beds of clay and sand, far beneath the surface. This lava hardened into the strong brittle black 

 trap rock which now forms the Palisades, the Watchung Mountains of New Jersey, and some 

 of the ridges in the Connecticut Valley. 



As this huge trough filled up with sediments and lava it sank still farther and it was 

 not until near the close of Triassic time, that the deposition and the sinking stopped. Earth 

 forces were then released which compressed the trough and arched up the beds within it. 

 At the same time the crust beneath all or most of the eastern part of the United States was 

 also raised, although there is little evidence that it was folded or crushed. Then began another 

 long period of slow wearing down of the land surface until, some millions of years later, it 

 had once more been reduced to a nearly level featureless plain at or near sea level. This plain 

 has been named the Fall Zone peneplain. 



The slow erosion which formed the Fall Zone peneplain extended through approximately 

 all of the Jurassic period into the early part of the next time interval, the Cretaceous period. 

 This protracted wearing down of the land removed a large part of the Triassic red beds 

 and trap as well as vast amounts of the older bedrock. The products of this erosion were 

 carried far from their source and their final resting place has not yet been discovered. It is 

 possible that they are now covered by the ocean. Remnants of the Triassic rocks are now 

 found on the west bank of the Hudson River in New Jersey, and in a long belt running 

 north from New Haven, Connecticut. These two patches, large as they are, are all that remains 

 of a much more extensive body of similar material. There is every reason in fact to believe that 

 the Triassic rocks once covered all of the area that is now Long Island. It is not known whether 

 they were entirely removed from this area during the formation of the Fall Zone peneplain, and 

 it is possible that the belt of Triassic rocks exposed at New Haven passes under the Sound and 

 underlies central Long Island in the area between Port Jefferson and Riverhead in Suffolk 

 County. Granite gneiss however was encountered by test wells drilled at the Brookhaven 

 National Laboratory at Upton in central Suffolk County. The presence of Triassic rock, either 

 red beds or trap, would have no direct influence on the ground-water resources of Long Island 

 for these rocks are not highly permeable. Of more importance is the present distribution of the 

 two Triassic remnants mentioned above, for, as explained below, they provide a clue to the origin 

 of the Jameco gravel, an important Pleistocene aquifer. 



While the history of the Triassic rocks themselves has only an indirect bearing on the hy- 

 iJrologic regimen of Long Island, the Fall Zone peneplain is a factor of much importance since 

 it is the foundation on which was deposited the younger water-bearing formations. It was, as 

 stated above, slowly worn down to a nearly level surface by the action of rain, frost, and run- 

 ning water during what must have been a very long time. At the end of this process, low hills 

 rose above the general plain to a height of roughly a hundred feet. Through the slow attack of 

 percolating ground water, chemical decomposition of the bedrock was probably extended a 

 hundred feet or so below the surface and over wide areas left nothing but quartz and clay in the 

 upper layers. We know nothing now of the pattern of hills or valleys which may have existed 

 at that time, nor of the factors which controlled the depth of the chemical attack. We do know 

 fairly well the general position of the buried bedrock surface for drilling operations today strike 

 bedrock usually within 50 feet of its predicted position. 



One of the most important chapters in the history of Long Island geology began in the 

 middle of Cretaceous time, when the previously nearly level surface of the Fall Zone peneplain 

 was slowly and gently tipped toward the southeast. This was part of a slow gentle arching of the 

 earth's crust which extended north and south along the eastern part of North America. The 

 center line of this arch runs roughly along the crest of the Appalachian Mountains, and in fact 

 it was this arching which gave them their present elevation. 



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