The Poundridge area in northeastern Westchester County, New York, is underlain by the Poundridge and Siscowit granites, the Fordham gneiss, Inwood marble, and the Manhattan formation, comprising the New York City group. Field evidence, including the fact that the Lowerre quartzite does not seem to exist, leads to the conclusion that the New York City group is a conformable sequence either of entirely Precambrian or Paleozoic (pre-upper Ordovician) age. It thus cannot be correlated with the Precambrian Highland gneiss and lower Paleozoic Poughquag quartzite-Wappinger limestone-Hudson River shale sequence of Dutchess County to the north.
The area has been subjected to regional metamorphism of the sillimanite-almandite subfacies of the amphibolite facies and lies within the sillimanite zone. The sedimentary antecedent of the Fordham gneiss was probably a graywacke; the Inwood marble, a cherty limestone; and the Manhattan formation, a siliceous and argillaceous shale.
Alkaline metasomatism has altered the Manhattan formation and an associated basic igneous body to the Bedford augen gneiss. It has also extensively affected the Inwood marble and has granitized an interlayered argillaceous bed. A migmatite phase of the Fordham gneiss, intermediate in composition between the Poundridge granite and the normal Fordham gneiss, appears to be a result of metasomatism. The Poundridge granite may be a product of the same metasomatic process in a more intense form or may be magmatic. In which case the migmatite could have resulted from a partial conversion of the gneiss necessitated by the equilibrium requirements of the PT environment induced by the granitic intrusion. Although not necessarily magmatic, the Poundridge granite is a synkinematic body contemporaneous with the last major deformation of the area.
Metamorphism has obliterated intraformational bedding planes. Foliation is the most prominent structural feature in all rock units. Its strike swings in a large arc around the central Poundridge mass. Lineation, consisting of alignment of inequate minerals and the axes of folds of differing amplitudes, shows a strong preferred orientation in its consistently northwestward plunge.
The Manhattan formation crops out in a belt which also forms a large arc through the area flanked on either side by successive belts of Inwood marble and Fordham gneiss. The conformable nature of the Fordham-Inwood-Manhattan sequence allows this map pattern to be interpreted as a secondarily folded syncline herein called an axial-plane fold. This interpretation is supported by additional structural evidence.
This structural complex resulted from the broad folding of pre-existing isoclinal folds which lay between the present Highland gneiss fault and the Siscowit granite fault. These folds were squeezed into axial-plane folds when the Highland gneiss block was thrust southeastward toward the Siscowit granite, which acted as a buttress but which may have been displaced to the southeast.