GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 99 



belonging normally to the gneisses and granites already described in 

 this bulletin, especially the Storm King granite and the gneisses 

 associated with the Canada Hill type of granite which is more fully 

 developed toward the south. Some of the complexities of relation 

 and variety were beautifully exposed in this tunnel, but there was 

 nothing of extraordinary interest or significance. All are covered 

 at the present time with the usual concrete lining. No unusual 

 engineering difficulties were encountered. The ground stood well 

 and the work was prosecuted with no greater difficulty than in the 

 average granite. 



Foundry brook section. At Foundry brook, which occupies the 

 depression immediately south of Bull Hill, the ground lies too low 

 to carry the aqueduct at grade, and in the beginning explorations 

 were made with the purpose of determining whether a pressure 

 tunnel in rock could be constructed. It was finally decided to use 

 steel pipe in crossing this valley, but the explorations that were made 

 discovered certain conditions that are of general geologic interest 

 and developed uncertainties which exerted an influence on the choice 

 of plan finally adopted. 



The usual heavy drift cover was encountered, but this in itself 

 was not a serious matter. The condition that was considered more 

 questionable was the discovery of a badly decayed condition of the 

 crystalline rock, in one of the borings, to a depth of several hundred 

 feet. At first, this was thought to indicate considerable extent of 

 bad ground which might give much trouble in construction, especi- 

 ally as artesian or flowing water was furnished by one of the borings 

 penetrating this zone. It was thought also that an old buried stream 

 channel had been discovered that went to much greater depth than 

 had been expected, but this was entirely disproved, and later explora- 

 tions indicated that the decayed zone is probably very narrow and 

 that it is in reality nothing more than a crush zone following a 

 fault line along the east side of the mountain. The questionable 

 boring passed through drift and then through a portion of the solid 

 hanging-wall side of this crush zone for about 75 feet and then ran 

 into the soft decayed rock of the crush zone, keeping within it from 

 that point on to the bottom of the hole, probably bending out of its 

 true course to keep within the softer ground. 



No considerable trouble ought to be expected in such a zone in 

 spite of the depth to which decay was found to extend. It would 

 soon be passed through in a tunnel and would probably give no 

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