GEOLOGY OF THE NEW YORK CITY AQUEDUCT I3I 



tested this zone so well that it is practically certain that the feature 

 must be regarded as a fault of some type with a displacement 

 of nearly 200 feet. The striking physiographic feature is the 

 development and preservation of the escarpment on the downthrow 

 side. This occurrence is certainly a very unusual case in that 

 regard [see fig. 19]. 



Because of the intention to construct the tunnel deep enougn in 

 bed rock to reach safe rock conditions the question of depth of 

 buried gorge becomes an important one. As soon as it was dis- 

 covered that a fault existed there the problem became of sufficient 

 prominence to demand more detailed exploration. If the faulting 

 is accompanied by a broken zone in condition favorable to more 

 ready erosion, it would be possible that the original stream in work- 

 ing down this dip slope might become entrenched in the fault zone 

 aiid at that point begin to cut a narrow gorge instead of continuing 

 the sapping process. In fact, it would undoubtedly do this very 

 thing if there is such a crushed zone of any consequence and if the 

 erosion process were allowed to continue long after reaching this 

 critical point. 



As a matter of fact explorations have shown that there is a thin 

 layer of Hamilton shales still remaining on the Onondaga and the 

 deepest point found is on the Hamilton shales side. These facts 

 in connection with the failure to find any deep notch indicate that 

 there is probably no zone of much greater weakness than the shale 

 member itself. It is reasonable to conclude that the rock floor can 

 be safely regarded as not much lovver than 88 feet A. T. and that the 

 rock condition is not especially bad for tunnel construction^ even in 

 the fault zone. 



Rondout creek section. This is the central portion of the 

 valley including the depression occupied by the present Rondout 

 and the exposed edges of the series of shales and Helderberg lime- 

 stone. The repetition of the dip slope and escarpment, together 

 with the heavy drift filling and the occurrence of so many forma- 

 tions together make this an important section. All formations from 

 the Shawangunk conglomerate to the Port Ewen shaly limestone 

 occur at this point, and although there is little outward evidence of 

 disturbance it is certain that whatever difficulty is to be found in 

 this variable series is likely to be met here. It is therefore a sec- 

 tion that requires exploration both for depth of preglacial channel 

 and for quality of rock. 



1 In construction this ground has proven to be good and sound throughout. 



5 



