242 



NEW YORK STATE MUSEUM 



3 More solid rock begins at 164 feet. 



4 The upper portion of the rotten rock (114-35 feet) is calcareous 



enough to belong to the Inwood limestone formation. The 

 lower 9 feet (164-73 f^et) is typical Manhattan schist. The 

 intermediate ground 135-64 feet is transition variety. 



5 The drill has cut the contact between Inwood and Manhattan 



formation. 



6 If this identification of the badly decayed matter is correct, the 



contact at this point dips steeply eastward, i. e. it is overturned. 



7 Both types of rock are shown to be extensively decayed. 



8 The worst (deepest) decay zone probably lies still a little farther 



east, and follows the dip of the micaceous limestone near the 

 contact. 



These conditions are indicated on the accompanying cross section 

 [see pi. 35]. 



The conditions indicated by this one hole are consistent with 

 those known for the New Croton aqueduct tunnel 2000 feet farther 

 north where, according to the engineers' drawings, the formations 

 also are overturned. Fifty feet of decayed rock is shown in this 

 hole. The contact is undoubtedly decayed considerably to a depth 

 of more than 200 feet below water level. 



ELEV. MANHATTAN i: 

 100 



•200 



Fig. 39 Harlem river crossing — New Croton aqueduct 



Another boring put down to test conditions at still greater depth 

 nearby explored the rock to -442.7 feet. Because of the informa- 

 tion it gives about the deeper bed rock, a summary of the record 

 based upon examination of the material is given: ^,-c^ 



^'JX& 



