GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 73 



features depending largely on dynamic influence are simply different 

 expressions of metamorphic history. 



Unconfonnities. Pre-cambrian unconformity. Only one well- 

 marked unconformity has been demonstrated in this area. This is 

 the unconformity between the gneisses of the Highlands and the 

 Cambro-Ordovician series of sediments. In a few places it is clearly 

 shown that the whole complex structural habit exhibited by the 

 gneisses was developed and exposed to erosion before the Poughquag 

 quartzite was laid down. The form of surface is much modified by 

 subsequent deformation, but the condition found at the best occur- 

 rences indicates a comparatively smooth cleanly swept erosion sur- 

 face so that the first layers of quartzite are in contact with fresh, 

 unweathered gneiss. The quartzite is simple and comparatively pure 

 right from the start and this condition requires not only that 

 erosion should take place under such conditions that the bedrock of 

 that time could be completely denuded, but there must have been 

 unusually perfect assorting and selective concentration to produce 

 such a rock immediately on a granite floor. 



The finding of trilobite fragments in certain beds of the Pough- 

 quag quartzite shows beyond question that this formation was devel- 

 oped in the sea margin, but it is a striking thing that it has no 

 conglomeratic habit and practically no arkosic composition. It is 

 difficult to see how it would be possible to make such a formation 

 directly from the disintegration of the gneisses. One Vv^ould expect 

 a history involving the destruction of earlier sandstones of less 

 purity which themselves might have been derived directly from the 

 gneisses. Perhaps there has been such a history, but there is no 

 evidence of it at this point beyond the fact of the purity of the 

 quartzite and the lack of conglomeratic facies. If the Manhattan- 

 Inwood-Lowerre series, however, is, as we now think, older than the 

 Poughquag, it may be that the destruction of members of that 

 series furnished the material of the Poughquag. If that source is 

 not the right one it is difficult to avoid the belief that some other 

 Cambrian or Precambrian formation now entirely missing in this 

 region was destroyed in the making of the Poughquag. 



In any case the Cambrian unconformity is a very profound one 

 and undoubtedly marks a great geologic hiatus. 



The actual unconformity can best be seen near the north margin of 

 the quadrangle where, in a few places, the quartzite still lies only 

 slightly tilted on the old erosion surface, while the adjacent outcrops 

 of gneiss show their itsual steep inclination. No actual observation 



