﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 37 



Shawangunk overthrust. At the extreme eastern side of the 

 Rondout valley near the point where the surface reaches hydraulic 

 grade again, the surface outcrops pass from High Falls shale to 

 Shawangunk conglomerate to Hudson River shale in the normal 

 order but with entirely too small an area of conglomerate consid- 

 ering the character of the formations. The higher ground is all 

 Hudson River in the vicinity, and there is abundant evidence of 

 crushing and disturbance. It is evident that a thrust fault is again 

 encountered here, one of sufficient throw to bring the Hudson River 

 slates above the Shawangunk conglomerate — probably a lateral 

 displacement of very great extent. Explorations have fully proven 

 the existence of this fault. The accompanying diagram shows a 

 cross section as now outlined by complete penetration of two 

 borings. 



Two trial tunnels were run to test working quality of Hudson 

 River slates compared to Shawangunk conglomerate at this locality. 

 Both are within the influence of the fault zone. Both are there- 

 fore more broken than the normal with the result that the Hudson 

 River slates probably show poorer condition than usual and more 

 troublesome working, while Shawangunk conglomerate probably 

 shows easier working than usual. It is believed that normally the 

 two rocks would present a greater difference than was found in 

 this test. 



Special features 



Several questions, some of which have a practical bearing, have 

 been raised as separate features during the exploration of the 

 Rondout valley. 



Caves. One of these is in regard to the possible existence of 

 underground caverns. This was given a special prominence early 

 in the work by the experience of one of the drills. After pene- 

 trating the limestone series near High Falls to a depth of over 

 200 feet, the drill seemed to leave the rock and enter a space 

 allowing the rods to drop 28 feet before being arrested by solid 

 material. The further attempt to work in this hole resulted in 

 the breaking of the rod down at this point and the subsequent 

 failure to recover the diamond bit which is still in the bottom of 

 the hole. The question is as to the meaning of this occurrence. 

 Is it a cavern? 



" Pompey's cave " has been referred to in an earlier paragraph. 

 This is clearly not much of a cave. It is essentially an enlarged 

 joint or series of joints by solution along the bed of a surface 



