Geologic units in Connecticut (state in United States)

Additional scientific data in this geographic area

Portland Arkose (Lower Jurassic) at surface, covers 8 % of this area

Reddish-brown to maroon micaceous arkose and siltstone and red to black fissile silty shale. Grades eastward into coarse conglomerate (fanglomerate).

New Haven Arkose (Upper Triassic; possibly Lower Jurassic at top) at surface, covers 6 % of this area

Red, pink, and gray coarse-grained, locally conglomeratic, poorly sorted and indurated arkose, interbedded with brick-red micaceous, locally shaly siltstone and fine-grained feldspathic clayey sandstone.

Hebron Gneiss (Silurian and Ordovician) at surface, covers 5 % of this area

Interlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar, and greenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite. Local lenses of graphitic two-mica schist.

Ratlum Mountain Schist (Lower? Ordovician) at surface, covers 4 % of this area

Gray, medium-grained, interlayered schist and granofels, composed of quartz, oligoclase, muscovite (in the schist), biotite, and garnet, also staurolite and kyanite in the schist. Numerous layers and lenses of amphibolite; also some of quartz-spessartine (coticule) and calc-silicate rock.

Brimfield Schist (Includes Hamilton Resevoir Formation) (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Tatnic Hill Formation (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Medium- to dark-gray, medium-grained gneiss or schist composed of quartz, andesine, biotite, garnet, and sillimanite, locally kyanite, muscovite, or K-feldspar, interlayered with locally mappable units and thinner layers of rusty-weathering graphitic pyrrhotitic two-mica schist, amphibolite, and calc-silicate rock.

Monson Gneiss (Middle or Lower Ordovician?) at surface, covers 3 % of this area

(May be equivalent to part of Waterford Group) - Interlayered light to dark, mostly medium to coarse-grained gneiss and amphibolite; gneiss composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; traces of garnet, epidote, and magnetite.

Ordovician? granitic gneiss (Middle Ordovician?) at surface, covers 2 % of this area

(Including local terms Ansonia, Mine Hill, "Tyler Lake," "Siscowit") - White, light-gray, buff, or pink, generally foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and biotite, and locally garnet or sillimanite. Commonly contains numerous inclusions or layers of mica schist and gneiss.

Hope Valley Alaskite Gneiss (Proterozoic Z?) at surface, covers 2 % of this area

Light-pink to gray, medium- to coarse-grained, locally porphyritic, variably lineated and foliated alaskitic gneiss, composed of microcline, quartz, albite or oligoclase, and minor magnetite, and locally biotite and muscovite. Lineation formed by rods of quartz. Locally contains quartz-sillimanite nodules.

The Straits Schist (Goshen Formation of Massachusetts) (Devonian or Silurian or both) at surface, covers 2 % of this area

Silvery to gray, non-rusty, coarse- to very coarse grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, garnet, and commonly with staurolite and kyanite or sillimanite; graphitic almost throughout. Distinctive sequence of metamorphic strata overlying the Collinsville Formation in the Bristol quad., is here correlated with the Straits Schist of Fritts (1963) in the Southington quad. Consists mainly of rusty-weathering, medium- to coarse-grained, commonly crenulated, garnet-bearing, graphitic, muscovite-rich, plagioclase-quartz schist with graded bedding. Unit is called The Straits Schist following the original usage of Rodgers and others (1959). In the southern part of the Bristol quad., unit includes the Southington Mountain Member (here reduced in rank) in its upper part. Age of The Straits is inferred to be Silurian or Devonian. Unit is tentatively correlated with the Goshen Formation based on lithologic similarities and a possible unconformity at the base (Simpson, 1990).

Manhattan Schist (Including Waramaug Formation and Canaan Mountain Schist) (Cambrian?) at surface, covers 2 % of this area

Dark-gray to silvery, rusty-weathering, generally coarse grained, foliated but poorly layered to massive gneiss or schistose gneiss, composed of quartz, oligoclase, microcline, biotite, and muscovite, and generally sillimanite and garnet. Amphibolite layers locally, especially near base where in places separately mapped as unit CAma. Cambrian Manhattan Schist in this report includes four informal members: Warren member, garnetiferous biotite schist member, Shepaug member, and schistose granulite member. The Warren member occurs only in the Above All thrust sheet, the Shepaug member only in the Rabbit Hill thrust sheet. The schist member occurs in both. The granulite member occurs in the Rabbit Hill thrust sheet and the Lake Waramaug thrust sheet. The Warren includes mainly interbedded, dark-gray, muscovite-garnet-chlorite-plagioclase-biotite-quartz schist, amphibolite lenses, siliceous granulite, and finely-layered schistose gneiss. The Shepaug consists of interbedded orthoclase-garnet-plagioclase-biotite-quartz schistose gneiss, rusty-weathering, schistose gneisses and schists with sillimanite rods, and subordinate granulite beds with sillimanite nodules similar to those in the schistose granulite member (Panish, 1992).

Quinebaug Formation (Middle Ordovician or older) at surface, covers 2 % of this area

Medium- to dark-gray, commonly greenish, medium-grained, well-layered gneiss, composed of hornblende, andesine, biotite, and epidote, commonly with quartz or garnet, interlayered with amphibolite.

Layered gneiss (Proterozoic Y) at surface, covers 2 % of this area

Gray, medium-grained, well-foliated and generally well layered, light and dark, but locally wispy gneiss, composed of quartz and plagioclase, with microcline locally in the light layers and abundant biotite and common hornblende in the dark layers; garnet or epidote locally. Layers and lenses of calc-silicate rock and amphibolite in some areas.

Rowe Schist (Lower Ordovician or Cambrian or both) at surface, covers 2 % of this area

Light-gray to silvery, fine- to medium-grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, and generally garnet, staurolite, and kyanite or sillimanite. Layers of granofels common; also some layers of amphibolite, quartz-spessartine rock (coticule), and calc-silicate rock.

Glastonbury Gneiss (Middle? Ordovician) at surface, covers 2 % of this area

Gray, medium- to coarse-grained, massive to well-foliated granitoid gneiss composed of oligoclase, quartz, microcline, and biotite (as patches), also epidote and hornblende in many areas, commonly associated with layers of amphibolite; elsewhere minor muscovite and garnet.

Plainfield Formation (Proterozoic Z?) at surface, covers 2 % of this area

Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopsite-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules.

Potter Hill Granite Gneiss (Proterozoic Z?) at surface, covers 1 % of this area

Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. The Sterling, as defined here, includes only the Hope Valley Alaskite Gneiss and the Potter Hill Granite Gneiss. These rocks intrude the Plainfield Formation and the Waterford Group in southeastern CT and western Rhode Island and are Late Proterozoic in age. The gneiss of the Potter Hill is mainly weathered, deeply stained, somewhat crumbly, and generally slabby. Contains xenoliths of the Plainfield Formation. Is distinguished from Hope Valley by its higher biotite content (Skehan and Rast, 1990).

Brookfield Gneiss (Middle? Ordovician) at surface, covers 1 % of this area

Brookfield (dioritic and granodioritic) Gneiss (including Newtown Gneiss of Crowley, 1968) - Dark and light, commonly speckled or banded, medium- to coarse-grained, massive to poorly foliated gneiss, composed of plagioclase, biotite, and hornblende, generally with quartz and K-feldspar, the latter commonly as megacrysts 1 to 3 cm across (also plagioclase megacrysts in darker rocks), locally associated with amphibolite or hornblende schist.

Canterbury Gneiss (Devonian) at surface, covers 1 % of this area

(May be equivalent to Ayer Granite of Massachusetts) - Light-gray, medium-grained, variably foliated, locally strongly lineated gneiss, composed of quartz, oligoclase, microcline, and biotite, locally also muscovite, or epidote, and generally with megacrysts 1 to 2 cm long of either or both feldspars.

East Berlin Formation (Lower Jurassic) at surface, covers 1 % of this area

Maroon siltstone, silty and sandy shale, and fine-grained silty sandstone, generally well laminated and commonly well indurated, alternating with dark fissile shale; dolomitic carbonate common in cement, concretions, and thin argillaceous laminae. Local arkose; grades eastward into coarse conglomerate close to eastern border fault. The East Berlin Formation of the Hartford basin contains eight facies: trough cross-bedded sandstones, horizontally stratified sandstones, interbedded sandstones and mudrocks, ripple cross-laminated siltstones, black shales, stratified mudrocks, disrupted shales, and disrupted mudstones. These facies are interpreted as a continental depositional system and are divided into two assemblages. Sandflat/alluvial plain facies assemblage (sandstones and siltstones) is composed of sheet-flood deposits. The lacustrine assemblage (shales and mudrocks) represents a saline lake-playa system (Gierlowski-Kordesch and Rust, 1994).

Pink granitic gneiss (Proterozoic Y) at surface, covers 1 % of this area

Light-pink to gray, medium- to coarse-grained, foliated but generally massive or poorly layered granitic gneiss, composed of quartz, microcline, oligoclase, and either biotite or muscovite or both, also locally amphibole or epidote.

Holyoke Basalt (Lower Jurassic) at surface, covers 1 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to coarse-grained, grading from basalt near contacts to gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass.

Hoosac Schist (Cambrian?) at surface, covers 1 % of this area

Light- to medium-gray, rusty-weathering, fine- to medium-grained schist and poorly layered schistose gneiss, composed of quartz, biotite, plagioclase, muscovite, and generally garnet and sillimanite or kyanite.

Collinsville Formation (Middle Ordovician) at surface, covers 1 % of this area

Mixture of rock types as described for the two members; in many areas felsic and mafic striped metavolcanic rocks predominate.

Trap Falls Formation plus Ordovician? granitic gneiss (Middle or Lower Ordovician) at surface, covers 1 % of this area

Trap Falls Formation (may be equivalent in part to Golden Hill Schist) - Gray to silvery, partly rusty weathering, medium-grained generally well layered schist, composed of quartz, sodic plagioclase, biotite, muscovite, and garnet, locally with sillimanite or kyanite, interlayered with two-mica gneiss and granulite and with amphibolite. Ordovician? granitic gneiss (including local terms Ansonia, Mine Hill, "Tyler Lake," "Siscowit") - White, light-gray, buff, or pink, generally foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and biotite, and locally garnet or sillimanite. Commonly contains numerous inclusions or layers of mica schist and gneiss.

Middletown Formation (Middle Ordovician) at surface, covers 1 % of this area

( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules.

Rope Ferry Gneiss (Proterozoic Z?) at surface, covers 1 % of this area

(May be equivalent in part to Monson Gneiss) - Interlayered (but layers commonly lenticular to indistinct) light- to dark-gray, fine- to medium-grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; local layers of amphibolite. Rope Ferry described as locally massive, gray-colored, lenticularly layered hornblende-biotite-quartz-plagioclase gneiss. Thickness varies; averages 1,130 m. U-Pb analysis of zircon and sphene from the Rope Ferry yields a 620+/-3 Ma age. Protolith consisted primarily of mafic metavolcanic rocks. Unconformably underlies Potter Hill Granite Gneiss of Sterling Plutonic Suite (Skehan and Rast, 1990).

Rusty mica schist and gneiss (Proterozoic Y; may contain some older rocks) at surface, covers 1 % of this area

(Equivalent in part to Washington Gneiss of Massachusetts) - Dark-gray, rusty-weathering, well-foliated and well- to poorly layered schist and gneiss composed of quartz, plagioclase, biotite, muscovite, sillimanite, and locally garnet; some layers of feldspathic quartzite and garnetiferous amphibolite.

Scotland Schist (Devonian or Silurian or both) at surface, covers 1 % of this area

Gray to silvery, locally rusty, fine- to medium-grained schist, composed of quartz, muscovite, biotite, staurolite, and oligoclase, locally with kyanite or sillimanite; interlayered, especially below and to the west, with quartz-oligoclase-biotite schist and granofels and locally with quartzite.

Waterford Group (Proterozoic Z?) at surface, covers 0.9 % of this area

(May be equivalent in part to Monson Gneiss) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. Usage of Waterford Group follows Goldsmith (1980; 1985). Described as a sequence of metavolcanic and metaplutonic plagioclase gneisses and amphibolites that unconformably overlies the Plainfield Formation in the Hope Valley terrane. (Hope Valley and Esmond-Dedham terranes compose the Avalon superterrane of this report.) Thickness is variable; ranges to 3,100 m. Subdivided (ascending) into Mamacoke Formation with its upper Cohanzie Member (first used?), New London Gneiss, and Rope Ferry Gneiss. Age is Late Proterozoic based on U-Pb analyses of zircon and sphene in the Rope Ferry Gneiss (620+/-3 Ma, Wintsch and Aleinikoff, 1987) (Skehan and Rast, 1990).

Waterbury Gneiss (Proterozoic Z or Cambrian or both) at surface, covers 0.9 % of this area

Medium- to dark-gray, fine- to medium-grained, generally irregularly foliated and lenticular rather than regularly layered schist and schistose gneiss, composed of biotite, quartz, oligoclase, kyanite (or sillimanite), and garnet, also locally microcline, irregularly mixed with granitoid gneiss, composed of oligoclase or andesine, quartz, biotite, and commonly microcline and muscovite.

Stockbridge Marble (Including Inwood Marble) (Lower Ordovician and Cambrian) at surface, covers 0.9 % of this area

White to gray, massive to layered marble, generally dolomitic but containing calcite marble in upper part, locally interlayered with schist or phyllite and with calcareous siltstone or sandstone.

Taine Mountain Member (Lower? Ordovician) at surface, covers 0.9 % of this area

Gray, medium-grained, generally fairly well layered to well-laminated ("pin-stripe") gneissic or schistose granofels, composed of quartz, oligoclase, biotite, muscovite, and garnet, and locally staurolite and kyanite or sillimanite. The Taine Mountain Formation of Stanley (1964) is here adopted in the Collinsville and Bristol quads, CT. Includes 3 pinstriped units: the Wildcat, Scranton Mountain, and Whigville Members, also adopted from Stanley (1964). Underlies Bristol Gneiss (revised). Correlates with the Savoy Schist of Emerson (1898), Missisquoi Schist or Group of Richardson (1919, 1924), in MA; and the Moretown Formation of Cady (1956) in MA and VT. Age is Middle Ordovician (Simpson, 1990).

Harrison Gneiss (including Prospect Gneiss) (Middle? Ordovician) at surface, covers 0.9 % of this area

Interlayered dark- and light-gray, medium-grained, well-foliated gneiss, composed of andesine, quartz, hornblende, and biotite (also locally K-feldspar as megacrysts 1 to 5 cm long). Thought to be metavolcanic equivalent of unit Ob. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields ages of 428+/-2 Ma for the Pumpkin Ground and 446+/-2 Ma for the Beardsley. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).

Collins Hill Formation (Upper? and Middle Ordovician) at surface, covers 0.8 % of this area

( = Partridge Formation of New Hampshire) - Gray, rusty-weathering, medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, muscovite, biotite, and garnet, and commonly staurolite, kyanite, or sillimanite, generally graphitic, interlayered with fine-grained two-mica gneiss, especially to the west, and with calc-silicate and amphibolite layers, also rare quartz-spessartine (coticule) layers.

Yantic Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 0.8 % of this area

Medium- to dark-gray, fine- to medium-grained schist, composed of quartz, oligoclase, biotite, and muscovite, some layers with garnet, staurolite, and kyanite or garnet and sillimanite, local epidote, or K-feldspar; some layers of rusty-weathering graphitic, pyrrhotitic, two-mica schist.

Southbridge Formation (Silurian or Ordovician or both) at surface, covers 0.8 % of this area

Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist, and amphibole, calc-silicate minerals, K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist.

Hornblende gneiss and amphibolite (Proterozoic Y) at surface, covers 0.8 % of this area

Dark-gray to mottled, fine- to medium-grained, massive to foliated amphibolite and gneiss, composed of hornblende and plagioclase, also commonly biotite and minor quartz; commonly interlayered with banded felsic gneiss. Locally contains calc-silicate rock or diopsidic calcite marble.

Wepawaug Schist (Devonian or Silurian or both) at surface, covers 0.8 % of this area

Medium- to dark-gray, medium- to fine-grained, well-layered schist or phyllite and metasiltstone, composed of quartz, muscovite or sericite, plagioclase, biotite, and in appropriate metamorphic zones chlorite, garnet, staurolite, and kyanite. Schist or phyllite generally graphic.

Dalton Formation (Lower Cambrian and perhaps partly older) at surface, covers 0.8 % of this area

(Including Poughquag and Lowerre Formation) - Gray, tan-weathering, medium-grained, generally well layered gneiss or feldspathic quartzite, composed of quartz, microcline, plagioclase, muscovite, biotite, and generally tourmaline; some schistose micaceous layers have sillimanite; commonly as quartz-sillimanite nodules rimmed with muscovite. Layers of purer quartzite in many areas, especially near the top or where the formation is thin.

Mamacoke Formation (Proterozoic Z?) at surface, covers 0.8 % of this area

Interlayered (but layers locally indistinct) light- to dark-gray, medium-grained gneiss, composed of plagioclase, quartz, and biotite; sillimanite, garnet, hornblende, or microcline in certain layers; in upper part locally contains quartz-sillimanite nodules or thin layers of quartzite, amphibolite, or calc-silicate rock.

Scituate Granite Gneiss (Proterozoic Z?) at surface, covers 0.7 % of this area

Light-pink to gray., medium- to coarse-grained, generally porphyritic, well-lineated and locally foliated granitic gneiss, composed of microcline, quartz, albite or orthoclase, biotite, hornblende, and magnetite. Megacrysts of microcline up to 3 cm long; lineation formed by splotches of biotite or by rods or quartz.

Gneiss of Highlands massifs (including Fordham Gneiss) (Proterozoic Y; may contain some older rocks) at surface, covers 0.7 % of this area

Mixture of rock types described below, where not separately mapped.

Lower member [of Bigelow Brook Formation] (Silurian or Ordovician or both) at surface, covers 0.7 % of this area

Chiefly gray, medium-grained, well-layered granofels, composed of quartz, oligoclase, and biotite, commonly with garnet and sillimanite, interlayered with thinly fissile sillimanitic, graphitic, pyrrhotite schist.

Quartzite unit [in Plainfield Formation] (Proterozoic Z?) at surface, covers 0.6 % of this area

Light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules.

Nonewaug Granite (Devonian) at surface, covers 0.6 % of this area

White to pink, fine- to very coarse-grained (commonly pegmatitic), massive to layered granite composed of albite, microcline, quartz, and muscovite, with minor biotite and garnet. Microcline commonly graphic; quartz and muscovite commonly in plumose aggregates.

Basal marble member [of Walloomsac Schist] (Middle Ordovician) at surface, covers 0.6 % of this area

Dark-gray to white, massive to layered schistose or phyllitic calcite-phlogopite marble.

Unit c [of Stockbridge Marble] (Upper Cambrian) at surface, covers 0.6 % of this area

Gray, generally massive dolomite marble, commonly contains quartz grains, locally beds of sandstone; may be calcitic near top.

Eastford gneiss phase [of Canterbury Gneiss] (Devonian) at surface, covers 0.6 % of this area

Mainly light-gray, medium-grained, foliated to strongly lineated gneiss, composed of quartz, microcline, oligoclase, or albite, biotite, and muscovite.

Walloomsac Schist (Middle Ordovician) at surface, covers 0.6 % of this area

Black to dark-or silvery-gray, rarely layered schist or phyllite, composed of quartz, albite, and commonly garnet and staurolite or sillimanite (locally strongly retrograded to chlorite and muscovite). Locally feldspathic or calcareous near the base.

Carringtons Pond Member [of Trap Falls Formation] (Middle or Lower Ordovician) at surface, covers 0.5 % of this area

Interlayered medium- to dark-gray, rusty-weathering, medium-grained schist and light-gray, fine- to medium-grained gneiss, composed of quartz, sodic plagioclase, biotite, muscovite, and garnet; schist locally contains sillimanite or kyanite; gneiss locally contains K-feldspar; amphibolite layers common.

Beardsley Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers 0.5 % of this area

Medium- to dark-gray, medium-grained, well-layered and lineated gneiss, composed of plagioclase, quartz, microcline, hornblende, biotite, and epidote. Microcline may occur as megacrysts 1 to 3 cm across. Minor layers of garnetiferous schist and rarely of calc-silicate rock or marble. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields crystallization ages of 428+/-2 Ma (Early Silurian) for the Pumpkin Ground and 446+/-2 Ma (Late Ordovician) for the Beardsley. Age of the Beardsley based on analysis of seven zircon and two sphene fractions. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).

Schist and granulite member [of Trap Falls Formation] (Middle or Lower Ordovician) at surface, covers 0.5 % of this area

Interlayered gray to silvery, medium- to coarse-grained schist and fine-grained granofels, composed of quartz, sodic plagioclase, biotite, and muscovite; garnet common in schist.

Pumpkin Ground Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers 0.5 % of this area

Medium- to light-gray, medium- to coarse-grained, well-layered and foliated gneiss, composed of oligoclase, microcline, quartz, and biotite; some layers have numerous microcline megacrysts 1 to 5 cm across; others have hornblende. Minor layers of garnetiferous schist and gneiss. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields ages of 428+/-2 Ma (Early Silurian) for the Pumpkin Ground and 446+/-2 Ma (Late Ordovician) for the Beardsley, accepted by authors as crystallization ages. Pumpkin Ground intrudes the Trap Falls Formation. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).

Amphibolite-bearing unit of Manhattan Schist (Cambrian?) at surface, covers 0.5 % of this area

Like Manhattan Schist but with numerous lenses and layers of amphibolite.

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 0.5 % of this area

Light-gray, medium-grained, layered to massive calc-silicate gneiss, composed of andesine, quartz, hornblende or actinolite, epidote, and commonly diopside, biotite, and scapolite; some layers are calcitic.

Shuttle Meadow Formation (Lower Jurassic) at surface, covers 0.5 % of this area

Maroon to dark-gray, silty shale, siltstone, and fine-grained silty sandstone, generally well and thinly laminated. In the southern part of the State includes a layer, up to 5 m thick, of blue, commonly sandy, fine-grained limestone or dolomitic limestone, grading laterally into calcareous siltstone. Coarser and more arkosic to east and south, grading into conglomerate near the eastern border fault.

Basal member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 0.5 % of this area

Around Waterbury dome - Differs from rest of Taine Mountain Formation in being especially well layered and generally less micaceous and schistose.

Littleton Formation (Devonian) at surface, covers 0.4 % of this area

Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist.

Bristol Gneiss (Middle? Ordovician) at surface, covers 0.4 % of this area

Light, medium-grained, massive to well-layered gneiss, composed of plagioclase, quartz, and biotite, also muscovite and garnet in many layers, interlayered in places with dark amphibolite. Bristol Member of Collinsville Formation (Stanley, 1964) is here raised in rank and revised as Bristol Gneiss in the report area. Garnet- and epidote-bearing amphibolite and hornblende gneiss assigned by Stanley to the upper part of his Bristol Member is here assigned to base of the overlying Collinsville Formation and composes the unnamed hornblende gneiss member. As defined here, the Bristol Gneiss consists of plagioclase-quartz gneiss characterized by foliae of muscovite or biotite, or locally chlorite, which in places give the unit a striking pinstriping; contains scattered small garnets and large pods of amphibolite. Average thickness may be about 2,000 ft; thickness varies due to local intense folding. Probably correlates with the Moretown Formation of central MA, in which case it would constitute a fourth member of Stanley's Taine Mountain Formation. Formational status assigned in this report based on its unique textural and lithological character. Inferred age is Middle Ordovician (Simpson, 1990).

Hornblende gneiss member [of Collinsville Formation] (Middle Ordovician) at surface, covers 0.4 % of this area

Dark, fine- to medium-grained, well-layered amphibolite and hornblende gneiss, composed of hornblende and plagioclase, commonly with biotite, garnet, or epidote, interlayered with light-gray felsic gneiss and pink quartz-spessartine rock (coticule). Grades into Bristol Gneiss.

Unit b [of Stockbridge Marble] (Upper and Middle? Cambrian) at surface, covers 0.4 % of this area

White, pink, cream, and light-gray, generally well bedded dolomitic marble interlayered with phyllite and schist and with siltstone, sandstone, or quartzite, commonly dolomitic.

Oronoque Schist (Lower? Ordovician) at surface, covers 0.4 % of this area

Gray to silver, medium- to fine-grained, well-layered to laminated schist and granofels, composed of quartz, oligoclase, or albite, muscovite or sericite, biotite, or chlorite, and in western belt local garnet, staurolite, and kyanite. Small lenses of amphibolite or greenstone.

Upper slice of Canaan Mountain Schist (Cambrian?) at surface, covers 0.4 % of this area

Dark-gray to silvery, generally rusty weathering, medium- to coarse-grained, well-foliated, massive to well-layered schist and schistose gneiss, composed of quartz, plagioclase, biotite, muscovite, and generally garnet and sillimanite; also layers of amphibolite.

Cobble Mountain Formation (Middle Ordovician) at surface, covers 0.4 % of this area

Gray to silver (not rusty), medium- to coarse-grained, generally layered schist and granofels, composed of quartz, oligoclase, muscovite, biotite, and garnet, and locally kyanite and staurolite or sillimanite. Some amphibolite layers.

New London Gneiss (Proterozoic Z?) at surface, covers 0.4 % of this area

New London consists of a layered facies and a massive facies. Layered facies described as alternating layers of light-colored biotite-quartz-plagioclase gneiss and amphibolite. Massive facies described as a granodiorite gneiss with a uniform texture, grain size, and color. Contains shiny black biotite plates and distinctive magnetite grains. New London consists of a layered facies and a massive facies. Layered facies described as alternating layers of light-colored biotite-quartz-plagioclase gneiss and amphibolite. Massive facies described as a granodiorite gneiss with a uniform texture, grain size, and color. Contains shiny black biotite plates and distinctive magnetite grains (Skehan and Rast, 1990).

Southington Mountain Member [of The Straits Schist] (Devonian or Silurian or both) at surface, covers 0.4 % of this area

Gray to silvery, non-rusty, medium-grained, well-layered alternating schist and granofels, composed of quartz, oligoclase, muscovite, biotite, and garnet, commonly with staurolite and kyanite (or sillimanite); schist commonly graphitic. Metamorphosed strata named Southington Mountain Schist by Fritts (1963) is traceable into southern Bristol quad., and is here revised as Southington Mountain Member of The Straits Schist. Consists of alternating bands of quartz-feldspar granulite and of graphitic, muscovite-biotite schist. Unit is characterized by distinct, widespread, graded bedding and locally abundant staurolite. Unit may correlate with the Goshen Formation of the Heath quad, MA-VT, of Middle Silurian to Early Devonian age (Simpson, 1990).

Gneiss (metavolcanic) member [of Brimfield Schist] (Upper? and Middle Ordovician) at surface, covers 0.4 % of this area

Medium-gray, medium-grained, layered gneiss and schist, composed of oligoclase, quartz, and biotite; some gneiss and most schist layers contain garnet and sillimanite; some gneiss layers contain garnet, hornblende or pyroxene or grade into amphibolite or calc-silicate rock. Probably includes metavolcanic rocks.

Upper member [of Bigelow Brook Formation] (Silurian and perhaps Ordovician) at surface, covers 0.4 % of this area

Chiefly gray, rusty-weathering, medium-grained, generally well layered and locally fissile schist, composed of plagioclase, quartz, biotite, garnet, and sillimanite, locally with K-feldspar or cordierite, fissile layers commonly with graphite and pyrrhotite, interlayered with quartzose granofels with less biotite but with calc-silicate minerals.

Hampden Basalt (Lower Jurassic) at surface, covers 0.4 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass.

Shelton Member [of Trap Falls Formation] (Middle or Lower Ordovician) at surface, covers 0.3 % of this area

White, light-gray, or buff, fine- to medium-grained, generally well foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and garnet (in tiny almost ubiquitous grains), also commonly minor biotite; generally interlayered with mica schist, biotite gneiss, and calc-silicate rock. Thought to be metavolcanic equivalent of unit Og. Shelton Member of Trap Falls Formation (Rodgers, 1985) is here referred to as Shelton muscovite granite. On the basis of field and laboratory studies, Ansonia, Beardsley, Pumpkin Ground, and Shelton gneisses, previously considered stratigraphic units, are reinterpreted as plutonic. Shelton is a foliated, medium-grained, garnet-bearing muscovite leucogranite with a conspicuous white color and abundant garnets. Age of crystallization determined from U-Pb garnet analysis is 380+/-3 Ma (Middle Devonian). Southeast margin of the Shelton is in contact with the Trap Falls Formation (Sevigny and Hanson, 1993).

Trap Falls Formation (Middle or Lower Ordovician) at surface, covers 0.3 % of this area

(May be equivalent in part to Golden Hill Schist) - Gray to silvery, partly rusty weathering, medium-grained generally well layered schist, composed of quartz, sodic plagioclase, biotite, muscovite, and garnet, locally with sillimanite or kyanite, interlayered with two-mica gneiss and granulite and with amphibolite.

Ponaganset Gneiss (Proterozoic Z?) at surface, covers 0.3 % of this area

Dark-gray, coarse-grained, porphyritic, well-foliated gneiss, composed of oligoclase, quartz, microcline (mostly as megacrysts up to 8 cm long), biotite, magnetite, and generally hornblende; also garnet and muscovite where hornblende is absent. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. Ponaganset lies east of the Hope Valley shear zone and extends from RI into MA. [Apparently does not occur in CT.] Predominantly granite, ranges to tonalite. Gray to light gray, some pink, generally medium- to coarse-grained, but ranges from fine-grained to porphyritic. Alkali K-feldspar, plagioclase, quartz, and biotite are major minerals. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. Strong lineation is defined by elongate aggregates of quartz and feldspar and trains of biotite and hornblende. Age is Late Proterozoic based on correlation with Northbridge Gneiss (MA), which has been dated at 557+/-4 Ma by Zartman and Naylor (1984) (Skehan and Rast, 1990).

Taine Mountain and Collinsville Formations undivided (Middle to Lower? Ordovician) at surface, covers 0.3 % of this area

See Ot and Oc.

Golden Hill Schist (Lower? Ordovician) at surface, covers 0.3 % of this area

(May be equivalent to part of Trap Falls Formation) - Gray to silvery, medium- to coarse-grained, generally layered schist and granofels, composed of quartz, muscovite, biotite, plagioclase, and garnet.

Lebanon Gabbro (Devonian) at surface, covers 0.3 % of this area

Dark, speckled, coarse-grained, massive but locally sheared gabbro, composed of hornblende, labradorite, and opaques. Some bodies contain biotite, and quartz; some smaller ones are nearly pure hornblende with local augite.

Plainfield Formation plus Potter Hill Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.3 % of this area

Plainfield Formation - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopsite-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules. Potter Hill Granite Gneiss - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. Narragansett Pier Granite - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Southbridge Formation (Silurian or Ordovician or both) at surface, covers 0.3 % of this area

Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist, and amphibole, calc-silicate minerals, K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist.

Ratlum Mountain Schist (Lower Ordovician) at surface, covers 0.3 % of this area

Gray, medium-grained, interlayered schist and granofels, composed of quartz, oligoclase, muscovite (in the schist), biotite, and garnet, also staurolite and kyanite in the schist. Numerous layers and lenses of amphibolite; also some of quartz-spessartine (coticule) and calc-silicate rock.

Upper member [of Middletown Formation] (Middle Ordovician) at surface, covers 0.2 % of this area

Light-gray, generally rusty weathering, well-layered gneiss and granofels, composed of oligoclase, quartz, biotite, and amphibole (cummingtonite, anthophyllite, gedrite, or hornblende, or several of these), also garnet and chlorite. Many layers of amphibolite and biotite gneiss throughout.

Talcott Basalt (Lower Jurassic) at surface, covers 0.2 % of this area

Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass. Pillows in many places; volcanic breccia with fragmentary pillows in others.

Everett Schist (Cambrian?) at surface, covers 0.2 % of this area

Grayish to greenish (some rusty-weathering), fine- to medium-grained, foliated but poorly layered schist or phyllite, composed of quartz, albite or oligoclase, muscovite, garnet, staurolite or chloritoid, and generally chlorite. Local layers are dark-gray to silvery schist or phyllite.

Preston Gabbro (Middle Ordovician or older) at surface, covers 0.2 % of this area

Dark, medium- to coarse-grained, mainly massive gabbro, composed of labradorite, augite, and opaques, generally with hornblende, locally hypersthene, or olivine or both.

Buttress Dolerite (Middle? Jurassic) at surface, covers 0.2 % of this area

Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Lower member [of Middletown Formation] (Middle Ordovician) at surface, covers 0.2 % of this area

Dark- to medium-gray amphibolite and hornblende gneiss, commonly with garnet, diopside, or epidote, interlayered with light-gray gneiss composed of oligoclase, quartz, biotite, and generally one or more amphiboles, also garnet.

Porphyritic phase [of Potter Hill Granite Gneiss] (Proterozoic Z?) at surface, covers 0.2 % of this area

Light- to medium-gray, fine- to medium-grained, porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline (much of it as megacrysts up to 4 cm long), quartz, oligoclase, biotite and magnetite. Sterling Plutonic Suite is here restricted to the Hope Valley terrane. (The Hope Valley together with the Esmond-Dedham terrane make up the Avalon superterrane of this report.) The Ponaganset Gneiss and the Ten Rod Granite Gneiss lie within the Esmond-Dedham terrane and are therefore removed from the Sterling. The Ponaganset is interpreted as an intensely deformed phase of the Esmond Plutonic Suite. The Sterling, as defined here, includes only the Hope Valley Alaskite Gneiss and the Potter Hill Granite Gneiss. These rocks intrude the Plainfield Formation and the Waterford Group in southeastern CT and western Rhode Island and are Late Proterozoic in age. The gneiss of the Potter Hill is mainly weathered, deeply stained, somewhat crumbly, and generally slabby. Contains xenoliths of the Plainfield Formation. Is distinguished from Hope Valley by its higher biotite content (Skehan and Rast, 1990).

Unit a [of Stockbridge Marble] (Lower Cambrian) at surface, covers 0.2 % of this area

White to pale-gray, massive, smooth-weathering dolomite marble.

Black Hill Member [of Quinebaug Formation] (Middle Ordovician or older) at surface, covers 0.2 % of this area

Gray, medium- to fine-grained, well-layered schist and granofels, composed of oligoclase, quartz, and biotite, commonly with hornblende or muscovite, and locally with calcite, garnet, or epidote.

Hawley Formation (Middle Ordovician) at surface, covers 0.2 % of this area

(Carbonaceous schist facies) - Gray, rusty-weathering, fine- to medium-grained, generally layered schist and granofels, composed of quartz, oligoclase, and biotite; some muscovite and graphite, rare garnet and kyanite or sillimanite. Layers of quartz-spessartine rock (coticule) common.

Lower slice of Canaan Mountain Schist (Cambrian?) at surface, covers 0.1 % of this area

Dark-gray, rusty-weathering, coarse-grained, well-foliated and moderately well layered schist composed of quartz, plagioclase, biotite, muscovite, and generally garnet and sillimanite (or minor staurolite). Amphibolite layers rather rare.

West Rock Dolerite (Lower Jurassic) at surface, covers 0.1 % of this area

Dark-gray to greenish-gray (weathers bright orange to brown), medium- to fine-grained, grading from basalt near contacts to fine-grained gabbro in the interior, generally massive with well-developed columnar jointing, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Augen gneiss (Proterozoic Y) at surface, covers 0.1 % of this area

(Including local term "Danbury Gneiss;" equivalent in part to Tyringham Gneiss of Massachusetts) - Medium-gray to spotted, fine- to medium-grained, porphyritic, foliated and lineated granitic gneiss, composed of microcline (largely as megacrysts or augen up to 10 cm long), quartz, albite, or oligoclase, biotite and minor hornblende.

Sweetheart Mountain Member [of Collinsville Formation] (Middle Ordovician) at surface, covers 0.1 % of this area

Gray and silvery (not rusty), medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, biotite, muscovite, and garnet, and in places kyanite or sillimanite. Amphibolite layers common; also layers of quartz-spessartine rock (coticule). In the Bristol quad., CT, Collinsville Formation is revised to include a basal unnamed hornblende gneiss member (was upper part of Stanley's (1964) Bristol Member), a middle unnamed metaquartzite member, and the upper Sweetheart Mountain Member. Bristol Member of Stanley (1964) is raised in rank to Bristol Gneiss in the report area. Collinsville unconformably underlies The Straits Schist. Inferred age is Middle Ordovician (Simpson, 1990).

Mylonite along Paleozoic faults (Upper or Middle Paleozoic) at surface, covers 0.1 % of this area

Mylonite, blastomylonite, and blastomylonitic gneiss, composed of intensely granulated quartz, plagioclase, biotite, and epidote, in places with hornblende or microcline and commonly with secondary minerals. In places has later been silicified (compare unit Jsi)..

Waterford Group plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.1 % of this area

Waterford Group (may be equivalent in part to Monson Gneiss) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Units e and d [of Stockbridge Marble] (Lower Ordovician) at surface, covers 0.1 % of this area

White to gray massive calcite marble, commonly mottled with dolomite and locally interlayered with dolomite marble and calcareous siltstone and sandstone.

Metavolcanic member [of Collins Hill Formation] (Upper? and Middle Ordovician) at surface, covers 0.1 % of this area

Ranges from mafic to felsic, from dark layered amphibolite and hornblende schist, locally with garnet or epidote, to light-gray (in places purplish), laminated gneiss, composed of quartz, oligoclase, and biotite, in which some layers contain garnet (generally manganiferous) and hornblende or cummingtonite.

Brimfield Schist (Includes Hamilton Resevoir Formation) (uncertain) (Upper? and Middle Ordovician) at surface, covers 0.1 % of this area

Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Waterford Group and Branford Gneiss (Proterozoic Z?) at surface, covers 0.1 % of this area

Waterford Group (may be equivalent in part to Monson Gneiss) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. Branford Gneiss - Gray to white but rarely pink., medium-grained, well-foliated granitic gneiss, composed of oligoclase, K-feldspar, quartz, biotite, garnet, magnetite, and muscovite.

Plainfield Formation plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers < 0.1 % of this area

Plainfield Formation - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-grey or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopside-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Lower part [of Maltby Lakes Metavolcanics] (Middle? Ordovician) at surface, covers < 0.1 % of this area

Gray-green to green, fine-grained, generally well foliated greenschist, greenstone, and schist or phyllite, composed of albite and chlorite, plus quartz and sericite or epidote and actinolite. Mixed metavolcanics and metasedimentary rocks.

Felsic gneiss member [of Quinebaug Formation] (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Light- to medium-gray, fine- to medium-grained gneiss, composed of plagioclase, quartz, biotite, and muscovite, commonly with K-feldspar.

Ratlum Mountain Schist plus Amphibolite unit [in Ratlum Mountain Schist] (Lower Ordovician) at surface, covers < 0.1 % of this area

Ratlum Mountain Schist - Gray, medium-grained, interlayered schist and granofels, composed of quartz, oligoclase, muscovite (in the schist), biotite, and garnet, also staurolite and kyanite in the schist. Numerous layers and lenses of amphibolite; also some of quartz-spessartine (coticule) and calc-silicate rock. Amphibolite unit [in Ratlum Mountain Schist] (Lower? Ordovician) - Black or mottled, generally massive amphibolite and hornblende gneiss, composed of hornblende and andesine, commonly with minor quartz and magnetite, and locally with garnet, biotite, and epidote.

Clough Quartzite (Silurian) at surface, covers < 0.1 % of this area

White, medium-grained, glassy to granular, well-layered quartzite and muscovitic quartzite, locally with garnet; conglomeratic (commonly with tourmaline) in lower part.

Whigville Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers < 0.1 % of this area

Gray, medium-grained, generally fairly well layered to well-laminated ("pin-stripe") gneissic or schistose granofels, composed of quartz. oligoclase, biotite, muscovite, and garnet, and locally staurolite and kyanite or sillimanite. The Taine Mountain Formation of Stanley (1964) is here adopted in the Collinsville and Bristol quads., CT. Includes three pinstriped units, the Wildcat, Scranton Mountain, and Whigville Members of Stanley (1964), also adopted. Whigville Member consists of plagioclase-quartz gneiss marked by distinctive thin layering (pinstriping) of muscovite and biotite. Thickness is 3,000 ft. Overlies the Scranton Mountain Member; underlies the Bristol Gneiss (revised). The Taine Mountain correlates with the Savoy Schist of Emerson (1898), Missisquoi Schist or Group of Richardson (1919, 1924), in MA; and the Moretown Formation of Cady (1956) in MA and VT. Age is Middle Ordovician (Simpson, 1990).

Basal member of upper slice of Canaan Mountain Schist (Cambrian?) at surface, covers < 0.1 % of this area

Gray, generally rusty- or tan-weathering gneiss, composed of quartz, plagioclase, microcline, biotite, and muscovite, interlayered with feldspathic quartzite.

Allingtown Metavolcanics (Middle? Ordovician) at surface, covers < 0.1 % of this area

Green, fine-grained, massive greenstone, composed of epidote, actinolite, albite, and chlorite, commonly with abundant megacrysts of saussurite, interlayered with minor green phyllite, generally containing quartz and sericite. Dark amphibole in western outcrops.

Nodular member [of Harrison Gneiss] (Middle? Ordovician) at surface, covers < 0.1 % of this area

Harrison Gneiss containing prominent quartz-sillimanite nodules.

Dioritic phase [of Preston Gabbro] (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Medium- to dark-gray, streaked or speckled, medium-grained diorite and quartz diorite, gneissic where sheared near contact, composed of plagioclase, hornblende, and biotite, and locally quartz and relic pyroxene.

Light House Gneiss (Proterozoic Z?) at surface, covers < 0.1 % of this area

Light-pink or gray to red, medium-grained, generally well foliated granitic gneiss, composed of K-feldspar, oligoclase, quartz, biotite, and magnetite, with local muscovite but no garnet.

Maromas Granite Gneiss (Devonian?) at surface, covers < 0.1 % of this area

Light-gray to buff, medium- to fine-grained granitic gneiss, composed of quartz and microcline with minor plagioclase and biotite. Central body is massive, but outlying strips are foliated and have accessory hornblende or garnet. Massive parts may be young anatectic intrusive rocks; foliated parts may include older felsic metavolcanic rocks belonging to unit Ochv. Pegmatite bodies are common in the vicinity.

Amphibolite unit [in Ratlum Mountain Schist] (Lower? Ordovician) at surface, covers < 0.1 % of this area

Black or mottled, generally massive amphibolite and hornblende gneiss, composed of hornblende and andesine, commonly with minor quartz and magnetite, and locally with garnet, biotite, and epidote.

Dioritic phase [of Lebanon Gabbro] (Devonian) at surface, covers < 0.1 % of this area

White to black, streaked, medium-grained, foliated or sheared gneiss, composed of plagioclase, biotite, quartz, and generally hornblende.

Scranton Mountain Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers < 0.1 % of this area

Gray, rusty-weathering, medium-grained schist, composed of quartz, muscovite, biotite, plagioclase, garnet, and generally kyanite. The Taine Mountain Formation of Stanley (1964) is here adopted in the Collinsville and Bristol quads, CT. Includes three pinstriped units, the Wildcat, Scranton Mountain, and Whigville Members of Stanley (1964), also adopted. Correlates with the Savoy Schist of Emerson (1898), Missisquoi Schist or Group of Richardson (1919, 1924), in MA; and the Moretown Formation of Cady (1956) in MA and VT. Age is Middle Ordovician (Simpson, 1990).

Mount Pisgah Member of Littleton Formation (Devonian) at surface, covers < 0.1 % of this area

Gray, medium-grained, well-layered (locally graded) granofels or micaceous quartzite with some schist, composed of quartz, oligoclase, biotite, garnet, and sillimanite.

Joshua Rock Member [of New London Gneiss] (Proterozoic Z?) at surface, covers < 0.1 % of this area

Medium-gray (weathers with red spots of hematite), medium-grained, foliated gneiss composed of microperthite, quartz, albite, aegerine-augite, and magnetite; rare riebeckite.

Foliated quartz diorite (Devonian in part, probably Ordovician in part) at surface, covers < 0.1 % of this area

Mainly dark-gray, medium-grained, well-foliated gneiss (locally strongly sheared, especially near contacts), composed of plagioclase, quartz, biotite, and hornblende, locally also pyroxene.

Upper part [of Maltby Lakes Metavolcanics] (Middle? Ordovician) at surface, covers < 0.1 % of this area

Green to gray-green, fine-grained, layered and foliated to massive greenstone and greenschist, composed of epidote, albite, actinolite, and chlorite, and locally minor quartz, sericite, garnet, pyrite, or calcite. Mainly metavolcanic.

Wildcat Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers < 0.1 % of this area

Gray, medium-grained, generally fairly well layered to well-laminated ("pin-stripe") gneissic or schistose granofels, composed of quartz, oligoclase, biotite, muscovite, and garnet, and locally staurolite and kyanite or sillimanite. The Taine Mountain Formation of Stanley (1964) is here adopted in the Collinsville and Bristol quads., CT. Includes three pinstriped units, the Wildcat, Scranton Mountain, and Whigville Members of Stanley (1964), also adopted. Wildcat Member is the basal pinstriped gneiss unit of the Taine Mountain, which correlates with the Savoy Schist of Emerson (1898), Missisquoi Schist or Group of Richardson (1919, 1924) in MA; and the Moretown Formation of Cady (1956) in MA and VT. Inferred age is Middle Ordovician (Simpson, 1990).

Rowe Schist plus Amphibolite unit [in Rowe Schist] (Lower Ordovician or Cambrian or both) at surface, covers < 0.1 % of this area

Light-gray to silvery, fine- to medium-grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, and generally garnet, staurolite, and kyanite or sillimanite. Layers of granofels common; also some layers of amphibolite, quartz-spessartine rock (coticule), and calc-silicate rock. Amphibolite unit [in Rowe Schist] - Black or mottled, generally massive amphibolite and hornblende gneiss, composed of hornblende and andesine.

Basal member [of The Straits Schist] (Silurian) at surface, covers < 0.1 % of this area

( = Russell Mountain Formation of Massachusetts) - Distinguished by presence of layers of amphibolite, marble, calc-silicate rock, and quartzite within more uniform schist like that on either side. Minor, unevenly distributed mineralization in W, Bi, Cu, Ni, and other metals.

Potter Hill Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers < 0.1 % of this area

Potter Hill Granite Gneiss - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Harrison Gneiss (Middle? Ordovician) at surface, covers < 0.1 % of this area

Interlayered dark- and light-gray, medium-grained, well-foliated gneiss, composed of andesine, quartz, hornblende, and biotite (also locally K-feldspar as megacrysts 1 to 5 cm long). Thought to be metavolcanic equivalent of unit Ob.

Golden Hill Schist (Lower? Ordovician) at surface, covers < 0.1 % of this area

(May be equivalent to part of Trap Falls Formation) - Gray to silvery, medium- to coarse-grained, generally layered schist and granofels, composed of quartz, muscovite, biotite, plagioclase, and garnet.

Rowe Schist (Lower Ordovician or Cambrian or both) at surface, covers < 0.1 % of this area

Light-gray to silvery, fine- to medium-grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, and generally garnet, staurolite, and kyanite or sillimanite. Layers of granofels common; also some layers of amphibolite, quartz-spessartine rock (coticule), and calc-silicate rock.

Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers < 0.1 % of this area

Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Litchfield Norite (Middle? Ordovician) at surface, covers < 0.1 % of this area

Dark, medium- to coarse-grained, mostly massive mafic rock (olivine norite, quartz norite, hypersthene pyroxenite), composed of labradorite, hypersthene, augite, and olivine in varying proportions, also hornblende and biotite (and minor quartz in quartz norite). Associated with small mineral deposits of pyrrhotite, pentlandite, and chalcopyrite.

Maltby Lakes Metavolcanics (Middle? Ordovician) at surface, covers < 0.1 % of this area

Green to gray-green, fine-grained, massive to well-foliated and layered greenstone, greenschist, and schist; also dark amphibolite to west and southwest.

Allingtown Metavolcanics plus Maltby Lakes Metavolcanics (Middle? Ordovician) at surface, covers < 0.1 % of this area

Allingtown Metavolcanics - Green, fine-grained, massive greenstone, composed of epidote, actinolite, albite, and chlorite, commonly with abundant megacrysts of saussurite, interlayered with minor green phyllite, generally containing quartz and sericite. Dark amphibole in western outcrops. Maltby Lakes Metavolcanics - Green to gray-green, fine-grained, massive to well-foliated and layered greenstone, greenschist, and schist; also dark amphibolite to west and southwest.

Quartzite unit [in Plainfield Formation] plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers < 0.1 % of this area

Quartzite unit [in Plainfield Formation] - light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Porphyritic member [of Southbridge Formation] (Silurian or Ordovician or both) at surface, covers < 0.1 % of this area

Light- to medium-gray, fine-grained porphyritic massive to layered gneiss, composed of quartz, oligoclase, microcline, and biotite, with megacrysts 1 to 2 cm long of microcline.

Mafic phase [of Narragansett Pier Granite] (Permian) at surface, covers < 0.1 % of this area

Gray to reddish, medium-grained, generally massive granite, like Pn but with more biotite and locally hornblende.

Erving Formation (Devonian) at surface, covers < 0.1 % of this area

Gray, medium-grained, well-foliated and generally well layered granofels and schist, composed of quartz, plagioclase, and biotite, also muscovite in schist, and accessory garnet and kyanite.

Pinewood Adamellite (Permian) at surface, covers < 0.1 % of this area

Light-gray, medium-grained, massive adamellite, composed of microcline, albite, quartz, and muscovite with accessory fluorite. High radioactivity.

Hartland Formation (Ordovician?) at surface, covers < 0.1 % of this area

Basal amphibolite overlain by pelitic schists.

Silicified rock and mylonite along Mesozoic faults (probably mainly Jurassic) at surface, covers < 0.1 % of this area

Close network of quartz veins and veinlets cutting each other and older rock, which is mostly replaced by very fine grained quartz. In places, incompletely replaced rock shows strongly mylonitic texture.

Littleton Formation (uncertain) (Devonian) at surface, covers < 0.1 % of this area

Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist.

Manhattan Formation, undivided (Ordovician?) at surface, covers < 0.1 % of this area

Pelitic schists, amphibolite; Units ?Omb, ?Omc, and ?Omd may be Cambrian eugeosynclinal rocks thrust upon Oma; ?Omd - sillimanite-garnet-muscovite-biotite-plagioclase-quartz gneiss; ?Omc - sillimanite-garnet-muscovite-biotite-quartz-plagioclase schistose gneiss, sillimanite nodules, local quartz-rich layers; ?Omb - discontinous unit of amphibolite and ?Omc-type schist.

Massive mafic rock (in Middletown Formation) (Middle Ordovician) at surface, covers < 0.1 % of this area

Dark, coarse-grained, massive amphibolite and metagabbro, composed of hornblende and plagioclase; in places with quartz and epidote, in others with patches of actinolite or anthophyllite, chlorite, and epidote or garnet. May be intrusive.

Fitch Formation (Silurian) at surface, covers < 0.1 % of this area

Gray, fine- to medium-grained calc-silicate rock, composed of quartz, biotite, calcite, actinolite, diopside, microcline, and locally garnet, scapolite, or epidote, interlayered with two-mica schist.

Stockbridge Marble (Cambrian - Lower Ordovician) at surface, covers < 0.1 % of this area

Stockbridge Marble.

Narragansett Pier Granite (Permian) at surface, covers < 0.1 % of this area

Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite, and accessory muscovite and magnetite. Considerable associated pegmatite.

Paxton Formation (Silurian) at surface, covers < 0.1 % of this area

Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Units g and f [of Stockbridge Marble] (Lower Ordovician) at surface, covers < 0.1 % of this area

White to gray massive calcite marble with layers and laminae of dolomitic marble.

Interlayered metasedimentary rock and granitic gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Interlayered metasedimentary rock and granitic gneiss.

New Haven Arkose (Upper Triassic) at surface, covers < 0.1 % of this area

Red, pink, and gray coarse-grained, locally conglomeratic arkose interbedded with brick-red shaley siltstone and fine-grained arkosic sandstone; boundary between Lower Jurassic (Jn) and Upper Triassic (TRn) parts is arbitrarily drawn through clastic rocks of similar lithology below gray mudstone containing Lower Jurassic palynofloral zone; TRn is continuous with and lithically similar to TRs near Northampton. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Middle member [of Bigelow Brook Formation] (Silurian or perhaps Ordovician) at surface, covers < 0.1 % of this area

Greenish-gray, medium-grained calc-silicate rock, composed of plagioclase, quartz, and diopside (locally hornblende and scapolite), interbedded with schist and granofels composed of plagioclase, quartz, biotite, and commonly garnet and sillimanite.

Westerly Granite (Permian) at surface, covers < 0.1 % of this area

Light-gray, pink-weathering, fine-grained, massive, aplitic granite, composed of oligoclase or albite, quartz, and K-feldspar, with minor biotite and accessory muscovite, magnetite, allanite, and sphene.

Buttress Dolerite (uncertain) (Middle? Jurassic) at surface, covers < 0.1 % of this area

Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Sterling Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals. Includes some rock mapped formerly as Ten Rod Granite Gneiss, Hope Valley Alaskite Gneiss, Potter Hill Granite Gneiss, and Scituate Granite Gneiss.

Foliated granitic gneiss (Devonian?) at surface, covers < 0.1 % of this area

Light-gray, coarse-grained, strongly to weakly foliated gneiss, composed of phenocrysts of K-feldspar in a groundmass of plagioclase, quartz, K-feldspar, and biotite, with accessory sillimanite and garnet.

Preston Gabbro plus Quinebaug Formation (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Preston Gabbro - Dark, medium- to coarse-grained, mainly massive gabbro, composed of labradorite, augite, and opaques, generally with hornblende, locally hypersthene, or olivine or both. Quinebaug Formation - Medium- to dark-gray, commonly greenish, medium-grained, well-layered gneiss, composed of hornblende, andesine, biotite, and epidote, commonly with quartz or garnet, interlayered with amphibolite.

Amphibolite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Pyroxenic amphibolite, hornblende gneiss, commonly biotitic, garnetiferous; subordinate calcsilicate rock.

Fordham Gneiss, undivided (Precambrian - Middle Proterozoic) at surface, covers < 0.1 % of this area

fe: garnet-biotite-quartz-plagioclase gneiss, and amphibolite; fd: sillimanite-garnet schistose gneiss, quartzite; fc: biotite-hornblende-quartz-plagioclase gneiss, quartz-feldspar lenses, amphibolite, biotite and/or hornblende-quartz-feldspar gneiss; fb: amphibolite, biotite and/or hornblende-garnet-quartz-plagioclase gneiss; fa: garnet-biotite-quartz-plagioclase gneiss, amphibolite, biotite-hornblende-quartz-plagioclase gneiss, quartz-feldspar granulite.

Everett Schist (Cambrian?) at surface, covers < 0.1 % of this area

Minor meta-graywacke lenses. Includes Greylock Schist in Massachusetts.

Inwood Marble (Early Cambrian - Lower Ordovician) at surface, covers < 0.1 % of this area

Dolomite marble, calc-schist, granulite, and quartzite, overlain by calcite marble; grades into underlying patchy Lowerre Quartzite of Early Cambrian age.

Quartzite unit [in Scotland Schist] (Devonian or Silurian or both) at surface, covers < 0.1 % of this area

Quartzite, generally micaceous, interlayered with mica schist.

Granitoid Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Biotite ferrohastingsite granodioritic and granitic gneiss with large schlieren of biotite, locally contains garnet and muscovite.

Walloomsac Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Phyllite, schist, metagraywacke.

New Haven Arkose plus Buttress Dolerite (Upper Triassic; possibly Lower Jurassic at top plus Middle? Jurassic) at surface, covers < 0.1 % of this area

New Haven Arkose - Red, pink, and gray coarse-grained, locally conglomeratic, poorly sorted and indurated arkose, interbedded with brick-red micaceous, locally shaly siltstone and fine-grained feldspathic clayey sandstone. Buttress Dolerite (Middle? Jurassic) - Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Washington Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Well-layered, rusty-tan weathering muscovite-biotite plagioclase-microcline-quartz granofels containing layers of rusty sulfidic calc-silicate rocks.

Ultramafic rock (Ordovician or older) at surface, covers < 0.1 % of this area

Ultramafic rock, originally composed of olivine and pyroxene, now generally altered to tremolite, talc, chlorite, or serpentine.

Fordham Gneiss (C and D member) (Precambrian - Middle Proterozoic) at surface, covers < 0.1 % of this area

fc: biotite-hornblende-quartz-plagioclase gneiss, quartz-feldspar lenses, amphibolite, biotite and/or hornblende-quartz-feldspar gneiss; fd: sillimanite-garnet schistose gneiss, quartzite.

Walloomsac Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Slate, phyllite, schist, metagraywacke.

Portland Formation (Lower Jurassic) at surface, covers < 0.1 % of this area

Reddish-brown to pale red conglomerate and arkose.

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Pale pink, orange, or gray, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition is granite with generally less than 3% dark minerals. Composed of sodic plagioclase, quartz, microcline/orthoclase, minor biotite, and opaque minerals; minor muscovite (in part secondary), and rare garnet and sphene in some rocks; accessory apatite and zircon; secondary chlorite. Varies from massive to layered. Strongly foliated and locally well lineated. Includes most rock mapped formerly as Hope Valley Alaskite Gneiss.

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Mafic-poor gneissic granite, locally muscovitic. Gradational with Zsg. Late Proterozoic Hope Valley Alaskite Gneiss occurs as one of several plutonic rocks in Milford antiform. Forms tabular masses along west side of Rhode Island anticlinorium from southern RI and eastern CT to northwestern RI; flanks west side of Milford anticlinorium and terminates at north end of anticlinorium in MA. Color is light pink to tan. Intrudes Plainfield Formation in CT and Blackstone Group rocks in RI. Isotopic age of 630 Ma by U/Pb methods on zircon is reported by Zartman and Naylor (1984) from a sample in MA. Age of 601 +/-5 Ma by U/Pb methods on zircon is reported by Hermes and Zartman (1985) from a sample in RI (Wones and Goldsmith, 1991).

Ayer Granite (Lower Silurian) at surface, covers < 0.1 % of this area

Granite to tonalite, partly porphyritic; locally gneissic, locally muscovitic; may include rocks older than Silurian; intrudes Sb and So. Ayer Granite is divided into the Clinton facies and the Devens-Long Pond facies (Gore, 1976). In addition, there are some masses not assigned to either facies that intrude Berwick Formation west and northwest of Lawrence, and that intrude Paxton and Oakdale Formations south of Worcester and west of probable southern continuation of Wekepeke fault. Radiometric ages obtained for facies of Ayer pose problems in assigning ages to unfossiliferous sedimentary rocks they intrude. Clinton facies has a well-defined U-Pb zircon age of 433 +/-5 Ma (Zartman and Naylor, 1984) that authors cite as Early Silurian; Devens-Long Pond facies has a similar age. This age greatly compresses the time available for deposition, burial, deformation, and metamorphism of Berwick and Paxton if these units are truly Silurian. Some of the Ayer not assigned to a facies may have been more properly correlated with Early Devonian Chelmsford Granite and muscovite-biotite granite at Millstone Hill. Bodies south of Worcester may be more properly correlated with Canterbury Gneiss of CT, which lies on strike with Ayer and has Early Devonian age of 329 +/-9 Ma (Zartman and Naylor, 1984). Zartman and Naylor (1984) believe Ayer Granite has same age range as Newburyport Complex. It is quite possible, based on textural and mineralogical differences that the two facies should be separate units, representing different magmatic events (Wones and Goldsmith, 1991).

Bedford Gneiss (Ordovician?) at surface, covers < 0.1 % of this area

Biotite-quartz-plagioclase gneiss and interlayered amphibolite; in part with augen of andesine and microcline.

Tatnic Hill Formation (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Sulfidic sillimanite schist, sillimanite schist and gneiss, biotite gneiss; minor amphibolite, calc-silicate gneiss and marble.

Biotite granite gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Overprint signifies inequigranular texture.

Harrison/Ravenswood Gneiss (Ordovician?) at surface, covers < 0.1 % of this area

Harrison Gneiss - in New York and Connecticut, Brookfield diorite gneiss in Connecticut, and Ravenswood Gneiss in Brooklyn - biotite-hornblende-quartz-plagioclase gneiss with accessory garnet and sphene; plagioclase commonly occurs as augen.

Collinsville Formation (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Plagioclase gneiss and minor amphibolite.

Stockbridge Formation (Lower Cambrian) at surface, covers < 0.1 % of this area

Beige, tan, and dark-gray weathering quartzose dolomite marble containing interbeds of black, green and maroon phyllite and punky weathering blue quartz pebble quartzite.

Cheshire Quartzite (Lower Cambrian) at surface, covers < 0.1 % of this area

Mainly pure, white, glassy, tough quartzite.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers < 0.1 % of this area

Tan weathering, muscovite-microcline quartzite and feldspathic quartzite rich in black tourmaline, locally includes thin beds of other rock types listed below.

Plainfield Formation (Late Proterozoic? or older?) at surface, covers < 0.1 % of this area

Pale-tan to gray, fine-grained quartzite, and pale- to medium-gray quartz-mica schist. Quartzite is massive to layered (bedded?). Includes Plainfield Formation and some rock mapped formerly as Blackstone Series.

Quinebaug Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers < 0.1 % of this area

Amphibolite, biotite and hornblende gneiss, felsic gneiss, and calc-silicate gneiss.

Ordovician? granitic gneiss (Middle Ordovician?) at surface, covers < 0.1 % of this area

(Including local terms Ansonia, Mine Hill, "Tyler Lake," "Siscowit") - White, light-gray, buff, or pink, generally foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and biotite, and locally garnet or sillimanite. Commonly contains numerous inclusions or layers of mica schist and gneiss.

Stockbridge Formation (Lower Cambrian) at surface, covers < 0.1 % of this area

White to light-powdery-blue-gray dolostone with disseminated grains of quartz and prominent sprays of tremolite in higher-grade areas.

Tyringham Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Light pinkish-gray ferrohastingsite-biotite, quartz-rodded granodioritic to quartz monzonitic gneiss, coarsely porphyritic, locally having fine-grained aplitic border. Intrudes all Berkshire Proterozoic Y units.

Poughquag Quartzite (Cambrian) at surface, covers < 0.1 % of this area

(includes local Dalton Formation at base)-locally conglomeratic.

Rusty and gray biotite-quartz-feldspar gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Rusty facies contains variable amounts of garnet, sillimanite, cordierite, graphite, sulfides; minor marble and calcsilicate rock.

Gray, well-layered biotite-plagioclase-quartz gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Containing beds of amphibolite, aluminous schist, quartzite, and calc-silicate gneiss.

Middletown Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules.

Esmond Igneous Suite - granite gneiss (Late Proterozoic) at surface, covers < 0.1 % of this area

Pale-gray to pale-pink, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition ranges from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene and hornblende present in some rocks; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly associated with, and in part gradational into, augen gneiss. Includes rock mapped formerly in part as Ponaganset Granite Gneiss, Scituate Granite Gneiss, Ten Rod Granite Gneiss, and Hope Valley Alaskite Gneiss.

Littleton Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Hornblende norite (Devonian?) at surface, covers < 0.1 % of this area

Dark, coarse-grained, massive rock, composed of bytownite, hornblende, and hypersthene.

Cobble Mountain Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Thick-bedded (15 to 40 cm), nonrusty-weathering, silvery-gray, medium- to coarse-grained mica gneiss interlayered with nonrusty-weathering mica schist and minor amphibolite.

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers < 0.1 % of this area

Sulfidic mica schist and subordinate amphibolite.

Portland Formation (Lower Jurassic) at surface, covers < 0.1 % of this area

Reddish-brown to pale red arkose and siltstone, and gray sandstone, gray siltstone, and black shale interpreted as lake beds.

Walloomsac Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Orange-brown weathering, graphite-albite-biotite calcitic marble and schistose marble and interbedded black phyllite.

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers < 0.1 % of this area

Undifferentiated Hoosac Formation.

Walloomsac Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Dark-gray, graphitic quartz phyllite and schist containing minor lenses of limestone.

Paxton Formation (Silurian) at surface, covers < 0.1 % of this area

Diopside calc-silicate granofels. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers < 0.1 % of this area

Orangish-gray, gray, and light-greenish-gray muscovite-quartz schist and interlayered feldspathic quartzite and quartz conglomerate; minor beds of rusty albitic schist.

Plainfield Formation (Proterozoic Z) at surface, covers < 0.1 % of this area

Quartzite, pelitic schist, minor calc-silicate rock and amphibolite. Plainfield Formation extend into MA in Webster-Oxford area from eastern CT and western RI along west flank of Rhode Island batholith, along same strike as elongate lenses of Westboro Formation (as shown on State bedrock map of Zen and others, 1983). Lithology of Plainfield in New London area of southeastern CT (Goldsmith, 1966, 1976) and in eastern CT (Harwood and Goldsmith, 1971), and its structural and stratigraphic position indicate Plainfield and Westboro are equivalent. Plainfield is truncated along CT-RI border by Lake Char fault; however, in New London area, Plainfield is overlain by suite of largely mafic metavolcanic rocks (Waterford Group of Goldsmith, 1980), a relationship similar to that of the Westboro Formation and overlying metavolcanic rocks. Base of Plainfield is unknown, but gneiss and schist in center of Lyme dome may lie below it. Probably equivalent to quartzite and schist of Blackstone Group on basis of similar lithology and structural relations with Rhode Island batholith rocks (Goldsmith, 1991).

Yonkers Gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Biotite and/or hornblende-quartz-feldspar gneiss.

Washington Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Rusty-weathering, muscovite-biotite-sillimanite and/or kyanite-garnet schist; blue-quartz ribbed conglomerate, interlayered garnet-plagioclase-quartz metadacite.

Glastonbury Gneiss (Ordovician) at surface, covers < 0.1 % of this area

Massive granitic gneiss in core of Glastonbury dome and in adjacent areas.

Washington Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Coarse- to medium-grained hornblende-garnet amphibolite, hornblende-plagioclase gneiss and phlogopite-hornblende-plagioclase amphibolite (metabasalt).

Partridge Formation (includes Brimfield Schist of Emerson, 1917) (Middle Ordovician) at surface, covers < 0.1 % of this area

Mafic and felsic gneisses of volcanic derivation with calc-silicate granofels.

Cheshire Quartzite (Lower Cambrian) at surface, covers < 0.1 % of this area

White, massive vitreous quartzite.

Stockbridge Formation (Lower Ordovician) at surface, covers < 0.1 % of this area

White to blue-gray and white layered calcite marble.

Erving Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Granofels and schist where mapped separately.

Goshen Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Well-bedded micaceous quartzite or quartz schist grading upward into light- to dark-gray, carbonaceous aluminous schist in beds 5 to 15 cm thick.

Ammonoosuc Volcanics (Middle Ordovician) at surface, covers < 0.1 % of this area

Amphibolite, felsic gneiss, garnet-amphibole quartzite, and marble too thin to show separately at map scale. Gedrite, anthophyllite, cummingtonite locally abundant in amphibolite layers.

Porphyry (dacite or rhyolite) (Permian) at surface, covers < 0.1 % of this area

Light-colored, very fine grained, massive porphyry with phenocrysts of quartz, feldspar, and biotite; muscovite and accessory fluorite in groundmass.

West Rock Dolerite (uncertain) (Lower Jurassic) at surface, covers < 0.1 % of this area

Dark-gray to greenish-gray (weathers bright orange to brown), medium- to fine-grained, grading from basalt near contacts to fine-grained gabbro in the interior, generally massive with well-developed columnar jointing, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Goshen Formation (Lower Devonian) at surface, covers < 0.1 % of this area

Poorly bedded carbonaceous schist and quartz schist.

Tatnic Hill Formation - Yantic Member (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Gray mica schist.

Cobble Mountain Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Rusty-weathering mica schist.

Tatnic Hill Formation - Fly Pond Member (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area

Calc-silicate gneiss and marble.

Collinsville Formation (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Brown to rusty-brown schist containing coticule and locally massive amphibolite at base.

Syenite (Permian) at surface, covers < 0.1 % of this area

Medium-gray, massive syenite, composed of microcline, amphibole (arfvedsonite), and biotite with accessory apatite and sphene.

Well-layered hornblende-biotite gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Well-layered hornblende-biotite gneiss .

Scituate Granite Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Gneissic granite containing biotite in small clots. Equivalent to part of former Northbridge Granite Gneiss (usage now abandoned). Gradational with Zhg.

Holyoke Basalt (Lower Jurassic) at surface, covers < 0.1 % of this area

Thick, columnar quartz tholeiite containing local gabbroic segregations, thinks eastward; interpreted as one or more thick ponded lava flows. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Collinsville Formation (Middle Ordovician or older) at surface, covers < 0.1 % of this area

Amphibolite and minor plagioclase gneiss. Magnetite-hornblende granofels near top.

Biotite-hornblende diorite and quartz-bearing diorite (Devonian) at surface, covers < 0.1 % of this area

Mostly foliated; intrudes Dl.

Cobble Mountain Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Nonrusty-weathering coarse-grained aluminous schist.

Everett Formation (Lower Cambrian and Proterozoic Z) at surface, covers < 0.1 % of this area

Light-green and greenish-gray chlorite-muscovite-albite or chloritoid-rich phyllite. Predominantly dark-gray chloritoid-rich schist in Lenox Mountain.

Cobble Mountain Formation (Middle Ordovician) at surface, covers < 0.1 % of this area

Brown- or rusty-weathering thin-bedded feldspathic gneiss and mica schist.

Shuttle Meadow Formation (Lower Jurassic) at surface, covers < 0.1 % of this area

Reddish-brown to pale red arkosic sandstone and siltstone, and gray sandstone, gray mudstone, and black shale; interpreted as lake beds. The Shuttle Meadow Formation is assigned to Newark Supergroup and is extended into MA in the Hartford basin. It consists of sandstone strata containing one interval of gray mudstone beds. The unit grades eastward along strike into a conglomeratic facies. It overlies the New Haven Arkose or Hitchcock Volcanics and underlies the Holyoke Basalt (Robinson and Luttrell, 1985).

Lamprophyre (Devonian?) at surface, covers < 0.1 % of this area

Dark-gray to greenish, fine-grained, badly altered dike rock, composed of biotite, augite, K-feldspar, and accessory apatite and sphene, plus secondary minerals.

New Haven Arkose (Lower Jurassic) at surface, covers < 0.1 % of this area

Red, pink, and gray coarse-grained, locally conglomeratic arkose interbedded with brick-red shaley siltstone and fine-grained arkosic sandstone; boundary between Lower Jurassic (Jn) and Upper Triassic (TRn) parts is arbitrarily drawn through clastic rocks of similar lithology below gray mudstone containing Lower Jurassic palynofloral zone. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Biotite granitic gneiss (Devonian) at surface, covers < 0.1 % of this area

Biotite granitic gneiss.

Stockbridge Formation (Lower Cambrian) at surface, covers < 0.1 % of this area

Massive to finely laminated steel-gray calcitic dolomite marble containing a prominent zone of white quartz nodules near top.

East Berlin Formation (Lower Jurassic) at surface, covers < 0.1 % of this area

Reddish-brown to pale red arkosic sandstone and siltstone, and gray sandstone, gray mudstone, and black shale; interpreted as lake beds. Assigned to Newark Supergroup (Robinson and Luttrell, 1985). The East Berlin Formation of the Hartford basin contains eight facies: trough cross-bedded sandstones, horizontally stratified sandstones, interbedded sandstones and mudrocks, ripple cross-laminated siltstones, black shales, stratified mudrocks, disrupted shales, and disrupted mudstones. These facies are interpreted as a continental depositional system and are divided into two assemblages. Sandflat/alluvial plain facies assemblage (sandstones and siltstones) is composed of sheet-flood deposits. The lacustrine assemblage (shales and mudrocks) represents a saline lake-playa system (Gierlowski-Kordesch, and Rust, 1994).

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Fine- to medium-grained, well-layered and foliated amphibolite; epidote-rich layers locally abundant. Includes its typical Chester Amphibolite Member at Chester, Massachusetts.

Clough Quartzite (Upper Silurian) at surface, covers < 0.1 % of this area

Quartz-pebble conglomerate, quartzite, and minor mica schist and calc-silicate rocks. Fossils at Bernardston are similar to those at Croyden Mountain, New Hampshire which indicate late Llandoverian age. Parts of the Littleton and Partridge Formations, and Clough Quartzite in MA are here reassigned to the Rangeley Formation [here geographically extended to MA]. The four mapped areas of Clough Quartzite in the Amherst area west of the Connecticut Valley border fault are now interpreted as conglomerate lenses in the Rangeley. Clough is considered the key stratigraphic unit in Bronson Hill anticlinorium because 1) it is dominated by distinctive, readily recognizable rock types, 2) where present, it is base of Silurian-Devonian sequence, resting with detectable unconformity on older rocks, and 3) it contains late Llandoverian fossils at several localities in western NH and adjacent VT, and at Bernardston, MA. Consists mostly of quartz-pebble conglomerate in which pebbles are typically deformed; other lithologies are quartz grit or white to pink, well-bedded quartzite. Locally contains some mica schist beds. On the MA State bedrock geologic map (Zen and others, 1983), thickness is locally exaggerated because at many localities, the unit was only a few meters or less thick and could not be shown at a scale of 1:250,000. Maximum thickness is 200 m on west limb of Northfield syncline. Unconformably overlies Fourmile Gneiss in Pelham dome and in Kempfield anticline, or Ammonoosuc Volcanics over most gneiss domes. Partridge Formation occurs along Clough-Ammonoosuc contact as lenses in many areas (Hatch and others, 1988).

White to gray and black-spotted muscovite-biotite granite and granodiorite (Ordovician) at surface, covers < 0.1 % of this area

Intruded near or along thrust faults. Intrudes CAZh and Proterozoic Y gneisses.