County the clay extends inland a short distance and wedges out over the z-ising Cretaceous 

 highland. In the Peconic River valley near Riverhead in eastern Suffolk County, the thick 

 clays reported in driller's logs have been correlated as Upper Pleistocene valley filling deposits 

 but some of these clays may actually be of Gardiners age. 



The surface of the Gardiners clay lies about 65 feet belov^^ sea level at the Brookhaven 

 National Laboratory and about 100 feet below sea level in the shore areas to the south. At Gar- 

 diners Island, the Gardiners clay has been folded and lies above sea level. Its thickness is quite 

 variable. It is only about 10 feet thick in some wells at the Brookhaven Laboratory and may be 

 up to 50 feet thick along the south shore. The maximum thickness is unknown since the south 

 shore beds have not been satisfactorily correlated. 



An unusual depth for the Gardiners clay is indicated at well S 184 near Sag Harbor in the 

 southeastern part of Suffolk County. At that locality, Lohman (33) has identified Pleistocene 

 diatoms at a depth of about 260 feet below sea level. 



Upper Pleistocene deposits 



The remaining glacial deposits not assigned to one of the previously described Pleistocene 

 formations are referred to in this report as upper Pleistocene. It includes the Manhasset for- 

 mation which Fuller (15) correlated with the Illinoian stage of the Pleistocene glaciation; and 

 the till, terminal moraines, and outwash deposits of Wisconsin stage. (9) (15). 



A wide variety of materials is found in the upper Pleistocene of Long Island. The deposits 

 include beds of fine to coarse stratified sand and gravel, boulder clays or tills consisting of un- 

 stratified mixtures of clay and boulders, and some fresh water lake deposits composed of silt 

 and clay. The upper Pleistocene consists of a heterogeneous mixture of rock fragments of all 

 types, quartz, biotite, muscovite, amphibole, pyroxene, feldspar, and limonite. The outwash de- 

 posits in Nassau and Suffolk Counties are frequently low in rock and mineral particles and con- 

 sist chiefly of yellow-stained and clear quartz. The clayey till deposits are best developed in 

 northern Kings and Queens Counties where the imperviousness of the till occasionally requires 

 drilling into bedrock to obtain water. The terminal moraine deposits composed of unstratified 

 sand, boulders and clay, underlie most of the elevated areas of the Island. The youngest moraine, 

 the Harbor Hill moraine, extends along the north shore to the eastern extremity of the north 

 fluke. The older Ronkonkoma moraine forms a ridge across the central part of the Island and 

 extends to the eastern end of the south fluke. The outwash deposits are best developed south of 

 the Ronkonkoma moraine but also occur in the area between the moraines particularly in Suf- 

 folk County. 



The upper Pleistocene deposits are commonly thickest beneath the moraines. They may be 

 as much as 200 feet thick in Kings and Queens Counties, and probably attain a thickness of 

 about 300 feet in Suffolk County beneath the highest points of the Ronkonkoma moraine. The 

 thickness of the Upper Pleistocene deposits is considerably less than the figures mentioned above 

 in those parts of Nassau and Suffolk Counties underlain by a core of high Cretaceous deposits. 

 The upper Pleistocene deposits extend to a depth of about 150 feet below sea level in parts of 

 Kings County and may occur at greater depth in some of the north shore embayments. 



Most of the wells drilled on Long Island are screened in the upper Pleistocene deposits 

 where unconfined water table conditions exist except locally where clayey till deposits support 

 perched water tables. 



Recent deposits consisting of beach sands, river and bay silts, and muds are included in the 

 Correlation Tables under the heading, "Recent and Upper Pleistocene deposits". These deposits 

 are not very thick in most places and for that reason are not classified separately. These recent 

 bods of clays and silts help to protect the underlying permeable deposits from the encroach- 

 ment of salt water which completely surrounds the entire Island. 



24 



