244 NEW YORK STATE MUSEUM 



slightly at both contacts. The nature of these contacts makes it 

 seem probable that there is very little of the limestone squeezed or 

 cut out by movement. Therefore this crossing gives a fairly ac- 

 curate measure of the thickness of the Inwood. This is approxi- 

 mately 750 feet. Xo section about Xew York city is more accurately 

 determined. 



2 Manhattan ville cross valley 



In northern ^lanhattan the schist ridge which forms the back- 

 bone of the island and has a reHef of more than 100 feet, is cut 

 across by a prominent valley that extends from the Hudson at 

 130th street eastward to the Harlem Flats and East river. This 

 valley is nowhere more than 25 or 30 feet above the sea level and 

 is drift filled. Previous to the recent boring explorations of the 

 Board of AA'ater Supply its true depth to rock floor was unknown. 

 The few borings recorded, however, indicated a depth of more than 

 a hundred feet. One such boring at 129th street and Amsterdam 

 avenue is reported as penetrating 109 feet from surface without 

 touching rock. Another of similar results is located at 125th and 

 ^Manhattan streets where a depth of 204 feet failed to touch rock. 



Besides determining rock floor in the present case, it was im- 

 portant to determine rock structure and conditions. It appears 

 from surface features that this cross valley probably follows a 

 fault zon^ along which there has been weakening of the rock and 

 consequent disintegration and decay. If this is so it would be ad- 

 vantageous to find the limits of it and determine Avhat displace- 

 ment effects were produced. It has been surmised by all students 

 of local geology that such cross faults may lift the blocks on the 

 south side of them, one of the chief indications being the fact 

 that in spite of a strong southerly pitch in all the formations they 

 do not rapidly disappear below sea level. 



The accompanying profile and explanatory section indicates the 

 principal results of exploration [^e^^:' pi. 36]. Badly crushed 

 ground has been found in the holes near the north end of ^lorning- 

 side Park but the rock, when found, is not very badly decayed. 

 The rock floor is very low, almost 200 feet below sea level at the 

 lowest. It appears that it the drift were stripped oft' from this 

 valley the Hudson and Long Island sound would unite across the 

 Harlem Flats and ^vlanhattanville forming a channel and outlet 

 much deeper than the present East river course. 



The glacial drift of this valley is prevailingly fine modified drift 

 some of which is probably stratified and fairly well assorted. 



