developed in late Tertiary time and attempts to do so, based on present information, would 

 probably fall into error. One or two generalizations however may be made. The southern slope 

 of the buried late Tertiary surface in Nassau County and in much of Queens County is smooth 

 or gently rolling, and has a gradient to the south of roughly 10 feet to the mile. Over wide 

 areas the depth to this surface can be predicted to within 10 or 20 feet. The northern slope 

 of the late Tertiary surface however was far more uneven and has a much steeper slope, its 

 gradient being about 20 to 50 feet to the mile. Prediction of position of the upper surface of 

 the Cretaceous is therefore less accurate in this area. 



The late Tertiary surface slopes irregularly downward to the east from its high point near 

 the Nassau-Suffolk County line, but the details of its shape cannot be closely shown as there 

 are too few deep wells in this area. About at the site of the Brookhaven National Laboratory 

 a central valley running east begins to take shape in the present surface and it is possible, 

 although not likely, that this reflects the shape of the buried surface of the Cretaceous beds. 

 This valley, which is now followed by the Peconic River and also, where drowned, forms Peconic 

 Bay, may be due only to the deposition of the low ridges formed to the north and south of it 

 by the Ronkonkoma and Harbor Hill moraines. Its presence gives the eastern end of the Island 

 a markedly "drowned" appearance, but it does not necessarily indicate that the eastern end 

 has been more deeply submerged than the western, or that the Island has tilted. 



PLEISTOCENE HISTORY 



The oldest Pleistocene formation on Long Island is the little known Mannetto gravel, which 

 lies in the broad flat summit areas of the Mannetto and Wheatley hills and has been tentatively 

 identified at lower elevations in some of the peninsulas of the north shore. These deposits 

 resemble the coarser parts of the younger beds of Pleistocene glacial outwash except that they 

 consist almost entirely of quartz sand and contain relatively few pebbles or boulders of gneiss 

 or schist. These few erratics, though water worn and rounded, are in places all deeply 

 weathered, so much so that most of them can be very easily broken and crumbled. Since these 

 pebbles could not have been transported in this weak condition, they must have weathered 

 in place, and the degree and depth of weathering possibly imply an age appreciably greater 

 than beds of Jameco gravel. Existing evidence suggests strongly that the Mannetto is early or 

 mid-Pleistocene in age but does not enable one to date it more exactly. 



Most of the students of Long Island geology have classified the Mannetto as glacial outwash 

 because it contains pebbles and small boulders of material other than quartz or chert. These 

 are not present however in such amounts as to demand the work of ice. The bedding is well 

 marked and the formation was certainly deposited by running water which had at times at 

 least sufficient force to move cobbles as much as six inches in diameter. Most of the material 

 however is sand and small pebbles and there is considerable clay and silt, particularly in the 

 lower part of the formation. It contains no beds of till and if the material is a glacial deposit 

 the ice sheet has left no unmistakeable record of its presence, although the deposits do resemble 

 glacial outwash. If this is what they are then the ice sheet which supplied the water which 

 formed them advanced to a line somewhere north of the north shore of Long Island but did 

 not, as far as is known, reach Long Island itself. The depression which is now Long Island 

 Sound probably existed at this time but may well not have been as deep as it is at present. 

 Certainly there has been much erosion since the deposition of the Mannetto for the beds now 

 found are but the remnants of a once much more extensive deposit. The original extent of the 

 Mannetto is not known, and only a few poorly exposed patches of it have been found. 



More progress has been made in the study of the glacial geology of New Jersey than has 

 been possible on Long Island. Three separate periods of glaciation have been recognized there ; 



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