GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 35 



shallow holes and the supply comes from near the contact between 

 High Falls shale and Sliawangunk conglomerate. 



It is certain from these observations and tests therefore that the 

 Binnewater sandstone and High Falls shale are more porous than 

 the other formations, and because of the serious difficulties arising 

 from so heavy inflow of water from them the tunnel grade should 

 be shifted so as to avoid these formations as much as possible. A 

 comparison of the accompanying cross section, which is drawn to 

 scale [fig. 20], will show that a tunnel on one level would neces- 

 sarily run for a long distance in these beds because of the gentle 

 syncline. Furthermore, they lie at about the depth that would 

 otherwise be a safe depth below the buried gorge. But a tunnel 

 with a step-down, i. e. one run at two different levels could avoid 

 most of this poor ground. By approaching at a level of about — 50 

 feet or — 100 feet in the limestone beds to station 600 (hole no. 34), 

 then stepping down to — 250 feet, the line in a very short distance 

 crosses these two porous formations and enters the Shawangunk 

 conglomerate which is more substantial, and, all things considered, 

 one that seems most advantageous for successful construction. It 

 will have to maintain a head of more than 700 feet as the difference 

 between hydraulic grade and the tunnel level in this section. Under 

 these conditions rock quality and condition are of greatest impor- 

 tance and there is no doubt about the advisability of avoiding the 

 poorest formations in some such manner. 



Coxing kill section. On the line of exploration the Coxing 

 kill flows OA^er Shawangunk conglomerate and High Falls shale. 

 Both dip plainly eastward, and a hole no. 11 located on the east 

 side of the brook penetrates about 70 feet of drift and shale. But 

 only a hundred feet to the east Shawangunk conglomerate outcrops 

 at the surface dipping the same way. It is certain therefore that 

 a fault occurs here. The dip of the fault plane is indeterminate 

 from the surface, but the relations and surroundings indicate a 

 fault of the thrust type. 



Later explorations indicate that the fault plane is rather flat 

 [see cross section fig. 21] so that the shales are repeated above 

 and below a tongue of conglomerate. Boring no. 11 has also an 

 artesian flow of considerable volume coming from near the bottom 

 of the conglomerate. It is a mineral water. 



The chief importance of this section as a problem in applied 

 geology lies in the influence of the fault and the maximum de- 

 pression of the conglomerate. If the tunnel, which enters Hud- 

 son River slates at the Rondout creek section at — 250 feet can 

 be kept within that formation throughout the rest of its course, 



