Metamorphosed strata of sedimentary origin underlie the upland surface of western Connecticut, and nonmetamorphosed sedimentary strata underlie the adjacent lowland surface of central Connecticut. The metamorphic rocks consist chiefly of micaceous schists and gneisses, with minor amounts of pegmatite, amphibolite, and metaquartzite. These rocks represent three formations. They are, in order of decreasing age, the Taine Mountain Formation, Collinsville Formation, and Straits Schist Formation; together they comprise seven members. The formations probably correlate with the Moretown Formation, Hawley Formation, and Goshen Formation of central western Massachusetts, and Vermont. The originally nonmetamorphosed sediments of the formations thus are inferred to range in geologic age from Middle Ordovician to Late Silurian and Early Devonian. Regional metamorphism of the strata may have occurred during the Acadian orogeny at or near the end of Early Devonian time. The nonmetamorphosed strata of the lowland are mainly sandstone; they are correlated with the New Haven Arkose of Middle Triassic age in central Connecticut. Gravimeter traverses across the upland-lowland boundary suggest the presence of a normal fault of a few hundred feet displacement, downthrown on the east. Traces of copper mineralization are found locally in the vicinity of the boundary, and at least one mine produced copper ore during the 1800's. Jointing and foliation planes in the metamorphic rocks may affect slope stability and the ease of excavation. The sedimentary strata are inclined, commonly massive, and poorly exposed.
