Evidence from detailed structural and petrographic studies in western Massachusetts shows that the Cambro-Ordovician Rowe Schist and the Middle Ordovician Moretown Formation have different depositional, structural, and metamorphic histories and are separated by a pre-Taconic angular unconformity. The Rowe Schist (pelitic and semi-pelitic schists and minor lenticular amphibolites) underwent multiple periods of deformation and amphibolite facies grade of metamorphism before the deposition of the Moretown Formation. The latter is composed of the Chester Amphibolite, chlorite–muscovite schist, and staurolite–garnet–biotite schists. The predominant structures and metamorphism in the Moretown Formation are those of the Acadian Orogeny. Both the Rowe Schist and the Moretown Formation are transected by a foliated pegmatitic granite (Middlefield – Blair Pond Pluton). A dyke of the granite cuts the composite tectonic fabric in the Rowe Schist (which it post-dates), but it is generally deformed by the earliest and regional schistosities found in the Moretown Formation (which it pre-dates). The base of the Moretown Formation is defined by the Chester Amphibolite. It represents a recrystallized basic volcanic rock associated with an island arc system.