A thick, lithologically varied sequence of high-grade metamorphic rocks, pre-Acadian in age, has been delineated in the Southbridge, Wales, Westford, Eastford, Stafford Springs, and Monson quadrangles of south-central Massachusetts and adjacent Connecticut. The sequence consists largely of metamorphosed aluminous mudstone and siliceous siltstone. Metamorphosed limy mudstone, euxinic mudstone, fine-grained volcaniclastic debris, and lava form marker units that may extend tens of kilometres laterally. The sequence is divided into four formations, each containing many lenticular units. Three formations in the southeastern part of the area are separated by thrust faults and apparently top westward in an imbricate homoclinal sequence. The fourth and youngest formation is in isoclinal synclines in a faulted block that parallels the eastern border of the Monson Gneiss. The observed rock sequences contain rocks similar in gross lithology to sequences present in east-central Massachusetts and southern New Hampshire.