2IO NEW YORK STATE MUSEUM 



which had a length of no feet on the axis of the tunnel and ex- 

 tended for a short distance below the invert of the conduit. The 

 soft material, consisting of sand, gravel, clay and decomposed rock 

 had a depth of about i6o feet from the surface to the top of the 

 tunnel. It exerted such a pressure against the timber bulkhead that 

 the 24-inch oak logs used as " rakers " (braces) became crushed in 

 24 hours and had to be continually renewed. 



The chief points of present interest are that the tunnel, at a depth 

 of about 160 feet from the surface, and after passing through sev- 

 eral hundred feet (407 feet) of good dolomite, came into rotten rock 

 and soft ground no feet across on the line. It was so soft that 

 it ran into the tunnel in great quantities and exerted such pressure 

 as to make progress in it a very troublesome and costly miatter, 

 taking " 60 weeks to advance the tunnel 85 feet " and costing " $539 

 per foot." The material caved in so freely as to form a pit on the 

 surface. 



Statement of geologic conditions 



It is not possible to interpret the conditions at this locality as 

 fully as one would wish because of the vagueness of some of the 

 statements, but the following facts and explanation are essentially 

 correct : 



1 The rock is the Inwood limestone, the same kind and same 

 general conditions as all of the limestone belts that occur in the 

 region of the Southern aqueduct. 



2 The soft ground penetrated at the point in question — 407 feet 

 south of shaft 13 — called in the Carson report and others ''a fis- 

 sure " or '' pocket,'' etc., is in reality a fault crush zone. The fault 

 plane probably dips steeply southeast and strikes n. 50° e. cutting 

 the tunnel line at an angle of something like 20°. 



3 The point is well up on the side of the valley more than a 

 hundred feet above Saw Mill river, and the strike of the fault zone 

 in its southwesterly extension cuts into the lower portion of the 

 valley, so that underground circulation would be encouraged along 

 the zone in this direction. 



4 The limestone outcrops very near by on the west side of the 

 line and the Manhattan schist occurs near by on the east. The atti- 

 tude of the beds is such as to indicate a fault of the thrust type, 

 The accompanying figure illustrates this relationship in a cross sec- 

 tion at right angles to the axis of the tunnel [see fig. 37]. 



