi in western Tennessee. The formation consists primarily of lenticular beds of very fine to coarse sand, silt, clay, and lignite. The Cockfield Formation has been extensively eroded, and the original thickness is preserved only in a few areas where the formation ranges from 235 to 270 ft in thickness. Recharge to the Cockfield aquifer is from precipitation on sparse outcrops or by downward infiltration of water from the overlying fluvial deposits of Tertiary and Quaternary age and alluvium of Quaternary age or, where present, the overlying Jackson Formation of Tertiary age. Data from two observation wells indicate that water levels have risen at average rates of about 0.5 and 0.7 ft/year during the period 1980-85. Water from the Cockfield aquifer is a calcium bicarbonate type that contains low concentrations of most major constituents, and generally is suitable for most uses. Dissolved-solids concentrations range from 44 to 218 mg/L. Data from two aquifer tests indicate transmissivities of 2,500 and 6 ,000 sq ft/day and storage coefficients of 0.0003 and 0.0007, respectively. The Cockfield aquifer presently provides small to moderate quantities of water for several public and industrial water supplies and small quantities to numerous domestic and farm wells. Withdrawals for public and industrial supplies in 1983 averaged about 3.3 million gal/day. (USGS)