Fossil marine faunas from the Pleistocene sediments of eastern Long Island, New York, and Nantucket Island, Massachusetts, previously attributed to the Sangamonian interglacial, appear to transcend the Sangamonian-Wisconsinan boundary.

The Gardiners Clay in its type area on Gardiners Island, New York, is of questionable age and is not correlative with the rhythmically bedded glaciolacustrine Gardiners Clay of Wisconsinan age on Long Island. The Wisconsinan Jacob Sand at its type area (Jacobs Hill, Long Island) is a nonfossiliferous glaciolacustrine or glaciofluvial unit. It is not correlative with the fossiliferous marine Jacob Sand of Gardiners Island. The Jacob Sand and the underlying glaciolacustrine Gardiners Clay are simply fluvial and lacustrine facies of the outwash that developed as part of the depositional system of the Montauk ice sheet.

The Jacob Sand or the Tobaccolot Bay fauna of Gardiners Island contains cold-water marine species extant north of Cape Cod, Massachusetts. This fauna suggests that Long Island waters were once 3° to 7°C colder than at present.

The marine fossil fauna from Bridgehampton, Long Island, contains species extant farther south, suggesting that Long Island waters were previously 2° to 11°C warmer than at present. The sparse fauna of the Gardiners Clay of Gardiners Island prevents its correlation with any of the other fossil-bearing units. The fauna of the lower unit of the Sankaty Beds exposed on Nantucket Island is extant in the Nantucket area, while the fauna of the upper unit is extant in the Gulf of Maine. This suggests that water in the Nantucket area was 2° to 7°C cooler when the upper Sankaty Beds were deposited.

It is thought that the warm-water Bridgehampton fauna is Sangamonian in age and that the marine waters of that time were as much as 6°C warmer than the present. The cold-water faunas from the upper Sankaty Beds and Tobaccolot Bay are Wisconsinan in age and suggest a cooling of approximately 5°C during the last glacial stage.

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