﻿l86 NEW YORK STATE MUSEUM 



ning nearly east and west. It is believed that similar movements 

 have affected the rock in the Croton valley itself, modifying its con- 

 dition so much as to control the course of the stream. The only 

 immediate bearing upon the problem of the Croton crossing is the 

 question that it raises about the quality of rock and the necessity 

 that is introduced of trying to determine whether or not there is 

 shattering enough to be very objectionable. 



Explorations and data 



Six drill holes have been made on this proposed Croton lake 

 crossing — one on either side just at the margin and four others 

 within the intermediate space of 1400 feet. These inner four have 

 been made from rafts floated on the lake and have penetrated water, 

 drift cover, and rock [see accompanying profile and cross section, 

 pi. 27]. 



Rock floor. The depth of the .preglacial Croton valley is 

 pretty accurately determined at o feet or sea level. There is no 

 reason to expect a gorge or inner channel of any consequence. 



The drills have penetrated only one formation, i. e. Manhattan 

 schist. These test holes are believed to be near enough together to 

 eliminate the possibility of any other formation appearing at tunnel 

 grade. 



Rock condition. The two varieties of schist (1) the coarse 

 garnetiferous quartz-mica rock, which is a metamorphosed former 

 sediment, and (2) the darker, close grained hornblendic rock that 

 is believed to represent an igneous intrusion, both occur in the cores 

 brought up by the drill. Either under normal conditions is a 

 good rock. But there are considerable differences in the physical 

 condition of the rock. Holes no. 3 and no. 4 at the two extremes, 

 on the lake borders, show sound rock that comes up in large cores 

 with very high percentage recovery. This is confidently believed to 

 represent the average condition of the rock in this vicinity at the 

 sides of the valley. 



The central holes, however, nos. 1, 2, 5 and 15, all show more 

 broken ground. Of these holes no. 2 is much the most broken, the 

 core recovery being only 14.8$. The pieces are small and many 

 are smoothed (slickensided) by movement. The hole penetrates a 

 typical crush zone resulting from slight faulting movements, and 

 the low saving is due to the fact that the incipient fractures are not 

 well bound together (rehealed) by later mineral change. They are 

 probably connected with the latest movements of this kind. 



