The Manhattan Prong in southeastern New York consists primarily of a Cambrian-Ordovician sequence (Lowerre Quartzite, Inwood Marble, Manhattan Schist) that unconformably overlies the Fordham Gneiss of uncertain age. Rb-Sr whole-rock analyses of the Pound Ridge Granite Gneiss, a rock unit within the Fordham Gneiss, indicate that the granite gneiss formed 596 ± 19 m.y. ago, with an initial 87Sr/86Sr ratio of 0.7287 ± 0.0130. Field relations suggest that the Pound Ridge Granite Gneiss was an Avalonian volcanic rock or accumulation of migmatitic fluids. As such, this Rb-Sr age determination is a minimum age for the Fordham Gneiss.