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Geologic units in Hampden county, Massachusetts

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Mafic and felsic gneisses of volcanic derivation with calc-silicate granofels.

Lithology: mafic gneiss; felsic gneiss; granofels

Goshen Formation (Lower Devonian) at surface, covers 5 % of this area

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

Lithology: schist; quartzite

Tyringham Gneiss (Proterozoic Y) at surface, covers 3 % 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

Diabase dikes and sills (Lower Jurassic) at surface, covers < 0.1 % of this area

Diabase dikes and sills.

Lithology: diabase

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

Serpentinite and/or talc rock (Precambrian to Phanerozoic) at surface, covers < 0.1 % of this area

Serpentinite and/or talc rock - Interpreted as tectonic slivers.

Lithology: serpentinite

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.2 % of this area

Hoosac Formation - Undifferentiated Hoosac Formation.

Lithology: schist; phyllite; gneiss; amphibolite; quartzite; granofels; calc-silicate rock

Washington Gneiss (Proterozoic Y) at surface, covers 0.9 % 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

Belchertown Complex (Devonian) at surface, covers < 0.1 % of this area

Belchertown Complex (intrudes De) - Inclusion of hornfelsed dacite porphyry.

Lithology: dacite

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

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

East Berlin Formation - 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).

Lithology: sandstone; siltstone; mudstone; black shale

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

Cobble Mountain Formation - Serpentinite and/or talc rock, interpreted as large clasts.

Lithology: serpentinite

Diorite at Goff Ledges (Ordovician) at surface, covers 0.2 % of this area

Diorite at Goff Ledges - Diorite at Goff Ledges (Ogd) and similar diorite (Od) in the Windsor quadrangle. Very coarse-grained to pegmatitic, slightly foliated hornblende-plagioclase diorite, minor hornblende pyroxenite. Ogd intrudes CAZh and CAZhk. Od intrudes the Rowe Schist of the Rowe-Hawley zone.

Lithology: diorite; pyroxenite

Meladiorite and norite (Devonian) at surface, covers < 0.1 % of this area

Meladiorite and norite - Weakly to moderately foliated; intrudes Dl.

Lithology: diorite; norite

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.3 % of this area

Hoosac Formation - Aluminous rusty-weathering kyanite schist with distinctive quartz lenses and minor thin beds of calc-silicate rocks.

Lithology: schist; calc-silicate rock

New Haven Arkose (Upper Triassic) at surface, covers 21 % 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 0.2 % of this area

Hampden Basalt - Thin flows of quartz tholeiite, locally intimately associated with Granby Basaltic Tuff.

Lithology: tholeiite

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

Hawley Formation (Middle Ordovician) at surface, covers 0.7 % of this area

Hawley Formation - Black, fine-grained, splintery, rusty-weathering schist and thin dark quartzite; interlayered amphibolite commonly has plagioclase megacrysts. As used here the Hawley includes amphibolite, sulfidic rusty schists, abundant coticules, silvery schists, quartzites and quartz conglomerates, and quartz, feldspar, biotite granulites. The quartzites and quartz conglomerates occur at two positions in rocks here assigned to the Hawley. Those occurring near the top have been mapped previously as Russell Mountain Formation or as Shaw Mountain Formation. The Hawley overlies the Ordovician Barnard Gneiss and underlies Silurian and Devonian "calciferous schists" that include the westernmost Goshen Formation in MA and Northfield Formation in southern VT, the central Waits River Formation and the eastern Gile Mountain Formation. Authors believe that the Goshen, Northfield, and Waits River are facies equivalents, while the Gile Mountain is slightly younger. Map symbol indicates that Hawley is Ordovician and Silurian. 40Ar/3Ar hornblende release spectrum date of 433+/-3 Ma obtained by Spear and Harrison (1989) (Trzcienski and others, 1992).

Lithology: schist; quartzite; amphibolite

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

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

Lithology: amphibolite; gneiss; granofels

Waits River Formation (Lower Devonian) at surface, covers 0.3 % of this area

Waits River Formation - Interbedded medium- to dark-gray, moderately rusty weathering, highly contorted, unbedded schist and punky-weathering calcareous granofels or quartzose marble, and pods and stringers of vein quartz.

Lithology: schist; granofels; marble

Belchertown Complex (Devonian) at surface, covers 0.9 % of this area

Belchertown Complex (intrudes De) - Zone containing primary igneous orthopyroxene-augite quartz monzodiorite.

Lithology: quartz monzodiorite

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

Erving Formation - Mixed mica schist and amphibolite where mapped separately.

Lithology: mica schist; amphibolite

Portland Formation (Lower Jurassic) at surface, covers 13 % of this area

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

Lithology: arkose; siltstone; sandstone; black shale

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

Holyoke Basalt - 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).

Lithology: tholeiite; gabbro

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

Middletown Formation ( = 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.

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

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

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

Lithology: mafic gneiss

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

Moretown Formation - Green to dark-green greenstone or amphibolite.

Lithology: greenstone; amphibolite

Fourmile Gneiss (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 0.5 % of this area

Fourmile Gneiss - Layered to massive biotite-feldspar gneiss and amphibolite.

Lithology: biotite gneiss; amphibolite

Partridge Formation (Middle Ordovician) at surface, covers 6 % of this area

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sulfidic mica schist and subordinate amphibolite.

Lithology: mica schist; amphibolite

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Amphibolite where mapped separately.

Lithology: amphibolite

Belchertown Complex (Devonian) at surface, covers < 0.1 % of this area

Belchertown Complex (intrudes De) - Hornblendite.

Lithology: hornblendite

Moretown Formation (Middle Ordovician or older) at surface, covers 0.3 % of this area

Moretown Formation - Nubble garnet schist, pinstriped granofels, and fine-grained amphibolite in equal parts.

Lithology: schist; granofels; amphibolite

Middlefield Granite (Devonian) at surface, covers < 0.1 % of this area

Middlefield Granite - Moderately foliated, biotite-muscovite granite with microcline megacrysts. Intrudes OCAr and Om.

Lithology: granite

Clough Quartzite (Upper Silurian) at surface, covers 0.2 % of this area

Clough Quartzite - 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).

Lithology: conglomerate; quartzite; mica schist; calc-silicate rock

Goshen Formation (Lower Devonian) at surface, covers 2 % of this area

Goshen Formation - Light gray-brown micaceous quartzite and quartz-mica-garnet schist in beds as much as 6 m thick. Calc-silicate granofels and rare punky-weathering calcareous granofels.

Lithology: quartzite; schist; granofels

Belchertown Complex (Devonian) at surface, covers 1 % of this area

Belchertown Complex (intrudes De) - Outer zone of hornblende quartz monzodiorite gneiss.

Lithology: granitic gneiss

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 0.4 % of this area

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

Lithology: amphibolite

Biotite-hornblende diorite and quartz-bearing diorite (Devonian) at surface, covers 0.2 % of this area

Biotite-hornblende diorite and quartz-bearing diorite - Mostly foliated; intrudes Dl.

Lithology: diorite; quartz diorite

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

Southbridge Formation - 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.

Lithology: granofels; schist

Calc-silicate granofels and gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Calc-silicate granofels and gneiss - Including calcitic or dolomitic chondrodite-diopside marble, coarse hornblende-plagioclase-diopside and diopside rock, locally containing beds of lustrous muscovite-kyanite sillimanite-garnet schist.

Lithology: granofels; gneiss; marble; metamorphic rock; schist

Hatch Hill Formation (Upper Cambrian) at surface, covers < 0.1 % of this area

Hatch Hill Formation - Bluish-gray weathering black sulfidic slate and chert.

Lithology: slate; chert

Granby Basaltic Tuff (Lower Jurassic) at surface, covers 0.2 % of this area

Granby Basaltic Tuff - Friable, well-bedded dark tuff, commonly incorporating sediment fragments. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Lithology: tuff

Ammonoosuc Volcanics (Middle Ordovician) at surface, covers 2 % of this area

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

Lithology: amphibolite; felsic gneiss; quartzite; marble

Shuttle Meadow Formation (Lower Jurassic) at surface, covers 0.3 % of this area

Shuttle Meadow Formation - 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).

Lithology: sandstone; siltstone; mudstone; black shale

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

Belchertown Complex (Devonian) at surface, covers 0.7 % of this area

Belchertown Complex (intrudes De) - Intrusive breccia, mafic and ultramafic fragments in quartz diorite matrix.

Lithology: quartz diorite; breccia; mafic rock; ultramafic intrusive rock

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

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

Lithology: granofels; quartzite; schist

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 1.0 % of this area

Rowe Schist - Light-green to light-bluish-gray schist having thin granular quartz lenses and lamellae. Kyanite and staurolite typical at higher grades.

Lithology: schist

Silicified fault-breccia or strongly silicified metamorphic rocks (Lower Jurassic) at surface, covers < 0.1 % of this area

Silicified fault-breccia or strongly silicified metamorphic rocks.

Lithology: breccia; metamorphic rock

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

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

Russell Mountain Formation - Quartzite, calc-silicate granofels, and calc-silicate marble. Correlated with the fossiliferous Shaw Mountain Formation of eastern Vermont. In original definition of Russell Mountain Formation (Hatch and others (1970), calcareous granofels on Woronoco dome was included in unit. Subsequent mapping by Stanley and others (1982) indicates that rocks mapped on Woronoco dome are different from the rest of Russell Mountain Formation and are more logically included in overlying Lower Devonian sequence (mapped as an unnamed member of Goshen Formation in fig. 3). Other than this modification and mapping a few lenses of Russell Mountain Formation just north of Massachusetts Turnpike, original definition of Russell Mountain stands. Thickness does not exceed 35 m, but its correlative in CT, the basal member of The Straits Schist of Rodgers (1982, 1985), is locally at least twice as thick. Has not been shown to correlate with either Clough Quartzite or Fitch Formation to the east. Russell Mountain Formation is highly discontinuous except near Shelburne Falls dome. Locally overlies members A and D of Cobble Mountain Formation; overlain everywhere by Goshen Formation. Silurian age is based on correlation with discontinuous lenses of similar rocks at same stratigraphic position as Shaw Mountain Formation of VT. Recent field trips with J.B. Thompson, Jr. (Harvard University) and others has raised questions as to whether many of those rocks in southern VT are actually Shaw Mountain. Shaw Mountain Formation has been assigned a firm age of late Llandoverian to Gedinnian north of Albany, VT, based on HOWELLELA (Boucot and Thompson, 1963; Konig, 1961) (Hatch and others, 1988).

Lithology: quartzite; granofels; marble

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

Biotite granitic gneiss.

Lithology: granitic gneiss

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

Paxton Formation - Sulfidic magnesian biotite and magnesian cordierite schist and sillimanite quartzite. 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).

Lithology: schist; quartzite

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sulfidic mica schist and abundant amphibolite.

Lithology: mica schist; amphibolite

Monson Gneiss (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 6 % of this area

Monson Gneiss - Layered to massive biotite-plagioclase gneiss, amphibolite, microcline augen gneiss.

Lithology: biotite gneiss; amphibolite; augen gneiss

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Biotite gneiss.

Lithology: biotite gneiss

Brimfield Schist (Upper? and Middle Ordovician) at surface, covers < 0.1 % of this area

Brimfield Schist (includes Hamilton Resevoir Formation) - 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.

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

Coys Hill Porphyritic Granite Gneiss (Lower Devonian) at surface, covers 0.5 % of this area

Coys Hill Porphyritic Granite Gneiss - Coarse-grained porphyritic microcline granite gneiss, commonly containing garnet and sillimanite with or without muscovite; continuous with the Cardigan and Ashuelot plutons of Kinsman Quartz Monzonite in New Hampshire; appears to be an early quasi-concordant intrusion within Dl.

Lithology: granitic gneiss

Biotite-garnet-feldspar gneiss of Ragged Hill (Devonian) at surface, covers 0.2 % of this area

Biotite-garnet-feldspar gneiss of Ragged Hill - Although extremely narrow, shows local cross cutting relations with Ops, Sfs, and Dl.

Lithology: gneiss

Feldspar-quartz-muscovite pegmatite (Devonian) at surface, covers 0.1 % of this area

Feldspar-quartz-muscovite pegmatites - Partly associated with the Williamsburg Granodiorite.

Lithology: pegmatite

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

Cobble Mountain Formation (Middle Ordovician) at surface, covers 2 % of this area

Cobble Mountain Formation - Light-brown, fine- to medium-grained pelitic schist and granofels locally graded in beds less than 15 cm thick. Local amphibolite. Rare calc-silicate rock, feldspar gneiss, coticule and cummingtonite schist.

Lithology: pelitic schist; granofels; amphibolite; calc-silicate rock; felsic gneiss; schist

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

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

Cobble Mountain Formation - Rusty-weathering mica schist; thin (15 cm or less) beds of gneiss.

Lithology: mica schist; gneiss

Goshen Formation (Lower Devonian) at surface, covers 0.3 % of this area

Goshen Formation - Poorly bedded Dg schist containing beds, 0.5 to 2.5 m thick, of punky-weathering calcareous pale green granofels with calc-silicate granofels rims.

Lithology: schist; granofels

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

Cobble Mountain Formation - Nonrusty-weathering coarse-grained aluminous schist.

Lithology: schist

Glastonbury Gneiss (Ordovician) at surface, covers 3 % of this area

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

Lithology: granitic gneiss

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 0.4 % of this area

Rowe Schist - Gray to black, fine-grained , slightly rusty, moderately carbonaceous schist; minor fine- to medium-grained dark-gray to white quartzite. Minor lenses of rock identical to OCAr.

Lithology: schist; quartzite

Portland Formation (Lower Jurassic) at surface, covers 12 % of this area

Portland Formation - Reddish-brown to pale red conglomerate and arkose.

Lithology: conglomerate; arkose

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

Cobble Mountain Formation - Red rusty-weathering schist containing thin beds of white quartz-plagioclase granofels and vitreous quartzite. Local amphibolite and thin anthophyllite amphibolite.

Lithology: schist; granofels; quartzite; amphibolite

Hardwick Tonalite (Lower Devonian) at surface, covers < 0.1 % of this area

Hardwick Tonalite - Dark-gray, moderately to strongly foliated biotite tonalite to granodiorite gneiss; intrudes Dl.

Lithology: granitic gneiss

Well-layered hornblende-biotite gneiss (Proterozoic Y) at surface, covers 0.3 % of this area

Well-layered hornblende-biotite gneiss .

Lithology: mafic gneiss

Belchertown Complex (Devonian) at surface, covers 0.4 % of this area

Belchertown Complex (intrudes De) - Transition zone dominated by augite-hornblende quartz monzodiorite.

Lithology: quartz monzodiorite

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

Glastonbury Gneiss - 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.

Lithology: granitic gneiss; amphibolite

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

Cobble Mountain Formation - Nonrusty-weathering silvery-gray schist, similar to schist in Ocb.

Lithology: schist

New Haven Arkose (Lower Jurassic) at surface, covers 0.3 % 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. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Lithology: arkose; siltstone; mudstone; conglomerate

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

Moretown Formation - Light-greenish-gray to buff, fine-grained, pinstriped granofels and schist.

Lithology: granofels; schist

Paxton Formation (Silurian) at surface, covers 0.7 % of this area

Paxton Formation - 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).

Lithology: granofels; schist

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

Cobble Mountain Formation - 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.

Lithology: gneiss; mica schist; amphibolite

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

Hawley Formation - Interbedded amphibolite, greenstone, feldspathic schist and granofels. Coarse plagioclase in some amphibolite near top; local coarse hornblende blades or sprays. Sparse coticule (Emerson, 1917, p. 43). As used here the Hawley includes amphibolite, sulfidic rusty schists, abundant coticules, silvery schists, quartzites and quartz conglomerates, and quartz, feldspar, biotite granulites. The quartzites and quartz conglomerates occur at two positions in rocks here assigned to the Hawley. Those occurring near the top have been mapped previously as Russell Mountain Formation or as Shaw Mountain Formation. The Hawley overlies the Ordovician Barnard Gneiss and underlies Silurian and Devonian "calciferous schists" that include the westernmost Goshen Formation in MA and Northfield Formation in southern VT, the central Waits River Formation and the eastern Gile Mountain Formation. Authors believe that the Goshen, Northfield, and Waits River are facies equivalents, while the Gile Mountain is slightly younger. Map symbol indicates that Hawley is Ordovician and Silurian. 40Ar/3Ar hornblende release spectrum date of 433+/-3 Ma obtained by Spear and Harrison (1989) (Trzcienski and others, 1992).

Lithology: amphibolite; greenstone; schist; granofels

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

Monson Gneiss (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.

Lithology: gneiss; amphibolite

Erving Formation (Lower Devonian) at surface, covers 0.8 % of this area

Erving Formation - Granofels and schist where mapped separately.

Lithology: granulite; schist

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

Erving Formation - Amphibolite where mapped separately.

Lithology: amphibolite

Mylonite along Connecticut Valley border fault (Lower Jurassic) at surface, covers < 0.1 % of this area

Mylonite along Connecticut Valley border fault.

Lithology: mylonite

Goshen Formation (Lower Devonian) at surface, covers 0.3 % of this area

Goshen Formation - Poorly bedded carbonaceous schist and quartz schist.

Lithology: schist

Littleton Formation (Lower Devonian) at surface, covers 3 % of this area

Littleton Formation - Black to gray aluminous mica schist, quartzose schist, and aluminous phyllite.

Lithology: schist; phyllite

Goshen Formation (Lower Devonian) at surface, covers 0.2 % of this area

Goshen Formation - Similar to Dg but having scattered beds of calcareous granofels.

Lithology: quartzite; schist; granofels

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

Washington Gneiss (Proterozoic Y) at surface, covers 1.0 % 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 < 0.1 % 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.

Lithology: arkose; conglomerate; siltstone; sandstone

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