﻿\~2 NEW YORK STATE MUSEUM 



may have been removed in erosion. One of these types it resembles, 

 but it can not be the Wappinger (Cambro-Ordovicic) as was 

 pointed out by the writer in a former report. 1 The Wappinger, 

 wherever known to be such, is never intruded and always lies above 

 a thick quartzite (Poughquag). It does so even in the next valley 

 (Peekskill creek) less than a mile distant. With the interpretation 

 of this Sprout Brook limestone therefore is involved the correlation 

 and interpretation of the age of much greater areas. That the 

 Sprout Brook limestone is not Wappinger is clear enough, but it 

 cculd be either interbedded (Grenville) or Inwood. If it is Gren- 

 ville then of course it has no direct bearing on the Wappinger- 

 Inwood question and these two> might be equivalents. But if the 

 Sprout Brook limestone is not Grenville (interbedded) then it must 

 be Inwood and in that case the Inwood and Wappinger are not 

 equivalent — which means that there are two series above the 

 gneisses instead of one — an Inwood-Manhattan series, and a 

 Poughquag-Wappinger-Hudson River series. At the present time 

 it is not possible to give with certainty a final interpretation of the 

 Sprout Brook limestone. 



Explorations 2 



It was ait first believed that a pressure tunnel could be con- 

 structed advantageously at the point of crossing this valley and 

 borings were made to test rock conditions. The data gathered in 

 exploration are indicated on the accompanying geologic cross sec- 

 tion [fig. 32]. 



Borings indicate that the rock floor has been eroded to a few 

 feet below present sea level and that the gorge has a drift filling 

 of more than 15.0 feet. The central borings penetrate limestone 

 and indicate a total width of this type of more than 400 and less 

 than 600 feet. The best estimate on the basis of these explora- 

 tions is 500 feet. Whether this width represents one thickness 

 of the formation as would probably be the case if it is an inter- 

 bedded Grenville layer, or part of a double thickness due to infold- 

 ing, as would probably be the case if it is the Inwood, there is 

 no evidence. The thickness seems to be even greater farther south 

 in the same valley (it becomes l /± mile wide), but it can not be 



1 Structural and Stratigraphic Features of the Basal Gneisses of the 

 Highlands. N. Y. State Mus. Bui. 107 (1907)- P- 361-78. 



2 Explorations at Sprout brook are in charge of Mr. A. A. Sproul, division 

 engineer in charge of the Peekskill division. 



