Geologic units in Hartford county, Connecticut

Portland Arkose (Lower Jurassic) at surface, covers 40 % 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 19 % 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.

East Berlin Formation (Lower Jurassic) at surface, covers 5 % 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).

Holyoke Basalt (Lower Jurassic) at surface, covers 5 % 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.

Glastonbury Gneiss (Middle? Ordovician) at surface, covers 4 % 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.

The Straits Schist (Goshen Formation of Massachusetts) (Devonian or Silurian or both) at surface, covers 4 % 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).

Hornblende gneiss member [of Collinsville Formation] (Middle Ordovician) at surface, covers 2 % 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.

Bristol Gneiss (Middle? Ordovician) at surface, covers 2 % 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).

Cobble Mountain Formation (Middle Ordovician) at surface, covers 2 % 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.

Hebron Gneiss (Silurian and Ordovician) at surface, covers 2 % 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.

Hampden Basalt (Lower Jurassic) at surface, covers 1 % 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.

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.

Shuttle Meadow Formation (Lower Jurassic) at surface, covers 1 % 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.

Brimfield Schist (Includes Hamilton Resevoir Formation) (Upper? and Middle Ordovician) at surface, covers 1.0 % 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.

Ratlum Mountain Schist (Lower? Ordovician) at surface, covers 0.9 % 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.

Sweetheart Mountain Member [of Collinsville Formation] (Middle Ordovician) at surface, covers 0.7 % 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).

Layered gneiss (Proterozoic Y) at surface, covers 0.7 % 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.

Hawley Formation (Middle Ordovician) at surface, covers 0.6 % 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.

Littleton Formation (Devonian) at surface, covers 0.6 % 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.

Talcott Basalt (Lower Jurassic) at surface, covers 0.6 % 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.

Buttress Dolerite (Middle? Jurassic) at surface, covers 0.6 % 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.

Whigville Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 0.6 % 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).

Rowe Schist (Lower Ordovician or Cambrian or both) at surface, covers 0.5 % 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.

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

Monson Gneiss (Middle or Lower Ordovician?) at surface, covers 0.5 % 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.

Scranton Mountain Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 0.4 % 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).

Nonewaug Granite (Devonian) at surface, covers 0.4 % 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.

Wildcat Member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 0.4 % 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).

Collins Hill Formation (Upper? and Middle Ordovician) at surface, covers 0.3 % 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.

Rusty mica schist and gneiss (Proterozoic Y; may contain some older rocks) at surface, covers 0.2 % 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.

Pumpkin Ground Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers 0.1 % 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).

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.

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).

Hornblende gneiss and amphibolite (Proterozoic Y) at surface, covers < 0.1 % 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.

Trap Falls Formation (Middle or Lower Ordovician) at surface, covers < 0.1 % 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.

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.

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.

Beardsley Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers < 0.1 % 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).

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.

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

Plagioclase gneiss and minor amphibolite.

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.

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.

Harrison Gneiss (including Prospect 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. 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).

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

Reddish-brown to pale red conglomerate and arkose.

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.

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.

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

Undifferentiated Hoosac Formation.

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.

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.

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.

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.

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.

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

Poorly bedded carbonaceous schist and quartz schist.

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

Rusty-weathering mica schist.

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.

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).

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.

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.

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

Nonrusty-weathering coarse-grained aluminous schist.

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).

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).

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.

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

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

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

Well-layered hornblende-biotite gneiss .