﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 77 



2 Stratigraphy 



This is therefore the rock series of Peekskill creek. It is 

 the only locality on the south side of the Highlands where all 

 are represented in complete and simple form. There is no doubt 

 that it is the Poughquag-Wappinger-Hudson River series, in spite 

 of the complete absence of organic evidence. A similar though 

 not so complete and clear occurrence is to be found on the west 

 side of the Hudson near Stony Point and Tompkins Cove. That 

 is a part of the same structural syncline. It is probable also that 

 the phyllite so finely developed in the village of Peekskill in the 

 next small valley to the east is tne same. But outside of these 

 occurrences there are none that clearly represent this same series 

 as a whole and in the same condition. 



No more striking example of this fact can be found than the 

 adjacent Sprout brook described in an earlier section. There coarse 

 crystalline and injected and impure limestone occurs alone — no 

 phyllite and no quartzite. When one remembers that the two 

 occurrences so strongly contrasted, Sprout brook and Peekskill 

 creek, converge until they actually unite, and still preserve their 

 stratigraphic dissimilarity, without any adequate structural reason 

 for it, the only conclusion possible is that the two occurrences rep- 

 resent two entirely different series of formations. 



The Peekskill valley series is Cambro-Ordovicic in age ; what is 

 the other? It is older, at least that is certain. But is it (the Sprout 

 Brook limestone) as old as the oldest of the gneisses themselves 

 and therefore linterbedded with them representing locally the Gren- 

 ville ; or is it intermediate — Postgrenville and Preeambric — with 

 which possibly other occurrences of rocks of similar habit and 

 equally uncertain relations belong? 



It is on the general similarity of this occurrence to the Inwood 

 limestone as known throughout Westchester county and New York 

 city that a tentative intermediate series has been reoognized. This 

 is the Inwood-Manhattan series. Whether it is in reality a separate 

 older series is not regarded as proven. But for engineering and 

 practical purposes the distinction is a good one and eminently ser- 

 viceable. Further discussion may better be continued in a different 

 publication. 



3 Rock surface 



The bed rock surface is pretty well outlined by the borings. A 

 profile based upon them seems to leave no unexplored space of suf- 

 ficient extent to admit a gorge of great consequence to a lower level 



