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Geologic units in Massachusetts (state in United States)

Additional scientific data in this geographic area

Rhode Island Formation (Upper and Middle Pennsylvanian) at surface, covers 8 % of this area

Rhode Island Formation - Sandstone, graywacke, shale and conglomerate; minor beds of meta-anthracite. Fossil plants. Rhode Island Formation is thickest and most extensive formation in Narragansett basin. Does not extend to Norfolk basin. Consists of gray sandstone and siltstone and lesser amounts of gray to black shale, gray conglomerate, and coal beds 10 m thick. Interfingers with Wamsutta Formation in Narragansett basin. In places overlies Dedham Granite. Age is Middle and Late Pennsylvanian (Goldsmith, 1991).

Paxton Formation (Silurian) at surface, covers 5 % 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

Littleton Formation (Lower Devonian) at surface, covers 4 % of this area

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

Lithology: schist; phyllite

New Haven Arkose (Upper Triassic) at surface, covers 4 % 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

Partridge Formation (Middle Ordovician) at surface, covers 3 % of this area

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

Waits River Formation (Lower Devonian) at surface, covers 2 % 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

Berwick Formation (Silurian) at surface, covers 2 % of this area

Berwick Formation - Thin- to thick-bedded metamorphosed calcareous sandstone, siltstone, and minor muscovite schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Scituate Granite Gneiss (Proterozoic Z) at surface, covers 2 % of this area

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

Lithology: granite

Dedham Granite (Proterozoic Z) at surface, covers 2 % of this area

Dedham Granite - Light grayish-pink to greenish-gray, equigranular to slightly porphyritic, variably altered, granite south and west of Boston. Includes dioritic rock near Scituate and Cohasset and Barefoot Hills Quartz Monzonite of Lyons (1969) and Lyons and Wolfe (1971). Intrudes Zdi, Zgb, Zb, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).

Granite of the Fall River pluton (Proterozoic Z) at surface, covers 2 % of this area

Granite of the Fall River pluton - Light-gray, medium-grained, biotite granite, in part mafic-poor. Gneissic in New Bedford area. Includes Bulgarmarsh Granite (Proterozoic Z). Intrudes Zgs. Fall River pluton of the MA State map (Zen and others, 1983) is here referred to as the Fall River Granite. According to the authors, the change in name is meant to indicate the heterogeneous nature of the granite and the fact that it may consist of more than a single pluton. Included in this unit is the Bulgarmarsh Granite of Fall River and a mass of alaskitic gneiss shown on the State map south of Fall River. No type section is designated. Geologic map shows the Fall River present in the New Bedford area of MA and RI and bounded on the west by the Narragansett Bay Group. The Fall River was dated by Zartman and Naylor (1984) at about 600 Ma (U-Th-Pb zircon) (Murray and others, 1990).

Lithology: granite

Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 2 % of this area

Nashoba Formation - Sillimanite schist and gneiss, partly sulfidic, amphibolite, biotite gneiss, calc-silicate gneiss and marble. Nashoba Formation occurs in Nashoba zone of eastern MA. Consists of interlayered sillimanite-bearing, partly sulfidic schist and gneiss, calc-silicate gneiss, and subordinate quartzite and marble. Protoliths were probably volcanogenic sediments interlayered with limy marine sediments. Bell and Alvord (1976) divided Nashoba into 10 members on basis of lithology. Amphibolite is most abundant near presumed base, namely in Boxford Member. Skehan and Abu-Moustafa (1976) divided Nashoba into 30 members based on section in Wachusett-Marlborough tunnel. Although Bell and Alvord's and Skehan and Moustafa's sections contain similar lithologies, Bell and Alvord's is much thicker, and Boxford Member is not readily identified in Skehan and Abu-Moustafa's. Subdivision of Nashoba is conjectural south of Marlborough and Shrewsbury. On MA State bedrock map (Zen and others, 1983) only Boxford Member is separated out from the rest of the Nashoba because this unit was the only member clearly recognized in several area. A definite sequence of members probably does not exist anywhere in the Nashoba because of lenticularity of assemblages and repeated rock types, both of which could be accounted for by either sedimentary or tectonic processes. Although Castle (1965) considered Fish Brook to be either a premetamorphic intrusive rock or a core gneiss of intrusive or sedimentary ancestry, Bell and Alvord (1976) considered it to be volcanic or volcaniclastic in origin. Zircons in Fish Brook are certainly volcanic in origin and yield a date of 730 +/-26 Ma (Olszewski, 1980). If the rock were a core gneiss, that date would apply only to the Fish Brook and not to surrounding rocks; but, Bell and Alvord (1976) believe Fish Brook to be part of the Marlboro Formation-Nashoba Formation sequence and therefore the date does apply to the sequence. In addition, a 1500 Ma date for Shawsheen Gneiss [reference not given] helps bracket age of Marlboro-Nashoba sequence. An upper limit for the sequence was established from the 430 +/-5 Ma age of intruding Sharpers Pond Diorite and 450 +/-23 Ma age of the intruding Andover Granite (Zartman and Naylor, 1984). Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [no formal age change made in this report] (Goldsmith, 1991).

Hardwick Tonalite (Lower Devonian) at surface, covers 2 % of this area

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

Lithology: granitic gneiss

Andover Granite (Silurian or Ordovician) at surface, covers 2 % of this area

Andover Granite - Light- to medium-gray, foliated, medium-to coarse-grained -muscovite-biotite granite; pegmatite masses common. Includes Acton Granite (Silurian or Ordovician). Intrudes OZn.

Lithology: granite; pegmatite

Stockbridge Formation (Lower Cambrian) at surface, covers 2 % 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

Portland Formation (Lower Jurassic) at surface, covers 2 % of this area

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

Lithology: conglomerate; arkose

Portland Formation (Lower Jurassic) at surface, covers 2 % 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.

Diorite and gabbro (Proterozoic Z) at surface, covers 2 % of this area

Diorite and gabbro - Complex of diorite and gabbro, subordinate metavolcanic rocks and intrusive granite and granodiorite.

Stockbridge Formation (Lower Ordovician) at surface, covers 1 % of this area

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

Lithology: marble

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks (Proterozoic Z) at surface, covers 1 % of this area

Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks - Includes some diorite and gabbro north and northwest of Boston.

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

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

Lithology: phyllite; schist; limestone

Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers 1 % of this area

Cape Ann Complex - Alkalic granite to quartz syenite containing ferro-hornblende. Intrudes Zdigb. Most of Cape Ann Complex forms Cape Ann peninsula of northeastern MA. Consists of alkalic granite to quartz syenite (its main phase), Beverly Syenite, and Squam Granite, all of which form a pluton covering 385 sq km. Intrudes Late Proterozoic greenschist, diorite, and gabbro that earlier workers assigned to Marlboro Formation, Salem Gabbro-Diorite, or Middlesex Fells Volcanic Complex. Dennen (1975) considered masses of diorite and gabbro within and adjacent to Cape Ann Complex to be cogenetic with it and equivalent in age to Nahant Gabbro and gabbro at Salem Neck. Authors recommend that usage of term Salem Gabbro-Diorite be restricted to these masses of diorite and gabbro in and around Cape Ann pluton that are younger than Dedham Granite and cogenetic with Cape Ann Complex. Gabbro at Salem Neck is probably representative. Radiometric ages straddle Late Ordovician-Early Silurian boundary. Age is based on Rb-Sr whole rock isochron of 426 +/-6 Ma (Zartman and Marvin, 1971) and U-Pb zircon date of 450 +/-25 Ma (Zartman, 1977) (Wones and Goldsmith, 1991).

Oakdale Formation (Silurian) at surface, covers 1 % of this area

Oakdale Formation - Metamorphosed thin-bedded, pelitic and calcareous siltstone and muscovite schist, probably low-grade equivalent of Paxton Formation. The Oakdale Formation is here revised to include strata previously mapped in CT and adjacent MA as the Hebron Formation and the Scotland Schist. The Scotland Member (Pease, 1980) is renamed the Scotland Schist Member of the Oakdale. The Oakdale is a homogeneous, calcareous metasiltstone at the base of a thick stratigraphic sequence in a geosyncline terrane and extends from NH to the Honey Hill fault in eastern CT. In central eastern CT it underlies the Hebron Formation; in northeast CT and adjacent MA it underlies conformably the Dudley Formation of the Paxton Group; in central MA it underlies the Paxton Group undivided. The lower part of the Oakdale is cut out along the Clinton-Newbury fault zone. Thickness in type area is about 1500 m. Correlative with the Gove Member of the Berwick Formation in NH and the Gonic Formation in ME. Age is Late Proterozoic(?) based on intrusion of 440 Ma Hedgehog Hill gneiss in the upper part of the Brimfield Group at the top of the stratigraphic sequence, and an age of 1188 Ma for detrital zircons from the Paxton in north-central MA (Pease, 1989).

Gray, well-layered biotite-plagioclase-quartz gneiss (Proterozoic Y) at surface, covers 1 % of this area

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

Washington Gneiss (Proterozoic Y) at surface, covers 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.

Stockbridge Formation (Lower Cambrian) at surface, covers 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

Ponaganset Gneiss (Proterozoic Z) at surface, covers 1.0 % of this area

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

Lithology: granite

Porphyritic granite (Proterozoic Z) at surface, covers 1.0 % of this area

Porphyritic granite - Gray to gray-green, seriate to porphyritic biotite granite containing clots and streaks of biotite, epidote, and sphene. Mafic inclusions common. Gneissic in New Bedford area. Intrudes Zgs.

Lithology: granite

Tyringham Gneiss (Proterozoic Y) at surface, covers 1.0 % 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

Paxton Formation (Silurian) at surface, covers 1.0 % of this area

Paxton Formation - Sulfidic mica schist where mapped separately. 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: mica schist

Biotite granite (Proterozoic Z) at surface, covers 0.9 % of this area

Biotite granite - Light-gray to grayish-pink, biotite granite, locally foliated. Mafic minerals less prominent than in Milford Granite but granular quartz common. Includes mafic-poor granite similar to Zhg. Intrudes Zdi, Agb, and Zv.

Lithology: granite

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

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

Moretown Formation (Middle Ordovician or older) at surface, covers 0.9 % of this area

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

Lithology: granofels; schist

Cretaceous sediments (Cretaceous) at surface, covers 0.9 % of this area

Cretaceous sediments - Clay, silt, sand, and gravel, mostly of non-marine and nearshore marine origin, Campanian and older.

Lithology: clay or mud; silt; sand; gravel

Alaskite (Proterozoic Z) at surface, covers 0.9 % of this area

Alaskite - Light-gray, pinkish-gray to tan, mafic-poor granite commonly containing muscovite.

Diorite (Proterozoic Z) at surface, covers 0.8 % of this area

Diorite - Medium-grained hornblende diorite metamorphosed in part to amphibolite and hornblende gneiss.

Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 0.8 % of this area

Nashoba Formation - Boxford Member - Thin bedded to massive amphibolite, minor biotite gneiss. Of the 10 members of the Nashoba Formation defined by Bell and Alvord (1976), only amphibolitic Boxford Member, at the presumed base of Nashoba is separated out on MA State bedrock map of Zen and others (1983) because it is the only member clearly identified in several locations. Although age on MA State bedrock map is shown as Proterozoic Z or Ordovician (due to uncertainty regarding actual rocks sampled by Olszewski and a strong belief that rocks of Nashoba zone correlated with Ordovician rocks to the west), author now feels that rocks of Nashoba zone (except for Tadmuck Brook Schist) are all Proterozoic, but that they are unlike the Proterozoic rocks of neighboring Milford-Dedham zone. [No formal age change is made in this report.] (Goldsmith, 1991).

Dry Hill Gneiss (Proterozoic Z) at surface, covers 0.8 % of this area

Dry Hill Gneiss - Pink microcline-biotite and microcline-hornblende gneiss containing pink microcline megacrysts and minor quartzite.

Wamsutta Formation (Middle and Lower Pennsylvanian) at surface, covers 0.7 % of this area

Wamsutta Formation - Red to pink, well-sorted conglomerate, graywacke, sandstone, and shale; fossil plants. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).

Quincy Granite (Lower Silurian or Upper Ordovician) at surface, covers 0.6 % of this area

Quincy Granite - Alkalic granite containing riebeckite and aegirine. Intrudes CAbw and PZZc (?). Quincy Granite intrudes Middle Cambrian Braintree Argillite. Southern contact is with chemically and mineralogically similar Blue Hills Granite Porphyry. Fresh rock is dark gray to dark green, weathering to buff brown or salmon. Bounded on the northwest by Blue Hills thrust and on the west by Neponset fault. No clasts of Quincy observed in Early Pennsylvanian Pondville Conglomerate, but clasts of overlying and probably related Blue Hills Granite Porphyry are common in Pondville. Combined area of Quincy and Blue Hills rocks is 55 sq km. Small mass of "Quincy Granite" east of Woonsocket, RI, near RI-MA border is similar in texture to Quincy Granite at Quincy, MA, but is more peralkaline. The mass near Woonsocket was thought at the time of compilation of the MA State geologic map (Zen and others, 1983) to be same general age as granite at Quincy. Age of mass at Woonsocket has been more recently determined to be Devonian or possibly Carboniferous, rather than Late Ordovician and Silurian (Hermes and Zartman (1985) (Wones and Goldsmith, 1991).

Ayer Granite (Lower Silurian) at surface, covers 0.6 % of this area

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

Lithology: granite; tonalite

Ammonoosuc Volcanics (Middle Ordovician) at surface, covers 0.6 % 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.

Mattapan Volcanic Complex (Proterozoic Z or younger) at surface, covers 0.5 % of this area

Mattapan Volcanic Complex - Rhyolite, melaphyre, agglomerate, and tuff. Mattapan Volcanic Complex is found in west and southwest part of Boston basin and beyond, and to the south in Blue Hills. Similar in lithology to Lynn Volcanic Complex. Both units consist largely of partly porphyritic rhyolite and rhyodacite flows, welded ash-flow tuffs, vitric tuff, lapilli tuff, lithic tuff, flow breccias, breccia pipes, and extrusion domes. Mattapan's rhyolite and rhyodacites are thinner and less varied in composition and texture than Lynn's, and volcanic breccias are absent in Lynn. Both units are reported to lie nonconformably on Dedham Granite and unnamed plutonic-volcanic complex of eastern MA; however, Mattapan has been observed as dikes and stocks cutting Dedham Granite (Billings, 1976; Kaye and Zartman, 1980; Chute, 1966), and other workers have pointed out evidence that some of Mattapan may be penecontemporaneous with younger phases of Dedham batholith. Westwood Granite may be intrusive equivalent of Mattapan. Mattapan is conformably and fairly continuously overlain by Roxbury Conglomerate of Boston Bay Group. LaForge (19832) cautioned against identifying rocks interbedded within Mattapan that look like Roxbury, but are still part of Mattapan. Metavolcanic rocks in Blue Hills resembling Mattapan are assigned to Mattapan on State bedrock map (Zen and others, 1983); for years they were considered to be either Silurian and Devonian, or Carboniferous; however, they are chemically and mineralogically distinct from Ordovician and Silurian Blue Hills [sic] Granite Porphyry. Author follows usage of Chute (1966) who could find no difference between volcanic rocks in Blue Hills area and Mattapan rocks, and thus these rocks are assigned to Mattapan. Proterozoic Z age is based on U-Th-Pb zircon date of 602 +/-3 Ma (Zartman, in Kaye and Zartman, 1980). Although Billings (1979) questions reliability of zircon ages from volcanic rocks, discovery of Proterozoic Z acritarchs in overlying Cambridge Argillite indicates zircon age is appropriate (Goldsmith, 1991).

Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite (Silurian) at surface, covers 0.5 % of this area

Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite - Locally porphyritic. Intrudes Ssqd.

Lithology: granite; granodiorite

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.5 % 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

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.

Roxbury Conglomerate (Proterozoic Z to earliest Paleozoic) at surface, covers 0.5 % of this area

Roxbury Conglomerate - Conglomerate, sandstone, siltstone, argillite, and melaphyre. Consists of Brookline, Dorchester, and Squantum Members. Roxbury Conglomerate forms base of Boston Bay Group. Divided into Brookline, Dorchester, and Squantum Members. Conglomerate in Brookline Member contains clasts of Dedham Granite, quartzite (possibly from Westboro Formation), and volcanic rock from underlying Mattapan Volcanic Complex. Dorchester Member consists of interbedded argillite and sandstone and forms an intermediate unit between Brookline Member and overlying Cambridge Argillite. Uppermost Squantum Member is a distinctive diamictite which appears to pinch out in northern part of basin. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).

Granite, gneiss, and schist, undivided (Proterozoic Z) at surface, covers 0.5 % of this area

Granite, gneiss, and schist, undivided - Plutonic and metamorphic rocks of probable Proterozoic Z age. May include plutonic and volcanic rocks of Paleozoic or younger age.

Lithology: granite; gneiss; schist

Eliot Formation (Silurian) at surface, covers 0.4 % of this area

Eliot Formation - Phyllite and calcareous phyllite. In New Hampshire: Used as Eliot Formation of Merrimack Group. Consists of gray to green phyllite, calcareous quartzite, quartz-mica schist, and well-bedded calc-silicate. Includes Calef Member, mapped separately. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick Formation (of Merrimack Group) by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Lithology: phyllite

Hawley Formation (Middle Ordovician) at surface, covers 0.4 % 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).

Marlboro Formation (Ordovician, Cambrian, or Proterozoic Z) at surface, covers 0.4 % of this area

Marlboro Formation - Thinly layered amphibolite, biotite schist and gneiss, minor calc-silicate granofels and felsic granofels.

Sharpners Pond Diorite (Silurian) at surface, covers 0.4 % of this area

Sharpners Pond Diorite - Non-foliated, medium-grained equigranular biotite-hornblende tonalite and diorite. Intrudes Soagr, OZn, OZf.

Lithology: tonalite; diorite

Nassau Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.4 % of this area

Nassau Formation - Lustrous, soft green, yellowish-green and purple laminated chloritoid-chlorite phyllite (Mettawee Member).

Lithology: phyllite

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

Goshen Formation - Dg containing beds of punky-weathering calcareous granofels more than 15 cm thick near the contact with the Waits River Formation.

Lithology: schist; quartzite; granofels

Gile Mountain Formation (Lower Devonian) at surface, covers 0.4 % of this area

Gile Mountain Formation - Gray, slightly rusty, poorly bedded phyllite and schist containing 20 cm to 2 m beds of light-gray, fine-grained quartzite, local punky-brown weathering calcareous granofels or quartzose marble, and pods and stringers of vein quartz.

Stockbridge Formation (Lower Cambrian) at surface, covers 0.4 % 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.

Pauchaug Gneiss (Ordovician) at surface, covers 0.4 % of this area

Glastonbury Gneiss - Massive granitic gneiss in cores of Warwick and Vernon domes.

Lithology: granitic gneiss

Gneiss and schist near New Bedford (Proterozoic Z) at surface, covers 0.4 % of this area

Gneiss and schist near New Bedford - Hornblende and biotite schist and gneiss, amphibolite.

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

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

Lithology: granitic gneiss

Westwood Granite (Proterozoic Z) at surface, covers 0.4 % of this area

Westwood Granite - Light-gray to pinkish-gray, fine- to medium-grained granite. Intrudes Zdgr. Proterozoic Z Westwood Granite forms small lenses of light-colored granite intruding Proterozoic Z Dedham Granite and older rocks. Occurs within Dedham batholith. Some rocks resembling Westwood, such as those exposed in Plymouth quarries near Weymouth, have been mapped as part of Dedham Granite on MA State Geologic Map (Zen and others, 1983). Extensive intrusion breccias occur at contacts of Westwood with older mafic rocks. Contacts with Dedham are commonly abrupt; dikes of Westwood cut Dedham and rare inclusions of Dedham are found within Westwood (Chute, 1966). No reports of cobbles of Westwood within Roxbury Conglomerate; therefore, it is possible that Westwood is intrusive equivalent of Mattapan Complex that underlies Roxbury and that Westwood was not exposed to erosion at time of deposition of Roxbury. Also may be an intrusive equivalent of Lynn Volcanic Complex. A somewhat questionable Rb/Sr whole-rock age of 579 +/-28 Ma (Fairbairn and others, 1967) obtained from Westwood indicates it may be younger than Dedham and may be about same age as Mattapan (Wones and Goldsmith, 1991).

Lithology: granite

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

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

Glastonbury Gneiss (Ordovician) at surface, covers 0.4 % of this area

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

Lithology: granitic gneiss

Blackstone Group (Proterozoic Z) at surface, covers 0.4 % of this area

Blackstone Group - Undivided - Quartzite, schist, phyllite, marble, and metavolcanic rocks.

Milford granite (Proterozoic Z) at surface, covers 0.4 % of this area

Milford Granite - Light-gray to pale orange-pink biotite granite; biotite tends to be in clots or short streaks, quartz granular; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Lithology: granite

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.4 % of this area

Hoosac Formation - Undifferentiated Hoosac Formation.

Newburyport Complex (Silurian or Ordovician) at surface, covers 0.3 % of this area

Newburyport Complex - Gray, medium-grained tonalite and granodiorite. Newburyport Complex was divided into two facies, tonalitic granodiorite and granite, by Shride (1971). Tonalitic facies was originally termed Newburyport Quartz Diorite and included dioritic rocks north of Clinton-Newbury fault zone that are now called Sharpners Pond Diorite in Nashoba zone, and Topsfield Granodiorite in Milford-Dedham zone. These correlations are no longer tenable due to differences in age and composition. Therefore, Newburyport Complex is restricted to the two facies present in Newburyport area. Rocks formerly mapped as Newburyport Quartz Diorite and Salem Gabbro-Diorite, except for gabbros at Salem Neck, MA, are included in undifferentiated diorite and gabbro unit (Zdigb) on MA State Geologic Map (Zen and others, 1983), largely because they could not be mapped separately at 1:250,000 scale. Unit Zdigb also includes mafic dikes and sills that are probably younger or contemporaneous. Most of the dioritic rocks northeast of Boston previously assigned to Newburyport Quartz Diorite are now assigned to an undifferentiated diorite unit (Zdi) on MA State Geologic Map. Newburyport Complex forms a large mass near Newburyport and a small one to its west, both truncated by Clinton-Newbury fault. Tonalite and granodiorite facies occupies core of Newburyport Complex at Newburyport and is intruded to the north by granite facies; described as medium to dark gray in fresh rock, weathering to both green and red, fine to medium grained, and highly variable in mineralogy. A U-Pb zircon age of 455 +/-15 Ma was determined by Zartman and Naylor (1984) for the tonalite. Granite facies intrudes both the Kittery Formation and the tonalite and granodiorite facies and covers an area of about 45 sq km. Described as light gray to dark gray, buff weathering, and porphyritic. No radiometric ages available for granite facies, but it is conceivable that the two facies are different in age. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Wones and Goldsmith, 1991).

Lithology: tonalite; granodiorite

Cheshire Quartzite (Lower Cambrian) at surface, covers 0.3 % of this area

Cheshire Quartzite - White, massive vitreous quartzite.

Lithology: quartzite

Dighton Conglomerate (Upper Pennsylvanian) at surface, covers 0.3 % of this area

Dighton Conglomerate - Coarse conglomerate having sandy matrix; minor sandstone. Dighton Conglomerate occurs in Narragansett basin. Consists of gray conglomerate composed mainly of rounded quartzite cobbles to boulders containing subordinate rounded granite cobbles and slate pebbles; very little sand matrix; lenses of medium-grained sandstone form less than 20 percent of unit. Age is Pennsylvanian (Goldsmith, 1991).

Lithology: conglomerate; sandstone

Tadmuck Brook Schist (Silurian?, Ordovician, or Proterozoic Z) at surface, covers 0.3 % of this area

Tadmuck Brook Schist - Andalusite phyllite and sillimanite schist, partly sulfidic; local quartzite in upper part.

Lithology: phyllite; schist; quartzite

Washington Gneiss (Proterozoic Y) at surface, covers 0.3 % of this area

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

Worcester Formation (Lower Devonian and Silurian) at surface, covers 0.3 % of this area

Worcester Formation - Carbonaceous slate and phyllite and minor metagraywacke.

Cambridge Argillite (Proterozoic Z to earliest Paleozoic) at surface, covers 0.3 % of this area

Cambridge Argillite - Gray argillite and minor quartzite; rare sandstone and conglomerate. Contains acritarchs. Cambridge Argillite of Boston Bay Group contains sandy horizons which are in some places quartzite. Most prominent are Milton quartzite unit of Billings (1976), and Tufts Quartzite Member (described by Billings, 1929, and LaForge, 1932) in northern part of basin. Red sandstone and sandy argillite in Chelsea, Revere, and Milton-Quincy areas intertongue with green argillite (Kaye, 1980). Red beds lie above cleaner quartzites such as Tufts and Milton units. Core analysis by D.A. Ashenden (Metropolitan District Commission, 1980, written commun.) indicates that Cambridge and Braintree Argillites are identical. Age of Boston Bay rocks has been controversial and was once thought to be Cambrian to Pennsylvanian (the latter based on lithologic similarity to rocks of Narragansett basin and now discredited plant fossils). Age of Cambridge and of Boston Bay Group as a whole is Proterozoic Z and possibly Early Cambrian based on presence of acritarchs in Cambridge. Acritarchs are diagnostic species that ranges in age from Proterozoic Z to Early Cambrian, but is most abundant in Proterozoic Z time (Lenk and others, 1982; [also see Goldsmith and others, 1982]). Age is also supported by the following: 1) plant fossils so numerous in strata of Narragansett basin are absent in the Boston Bay Group strata, 2) Late Ordovician and Early Silurian Quincy Granite contains argillite inclusions that are on strike with Cambridge Argillite, and 3) Boston Bay Group stratigraphy is primarily marine, not similar to terrestrial stratigraphy of Narragansett basin (Goldsmith, 1991).

Topsfield Granodiorite (Proterozoic Z) at surface, covers 0.3 % of this area

Topsfield Granodiorite - Gray to gray-green, porphyritic granodiorite containing blue quartz; usually cataclastically foliated and altered. Intrudes Zrdi, Zv. Topsfield Granite occupies area of 80 sq km between Middleton and Newbury, eastern MA. Occurs as part of Dedham batholith. Composition ranges from granite to tonalite. Intrudes unnamed diorite and gabbro unit, and unnamed mafic and felsic metavolcanic rocks. Bounded on northwest by faults bordering Newbury and Middleton basins and by northwest extensions of Bloody Bluff (and Mystic?) faults, and on southeast by a splay(?) of Bloody Bluff fault. Probably overlain by Silurian and Devonian Newbury Volcanic Complex (Dennen, 1975), but contact is now a fault. Although not radiometrically dated, considered to be Proterozoic Z in age because it is similar in mineralogy to Dedham Granite and it intrudes Proterozoic Z mafic complex (Wones and Goldsmith, 1991).

Lithology: granodiorite

Westboro Formation (Proterozoic Z) at surface, covers 0.3 % of this area

Westboro Formation - Quartzite, schist, calc-silicate quartzite, and amphibolite. Consists of quartzite and argillite in Saugus and Lynnfield areas. Westboro Formation consists primarily of orthoquartzite and subordinate mica schist, calc-silicate rock, amphibolite, and quartzitic biotite gneiss and schist. Westboro as portrayed by Nelson (1974), Bell and Alvord (1976), and Hepburn and DiNitto (1978) are correlative [with varying certainty]. West and south of Boston, disconnected masses of quartzite and associated rocks are shown on State bedrock map by Zen and others (1983) as Westboro although not continuous with belts mapped by Nelson (1974) or Hepburn and DiNitto (1978). Includes isolated quartzite masses mapped by Castle (1964) in Reading area. On State bedrock map, arbitrarily includes thin quartzite mapped as Burlington Formation by Bell and Alvord (1976) because the units are similar and to reduce number of small units on State map. Rocks mapped as Rice Gneiss by Nelson (1974) were included in unnamed metamorphosed mafic and felsic volcanic unit on State bedrock map, but author now feels it should be either a part of Westboro, or a separate unit below it. Westboro in Framingham area and to the northeast is overlain by unnamed assemblage of metamorphosed mafic and felsic volcanic rocks. South of town of Westborough, Westboro is truncated by Bloody Bluff-Lake Char fault system. Intruded by Proterozoic Z batholithic rocks. Equivalent to Plainfield Formation of eastern CT because it lies in same strike belt, and is probably equivalent to Quinnville Quartzite and unnamed mica schist and phyllite of Blackstone Group. Contact between Westboro and Blackstone is arbitrary on State bedrock map on basis of proximity of isolated exposures of the two units to their respective type areas. No area of continuous exposure exists between Westboro and Blackstone (Goldsmith, 1991).

Lynn Volcanic Complex (Lower Devonian, Silurian, or Proterozoic Z) at surface, covers 0.3 % of this area

Lynn Volcanic Complex - Rhyolite, agglomerate and tuff.

Chelmsford Granite (Lower Devonian) at surface, covers 0.3 % of this area

Chelmsford Granite - Light-gray, even and medium-grained, muscovite-bearing granite; locally foliated; intrudes Sb.

Lithology: granite

Peabody Granite (Middle Devonian) at surface, covers 0.3 % of this area

Peabody Granite - Alkalic granite containing ferro-hornblende. Intrudes Zgb, Zdngr.

Ayer Granite (Lower Silurian and Upper Ordovician?) at surface, covers 0.3 % of this area

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

Lithology: granite; granodiorite

Granitoid Gneiss (Proterozoic Y) at surface, covers 0.3 % of this area

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

Lithology: granitic gneiss

Ayer Granite, Clinton facies (Lower Silurian) at surface, covers 0.3 % of this area

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

Lithology: granite

Hope Valley Alaskite Gneiss (Proterozoic Z) at surface, covers 0.3 % of this area

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

Fish Brook Gneiss (Ordovician or Proterozoic Z) at surface, covers 0.3 % of this area

Fish Brook Gneiss - Light-gray, biotite-plagioclase quartz gneiss; distinctive "swirl-form" foliation.

Lithology: granitic gneiss

Mount Toby Formation (Lower Jurassic) at surface, covers 0.3 % of this area

Mount Toby Formation - Breccia of amphibolite at Whitmore Ferry; interpreted as landslide deposit. The Mount Toby Conglomerate is here revised and renamed the Mount Toby Formation of the Newark Supergroup. It includes only the sedimentary strata in the Deerfield basin above the slump zone unconformity defined by Cornet (1977), or its projected equivalent at its contact with the underlying Turners Falls Sandstone. It includes conglomerates at the type locality, landslide deposits within the conglomerate, and sandstone and lake beds above the slump zone unconformity, which were formerly included in the Turners Falls Sandstone. Other rocks mapped as Mount Toby Conglomerate by Emerson (1898) in the Hartford, Deerfield, and Northfield basins have been assigned to the Portland, Sugarloaf, and Turners Falls Formations. Age is Sinemurian and Pliensbachian, based on the discovery by Cornet (1977) of palynoflora in these strata (Robinson and Luttrell, 1985).

Diorite and tonalite (Devonian and Silurian) at surface, covers 0.3 % of this area

Diorite and tonalite - Includes Dracut Diorite, tonalite near the Ayer Granite, and equivalents of the Exeter Diorite of New Hampshire; intrudes Sb.

Lithology: diorite; tonalite

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

Nassau Formation - Gray to dark-greenish-gray, siliceous phyllite with abundant beds of quartzite, olive-gray metasiltstone and subgraywacke (includes Bomoseen Graywacke Member and Zion Hill quartzite Member).

Dedham Granite (Proterozoic Z) at surface, covers 0.3 % of this area

Dedham Granite - Gray granite to granodiorite more mafic than Zdgr north of Boston. Intrudes Zw, Zv. Extensive calc-alkaline plutons separated by Boston basin have long been mapped as Dedham. Those to the north of Boston and studied in this report, are referred to as Dedham North. Crystallization ages for the Dedham North suite (based on titanites and zircons) have been determined at 607+/-4 Ma, while ages for the Lynn are slightly younger at 596+/-3 Ma. Both are clearly part of the Late Proterozoic magmatic event. Dates on two samples from Sheffield Heights indicate that the diorite and granite are part of the Dedham North suite. The Dedham south and west of Boston has been dated at 630+/15 Ma (Zartman and Naylor, 1984). Dedham North Granite has a compositionally highly variable suite ranging from leucogranites to granodiorites, tonalites, and quartz diorite. The granites originated by partial melting of a sedimentary protolith, while the intermediate members show a mixing of granitic magma and mafic magma (Hepburn and others, 1993).

Lithology: granite; granodiorite

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

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

Lithology: phyllite; schist

Milford granite (Proterozoic Z) at surface, covers 0.2 % of this area

Milford Granite - Mafic phase. Gray, seriate to sub-porphyritic granite to granodiorite, mafic minerals tend to be in clots; locally gneissic. Intrudes Zb. Occupies an area of about 100 sq km. Central mass near Milford is elliptical and is divided into and mapped as a light-colored phase and a dark-colored phase; dark-colored phase defines an irregular border for largest of light-colored plutons. Intrudes Blackstone Group rocks and Ponaganset Gneiss, but was deformed with them at some later unknown time. Isotopic age of 630 +/-15 Ma was determined using U-Pb methods on zircon by Zartman and Naylor (1984). Characterized by salmon-pink color, bluish quartz on weathered surfaces, and lineations defined by lenticular mosaics of quartz and oriented patches of biotite; texture contrast strongly with that of Dedham Granite (Wones and Goldsmith, 1991).

Lithology: granite; granodiorite

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

Hoosac Formation - Lustrous greenish-gray schist characterized by 1-1.5 cm garnets; resembles Gassetts Schist of Vermont.

Lithology: schist

Shawsheen Gneiss (Ordovician or Proterozoic Z) at surface, covers 0.2 % of this area

Shawsheen Gneiss - Sillimanite gneiss, sulfidic at base; minor amphibolite.

Lithology: gneiss; amphibolite

Cobble Mountain Formation (Middle Ordovician) at surface, covers 0.2 % 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.

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

Greylock Schist - Soft, lustrous green phyllite with minor beds of green quartzite and well laminated gray dolomitic phyllite; resembles Hoosac (CAZhg) and Nassau (CAZnp).

Lithology: phyllite; quartzite

Stockbridge Formation (Lower Ordovician) at surface, covers 0.2 % of this area

Stockbridge Formation - Blue and gray mottled limestone and calcite marble and beds of beige dolostone.

Diabase dikes and sills (Lower Jurassic) at surface, covers 0.2 % of this area

Diabase dikes and sills.

Lithology: diabase

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

Gile Mountain Formation - Like Dgm but having a higher percentage of quartzite.

Fitchburg Complex (Lower Devonian or younger) at surface, covers 0.2 % of this area

Fitchburg Complex - Light-gray, strongly foliated biotite-muscovite granite to granodiorite gneiss; common small to very large inclusions of Dl, some mapped separately.

Pinkish-gray, fine-grained, well-laminated felsic biotite-microcline-plagioclase-quartz gneiss (Proterozoic Y) at surface, covers 0.2 % of this area

Pinkish-gray, fine-grained, well-laminated felsic biotite-microcline-plagioclase-quartz gneiss - Probably metamorphosed rhyolite.

Lithology: felsic gneiss

Sharon Syenite (Proterozoic Z) at surface, covers 0.2 % of this area

Sharon Syenite - Gray to dark-gray syenite containing microperthite, oligoclase and clinopyroxene, mixed with ferro-gabbro. Proterozoic Z Sharon Syenite forms long narrow mass along southeast side of Norfolk basin. Consists of gray to dark-gray syenite and minor ferrogabbro. Both rock types are intruded by Proterozoic Z Dedham Granite, but relationship to other Proterozoic Z gabbros is unknown. Near Sharon, unit is intruded by unnamed Proterozoic Z diorite (Wones and Goldsmith, 1991).

Lithology: syenite; gabbro

Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers 0.2 % of this area

Cape Ann Complex - Beverly Syenite - Quartz-poor facies. Beverly Syenite is part of Cape Ann Complex. Forms subordinate elongate masses within main mass of Cape Ann; dikes of syenite also intrude gabbro at Salem Neck, as well as elsewhere in the Salem area, suggesting that Beverly is, at least in part, a differentiate. Described as cream colored, medium to coarse grained, and rich in alkali feldspar. Age is Late Ordovician and Early Silurian (Wones and Goldsmith, 1991).

Lithology: syenite

Massive to weakly foliated, pink and gray, fine- to medium-grained biotite granite (Pennsylvanian) at surface, covers 0.2 % of this area

Massive to weakly foliated, pink and gray, fine- to medium-grained biotite granite - In the Townsend area; commonly contains pink magnetite-bearing pegmatite identical to granite of Milford, New Hampshire; intrudes OZma and Sp.

Lithology: granite; pegmatite

Holyoke Basalt (Lower Jurassic) at surface, covers 0.2 % 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

Granodiorite of the Indian Head pluton (Precambrian to Paleozoic) at surface, covers 0.2 % of this area

Granodiorite of the Indian Head pluton - Light-gray to pinkish-gray, fine- to medium-grained biotite granodiorite, and gray fine-grained hornblende-biotite tonalite. Intrudes OZm.

Lithology: granodiorite; tonalite

Turner Falls Sandstone (Lower Jurassic) at surface, covers 0.2 % of this area

Turner Falls Sandstone - Reddish-brown to pale red arkosic sandstone, and gray sandstone, gray siltstone, and black shale interpreted as lake beds.

Paxton Formation (Silurian) at surface, covers 0.2 % of this area

Paxton Formation - Rusty-weathering sulfidic quartzite 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: quartzite; schist

Roxbury Conglomerate (Proterozoic Z to earliest Paleozoic) at surface, covers 0.2 % of this area

Roxbury Conglomerate - Melaphyre in the Roxbury Conglomerate. Brighton Melaphyre lies within Brookline Member and consists of mafic volcanic rocks (quartz keratophyre, keratophyre, and spilite). Roxbury clearly lies nonconformably on Dedham Granite near Hull, MA; can be traced continuously over Mattapan Volcanic Complex. Age is Proterozoic Z and possibly Early Cambrian (Goldsmith, 1991).

Biotite granitic gneiss (Devonian) at surface, covers 0.2 % of this area

Biotite granitic gneiss.

Lithology: granitic gneiss

Belchertown Complex (Devonian) at surface, covers 0.2 % of this area

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

Moretown Formation (Middle Ordovician or older) at surface, covers 0.2 % of this area

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

Newburyport Complex (Silurian) at surface, covers 0.2 % of this area

Newburyport Complex - Gray, medium-grained porphyritic granite with microcline phenocrysts; intrudes SOk. Newburyport Complex was divided into two facies, tonalitic granodiorite and granite, by Shride (1971). Tonalitic facies was originally termed Newburyport Quartz Diorite and included dioritic rocks north of Clinton-Newbury fault zone that are now called Sharpners Pond Diorite in Nashoba zone, and Topsfield Granodiorite in Milford-Dedham zone. These correlations are no longer tenable due to differences in age and composition. Therefore, Newburyport Complex is restricted to the two facies present in Newburyport area. Rocks formerly mapped as Newburyport Quartz Diorite and Salem Gabbro-Diorite, except for gabbros at Salem Neck, MA, are included in undifferentiated diorite and gabbro unit (Zdigb) on MA State Geologic Map (Zen and others, 1983), largely because they could not be mapped separately at 1:250,000 scale. Unit Zdigb also includes mafic dikes and sills that are probably younger or contemporaneous. Most of the dioritic rocks northeast of Boston previously assigned to Newburyport Quartz Diorite are now assigned to an undifferentiated diorite unit (Zdi) on MA State Geologic Map. Newburyport Complex forms a large mass near Newburyport and a small one to its west, both truncated by Clinton-Newbury fault. Tonalite and granodiorite facies occupies core of Newburyport Complex at Newburyport and is intruded to the north by granite facies; described as medium to dark gray in fresh rock, weathering to both green and red, fine to medium grained, and highly variable in mineralogy. A U-Pb zircon age of 455 +/-15 Ma was determined by Zartman and Naylor (1984) for the tonalite. Granite facies intrudes both the Kittery Formation and the tonalite and granodiorite facies and covers an area of about 45 sq km. Described as light gray to dark gray, buff weathering, and porphyritic. No radiometric ages available for granite facies, but it is conceivable that the two facies are different in age. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Wones and Goldsmith, 1991).

Lithology: granite

Undifferentiated Poplar Mountain and Dry Hill Gneisses (Proterozoic Z) at surface, covers 0.2 % of this area

Undifferentiated Poplar Mountain and Dry Hill Gneisses .

Hawley Formation (Middle Ordovician) at surface, covers 0.2 % 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).

East Berlin Formation (Lower Jurassic) at surface, covers 0.2 % 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).

Lee Gneiss (Proterozoic Y) at surface, covers 0.2 % of this area

Lee Gneiss - Hornblende-spotted plagioclase-hornblende-quartz gneiss and granofels and well-layered biotite-hornblende-plagioclase-quartz gneiss containing knots of diopside or epidote.

Lithology: mafic gneiss; granofels

Paxton Formation (Silurian) at surface, covers 0.2 % 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

Fitchburg Complex (Lower Devonian or younger) at surface, covers 0.2 % of this area

Fitchburg Complex -Dark-gray, strongly foliated biotite granodiorite to tonalite gneiss; resembles Dht; intrudes and contains inclusions of Dl, some mappable; locally cut by sills identical to Dfgrg.

Pondville Conglomerate (Lower Pennsylvanian) at surface, covers 0.2 % of this area

Pondville Conglomerate - Quartz conglomerate having abundant sandy matrix; boulder conglomerate, arkose; fossil plants. Pondville Conglomerate is present in Narragansett and Norfolk basins. Although Chute (1964, 1966, 1969) recognized a lower boulder conglomerate member and an upper sandstone to pebble conglomerate member in the Pondville in the northeast part of the basin, such a division is not readily made to the southwest because of facies changes. Upper member grades into and interfingers with Wamsutta Formation. Basal beds nonconformably overlie Dedham Granite in northern part of Narragansett basin. Age of deposits in Narragansett and Norfolk basins ranges from Early to Late Pennsylvanian; however, Skehan and Murray (1980) assigned lower part of Pondville to Mississippian. Further study may reveal even older Paleozoic sediments in Narragansett basin (Goldsmith, 1991).

Lithology: conglomerate; arkose

Cooleyville Granitic Gneiss (Devonian) at surface, covers 0.2 % of this area

Cooleyville Granitic Gneiss - Biotite tonalite to granite in composition, strongly foliated and lineated; contains inclusions of Dpgb; intrudes Dl.

Lithology: tonalite; granite; gabbro

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

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

Waits River Formation - Dw containing thick (1 m) beds of calcareous granofels. Mapped only in Colrain quadrangle; included in Dw elsewhere.

Lithology: schist; granofels; marble

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

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

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.

Dry Hill Gneiss (Proterozoic Z) at surface, covers 0.1 % of this area

Dry Hill Gneiss - Pelham Quartzite Member. White to buff quartzite and feldspathic quartzite commonly with biotite and/or actinolite.

Lithology: quartzite

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

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

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.

Poplar Mountain Gneiss (Proterozoic Z) at surface, covers 0.1 % of this area

Poplar Mountain Gneiss (Probably correlates with Mount Mineral Formation but is more feldspathic) - Biotite gneiss where mapped separately.

Lithology: biotite gneiss

Straw Hollow Diorite and Assabet Quartz Diorite undifferentiated (Silurian) at surface, covers 0.1 % of this area

Straw Hollow Diorite and Assabet Quartz Diorite undifferentiated - Gray, medium-grained, slightly-foliated biotite-hornblende diorite and quartz diorite. Intrudes OZn.

Lithology: diorite; quartz diorite

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

Erving Formation - Biotite-plagioclase granofels, minor mica schist and calc-silicate granofels, and layers of epidote amphibolite.

Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers 0.1 % of this area

Newbury Volcanic Complex - Porphyritic andesite, includes tuffaceous mudstone beds containing fossils of Late Silurian through Early Devonian age.

Lithology: andesite; mudstone

Boylston Schist (Silurian or Ordovician) at surface, covers 0.1 % of this area

Boylston Schist - Carbonaceous phyllite and schist, locally sulfidic; quartzite; calc-silicate beds.

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 0.1 % 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

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

Marlboro Formation - Homogeneous light-gray feldspathic gneiss.

Lithology: granitic gneiss

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

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

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

Collinsville Formation - Homogeneous garnetiferous biotite gneiss containing chlorite as clots and halos around garnet.

Lithology: biotite gneiss

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

Hoosac Formation - Green to gray-green chlorite-sericite-quartz phyllite; interbeds of chloritoid- or albite-rich schist and minor quartzite, locally rich in garnet and kyanite.

Lithology: phyllite; schist; quartzite

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

Dalton Formation - Tan to orangish-tan quartz and gneiss cobble and pebble conglomerate, rusty feldspathic schist, and lustrous greenish-gray muscovite quartz schist.

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 0.1 % 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

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

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

Lithology: mica schist; gneiss

Clough Quartzite (Upper Silurian) at surface, covers 0.1 % 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).

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

Dalton Formation - Black to dark-gray carbonaceous quartz schist.

Lithology: schist

Tower Hill Quartzite (Silurian) at surface, covers 0.1 % of this area

Tower Hill Quartzite - Quartzite and phyllite.

Lithology: quartzite; phyllite

Granite of Rattlesnake Hill pluton (Devonian) at surface, covers 0.1 % of this area

Granite of Rattlesnake Hill pluton - Coarse-grained biotite-granite and fine-grained riebeckite granite. Intrudes Zdgr.

Lithology: granite

Biotite gneiss near New Bedford (Proterozoic Z) at surface, covers 0.1 % of this area

Biotite gneiss near New Bedford - Layered feldspathic gneiss.

Lithology: biotite gneiss

Turner Falls Sandstone (Lower Jurassic) at surface, covers 0.1 % of this area

Turner Falls Sandstone - Pale red conglomerate and arkosic sandstone.

Lithology: conglomerate; sandstone

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

Dalton Formation - White to yellowish-gray weathering, flaggy quartzite interbedded in feldspathic quartzite.

Lithology: quartzite

Light-gray muscovite granite (Precambrian to Silurian) at surface, covers 0.1 % of this area

Light-gray muscovite granite.

Lithology: granite

Metamorphosed felsic metavolcanic rocks (Proterozoic Z) at surface, covers < 0.1 % of this area

Metamorphosed felsic metavolcanic rocks .

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

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

Erving Formation - Granofels and schist where mapped separately.

Lithology: granulite; schist

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

Well-layered hornblende-biotite gneiss .

Lithology: mafic gneiss

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

Greylock Schist - Light-green to gray, white albite-spotted phyllite and interbedded blue-quartz metagraywacke, salmon-pink dolostone, and blue-quartz conglomerate. Minor interbeds of black and white albite-spotted phyllite resembles Hoosac (CAZhga).

Wamsutta Formation (Middle and Lower Pennsylvanian) at surface, covers < 0.1 % of this area

Wamsutta Formation - Rhyolite and mafic volcanic rocks in the Wamsutta Formation. Wamsutta Formation occurs in Narragansett and Norfolk basins. Consists of conglomerate, lithic graywacke, sandstone, and shale. Also contains rhyolite and basalt horizons near Attleboro. Northwest of Attleboro, Wamsutta overlies Diamond Hill Felsite as used by Skehan and Murray (in Skehan and others, 1979). Volcanic rocks similar to Diamond Hill Felsite crop out west of Lake Pearl, between Franklin and Wrentham, on west flank of Norfolk basin. These are shown within Wamsutta Formation on MA State bedrock map of Zen and others (1983) because of their proximity to Diamond Hill. They also resemble Proterozoic Z Mattapan Volcanic Complex. Chute (1966) described lenses of carbonate rock in red and green shale in Wamsutta in Norwood quad. Limestone also observed in rocks mapped as Wamsutta adjacent to exposed Dedham Granite at Manchester Pond Reservoir (J.P. Schafer, 1982, oral commun.). Red and green shales may actually be Cambrian. Upper member of Pondville Conglomerate grades into and interfingers with Wamsutta; in turn, Wamsutta interfingers with Rhode Island Formation in northwest part of Narragansett basin. Nonconformably overlies Dedham Granite. Partly equivalent to Rhode Island Formation. Age is Early and Middle Pennsylvanian. Contains a few plant fossils (Goldsmith, 1991).

Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers < 0.1 % of this area

Newbury Volcanic Complex - Lower members. Basalt, andesite, rhyolite, and tuff.

Lithology: basalt; andesite; rhyolite; tuff

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

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

Lithology: diorite; quartz diorite

Wenham Monzonite (Middle Devonian) at surface, covers < 0.1 % of this area

Wenham Monzonite - Monzonite containing ferro-hornblende.

Lithology: monzonite

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

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

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

Mount Toby Formation - Reddish-brown to pale red arkosic sandstone, and gray sandstone, gray siltstone, and black shale interpreted as lake beds.

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

Mount Mineral Formation (Probably correlates with Poplar Mountain Gneiss but is more aluminous) - Aluminous schist, amphibolite, and quartzite, undifferentiated; locally rich in garnet and kyanite, and with relict sillimanite and orthoclase from pre-Middle Ordovician metamorphism.

Braintree Argillite and Weymouth Formation (Middle and Lower Cambrian) at surface, covers < 0.1 % of this area

Braintree Argillite and Weymouth Formation - Argillite, some with rare limestone; contains Middle and Early Cambrian faunae respectively.

Lithology: argillite; limestone

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

Deerfield Basalt - Well-jointed quartz tholeiite, locally vesicular and locally pillowed near base.

Lithology: tholeiite

Black and white, well-layered hornblende-biotite-plagioclase gneiss and amphibolite (Proterozoic Y) at surface, covers < 0.1 % of this area

Black and white, well-layered hornblende-biotite-plagioclase gneiss and amphibolite - Contains irregular pods of diopside or cummingtonite-talc rock or amphibole calc-silicate, epidote-layered quartz-plagioclase gneiss near Hinsdale.

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

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

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

Lithology: quartzite; schist; granofels

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

Moretown Formation - Buff to gray, medium- to coarse-grained, poorly bedded mica-rich schist showing dark-green chlorite clots. Some pinstriped granofels.

Lithology: mica schist; granofels

Poplar Mountain Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Poplar Mountain Gneiss (Probably correlates with Mount Mineral Formation but is more feldspathic) - Dark biotite gneiss containing white microcline megacrysts and beds of quartzite.

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

Berwick Formation - Mica schist. In New Hampshire: Used as Berwick Formation of Merrimack Group. Consists of purple biotite-feldspar granofels or schist. Contains interbeds of calcsilicate granofels and minor metapelites. Includes Gove Member, mapped separately. Stratigraphic sequence with respect to Eliot Formation is uncertain. Age of all formations in Merrimack Group changed to Ordovician(?) to Silurian(?) based on isotopic age determinations of approx 440 and 420 Ma from detrital zircons from Berwick by J.N. Aleinikoff (oral commun., 1994) (Lyons and others, 1997).

Lithology: mica schist

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Felsic gneiss of volcanic derivation and minor amphibolite.

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

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

Biotite-garnet-feldspar gneiss of Ragged Hill (Devonian) at surface, covers < 0.1 % 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

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

Hawley Formation - Medium-gray plagioclase-hornblende-chlorite schist containing megacrysts of plagioclase and angular fragments of feldspar granofels, epidote-plagioclase granofels, and dark-gray amphibolite. 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).

Fitchburg Complex (Lower Devonian or younger) at surface, covers < 0.1 % of this area

Fitchburg Complex - Light-gray to white, medium-grained, weakly foliated muscovite-biotite granite; commonly contains white pegmatite bearing muscovite and tourmaline; may include some granite of late Paleozoic age; locally intrudes Dfgrg, Dfgd, and Dl.

Lithology: granite; pegmatite

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

Gabbro - Hornblende gabbro and hornblende-pyroxene gabbro metamorphosed in part to hornblende gneiss and amphibolite.

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

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

Poplar Mountain Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Poplar Mountain Gneiss (Probably correlates with Mount Mineral Formation but is more feldspathic) - Basal quartzite, where thick enough to map; commonly feldspathic, containing biotite and actinolite or muscovite.

Lithology: quartzite

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.

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

Walloomsac Formation - Tan to dark-blue-gray laminated quartzite and calcareous quartzite previously mapped as Bellowspipe Quartzite.

Lithology: quartzite

Massabesic Gneiss Complex (Ordovician and Proterozoic Z) at surface, covers < 0.1 % of this area

Massabesic Gneiss Complex - Biotite-feldspar paragneiss of Proterozoic Z age intruded by potassium-feldspar-rich gneiss of Ordovician age.

Lithology: paragneiss; gneiss

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

Collinsville Formation - Interlayered amphibolite and felsic gneiss in beds less than 1 m thick. Local, coarse-grained magnetite-hornblende gneiss.

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

Stamford Granite Gneiss - White to gray biotite-oligoclase-microcline Rapakivi granite gneiss containing blue quartz. Intrudes Yb, Ybu.

Lithology: granitic gneiss

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

Collinsville Formation - Plagioclase gneiss and minor amphibolite.

Lithology: gneiss; amphibolite

Canaan Mountain Formation (Lower Ordovician and Proterozoic Z) at surface, covers < 0.1 % of this area

Canaan Mountain Formation - Rusty-weathering, coarse garnet schist and feldspathic schist.

Lithology: schist

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

Tower Hill Quartzite - Gray phyllite associated with the Tower Hill Quartzite.

Lithology: phyllite

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

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

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

Williamsburg Granodiorite (Devonian) at surface, covers < 0.1 % of this area

Williamsburg Granodiorite - Equigranular biotite-muscovite granodiorite. Overprinting used in area where sufficient metasedimentary outcrop allows continuity in stratigraphic mapped. Intrudes Dg, Dw, Dgm, Dl, and De.

Lithology: granodiorite

Granby Basaltic Tuff (Lower Jurassic) at surface, covers < 0.1 % 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

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

Erving Formation - Amphibolite where mapped separately.

Lithology: amphibolite

Diorite at Rowley (Proterozoic Z) at surface, covers < 0.1 % of this area

Diorite at Rowley - Dark, green-gray, medium-grained hornblende diorite.

Lithology: diorite

Intimately interfolded Littleton and Partridge Formations (Lower Devonian and Middle Ordovician) at surface, covers < 0.1 % of this area

Intimately interfolded Littleton and Partridge Formations - In areas of poor exposure and incomplete mapping.

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

Hoosac Formation - Greenish chlorite-albite-magnetite-sericite-quartz schist and granofels.

Lithology: mica schist; granofels

Blue Hill Granite Porphyry (Lower Silurian and Upper Ordovician) at surface, covers < 0.1 % of this area

Blue Hill Granite Porphyry - Microperthite-quartz porphyry. [Used mistakenly as Blue Hills Granite Porphyry; the plural Hills was not intended and should not be used.] Described as dark gray to dark blue, weathering buff brown to salmon. Assigned a Late Ordovician and Early Silurian age because of its close similarity, both chemically and mineralogically, to Quincy Granite. Cobbles of the rock are found in Early Pennsylvanian Pondville Conglomerate. Bounded by Quincy Granite to the north and east, and by Neponset fault on the west; unconformably overlain by Pondville Conglomerate to the south (Wones and Goldsmith, 1991).

Lithology: porphyry

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

Gneiss at Hallockville Pond - Light-gray foliated microcline-plagioclase-quartz biotite gneiss containing microcline megacrysts. Intrudes Om.

Lithology: granitic gneiss

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

Belchertown Complex (intrudes De) - Hornblendite.

Lithology: hornblendite

Coys Hill Porphyritic Granite Gneiss (Lower Devonian) at surface, covers < 0.1 % 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

Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers < 0.1 % of this area

Newbury Volcanic Complex - Upper members. Calcareous mudstone, red mudstone, and siliceous siltstone; fossils of Late Silurian through Early Devonian age.

Lithology: mudstone; siltstone

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

Prescott Complex - Hornblende gabbro (formerly Prescott Diorite of Emerson, 1917).

Lithology: gabbro

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

Nassau Formation - Dark-gray to black chloritoid-rich phyllite.

Lithology: phyllite

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

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

New Haven Arkose (Lower Jurassic) 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. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Austerlitz Phyllite (Cambrian?) at surface, covers < 0.1 % of this area

Austerlitz Phyllite - minor quartzite

Lithology: phyllite; quartzite

Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers < 0.1 % of this area

Cape Ann Complex - Squam Granite - Fine- to medium-grained monzodiorite. Squam Granite, though included in the Cape Ann Complex of this report, is probably not genetically related to the Cape Ann Granite and the Beverly Syenite, the dominant lithological types of the Complex. The origin of the granitic melt of the Cape Ann was likely under water-deficient conditions, but the origin of the Squam Granite, described here as a two-feldspar ferrohornblende biotite granite, remains obscure (Hon and others, 1993).

Lithology: monzodiorite

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

Hoosac Formation - Rusty-brown to dark-gray, albite-spotted muscovite-biotite schist or gneiss, with interlayered black garnet-biotite-albite-quartz schist near base; interfingers with Dalton Formation.

Lithology: mica schist; gneiss; schist

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

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sillimanite-feldspar augen gneiss.

Lithology: augen gneiss

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

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

Fitch Formation - Sulfidic calc-silicate and minor sulfidic schist. Although the text and figures of this report show the Fitch as Silurian, a footnote [added just before this report went to press] cites a change in age from Silurian to Early Devonian based on conodonts found at the Bernardston, MA, locality, as reported in Elbert and others (1988). In Bronson Hill anticlinorium in MA, Fitch occurs as lenses between Clough Quartzite and Littleton Formation. Most common rock types in MA are gray, massive to weakly bedded, quartz-labradorite-biotite granulite containing a moderate amount of some combination of calc-silicate minerals (calcic amphibole, zoisite or clinozoisite, diopside, sphene, and microcline); commonly interbedded with biotite-free granulite that contains same calc-silicate minerals. One small exposure consists of nearly pure calcite marble. Larger lenses of Fitch consist of varieties of schist, similar to Partridge Formation. Best exposures are in low hills west of village of Orange, northeast of junction of MA Hwys 2A and 78. As shown on MA State bedrock geologic map, Fitch everywhere overlies Clough Quartzite and is never in contact with Partridge. Fossils dating the Fitch as Pridolian (Harris and others, 1983) are all from Littleton, NH, area [however, see mention of footnote, above]. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Hatch and others, 1988).

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

Goshen Formation - Poorly bedded carbonaceous schist and quartz schist.

Lithology: schist

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

Cherry Hill Granite - Alaskite granite containing ferro-hornblende. Intrudes Dwn.

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

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

Lithology: tholeiite

Shuttle Meadow Formation (Lower Jurassic) at surface, covers < 0.1 % 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).

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

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

Lithology: granofels

Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers < 0.1 % of this area

Newbury Volcanic Complex - Undivided sedimentary and volcanic rocks.

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

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

Littleton Formation - Quartz-feldspar-garnet gneiss, probably originally felsic volcanic rocks.

Lithology: felsic gneiss

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

Hardwick Tonalite - Porphyritic microcline-biotite granite gneiss in sills intruding Dht.

Lithology: granitic gneiss

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

Paxton Formation - Amphibolite. 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: amphibolite

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).

Diorite at Goff Ledges (Ordovician) at surface, covers < 0.1 % 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

Biotite-muscovite granite (Devonian) at surface, covers < 0.1 % of this area

Biotite-muscovite granite - Slightly foliated.

Lithology: granite

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

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Biotite gneiss of volcanic derivation, minor amphibolite and sulfidic schist.

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

Washington Gneiss - Rusty-weathering diopside and sulfidic -rich calcite marble and calc-silicate rock.

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

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

Lithology: granofels; schist

Muscovite-bearing granite (Lower Devonian) at surface, covers < 0.1 % of this area

Muscovite-bearing granite - At Millstone Hill; intrudes So.

Lithology: granite

Rhode Island Formation (Upper and Middle Pennsylvanian) at surface, covers < 0.1 % of this area

Rhode Island Formation - Conglomerate, sandstone, and graywacke. Rhode Island Formation is thickest and most extensive formation in Narragansett basin. Does not extend to Norfolk basin. Consists of gray sandstone and siltstone and lesser amounts of gray to black shale, gray conglomerate, and coal beds 10 m thick. Interfingers with Wamsutta Formation in Narragansett basin. In places overlies Dedham Granite. Age is Middle and Late Pennsylvanian (Goldsmith, 1991).

Tectonic breccia (Ordovician) at surface, covers < 0.1 % of this area

Tectonic breccia - Consisting of completely mixed phyllites of Walloomsac Formation Taconic allochthon.

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

Walloomsac Formation - slate, phyllite, schist, metagraywacke.

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

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

Lithology: schist

Nahant Gabbro and gabbro at Salem Neck (Ordovician) at surface, covers < 0.1 % of this area

Nahant Gabbro and gabbro at Salem Neck - Labradorite-pyroxene gabbro, hornblende gabbro and hornblende diorite. Intrudes CAbw. Nahant Gabbro intrudes Early Cambrian Weymouth Formation at Nahant and thus is younger than gabbroic or dioritic rocks intruded by Late Proterozoic Dedham Granite or Topsfield Granite. Unrecognized equivalents to Nahant Gabbro at Nahant and at Salem Neck may be present within unnamed gabbro and diorite units shown on MA State geologic map (Zen and others, 1983). Mapped as three facies at Nahant, following usage of Bell (1977): pyroxene gabbro, olivine gabbro, and quartz diorite. Biotite from olivine gabbro was analyzed by Rb-Sr and K-Ar methods and yielded an Ordovician age (450 +/-13 Ma by Rb-Sr methods, 483 +/-21 Ma by K-Ar methods). Quartz diorite phase may be a more mafic phase of Dedham Granite (Wones and Goldsmith, 1991).

Lithology: gabbro; diorite

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

Sugarloaf Formation - Pale red conglomerate and arkosic sandstone, coarsens eastward.

Lithology: conglomerate; sandstone

Upper part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Upper part of Rangeley Formation - Rusty-weathering, pelitic schist, metasandstone, and local coarse-grained metasandstone lentils; calc-silicate pods common; minor coticule. Probably equivalent to member C of Rangeley Formation of Maine.

Red arkosic conglomerate, sandstone, and siltstone (Upper Triassic) at surface, covers < 0.1 % of this area

Red arkosic conglomerate, sandstone, and siltstone - In Essex County.

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

Gile Mountain Formation - Amphibolite, hornblende schist.

Hoppin Formation (Middle and Lower Cambrian) at surface, covers < 0.1 % of this area

Hoppin Formation - Quartzite, argillite, and minor limestone; contains Middle Cambrian fauna.

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

Stockbridge Formation - Tan-beige weathering quartzose calcite and dolomite marble; minor cross-laminated quartzite.

Lithology: marble; quartzite

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

Shuttle Meadow Formation - Pale red conglomerate and arkosic sandstone. 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: conglomerate; sandstone

Dry Hill Gneiss (Proterozoic Z) at surface, covers < 0.1 % of this area

Dry Hill Gneiss - Biotite-tourmaline schist and quartzite.

Lithology: schist; quartzite

Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers < 0.1 % of this area

Newbury Volcanic Complex - Micrographic rhyolite, intrudes DSnu, DSna, DSnl.

Lithology: rhyolite

Vaughn Hills Quartzite (Silurian or Ordovician) at surface, covers < 0.1 % of this area

Vaughn Hills Quartzite - Quartzite, phyllite, conglomerate, and chlorite schist.

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

Tatnic Hill Formation - Yantic Member - Gray mica schist.

Lithology: mica schist

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

Hoosac Formation - Rusty, gray, quartz-albite-mica (-chlorite) schist and gneiss. Locally conglomeratic.

Lithology: mica schist; gneiss

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

Fitch Formation - Calc-silicate granofels, biotite granofels, minor sulfidic schist and marble. Correlated with the fossiliferous Fitch Formation of western New Hampshire. Although the text and figures of this report show the Fitch as Silurian, a footnote [added just before this report went to press] cites a change in age from Silurian to Early Devonian based on conodonts found at the Bernardston, MA, locality, as reported in Elbert and others (1988). In Bronson Hill anticlinorium in MA, Fitch occurs as lenses between Clough Quartzite and Littleton Formation. Most common rock types in MA are gray, massive to weakly bedded, quartz-labradorite-biotite granulite containing a moderate amount of some combination of calc-silicate minerals (calcic amphibole, zoisite or clinozoisite, diopside, sphene, and microcline); commonly interbedded with biotite-free granulite that contains same calc-silicate minerals. One small exposure consists of nearly pure calcite marble. Larger lenses of Fitch consist of varieties of schist, similar to Partridge Formation. Best exposures are in low hills west of village of Orange, northeast of junction of MA Hwys 2A and 78. As shown on MA State bedrock geologic map, Fitch everywhere overlies Clough Quartzite and is never in contact with Partridge. Fossils dating the Fitch as Pridolian (Harris and others, 1983) are all from Littleton, NH, area [however, see mention of footnote, above] (Hatch and others, 1988).

Lithology: granofels; schist; marble

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

Hoosac Formation - Light-gray to tan, albite-muscovite-calcite spangled schist.

Lithology: schist

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

Collinsville Formation - Interlayered amphibolite and white felsic gneiss containing biotite, hornblende, and magnetite. Local calc-silicate beds. Coticule-bearing granofels, muscovite quartzite and amphibolite in upper part.

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

Collinsville Formation - Felsic gneiss containing scattered biotite, magnetite, garnet and hornblende. Local beds of amphibolite.

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.

Tectonic breccia (Ordovician) at surface, covers < 0.1 % of this area

Tectonic breccia - Zones of mixed inclusions of Stockbridge Formation, Walloomsac Formation, and phyllites of the Taconic allochthon.

Lithology: tectonic breccia

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

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

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

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

Hawley Formation - Pale-buff, light-green or white, medium-grained plagioclase gneiss. 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: gneiss

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.

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

East Berlin Formation - Pale red conglomerate and arkosic sandstone. Assigned to Newark Supergroup (Robinson and Luttrell, 1985).

Lithology: conglomerate; sandstone

White, magnetite-bearing alaskite and trondhjemite (Ordovician) at surface, covers < 0.1 % of this area

White, magnetite-bearing alaskite and trondhjemite - Associated with blastomylonite along thrust slices. Intrudes Proterozoic Y gneisses.

Harvard Conglomerate (Pennsylvanian) at surface, covers < 0.1 % of this area

Harvard Conglomerate - Conglomerate and chloritoid-hematite phyllite.

Lithology: conglomerate; phyllite

Green Lodge Formation of Rhodes and Graves (Upper Cambrian?) at surface, covers < 0.1 % of this area

Green Lodge Formation of Rhodes and Graves - Quartzite and slate; contains fossils of questionable Late Cambrian age.

Lithology: quartzite; slate

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

Tatnic Hill Formation - Fly Pond Member - Calc-silicate gneiss and marble.

Lithology: gneiss; marble

Granodiorite (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Granodiorite - Mostly nonfoliated.

Lithology: granodiorite

Coal Mine Brook Formation (Middle Pennsylvanian) at surface, covers < 0.1 % of this area

Coal Mine Brook Formation - Fossiliferous, carbonaceous slate and garnet phyllite with a lens of meta-anthracite; conglomerate and arkose.

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

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

Fourmile Gneiss - Muscovite quartzite.

Lithology: quartzite

Putney Volcanics (Lower Devonian) at surface, covers < 0.1 % of this area

Putney Volcanics - Light greenish-gray plagioclase-quartz-muscovite phyllite and granofels.

Lithology: phyllite; granofels

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

Hoosac Formation - Gray conglomerate containing pebbles of albite and blue quartz, and boulders of gneiss.

Lithology: conglomerate

Hitchcock Volcanics (Lower Jurassic) at surface, covers < 0.1 % of this area

Hitchcock Volcanics - Nested cones of basaltic breccia containing abundant fragments of New Haven Arkose (Jn, TRn), locally intrusive into arkose near base; overlain by lava flow of Holyoke Basalt (Jhb) and/or Shuttle Meadow Formation (Jsm).

Merrimack Group, Berwick Formation (Ordovician? - Silurian?) at surface, covers < 0.1 % of this area

Merrimack Group, Berwick Formation - Purple biotite-quartz-feldspar granofels or schist and interbeds of calc-silicate granofels and minor metapelites. Stratigraphic sequence with respect to Eliot Formation uncertain

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

Nassau Formation - Rensselaer Graywacke Member - Greenish-gray, plagioclase-rich, blue quartz pebble metagraywacke and minor gneiss-cobble conglomerate.

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

Nassau Formation- Greenish-gray albitic phyllite.

Lithology: phyllite

Spaulding Tonalite (Early Devonian) at surface, covers < 0.1 % of this area

Spaulding Tonalite (Spaulding Quartz Diorite of Fowler-Billings, 1949) - Weakly foliated to nonfoliated, spotted biotite quartz diorite, tonalite, granodiorite, and granite; garnet and muscovite may or may not be present.

Granite, granodiorite, and tonalite (Late Ordovician) at surface, covers < 0.1 % of this area

Granite, granodiorite, and tonalite.

Granite (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Granite - Mostly nonfoliated.

Lithology: granite

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

Everett Formation - Olive-gray to green, blue-quartz pebble metagraywacke and quartzite.

Belchertown Complex (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Belchertown Complex (intrudes De) - Inclusions of granofels (age uncertain).

Lithology: granofels

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

Stockbridge Formation - calcitic and dolomitic marble.

Lithology: marble

Fitchburg Complex (Lower Devonian or younger) at surface, covers < 0.1 % of this area

Fitchburg Complex - Dfgd containing many zones of foliated biotite-muscovite granite gneiss and inclusions of mica schist and feldspathic granulite.

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

Fitch Formation - Sulfidic mica schist. Although the text and figures of this report show the Fitch as Silurian, a footnote [added just before this report went to press] cites a change in age from Silurian to Early Devonian based on conodonts found at the Bernardston, MA, locality, as reported in Elbert and others (1988). In Bronson Hill anticlinorium in MA, Fitch occurs as lenses between Clough Quartzite and Littleton Formation. Most common rock types in MA are gray, massive to weakly bedded, quartz-labradorite-biotite granulite containing a moderate amount of some combination of calc-silicate minerals (calcic amphibole, zoisite or clinozoisite, diopside, sphene, and microcline); commonly interbedded with biotite-free granulite that contains same calc-silicate minerals. One small exposure consists of nearly pure calcite marble. Larger lenses of Fitch consist of varieties of schist, similar to Partridge Formation. Best exposures are in low hills west of village of Orange, northeast of junction of MA Hwys 2A and 78. As shown on MA State bedrock geologic map, Fitch everywhere overlies Clough Quartzite and is never in contact with Partridge. Fossils dating the Fitch as Pridolian (Harris and others, 1983) are all from Littleton, NH, area [however, see mention of footnote, above]. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] (Hatch and others, 1988).

Lithology: mica schist

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

Littleton Formation undivided - Gray metapelite and metawacke and subordinate metavolcanic rocks; generally, but not everywhere, conformable with underlying Fitch or Madrid Formations. Fossiliferous in western New Hampshire.

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

Littleton Formation - Orthopyroxene-biotite gneiss, probably originally intermediate volcanic rocks.

Lithology: mafic gneiss

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

Waits River Formation - Amphibolite or hornblende schist locally containing conspicuous hornblende or garnet megacrysts. Rocks mapped as Conway Schist by Emerson (1898, 1917) and subsequently subdivided by Segerstrom (1956) and Willard (1956) were mapped across the MA-VT State line as Waits River and Gile Mountain Formations by Doll and others (1961) on Centennial Geologic Map of Vermont. Although controversy still exists over relative ages, detailed reconnaissance mapping by authors and S.F. Clark, Jr., L.M. Hall, and J.W. Pferd shows that Waits River and Gile Mountain Formations are readily distinguished in the field. For these reasons, and to maintain continuity across the State line, authors chose to follow VT nomenclature on here and on MA State bedrock geologic map (Zen and others, 1983). Primary difference between Waits River and Gile Mountain is presence in Gile Mountain of beds of noncalcareous, commonly micaceous quartzite. Both formations contain conspicuous beds of punky brown-weathering impure marble or calcareous granulite, mostly in Waits River and less abundant in Gile Mountain. Predominant lithology of both formations is typically contorted gray, graphitic, locally very sulfidic, moderately aluminous mica schist containing quartz veins. Gradational but definitely significant boundary can be mapped between both formations. Amphibolite in both formations may correlate with Standing Pond Volcanics occurring at or near Waits River-Gile Mountain contact in VT. Report goes into great detail regarding informal subdivision of each formation. Rocks previously mapped as Waits River Formation northeast of Shelburne Falls dome by Hatch and Hartshorn (1968) are here reassigned to an unnamed member of Goshen Formation because the rocks are indistinguishable from the Goshen. Goshen-Waits River contact is defined as the line along which, going eastward, the schist changes from aluminous, planar-bedded, and virtually quartz-free (Goshen), to alumina-poor, contorted, and rich in quartz veins (Waits River) (Hatch and others, 1988).

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

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

Lithology: schist

Lower part of Rangeley Formation (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Lower part of Rangeley Formation - Gray, thinly laminated (5-25 mm) metapelite with local lentils of turbidites and thin quartz conglomerates in western New Hampshire. Sparse calc-silicate pods and coticule. Probably equivalent to member B of Rangeley Formation of Maine.

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

Moretown Formation - Light- to medium-gray, rusty, carbonaceous quartz-muscovite schist.

Lithology: mica schist

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

Littleton Formation - Thinly laminated calcitic marble.

Lithology: marble

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Lenses of ultramafic rock, commonly hornblendite with or without olivine, orthopyroxene, spinel, cummingtonite, anthophyllite, ilmenite and chlorite.

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

Partridge Formation, undivided - Black, rusty-weathering sulfidic-graphitic slate or schist and sparse to abundant metagraywacke. Lies stratigraphically between upper and lower parts of the Ammonoosuc Volcanics.

Hornblende-olivine gabbro (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Hornblende-olivine gabbro - Intrudes Dl.

Lithology: gabbro

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

Mount Toby Formation - Breccia of granitic gneiss at Taylor Hill; interpreted as landslide deposit. The Mount Toby Conglomerate is here revised and renamed the Mount Toby Formation of the Newark Supergroup. It includes only the sedimentary strata in the Deerfield basin above the slump zone unconformity defined by Cornet (1977), or its projected equivalent at its contact with the underlying Turners Falls Sandstone. It includes conglomerates at the type locality, landslide deposits within the conglomerate, and sandstone and lake beds above the slump zone unconformity, which were formerly included in the Turners Falls Sandstone. Other rocks mapped as Mount Toby Conglomerate by Emerson (1898) in the Hartford, Deerfield, and Northfield basins have been assigned to the Portland, Sugarloaf, and Turners Falls Formations. Age is Sinemurian and Pliensbachian, based on the discovery by Cornet (1977) of palynoflora in these strata (Robinson and Luttrell, 1985).

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

Nassau Formation - Dark-green to greenish brown, massive to well foliated stilpnomelane-chlorite-oligoclase-epidote metabasalt and basaltic tuff.

Lithology: meta-basalt; tuff

Grant Mills Granodiorite (Proterozoic Z) at surface, covers < 0.1 % of this area

Grant Mills Granodiorite - Medium- to coarse-grained porphyritic granodiorite. Gradational with Zegr.

Lithology: granodiorite

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

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

Belchertown Complex (intrudes De) - Inclusions of amphibolite (age uncertain).

Lithology: amphibolite

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

unknown

Lithology: indeterminate

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

Concord Granite - Gray two-mica granite, locally grading to tonalite.

Lithology: granite; tonalite

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

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

Lithology: diorite; norite

Hornblende-plagioclase gneiss (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Hornblende-plagioclase gneiss .

Lithology: mafic gneiss

Coys Hill Porphyritic Granite Gneiss (Lower Devonian) at surface, covers < 0.1 % of this area

Coys Hill Porphyritic Granite Gneiss - Hornblende gneiss inclusions in granitic gneiss of the Coys Hill.

Lithology: mafic gneiss

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

Littleton Formation - Gray slate and phyllite containing interbeds of gray schistose quartzite 1/4 inch to 6 inches thick. West of Guildhall are lustrous, light to dark gray biotite-garnet phyllite and schist, some slate, and subordinate quartzite and impure quartzite. South of Bellows Falls gray phyllite passes eastward into gray mica schist containing porphyroblasts of biotite, garnet, and staurolite.

Granites of southeastern Rhode Island - porphyritic granite (Late Proterozoic) at surface, covers < 0.1 % of this area

Granites of southeastern Rhode Island - porphyritic granite - Gray, pink, or greenish, generally massive, coarse-grained, porphyritic (microcline phenocrysts) granite. Contains microcline, perthite, plagioclase, quartz, and accessory biotite, hornblende, apatite, sphene, and opaque minerals; secondary chlorite, epidote, and sericite. Includes some rock mapped formerly as Bulgarmarsh Granite and Newport Granite Porphyry.

Lithology: granite

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

Monson Gneiss - Amphibolite where mapped separately.

Lithology: amphibolite

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Narragansett Bay Group - Rhode Island Formation - In northern Rhode Island, consists of gray to black, fine- to coarse-grained quartz arenite, litharenite, shale, and conglomerate, with minor beds of anthracite and meta-anthracite. In southern Rhode Island, consists of meta-sandstone, meta-conglomerate, schist, carbonaceous schist, and graphite. Plant fossils are common.

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

Granodiorite - Mostly nonfoliated, intrudes Dl.

Lithology: granodiorite

Rangeley Formation, Pink to green calc-silicate and purple biotite granofels (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Rangeley Formation, Pink to green calc-silicate and purple biotite granofels - Thinly bedded. Close to transition from lower to upper parts of the Rangeley Formation. Probably equivalent to part of Paxton Formation of Zen and others (1983) in Massachusetts.

Biotite-hornblende mafic dikes (Proterozoic Z) at surface, covers < 0.1 % of this area

Biotite-hornblende mafic dikes - Locally with relict subophitic texture. Intrudes Yw and Ygg.

Lithology: diorite

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

Monson Gneiss - Lenses of peridotite, variously altered.

Lithology: peridotite

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

Partridge Formation - Rusty weathering carbonaceous mica schist locally containing porphyroblasts of biotite, garnet, or staurolite. (Southeastern Vermont).

Lithology: mica schist

Rangeley Formation, undivided (Lower Silurian (Llandoverian)) at surface, covers < 0.1 % of this area

Rangeley Formation, undivided.

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

Belchertown Complex (intrudes De) - Biotite tonalite of marginal stocks.

Lithology: tonalite

Fine-grained hornblende diorite (Precambrian to Paleozoic) at surface, covers < 0.1 % of this area

Fine-grained hornblende diorite - In Connecticut River bed, near French King Rock.

Lithology: diorite

Gray biotite granite (Permian) at surface, covers < 0.1 % of this area

Gray biotite granite - Contains minor muscovite. Found in Milford quadrangle.

Lithology: granite

Kinsman Granodiorite (Early Devonian ) at surface, covers < 0.1 % of this area

Kinsman Granodiorite - (Kinsman Quartz Monzonite of Billings, 1955) - Foliated granite, granodiorite, tonalite, and minor quartz diorite; large megacrysts of potassium feldspar characteristic; garnet locally abundant.

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

Mount Mineral Formation (Probably correlates with Poplar Mountain Gneiss but is more aluminous) - Lenses of partially serpentinized harzburgite containing abundant veins of anthophyllite.

Quartz diorite (Precambrian to Phanerozoic) at surface, covers < 0.1 % of this area

Quartz diorite - Mostly nonfoliated.

Lithology: quartz diorite

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

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

Mylonite along Connecticut Valley border fault.

Lithology: mylonite

Serpentinized peridotite stocks (Ordovician to Proterozoic Z) at surface, covers < 0.1 % of this area

Serpentinized peridotite stocks - Intrudes Yw and Ygg.

Ammonoosuc Volcanics, Bimodal volcanic rocks (Middle - Upper Ordovician) at surface, covers < 0.1 % of this area

Ammonoosuc Volcanics, Bimodal volcanic rocks - Locally includes unmapped Oals.

Lithology: bimodal suite

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

Ammonoosuc Volcanics - Hornblendite (ultramafic rock).

Lithology: hornblendite

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

Ammonoosuc Volcanics - Basal quartzite and conglomerate.

Lithology: quartzite; conglomerate

Narragansett Bay Group - Wamsutta Formation (Pennsylvanian) at surface, covers < 0.1 % of this area

Narragansett Bay Group - Wamsutta Formation - Red sandstone, shale, and conglomerate, locally containing abundant volcanic detritus as clasts and matrix. Plant fossil localities occur in adjacent Massachusetts. Minor, but significant amounts of interstratified bimodal-composition volcanic lava flows consisting of alkalic basalt (locally pillowed) and rhyolite are present in adjacent Massachusetts.

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.

Narragansett Bay Group - Sachuest Arkose (Pennsylvanian) at surface, covers < 0.1 % of this area

Narragansett Bay Group - Sachuest Arkose - Gray, smoky-quartz granule-conglomerate, sandstone, and pebble to cobble conglomerate, interbedded with black carbonaceous phyllite. Includes some rock mapped formerly as Pondville Conglomerate.

Narragansett Bay Group - Dighton Conglomerate (Pennsylvanian) at surface, covers < 0.1 % of this area

Narragansett Bay Group - Dighton Conglomerate - Gray conglomerate consisting predominantly of quartz clasts set in a sand-sized matrix. Minor lenses of litharenite and arkosic sandstone.

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

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

Lithology: dacite

Fitchburg Complex (Lower Devonian or younger) at surface, covers < 0.1 % of this area

Fitchburg Complex - Inclusions of massive coarse-grained biotite-hornblende tonalite within Dfgd.

Lithology: tonalite

Newburyport Complex (Late Silurian) at surface, covers < 0.1 % of this area

Newburyport Complex - Medium-grained porphyritic granite.

Lithology: granite

Walloomsac Schist (Middle Ordovician) at surface, covers < 0.1 % 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

Diorite at Goff Ledges (Ordovician) at surface, covers < 0.1 % 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

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

Belchertown Complex (intrudes De) - Hornblende peridotite.

Lithology: peridotite

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.

Two-mica granite of northern and southeastern New Hampshire (Early - Late Devonian) at surface, covers < 0.1 % of this area

Two-mica granite of northern and southeastern New Hampshire - Similar to Concord Granite.

Lithology: granite

Sherman Marble (Proterozoic Y) at surface, covers < 0.1 % of this area

Sherman Marble - White, coarse-grained graphite dolomite-calcite marble at Sherman Reservoir at the State line.

Lithology: marble

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

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

Clough Quartzite - Orthoquartzite, quartz metaconglomerate, muscovite schist, minor polymictic metaconglomerate. Disconformable below Fitch Formation and unconformable on Ordovician formations. Equivalent, in part, to member C of Rangeley Formation of Maine. Fossiliferous.

Cambrian thru Middle Ordovician carbonate rock (Cambrian - Middle Ordovician) at surface, covers < 0.1 % of this area

Cambrian thru Middle Ordovician (Barneveld) carbonate rock - occuring as slivers caught along thrusts of later allochthones, or carbonate blocks in Taconic Melange.

Lithology: carbonate; melange

Unit b [of Stockbridge Marble] (Upper and Middle? Cambrian) at surface, covers < 0.1 % 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.

Ayer Granodiorite (Early Silurian) at surface, covers < 0.1 % of this area

Ayer Granodiorite - Gneissic granite to tonalite, locally coarsely porphyritic and muscovitic, southeastern New Hampshire.

Massabesic Gneiss Complex (Late Proterozoic) at surface, covers < 0.1 % of this area

Massabesic Gneiss Complex - Migmatite consisting of pink, foliated biotite granite intruding gneissic and granulose metasedimentary and metavolcanic rocks in southeastern New Hampshire.

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.

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).

Newburyport Complex (early Late Silurian) at surface, covers < 0.1 % of this area

Newburyport Complex - Gray, medium-grained tonalite and granodiorite.

Lithology: tonalite; granodiorite

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

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

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.

Mesoperthitic granite (Jurassic) at surface, covers < 0.1 % of this area

Mesoperthitic granite - Contains riebeckite and (or) hastingsite.

Lithology: granite

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?) at surface, covers < 0.1 % of this area

Other metasedimentary rocks - metaclastic rock, undivided - Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.

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

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