GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 8l 



has been thrust up to a much higher relative position with respect 

 to the bordering country, both north and south, in comparatively 

 recent geologic time. Part of the uplift certainly dates from the 

 Triassic, since which time there has been no folding in the region. 



It is the writers' belief that the Grenville structural habit and 

 attitude and distribution indicates folding of that formation dating 

 back to very ancient Precambrian time. There is no claim that 

 distinct repetition of beds can be detected, but the very definite 

 northeast-southwest trend and the mafked control over all igneous 

 intrusions resulting in a similar orientation of them indicate that 

 this formation, which is the oldest of the region, must have had this 

 structure before the igneous history began. The only way by which 

 a series of sedimentary strata can exhibit a regional trend is by 

 deformation and we regard this regional habit therefore as satis- 

 factory proof of the folding of the ancient Grenville. 



If the Manhattan-Inwood-Lowerre is also Grenville, as we are 

 now inclined to believe, much of its folding must also be Pre- 

 cambrian. 



The Post-Ordovician or Taconic folding must have affected the 

 region also and later the Permian or Appalachian folding was 

 imposed upon it. 



Thus it happens that the Cambro-Ordovician sediments to the 

 north and the down-faulted block in Peekskill valley, and the schists 

 and limestones of the southeast quarter are all much deformed by 

 folding, but to which of these periods the chief deformation should 

 be credited is a matter of much obscurity. It may even happen in 

 the case of the down-faulted block in Peekskill valley that its fold- 

 ing is in part connected with the Triassic faulting. On the whole, 

 the chief folding belonging to the members on the south margin of 

 the quadrangle is of comparatively ancient time, and that affecting 

 the Cambro-Ordovician of the north margin belongs chiefly to the 

 period of Appalachian folding. All the folds trend northeast-south- 

 west except for minor flexures and local pitches and the deformation 

 of every period seems to have had nearly the same orientation. Most 

 of the folds are asymmetric and sometimes overturned toward the 

 northwest, and the major structures are accompanied by all the nor- 

 mal minor folds and crumples of the second, third and fourth and 

 still higher orders which might be expected in a region of such 

 extensive deformation. 



