The metamorphic rocks of the Manhattan Prong (Fig. 1) include the Precambrian Fordham Gneiss, Yonkers and Pound Ridge granite gneisses, and the overlying Lowerre Quartzite, Inwood Marble, and Manhattan Schist. The depositional age of the basal unit of the Manhattan is estimated to be Middle Ordovician on the basis of fossil evidence (Ratcliffe and Knowles, 1968) and on the correlation of an unconformity beneath the Manhattan with a regionally developed Middle Ordovician unconformity (Hall, 1968a).
In the southern prong, two episodes of tight to isoclinal folding are followed by an episode of less intense, more open F3 folding (Bowes and Langer, 1968; Hall, 1968a, 1968b). In the Peekskill area, Ratcliffe (1968a, 1968b) and Mose and others (1976) have shown that the essentially undeformed 435-m.y.-old Cortlandt ultramafic complex (Long and Kulp, 1962) clearly crosscuts the F1 folding and postdates the regional metamorphism. Local F2 folding is contemporaneous with the intrusion and is interpreted as “shouldering aside” of the country rocks by the ultramafic intrusions (Ratcliffe, 1968a, 1968b; Mose and others, 1976) and is therefore genetically unrelated to the regionally developed F2 folding mapped elsewhere in the prong. The 360-m.y.-old Peekskill Granite (Mose and others, 1976) crosscuts the F3 folds, and is interpreted as syntectonic with the F3 folds (Ratcliffe, in Mose and others, 1976).