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Most of the bedrock of the Mystic and Old Mystic quadrangles is part of an upper Proterozoic gneissic, crystalline terrane extending from eastern Massachusetts through western Rhode Island to and across southeastern Connecticut north of Long Island Sound. This terrane, equated with the Avalonian terrane of Newfoundland (Rast and others, 1976; Skehan and Murray, 1980), is separated from a block of metamorphosed volcanic and sedimentary rocks of Ordovician or older age, the Quinebaug and Tatnic Hill Formations, by the ductile, northand west-dipping Honey Hill-Lake Char fault system which crosses the northern part of the Old Mystic quadrangle (figs. 1 and 2). The crystalline terrane, in the lower plate, consists predominantly of granitoid gneisses, the Sterling Plutonic Group of Late Proterozoic age (Day and others, 1980; Goldsmith, 1980), that intrude and are interleaved with a layered sequence consisting of the predominantly metaclastic Plainfield Formation below and the primarily metavolcanic and metaplutonic Waterford Group above. Regional metamorphism probably took place in the early Paleozoic and possibly in the Proterozoic, but metamorphic events continued, at least in the upper plate, during the middle Paleozoic. In the Silurian, the laccolithic Preston Gabbro was intruded in the developing Honey Hill fault zone. During the middle and into the late Paleozoic, deformation continued both along the Honey Hill fault zone and in the .gneissic terrane, where it produced a major foliation arch and a complex interference pattern. Late in the Paleozoic, lateto post-tectonic granite and quartz monzonite, including the Westerly and Narragansett Pier Granites, were intruded in the coastal area of Rhode Island and Connecticut. During the Mesozoic, brittle faulting accompanied by hydrothermal activity occurred in a north-south zone from Mystic River through the Preston Gabbro area. The prominent silexite mass at Lantern Hill formed at this time. The bedrock in the map area is partly covered by glacial deposits of Pleistocene age (Gaffney, 1966; Upson, 1971). Accordingly, bedrock exposures are discontinuous and irregularly distributed. South of the mainland, Fishers Island, Wicopesett Island (not shown on this map), Napatree Point, and Sandy Point are composed of thick unconsolidated glacial deposits, and no bedrock information could be obtained. ROCKS OF THE LOWER PLATE

Bedrock geologic map of the Old Mystic and part of the Mystic quadrangles, Connecticut, New York, and Rhode Island