Structural and petrographic data and K-Ar determinations on muscovite, biotite, orthoclase, and whole-rock samples in the Piedmont province of southeastern Pennsylvania show that: ultramafic intrusion and contact metamorphism took place at least 460 m.y. (million years) ago; biotite in small mafic intrusions at the ultramafic rock contacts is 460 m.y. old; apparent thrust slices at the Martic Line are older than 330 m.y.; a regional metamorphism occurred approximately 330 m.y. ago and is tentatively correlated with the second of three recognized deformations; and granitization of Peters Creek Schist in the vicinity of the ultramafic pluton occurred 330 m.y. ago or later. Ages between 460 m.y. and 330 m.y. and ages younger than 330 m.y. are believed to be anomalous because of argon leakage resulting from green-schist-facies metamorphism, cataclasis of muscovite, and incomplete recrystallization of muscovite in granitized gneiss and pegmatite.