GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 29 



at an obtuse instead of the usual acute angle. They are essentially 

 retrograde streams. 



Explorations. Systematic explorations and tests are repre- 

 sented chiefly by drill borings through drift into the rock floor. 

 These were supplemented by two test tunnels for working character 

 of material and a series of tests on the behavior of certain of the 

 drill holes, together with other tests on material. The results are 

 embodied in the accompanying cross sections and the additional 

 discussion of special features. 



Detail of local sections 



Kripplebush section. This from the first was regarded as one 

 of the critical sections because of the buried gorge along the base 

 of the Hamilton escarpment and because of the doubt as to the 

 behavior of the Onondaga limestone. On the accompanying section 

 the borings are plotted and the structure as now interpreted is 

 indicated. The dip slope formed by the Onondaga limestone is 

 covered by 200 to 250 feet of drift, mostly modified drift. The 

 strong valley character of the rock floor is almost wholly obscured 

 by the glacial deposits and the present brook, an insignificant stream 

 compared to the preglacial one, occupies a position above the escarp- 

 ment instead of above the old channel. 



After a couple of the central holes were finished, it became appar- 

 ent that the structure is not nearly so simple at this point as the 

 general surface features would lead one to expect. It was clear 

 that a simple dip such as was proven to prevail on the dip slope 

 would not account for the much greater depth attained by it in 

 the vicinity of station 500. The discovery of this additional feature' 

 raised two questions : ( i ) Is the structure a flexure or is it a fault, 

 and if a fault whether normal or thrust, and (2) what is the prob- 

 able effect of this structure on the position and depth of the pre- 

 glacial gorge? 



The habit of the district immediately east of the valley would 

 support the theory of a thrust fault. The nature of the immediate 

 area would suggest a simple flexure while it is manifestly possible 

 that a normal fault could easily occur. Later explorations^ have 



1 Since the above was written the tunnel has been completed through the 

 Kripplebush section. Although faulting is indicated by the borings and 

 actual occurrence of the beds it is very difficult to find the fault A part 

 of the displacement is accomplished by the steepening of the dip but this 

 will not account for more than half of it. 



