SEQUENCE OF MAPPING 



It is quite evident that paucity of data makes the mapping of any of these buried surfaces 

 most difficult. When it is necessary to contour some four surfaces lying between the roughly 

 known rock and the accurately surveyed present surface, the difficulties are greatly increased. 

 The chief trouble, of course, being to keep the contoured surfaces from intersecting each other. 

 Reliance for avoiding this error was largely placed on study of the sections. The contours were 

 first drawn, sections then made from them and inconsistencies disclosed by and corrected on 

 the sections where carried back to the maps and the contours made to conform. Frequently 

 special sections at odd locations and directions were often required for this purpose. 



As above explained, any attempt at drawing contours of these subterranean and invisible 

 surfaces requires certain assumptions as to the hydrology of the surface in question ; that is the 

 location and elevation of the ocean and of the ponds and streams existing on the surface. On 

 study of the situation, it appeared that the only one of these surfaces which gave promise of 

 being so little altered by a subsequent erosion that the stream valleys would be completely 

 masked is the upper Cretaceous surface, which over most of the island, is the top of the Mago- 

 thy. Accordingly, contours of this surface were first attempted by making the following assump- 

 tions : 



Sea level is assumed to be several hundred feet lower than at present, leaving Long Island 

 sound as a valley and exposing a considerable portion of the pre-glacial continental shelf, which 

 surface lies at considerable depth below the subsequent glacial fill. 



Hudson river, constricted between the Palisades and the rocky hills of Westchester, is 

 assumed to have been nearly in its present position, but at the bottom of a rock gorge several 

 hundred feet deeper than the present channel. This river ran out on the continental shelf, pre- 

 sumably in a deep valley ; the upper end of this depression was wide and fanned out to include 

 not only the present lower bay but most of Kings and part of Nassau counties. One lobe of this 

 ran up from the Coney island-Gravesend region towards Flushing bay and must have been 

 drained by a stream running into the Hudson in the lower bay. There should be a considerable 

 notch in the rock wall east of the Hudson where this stream passed over it, but it has not been 

 found. This lobe represents Crosby's Sound River valley. It is cut through to the sound or nearly 

 so but apparently never was occupied by a stream of any magnitude. Another lobe covered 

 Jamaica bay and headed towards Little Neck bay, stopping rather abruptly in Jamaica. The 

 drainage apparently was to the south into the Hudson gorge. This is Veatch's Sound River valley. 

 Further to the east on the south shore there must have been a number of streams tributary to 

 Hudson canyon, but these have been covered by the westerly drift of the beach sand and barrier 

 beaches formed as the ocean again rose and submerged this area. 



Long Island sound was a valley with gently sloping floor from the present Connecticut 

 shore to a stream running easterly near the present north shore of the island. This valley floor 

 presumably rested on rock at both ends, but could not have done so in the middle. On the south 

 it was bounded by a rather abrupt escarpment in soft material which probably was being under- 

 cut by the stream and eroded away. Along this escarpment, numerous small streams ran north- 

 ward to join Sound river. They deeply notched the escarpment and are now represented by the 

 existing north shore bays. The western end of the ridge now forming the island was low and 

 had little relief ; there was a group of hills of considerable height near the present Nassau- 

 Suffolk line. Information is insufficient to show the condition in the present Shelter island and 

 Flukes complex. Sound river perhaps ran out onto the shelf between Orient point and Block 



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