Geology of the "20-foot" clay and Gardiners clay in southern Nassau and southwestern Suffolk counties, Long Island, New York

Data from 1978-79 drilling was compiled with information from previous reports and historical records to prepare surface contour and isopach maps of the ' 20-foot ' clay and Gardiners Clay in southern Nassau and southwestern Suffolk Counties. These units are major confining layers in the upper part of the groundwater reservoir along Long Island 's south shore. Where present, they influence the groundwater flow patterns locally. The ' 20-foot ' clay, previously mapped in Nassau County only, was found in test borings in Suffolk County also. Its surface altitude ranges from 20 to 40 ft below NGVD (National Geodetic Vertical Datum of 1929); thickness ranges from 0 to 30 ft. The surface altitude of the Gardiners Clay ranges from 40 to 120 ft below NGVD; thickness ranges from 0 to 90 ft. Previously known discontinuities in both formations are more accurately delineated, and several new discontinuities have been inferred from the new data. The Matawan Group-Magothy Formation undifferentiated the Monmouth Group, and the Jameco Gravel directly underlie the Gardiners Clay. Revised surface altitudes of these formations are depicted on maps and cross sections of the south-shore area. (Author 's abstract)