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Mineral Resources > Online Spatial Data > Geology > by state > Connecticut Geology

Geologic units in Litchfield county, Connecticut

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

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

Lithology: quartzite

Litchfield Norite (Middle? Ordovician) at surface, covers 0.2 % of this area

Litchfield Norite - 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.

Lithology: norite; pyroxenite

Hawley Formation (Middle Ordovician) at surface, covers 0.3 % of this area

Hawley Formation (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.

Lithology: schist; granofels

Walloomsac Schist (Middle Ordovician) at surface, covers 2 % of this area

Walloomsac Schist - 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.

Lithology: schist; phyllite

Nonewaug Granite (Devonian) at surface, covers 3 % of this area

Nonewaug Granite - 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.

Lithology: granite; pegmatite

Cobble Mountain Formation (Middle Ordovician) at surface, covers 0.5 % of this area

Cobble Mountain Formation - 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.

Lithology: schist; granofels; amphibolite

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

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

Lithology: arkose; siltstone; shale; conglomerate

Unit a [of Stockbridge Marble] (Lower Cambrian) at surface, covers 0.9 % of this area

Unit a [of Stockbridge Marble] - White to pale-gray, massive, smooth-weathering dolomite marble.

Lithology: marble

Amphibolite unit [in Ratlum Mountain Schist] (Lower? Ordovician) at surface, covers 0.2 % of this area

Amphibolite unit [in Ratlum Mountain Schist] - 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.

Lithology: amphibolite; mafic gneiss

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

Holyoke Basalt - 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.

Lithology: basalt; gabbro

Rowe Schist plus Amphibolite unit [in Rowe Schist] (Lower Ordovician or Cambrian or both) at surface, covers 0.3 % of this area

Rowe Schist plus Amphibolite unit [in Rowe Schist] - Rowe Schist - 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.

Lithology: amphibolite; mafic gneiss

Rowe Schist (Lower Ordovician or Cambrian or both) at surface, covers 6 % of this area

Rowe Schist - 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.

Lithology: schist; granofels; amphibolite; calc-silicate rock

Ratlum Mountain Schist (Lower? Ordovician) at surface, covers 17 % 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.

Lithology: schist; granofels; amphibolite; calc-silicate rock

Stockbridge Marble (Lower Ordovician and Cambrian) at surface, covers 3 % of this area

Stockbridge Marble (including Inwood Marble) - 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.

Lithology: marble; schist; phyllite; siltstone; sandstone

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

Basal member [of The Straits Schist] ( = 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.

Lithology: schist; amphibolite; marble; calc-silicate rock; quartzite

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

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

Lithology: quartzite; conglomerate

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

Walloomsac Formation - slate, phyllite, schist, metagraywacke.

Lithology: slate; phyllite; schist; metasedimentary rock

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

New Haven Arkose - 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).

Lithology: arkose; siltstone; mudstone

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

Hampden Basalt - 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.

Lithology: basalt; gabbro

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

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

Lithology: gneiss; amphibolite; felsic metavolcanic rock; mafic metavolcanic rock; granulite

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

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

Lithology: ultramafic intrusive rock; serpentinite

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

Stockbridge Formation - White to blue-gray and white layered calcite marble.

Lithology: marble

Rusty mica schist and gneiss (Proterozoic Y; may contain some older rocks) at surface, covers 5 % of this area

Rusty mica schist and gneiss (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.

Lithology: schist; gneiss; quartzite; amphibolite

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

Cheshire Quartzite - Mainly pure, white, glassy, tough quartzite.

Lithology: quartzite

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

Stockbridge Formation - calcitic and dolomitic marble.

Lithology: marble

Ordovician? granitic gneiss (Middle Ordovician?) at surface, covers 3 % of this area

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.

Lithology: granitic gneiss; schist

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

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

Lithology: granofels; calc-silicate rock

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

Basal member of upper slice of Canaan Mountain Schist - Gray, generally rusty- or tan-weathering gneiss, composed of quartz, plagioclase, microcline, biotite, and muscovite, interlayered with feldspathic quartzite.

Lithology: gneiss; quartzite

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

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

Lithology: amphibolite; mafic gneiss; meta-basalt

Basal member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 0.6 % of this area

Basal member [of Taine Mountain Formation] around Waterbury dome - Differs from rest of Taine Mountain Formation in being especially well layered and generally less micaceous and schistose.

Lithology: granofels

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

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

Lithology: dolostone (dolomite)

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

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

Lithology: mica schist; quartzite; conglomerate; schist

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

Tyringham Gneiss - 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.

Lithology: granitic gneiss

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

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

Lithology: phyllite; schist; limestone

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.

Lithology: orthogneiss; amphibolite; mica schist

Lower slice of Canaan Mountain Schist (Cambrian?) at surface, covers 0.8 % of this area

Lower slice of Canaan Mountain Schist - 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.

Lithology: schist; amphibolite

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

White to gray and black-spotted muscovite-biotite granite and granodiorite - Intruded near or along thrust faults. Intrudes CAZh and Proterozoic Y gneisses.

Lithology: granite; granodiorite

Pink granitic gneiss (Proterozoic Y) at surface, covers 5 % of this area

Pink granitic gneiss - 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.

Lithology: granitic gneiss

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

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

Lithology: mica schist; conglomerate; felsic metavolcanic rock

New Haven Arkose (Upper Triassic; possibly Lower Jurassic at top) at surface, covers 0.3 % 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.

Lithology: arkose; conglomerate; siltstone; sandstone

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

Everett Schist - 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.

Lithology: schist; phyllite

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

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

Lithology: granitic gneiss

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

Cheshire Quartzite - White, massive vitreous quartzite.

Lithology: quartzite

Unit c [of Stockbridge Marble] (Upper Cambrian) at surface, covers 3 % of this area

Unit c [of Stockbridge Marble] - Gray, generally massive dolomite marble, commonly contains quartz grains, locally beds of sandstone; may be calcitic near top.

Lithology: marble; sandstone

Southington Mountain Member [of The Straits Schist] (Devonian or Silurian or both) at surface, covers 0.5 % of this area

Southington Mountain Member [of The Straits Schist] - 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).

Lithology: schist; granofels

Hornblende gneiss member [of Collinsville Formation] (Middle Ordovician) at surface, covers 0.2 % of this area

Hornblende gneiss member [of Collinsville Formation] - 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.

Lithology: amphibolite; mafic gneiss; metamorphic rock

Units e and d [of Stockbridge Marble] (Lower Ordovician) at surface, covers 0.7 % of this area

Units e and d [of Stockbridge Marble] - White to gray massive calcite marble, commonly mottled with dolomite and locally interlayered with dolomite marble and calcareous siltstone and sandstone.

Lithology: marble; dolostone (dolomite); siltstone; sandstone

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

The Straits Schist ( = Goshen Formation of Massachusetts) - 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).

Lithology: schist

Bristol Gneiss (Middle? Ordovician) at surface, covers 0.6 % of this area

Bristol Gneiss - 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).

Lithology: gneiss; amphibolite

Hornblende gneiss and amphibolite (Proterozoic Y) at surface, covers 2 % of this area

Hornblende gneiss and amphibolite - 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.

Lithology: amphibolite; gneiss; felsic gneiss; calc-silicate rock; marble

Manhattan Schist (Cambrian?) at surface, covers 8 % of this area

Manhattan Schist (including Waramaug Formation and Canaan Mountain Schist) - 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).

Lithology: gneiss; amphibolite

Hoosac Schist (Cambrian?) at surface, covers 5 % of this area

Hoosac Schist - 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.

Lithology: schist; gneiss

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

Talcott Basalt - 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.

Lithology: basalt; gabbro; volcanic breccia (agglomerate)

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

Well-layered hornblende-biotite gneiss .

Lithology: mafic gneiss

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

Interlayered metasedimentary rock and granitic, charnockitic, mangeritic, or syenitic gneiss.

Lithology: metasedimentary rock; gneiss

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

Shuttle Meadow Formation - 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.

Lithology: shale; siltstone; sandstone; limestone. conglomerate

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

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

Lithology: marble; phyllite

Upper slice of Canaan Mountain Schist (Cambrian?) at surface, covers 2 % of this area

Upper slice of Canaan Mountain Schist - 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.

Lithology: schist; gneiss; amphibolite

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

Everett Schist - minor meta-graywacke lenses. Includes Greylock Schist in Massachusetts.

Lithology: schist; metasedimentary rock

Layered gneiss (Proterozoic Y) at surface, covers 10 % of this area

Layered gneiss - 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.

Lithology: gneiss; calc-silicate rock; amphibolite

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

Units g and f [of Stockbridge Marble] - White to gray massive calcite marble with layers and laminae of dolomitic marble.

Lithology: marble

Gneiss of Highlands massifs (Proterozoic Y; may contain some older rocks) at surface, covers 0.5 % of this area

Gneiss of Highlands massifs, (including Fordham Gneiss) - Mixture of rock types described below, where not separately mapped.

Lithology: gneiss; amphibolite; schist

Taine Mountain Member (Lower? Ordovician) at surface, covers 1 % of this area

Taine Mountain Member - 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).

Lithology: granofels

Unit b [of Stockbridge Marble] (Upper and Middle? Cambrian) at surface, covers 2 % of this area

Unit b [of Stockbridge Marble] - White, pink, cream, and light-gray, generally well bedded dolomitic marble interlayered with phyllite and schist and with siltstone, sandstone, or quartzite, commonly dolomitic.

Lithology: marble; phyllite; schist; siltstone; sandstone; quartzite

Ratlum Mountain Schist plus Amphibolite unit [in Ratlum Mountain Schist] (Lower Ordovician) at surface, covers 0.5 % of this area

Ratlum Mountain Schist plus Amphibolite unit [in Ratlum Mountain Schist] - 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.

Lithology: schist; granofels; amphibolite; mafic gneiss; calc-silicate rock

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

Gray, well-layered biotite-plagioclase-quartz gneiss - Containing beds of amphibolite, aluminous schist, quartzite, and calc-silicate gneiss.

Lithology: granitic gneiss; amphibolite; schist; quartzite; gneiss

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

East Berlin Formation - 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).

Lithology: siltstone; shale; sandstone; dolostone (dolomite); arkose; conglomerate

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

Sweetheart Mountain Member [of Collinsville Formation] - 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).

Lithology: schist; amphibolite; metamorphic rock

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

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

Lithology: marble; phyllite; quartzite

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

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

Lithology: marble

Dalton Formation (Lower Cambrian and perhaps partly older) at surface, covers 2 % of this area

Dalton Formation (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.

Lithology: gneiss; quartzite

Basal marble member [of Walloomsac Schist] (Middle Ordovician) at surface, covers 2 % of this area

Basal marble member [of Walloomsac Schist] - Dark-gray to white, massive to layered schistose or phyllitic calcite-phlogopite marble.

Lithology: marble

Amphibolite-bearing unit of Manhattan Schist (Cambrian?) at surface, covers 3 % of this area

Amphibolite-bearing unit of Manhattan Schist - Like Manhattan Schist but with numerous lenses and layers of amphibolite.

Lithology: gneiss; amphibolite

Waterbury Gneiss (Proterozoic Z or Cambrian or both) at surface, covers 0.3 % of this area

Waterbury Gneiss - 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.

Lithology: schist; gneiss

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

Augen gneiss (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.

Lithology: augen gneiss