the Mannetto possibly lying below, and more recent glacial deposits above, are difficult to deter- 

 mine. In the southern half of Long Island it is quite impossible to distinguish between the Man- 

 hasset outwash and the younger outwash gravels. 



The belief that the Manhasset formation is of Illinoian age, and represents an earlier gla- 

 cial advance than the relatively recently vanished Wisconsin ice sheet, is based largely on the 

 supposed deep erosion of the Manhasset prior to the deposition of the Wisconsin till. The mag- 

 nitude of this erosion depends on how continuous and extensive a deposit one imagines the 

 Manhasset formation to have been at the time it was first deposited. At present it is cut off 

 abruptly along the margins of the bays and necks in Nassau and Suffolk Counties, and the bluffs 

 of the north shore to the east. If the Manhasset formation at one time filled these bays and 

 extended northward well into what is now Long Island Sound, then the amount of erosion has 

 been very great. But the most recent ice advances, the Ronkonkoma and the Harbor Hill, failed 

 to deposit substantial quantities of material in the bays and the Sound, and even appear to have 

 deepened them locally, and there is no reason to believe that the earlier ice sheet behaved in a 

 radically different fashion. 



The best evidence that two superimposed glacial deposits belong to two separate glacial per- 

 iods and not to two advances of the same ice sheet, is to find between them deposits formed 

 during a warm interglacial period. Granted that the Jameco is of glacial origin, then the Gar- 

 diners clay, a relatively warm water deposit, proves that the Jameco and the Manhasset forma- 

 tion must belong to separate glacial periods. But no such interglacial deposit separates the 

 Manhasset formation from the overlying till and outwash of the Ronkonkoma and Harbor Hill 

 ice sheets. Fuller (15) did indeed try to identify the muck, clay and fossil shells or wood in cer- 

 tain well records as representing such an interglacial deposit, but a critical examination of 

 these localities shows that the material is either Gardiners clay or may be explained as deposits 

 of Recent age. In fact the highly dubious nature of the data that Fuller was called upon to use 

 in trying to show the existence of an interglacial deposit between the Manhasset and the over- 

 lying till and outwash, suggests that such deposits do not exist. There is then a possibility, even 

 a probability, that the Jameco is late Illinoian in age, that the Gardiners clay is Sangamon, and 

 that the Manhasset formation and the overlying Ronkonkoma and Harbor Hill deposits were 

 all formed during the Wisconsin by fluctuations of the same ice sheet. Their common lack of 

 weathering strongly substantiates this conclusion. 



Since the material overlying the Gardiners clay is all classed as upper Pleistocene in the 

 geologic correlations of Long Island, it might wall appear that the details of their history and 

 formation could be of little importance in the hydrology of Long Island. In a general way this 

 is true. The upper Pleistocene over wide areas acts as a hydrologic unit, water passing easily 

 from one part of it to another. The upper Pleistocene is the source of most of the well water 

 pumped on Long Island and the intelligent development of this unit cannot help but be influ- 

 enced by a more complete understanding of the composition of its beds, and their distribution 

 and history. 



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