veloped. There may be buried valleys dating from this period in almost any part of the Island, 

 although the possibility of Long Island Sound having once drained westward now seems to be 

 ruled out by well records. But because these buried valleys may exist is no reason for assuming 

 that they do, and all of the known facts, including the overdeepening of Long Island Sound, can 

 be explained without them. These buried valleys, if they existed, would be of considerable im- 

 portance to the water supply of Long Island. 



During the Pleistocene we know, from records in other parts of the country, that the con- 

 tinental ice sheet advanced four separate times, but it is not certain that the ice reached as far 

 south as Long Island on each of these occasions. There are records of three widely separated ad- 

 vances, the Manneto, the Jameco, and the three here presumed Wisconsin fluctuations repre- 

 sented by the Montauk, the Ronkonkoma moraine, and the Harbor Hill moraine. For each of 

 these ice advances there must have been a drop in sea level and local erosion, as well as the 

 more obvious deposition of moraine and outwash. For each of the interglacial periods there 

 must have been a corresponding high sea level and presumably interglacial deposition. 



The actual record however is very fragmentary. The only interglacial deposit so far identi- 

 fied is the Gardiners clay, of importance hydrologically because over wide areas it forms the 

 bottom of the water table aquifer, and locally seals underlying sands and gravels, particularly 

 the Jameco gravel in Brooklyn. 



The rest of the glacial deposits however, except for the Jameco gravel, act as one hydro- 

 logic unit, and the deciphering of the history of late Pleistocene and advances and retreats of 

 the ice offers little hope of contributing materially to our understanding of the hydraulics of 

 ground-water resources of the Island. 



GEOLOGIC HISTORY OF THE BEDROCK 



Most of the bedrock which underlies Long Island is of pre-Cambrian age, to science. 

 Little is lost to us however from our lack of knowledge of these most distant ages, for 

 these rocks are all of low permeability and are not important sources of well water on Long 

 Island. We can properly start our history perhaps a billion years later when in Permian 

 time and in the early part of Triassic time, the previously mountainous surface of the land 

 was slowly worn away until it was a flat or gently undulating plain, at or near the sea level 

 of that time. This slow erosion effectively removed all traces of earlier mountains or valleys. 



The next major geologic event, which took place a short time later, was the formation of 

 the Triassic red beds and trap rock of New Jersey and Connecticut. The deposition of these 

 sediments and the formation of the related igneous rocks was preceded by a cracking of the crust 

 of the earth on a magnificent scale, the zone of fractures running from the Bay of Fundy south 

 to the Carolinas. Two fracture zones of this group are important to the geologic history of Long 

 Island. One of these ran north from New Haven and formed the east side of what is now the 

 Connecticut Valley, the other ran southwest through the north-central part of New Jersey 

 along a line marked roughly by Trenton and Bayonne. 



A large block of the crust of the earth, bounded in part by these two fracture zones, 

 slowly sank during the later part of the Triassic period and formed an immense depression. 

 Into this trough, as it was formed, was swept layer after layer of mud, silt, sand and gravel, 

 which for reasons that have never been entirely explained, are now largely bright brick-red 

 in color. While these materials were being deposited by running water, layers of rock were 

 also formed by intermittant outpourings of molten lava. Some of this lava reached the land 

 surface and spread out in great sheets, and some of it was forced between the recently formed 



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