﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 79 



rock forming essentially a great reservoir of supply that has ready 

 access to the almost vertical limestone beds. This will give a maxi- 

 mum water supply to holes that penetrate porous or broken por- 

 tions of bed rock. The attitude of all strata is especially favorable 

 for admitting an almost inexhaustible supply from a considerable 

 drift-covered area within which circulation is probably very rapid. 



6 Condition of rock 



All strata of this valley stand so nearly on edge that drills actually 

 explore a very limited portion of the whole series of beds. No very 

 great advantage is gained by excessively deep boring because the 

 drill follows necessarily almost the same bed from top to bottom. 

 At best only the immediately adjacent beds are penetrated. This 

 means that much of the total thickness of beds is untouched by 

 present explorations, and must be interpreted on the basis of their 

 general likeness to those more fully determined. The usual suc- 

 cession of beds is known to be quite uniform in quality and loca- 

 tions where they can be studied and there is no reason to expect 

 greater variation here. 



Deviations from such normal or uniform conditions are mostly 

 due (a) to local development of mica from recrystallization of im- 

 purities in the limestone, and (b) to crush zones developed in the 

 process of folding and faulting which has broken the rock or weak- 

 ened it enough to permit a more ready circulation of underground 

 water. Wherever either of these structural conditions prevail, the 

 rock has been excessively decayed, or disintegrated, or sufficiently 

 weakened in its binding matter or its sutures to crumble in the 

 hand or break down to a sand under ordinary boring manipulation. 

 This condition is known to reach to -297 feet. How much deeper 

 is not known. Probably the decay dates back in large part to pre- 

 glacial continental elevation at which time probably there was more 

 ready deep circulation with possible outlet in the Hudson gorge. 

 This action has been all the more effective by reason of the attitude 

 of the beds. They stand so nearly on edge that they present all 

 their weaknesses of bedding and sedimentation structures to the 

 destructive surface agents. They admit surface water readily and 

 favor abundant underground circulation. 



Considerable faulting occurs. The contact between the granite- 

 gneiss and quartzite is a fault contact. Wherever seen this is sound. 

 But a crush zone in limestone lies nearly central in the valley, cut 

 by holes no. 23 and no. 25, where the rock shows a finely brecciated 



