﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 33 



All of the formations dip westward wherever exposed, but the 

 dips vary somewhat, nearly all being of low angle. Occasional 

 minor inequalities of the nature of small rolls may be seen, as, for 

 example, the small fold in the gorge at High Falls [see pi. 24] . 



Explorations have shown, as indicated on the accompanying cross 

 section [fig. '20], that there is a deeper buried gorge here than at 

 Kripplebush. The deepest point discovered is a few feet below tide 

 level. The escarpment is steep and is formed by the Coeymans and 

 New Scotland formations. The dip slope is Shawangunk conglom- 

 erate, High Falls shale and Binnewater sandstone, with the Manlius 

 limestone forming the floor. 



Identification of the drill cores which penetrate the limestone 

 indicate that the dip slope is reversed on the west side of the gorge 

 and that the stream had really reached about the axis of the trough. 

 A discrepancy in thicknesses and depths in hole no. 34 by which it 

 appeared that the Coeymans formation was almost twice as thick 

 as usual and that it contained a broken or crushed zone leads to 

 the interpretation that there is a small thrust fault here which re- 

 peats the formation as shown on the accompanying cross section. 



Instead of a uniform westerly dip of all formations from the 

 Rondout westward it is proven that minor anticlinal rolls and even 

 thrust faults, as in this case, or such faults as in the Kripplebush 

 case are not to be excluded. 



This structural relation has a direct bearing upon the question of 

 the thickness of the Esopus shales. The Esopus is certainly not so 

 thick as would otherwise be supposed, by 200 or 300 feet at the 

 least. The true thickness is still an unknown quantity (estimated 

 at 800 feet). 



It is clear that the aqueduct tunnel will have to be constructed a 

 considerable depth below sea level at this section, probably not less 

 than minus 150 feet, 1 even if the character of the formations be 

 neglected. 



But the character or quality of these formations in view of their 

 structural relation constitutes the chief problem. Because of the 

 fact that every structure reaches the surface and eventually dips 

 gently to the west in such manner as to encourage water circulation, 

 their water-carrying capacity or general porosity becomes of great 

 importance. A great capacity is all the more serious because of 

 the heavy drift cover within the abandoned gorge, on top of which 



1 This portion of the tunnel and its continuation south to the Shawangunk 

 range has been constructed at 250 feet below sea level. 



