Geologic reconnaissance of an extensive clay unit in north-central Suffolk County, Long Island, New York

Recent geologic data indicate an extensive lacustrine clay unit within the glacial deposits in the area between the Harbor Hill and Ronkonkoma moraines in north-central Suffolk County, New York. The unit, locally known as the ' Smithtown clay unit ' (informal usage), is an integral part of the glacial aquifer in this area. The clay occurs at depths varying from 90 ft above to 150 ft below National Geodetic Vertical Datum of 1929 (NGVD) and attains a maximum thickness of 170 ft in the northern part of the Town of Smithtown. Its upper surface is mostly above NGVD and reaches a maximum altitude of 90 ft in the Town of Huntington. The unit is predominantly clay but locally contains lenses of sand, silt, and gravel. The upper part of the clay is generally light to dark brown; the lower part is light gray, which is characteristics of other clays in the glacial aquifer. (USGS)