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Geologic units in Essex county, Massachusetts

Sharpners Pond Diorite (Silurian) at surface, covers 10 % of this area
Sharpners Pond Diorite - Non-foliated, medium-grained equigranular biotite-hornblende tonalite and diorite. Intrudes Soagr, OZn, OZf.
Lithology: tonalite; diorite
Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks (Proterozoic Z) at surface, covers 2 % of this area
Metamorphosed mafic to felsic flow, and volcaniclastic and hypabyssal intrusive rocks - Includes some diorite and gabbro north and northwest of Boston.
Lithology: metavolcanic rock; mixed clastic/volcanic; diorite; gabbro
Diorite at Rowley (Proterozoic Z) at surface, covers 0.5 % of this area
Diorite at Rowley - Dark, green-gray, medium-grained hornblende diorite.
Lithology: diorite
Diorite and tonalite (Devonian and Silurian) at surface, covers 0.2 % 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
Cherry Hill Granite (Devonian) at surface, covers 0.3 % of this area
Cherry Hill Granite - Alaskite granite containing ferro-hornblende. Intrudes Dwn.
Lithology: alkali-granite (alaskite)
Berwick Formation (Silurian) at surface, covers 0.2 % 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
Eliot Formation (Silurian) at surface, covers 5 % 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
Mesoperthitic granite (Jurassic) at surface, covers < 0.1 % of this area
Mesoperthitic granite - Contains riebeckite and (or) hastingsite.
Lithology: granite
Wenham Monzonite (Middle Devonian) at surface, covers 0.9 % of this area
Wenham Monzonite - Monzonite containing ferro-hornblende.
Lithology: monzonite
Nahant Gabbro and gabbro at Salem Neck (Ordovician) at surface, covers 0.2 % 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
Newburyport Complex (Late Silurian) at surface, covers < 0.1 % of this area
Newburyport Complex - Medium-grained porphyritic granite.
Lithology: granite
Merrimack Group, Eliot Formation (Ordovician? - Silurian?) at surface, covers < 0.1 % of this area
Merrimack Group, Eliot Formation - Gray to green phyllite, calcareous quartzite, quartz-mica schist, and well-bedded calc-silicate.
Lithology: phyllite; quartzite; mica schist; calc-silicate rock
Newburyport Complex (Silurian or Ordovician) at surface, covers 4 % 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
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
Red arkosic conglomerate, sandstone, and siltstone (Upper Triassic) at surface, covers 0.2 % of this area
Red arkosic conglomerate, sandstone, and siltstone - In Essex County.
Lithology: conglomerate; sandstone; siltstone
Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers 0.3 % of this area
Newbury Volcanic Complex - Undivided sedimentary and volcanic rocks.
Lithology: sedimentary rock; volcanic rock (aphanitic)
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
Andover Granite (Silurian or Ordovician) at surface, covers 8 % 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
Chelmsford Granite (Lower Devonian) at surface, covers < 0.1 % of this area
Chelmsford Granite - Light-gray, even and medium-grained, muscovite-bearing granite; locally foliated; intrudes Sb.
Lithology: granite
Berwick Formation (Silurian) at surface, covers 14 % 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).
Lithology: metasedimentary rock; mica schist
Dedham Granite (Proterozoic Z) at surface, covers 1 % 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
Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers 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
Fish Brook Gneiss (Ordovician or Proterozoic Z) at surface, covers 2 % of this area
Fish Brook Gneiss - Light-gray, biotite-plagioclase quartz gneiss; distinctive "swirl-form" foliation.
Lithology: granitic gneiss
Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers < 0.1 % 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).
Lithology: schist; gneiss; amphibolite; biotite gneiss; marble
Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers 0.9 % of this area
Newbury Volcanic Complex - Lower members. Basalt, andesite, rhyolite, and tuff.
Lithology: basalt; andesite; rhyolite; tuff
Merrimack Group, Berwick Formation (Ordovician? - Silurian?) at surface, covers 0.2 % 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
Lithology: granofels; quartz-feldspar schist; meta-argillite
Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite (Silurian) at surface, covers 2 % of this area
Orange-pink, rusty-weathering, medium- to coarse-grained biotite granite to granodiorite - Locally porphyritic. Intrudes Ssqd.
Lithology: granite; granodiorite
Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers 15 % 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).
Lithology: alkali-granite (alaskite); quartz syenite
Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers 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
Cambridge Argillite (Proterozoic Z to earliest Paleozoic) at surface, covers 0.5 % 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).
Lithology: argillite; quartzite; sandstone; conglomerate
Peabody Granite (Middle Devonian) at surface, covers 3 % of this area
Peabody Granite - Alkalic granite containing ferro-hornblende. Intrudes Zgb, Zdngr.
Lithology: alkali-granite (alaskite)
Nashoba Formation (Ordovician or Proterozoic Z) at surface, covers 4 % 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).
Lithology: amphibolite; biotite gneiss
Newburyport Complex (Silurian) at surface, covers 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
Newburyport Complex (early Late Silurian) at surface, covers < 0.1 % of this area
Newburyport Complex - Gray, medium-grained tonalite and granodiorite.
Lithology: tonalite; granodiorite
Lynn Volcanic Complex (Lower Devonian, Silurian, or Proterozoic Z) at surface, covers 2 % of this area
Lynn Volcanic Complex - Rhyolite, agglomerate and tuff.
Lithology: rhyolite; volcanic breccia (agglomerate); tuff
Diorite and gabbro (Proterozoic Z) at surface, covers 10 % of this area
Diorite and gabbro - Complex of diorite and gabbro, subordinate metavolcanic rocks and intrusive granite and granodiorite.
Lithology: diorite; gabbro; metavolcanic rock; granite; granodiorite
Cape Ann Complex (Lower Silurian or Upper Ordovician) at surface, covers 0.4 % 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
Newbury Volcanic Complex (Lower Devonian and Upper Silurian) at surface, covers 0.4 % 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
Shawsheen Gneiss (Ordovician or Proterozoic Z) at surface, covers < 0.1 % of this area
Shawsheen Gneiss - Sillimanite gneiss, sulfidic at base; minor amphibolite.
Lithology: gneiss; amphibolite
Topsfield Granodiorite (Proterozoic Z) at surface, covers 4 % 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).
Lithology: quartzite; schist; amphibolite; argillite

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