The intimately associated Yonkers and Fordham Gneisses are the oldest formations in the Manhattan Prong. A whole-rock Rb-Sr isochron determined from the Yonkers Gneiss corresponds to an age of 575 ± 30 m.y., and initial Sr87/Sr86 = 0.725 ± 0.008. The Manhattan Prong rocks are thus distinctly younger than the adjacent Hudson Highland gneisses. The Fordham and Yonkers Gneisses were involved in major tectonism around 575 m.y. ago, but the nature of the event is obscured by more recent Paleozoic deformation and metamorphism.

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