GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 75 



It has been quite natural to look for it at the base of the Manhattan- 

 Inwood-Lowerre series and sometimes an overlap has been postulated 

 to care for some of the descrepancies. Thus far, however, no obser- 

 vations can be said to establish fully another unconformity or even 

 a definite overlap. As a matter of fact very little evidence on that 

 point can be gathered from this particular quadrangle because the 

 Manhattan-Inwood series is too much obscured by drift cover to 

 furnish reliable data. 



Pre-Triassic unconformity. Another great unconformity sep- 

 arates the Triassic from all the earlier series, but the structure itself 

 does not figure in this quadrangle since the sediments do not cross 

 its southern margin. It was the erosion work of that interval, how- 

 ever, which strapped some of the Cambro-Ordovician sediments from 

 the region, as may be seen by examination of the basal conglomerates 

 of that age just to the south. It appears, therefore, that the Pre- 

 Triassic erosion interval is a factor of some prominence in a full his- 

 torical statement although it is not possible in this quadrangle to 

 point out the actual unconformity. 



Post-Cretaceous unconformity. The next erosion surface which 

 served at one time as an unconformity was the Cretaceous peneplane 

 which is still to some degree preserved. After it was partly dis- 

 sected Tertiary deposits must have been laid down on it over this 

 area and still farther inland. These were all stripped from it in 

 preglacial time and the old unconformity (the Cretaceous peneplane) 

 subjected to additional erosion. The result is a somewhat more com- 

 plex surface forming the preglacial floor than would be possible from 

 a single erosion epoch. 



Glacial unconformity. The last unconformity is that between the 

 rock floor of glacial time and the glacial drift itself. 



Faults. The region is one of many faults representing several 

 different ages. It is doubtless impossible to determine the age of 

 some of these faults and it is also impossible to locate all of them. 

 The fairest statement that can be made on that point is that they are 

 much more numerous than any inspection of the grotmd will dis- 

 cover and doubtless many of the most ancient ones are completely 

 obscured by rehealing, injection and complete recrystallization. 

 Thus it happens that most of and perhaps all such structures of 

 Grenville age are lost. It is probable that very few Precambrian 

 faults can be detected with any certainty although evidence of 

 rehealed crush zones may be detected with the microscope. 



In one of the tunnels of the Catskill aqueduct, however, a typical 



