GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK lO? 



aqueduct. The boring already made was not only to be abandoned 

 but could be completely sealed. 



It forms a good illustration of the relation of geological factors 

 to some of the side questions growing out of engineering undertak- 

 ings. Numbers of such questions are encountered, some of which 

 have even less foundation for special claim and it is not at all an easy 

 matter to show what the real situation and relative responsibility is. 



Garrison tunnel. After leaving Foundry brook, the Catskill 

 aqueduct continues for some distance on the high ground with cut- 

 and-cover construction along the sides of the ridges back of Garri- 

 son toward the south and enters a tunnel through the higher ridge 

 which forms the divide between this drainage slope and Sprout 

 brook. Although this ridge is not very high, a total distance of over 

 2 miles requires tunneling. This section is known as the Garrison 

 tunnel. It cuts typical gneisses made up of remnants of Grenville 

 and injections of granite and diorite, with all the complications 

 characterising this belt. On the map this area is referred to the 

 Canada Hill type because of the fact that this kind of granite seems 

 to be the chief injection material and the largest single constituent. 

 Parts of the belt carry two or three symbols indicating a mixed 

 origin. 



It is possible in this tunnel to study the minor structural features 

 of the work in detail and get at something of the association of 

 the different parts making up the average -gneis sic belt. It is pos- 

 sible also in this case to plot the jointing with a great degree of 

 accuracy at great enough depth to be practically free from the 

 influence of simple weathering effects. These factors have been 

 plotted on the tunnel profile by the engineers in charge of this work 

 and. a typical section of this profile is reproduced as an illustration 

 and a typical section of this profile will ultimately be made available 

 through the final reports of the New York City Board of Water 

 Supply. 



None of these features, however, introduces any real problem in 

 the engineering work. Interest from that side attaches to quite 

 another matter, that of an excessive amount of decay of bedrock. 

 At the north end of the tunnel for a distance of about 500 feet the 

 ordinary gneisses forming the bedrock were found to be so com- 

 pletely decayed that the ground would not stand at all. In fact it 

 stood no better than ordinary drift and in some places was much 

 more difficult to keep from running into the tunnel. In this ground 

 the structure of the gneiss was almost perfectly preserved, but the 

 whole mass was soft and in places so completely leached of its quartz 

 content that it could be cut with a knife or picked out with the 



