INTRODUCTION 



In order to understand the complex interrelation of the geologic and hydrologic units found 

 on Long Island it is essential to understand clearly the geologic history of the Island and the 

 manner in which the events of that history determined the physical make-up of the various units 

 and their present distribution underground. This comprehensive viewpoint is required both to 

 interpret the data obtained from individual wells and other underground borings, and to pro- 

 vide a basis for comparison from well to well. 



The geologic history however must for the most part be based on and derived from the 

 same well records which one is also called on to interpret. For this reason the advantages of an 

 historical analysis becomes greater as more and more well records are made available for study. 

 In areas where wells are far apart, or where the data available are either lacking in detail or 

 are unreliable, little can be deduced as to the history of the area and a clarification of the record 

 is not possible. Where more information is available, a more nearly complete history may be 

 established, and this in turn may then be used to interpret new data, or to predict the un- 

 derground conditions in advance of drilling. Considerable judgment is required therefore in 

 interpreting the record, for a premature decision as to the history of an area, that is one which 

 is in error because it is based on inadequate data, will result in an incorrect evaluation and 

 analysis of later information obtained in that area, thus compounding the error and concealing 

 the truth. 



In the following history of the geology of Long Island an attempt is therefore made to dis- 

 tinguish between well established events from which deductions may safely be made, and theories 

 which although they serve to suggest answers to many of the problems are themselves problems, 

 the solution of which requires the critical evaluation of data which are not yet available. Unfor- 

 tunately, well established events are few in number. 



The geologic history up to the beginning of Cretaceous time is important for only two facts, 

 (1) the bedrock over a wide area had been worn down to a nearly level surface, and (2) con- 

 tained two long narrow patches of diabase. One of these now forms the Palisades of New Jersey, 

 ^while the other forms East Rock and West Rock at New Haven, and continues north to and along 

 the valley of the Connecticut River. Since the Palisades strip must have supplied the diabase 

 found so abundantly in the Pleistocene Jameco gravel, there is a suggestion that the Jameco 

 gravel was formed by a stream which had followed the Hudson Valley, and that the Jameco 

 gravel may have been deposited only in the western part of the Island. 



The Cretaceous history is one of deposition on the rock floor mentioned above. The Lloyd 

 sand and the clay member of the Raritan formation resemble the corresponding beds in New 

 Jersey, but the overlying formations, the so-called Magothy sands, are not comparable. The Long 

 Island Cretaceous deposits appear to be fluvial rather than marine, and individual beds can be 

 traced for only short distances. The important hydrologic problems are the hydraulic inter- 

 connection between aquifers, the method by which ground-water descends from the Pleistocene 

 water-table beds to recharge the artesian Magothy formation, and in particular the pattern of 

 flow through the Raritan clay into the underlying Lloyd sand. What little is known of the geo- 

 logic history of this period suggests alternative answers to these questions. 



There are no records of early Tertiary events on Long Island. As far as we know, the late 

 Tertiary and the early part of the Pleistocene were dominantly periods of erosion. A great deal 

 of material was eroded and removed, but virtually nothing is known of the erosional pattern de- 



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