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

Geologic units in Windham county, Vermont

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

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

Lithology: schist; granofels; amphibolite

Trondhjemite (Late Ordovician) at surface, covers < 0.1 % of this area

Trondhjemite - Contains biotite and hornblende.

Lithology: trondhjemite

Mount Holly Complex, calcite and dolomite marbles (Precambrian) at surface, covers < 0.1 % of this area

Mount Holly Complex, calcite and dolomite marbles - locally coarse grained; commonly contain phlogopite, actinolite, and diopside, and are interbedded with medium- to coarse-grained calc-silicate granulite; includes minor amounts of other types of Precambrian rock.

Lithology: marble; granulite; metamorphic rock

Hoosac Formation (Cambrian) at surface, covers 8 % of this area

Hoosac Formation - Quartz-sericite-albite-biotite-chlorite schist characterized by albite porphyroblasts - biotite and garnet porphyroblasts common southward; locally carbonaceous. (Southern and Central Vermont). First revision is restriction of Tyson Formation and its replacement by Hoosac Formation in this quad. Cover rocks overlying basement of West River antiformal sheath fold (in hanging-wall of Ball Mountain thrust) consist of albitic schist locally containing pods of white dolomite and discontinuous basal beds of vitreous quartzite and interbedded dolomite as much as 15 m thick. These rocks were previously mapped as Tyson Formation by Doll and others (1961, State geologic map) and Karabinos (1984) and are now mapped as Hoosac Formation in this quad because of the presence of quartzite and dolomite locally contained within rusty albitic schist and granofels rocks typical of Hoosac. Similarly, cover rocks of Jamaica antiformal sheath fold consist of a 10-m-thick basal and quite continuous belt of dolomite marble that contains thin beds of vitreous quartzite. This unit was also mapped as Tyson by Doll and others (1961) and by Karabinos (1984) and is now mapped as Hoosac. Second revision is that Turkey Mountain Member is formalized as a member of Hoosac Formation to include all metabasalts within the formation. Exposed for a distance of 3 km northwest of Townshend. Consists of a collection of massive black amphibolite layers, 1 to 2 m thick, interlayered with epidotitic and quartz-rich, laminated greenstones. Total thickness of interbedded amphibolite and associated metasedimentary rock on Turkey Mountain is as much as 200 m. Termination of Turkey Mountain Member northward appears to result from thinning to the north although fault truncation along its lower contact cannot be ruled out. To the south, Turkey Mountain Member also appears to thin by interbedding with enclosing metasedimentary rocks, and pinches out north of Townshend. Where several layers in a limited region can be mapped separately, they are each referred to informally by use of subscripts in the letter symbol; this, however, does not imply correlation of numbered layers between different areas of the map (Ratcliffe, in press).

Lithology: mica schist

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

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

Lithology: mica schist

Missisquoi Formation, Moretown Member (Ordovician) at surface, covers 11 % of this area

Missisquoi Formation, Moretown Member - Quartzite and quartz-plagioclase granulite, in layers 1/8 to several inches thick, separated by "pinstripe" partings that contain muscovite, chlorite, epidote, biotite, and locally garnet; also greenish quartz-sericite-chlorite phyllite and schist, and minor carbonaceous phyllite. Schist and phyllite commonly contain biotite and garnet porphyroblasts in southern Vermont.

Lithology: granulite; quartzite; phyllite; 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

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

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.

Lithology: meta-argillite; meta-conglomerate; calc-silicate rock

Hoosac Formation, Amphibolite and Greenstone (Cambrian) at surface, covers 0.3 % of this area

Hoosac Formation, Amphibolite and Greenstone - Amphibolite and actinolitic greenstone. (Southern and Central Vermont).

Lithology: amphibolite; greenstone

Pinney Hollow Formation, Ottauquechee, and Stowe Formations, Undifferentiated (Ordovician) at surface, covers 1 % of this area

Pinney Hollow Formation, Ottauquechee, and Stowe Formations, Undifferentiated - Includes quartz-muscovite-garnet-chlorite-biotite schist, rusty carbonaceous schist, amphibolite, and schistose quartzite. Schist locally contains porphyroblasts of staurolite and kyanite. On flanks of Chester and Athens domes.

Lithology: mica schist; schist; amphibolite; quartzite

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.

Lithology: slate; schist; metasedimentary rock

Cavendish Formation, Readsboro Member (Cambrian?) at surface, covers 4 % of this area

Cavendish Formation, Readsboro Member - Quartz-muscovite schist containing biotite or chlorite and characterized by conspicuous porphyroblasts of sodic plagioclase; less commonly quartz-muscovite-paragonite schist containing chlorite, garnet, or chloritoid, or, in Chester dome, quartz-muscovite-paragonite schist containing garnet, staurolite, and locally kyanite (Gassetts schist). The Cavendish Formation is reinstated and considered part of the Mount Holly Complex in VT. Usage follows Thompson (1950), but is extended to include some rocks on Star Hill, including inner and outer cover rocks assigned by Downie (1982) to Hoosac and Pinney Hollow Formations. Formation is divided into four map units: calc-silicate rock and gneiss, marble, feldspathic schist or granofels, and the Gassetts Schist Member. The Cavendish correlates with the Wilcox Formation of the Mount Holly Complex in the Green Mountain massif, and therefore, is of Middle Proterozoic age (Ratcliffe, in press).

Lithology: mica schist; schist

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

Hoosac Formation - Undifferentiated Hoosac Formation.

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

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.

Lithology: metasedimentary rock; metavolcanic rock

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

Bethlehem Granodiorite (Bethlehem Gneiss of Billings, 1955) - Gray, strongly foliated biotite-muscovite granodiorite and associated tonalite and granite.

Lithology: granodiorite; tonalite; granite

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

Clough Formation - Quartzite, quartz-conglomerate, and mica schist; lenses of fossiliferous calcareous quartzite in upper part. (Southeastern Vermont).

Lithology: conglomerate; quartzite; mica schist

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

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

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

Fitch Formation - Quartz-plagioclase-biotite granulite; actinolite-diopside granulite; impure limestone and dolomite; mica schist; the carbonate-rich beds are typically an inch or two thick and segmented so as to give the weathered outcrop a characteristic pitted appearance. (Southeastern Vermont).

Lithology: granulite; limestone; dolostone (dolomite); mica schist

Mount Holly Complex, quartzite and schist (Precambrian) at surface, covers 2 % of this area

Mount Holly Complex, quartzite and schist - Quartzite, locally in massive beds as much as 30 ft thick, micaceous quartzite, and quartz-mica schist that commonly contains garnet or pseudomorphs (largely chlorite) after garnet; schists are locally rusty weathered and contain conspicuous flakes of graphite; also includes amphibolite and minor hornblende gneiss, biotite gneiss, and pegmatite.

Lithology: quartzite; mica schist; amphibolite; mafic gneiss; biotite gneiss; pegmatite

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

Pinney Hollow Formation (Cambrian) at surface, covers 3 % of this area

Pinney Hollow Formation - Pale green quartz-sericite (muscovite-paragonite)-chlorite phyllite and schist with abundant magnetite, chloritoid phyllite and schist, quartz-sericite-albite-chlorite schist, and rare beds of carbonaceous and schistose quartzite; garnet porphyroblasts common south of Ottauquechee River. (Southern and Central Vermont).

Lithology: phyllite; mica schist; schist; quartzite

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

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

Lithology: schist; quartzite; amphibolite

Waits River Formation (Devonian) at surface, covers 13 % of this area

Waits River Formation - Gray quartzose and micaceous crystalline limestone weathered to distinctive brown earthy crust; interbedded and intergradational with gray quartz-muscovite phyllite or schist. Where more metamorphosed the limestones contain actinolite, hornblende, zoisite, diopside, wollastonite, and garnet, and the phyllite and schist, biotite, garnet, and locally andalusite, kyanite or sillimanite.

Lithology: limestone; phyllite; mica schist

Northfield Formation (Devonian - Silurian) at surface, covers 5 % of this area

Northfield Formation - Dark gray to black quartz-sericite slate or phyllite with fairly widely-spaced interbeds a few inches thick of siltstone and silty crystalline limestone like that of the Waits River Formation; calcareous slate north of Lamoille River; phyllite passes into gray quartz-sericite schist containing abundant porphyroblasts of biotite and garnet in southern Vermont.

Lithology: slate; phyllite; mica schist; siltstone; limestone

Ultramafic Rocks (Ordovician) at surface, covers 0.1 % of this area

Ultramafic Rocks - Serpentinite, carbonate rock, talc-carbonate rocks and steatite.

Lithology: serpentinite; intrusive carbonatite

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

Partridge Formation, Volcanics - fine-grained biotite gneiss and amphibolite. (Southeastern Vermont).

Lithology: biotite gneiss; amphibolite

Stowe Formation, greenstone and amphibolite (Cambrian-Ordovician) at surface, covers 0.2 % of this area

Stowe Formation, greenstone and amphibolite - Epidote-albite-chlorite rocks contain actinolite and hornblende where more metamorphosed.

Lithology: greenstone; amphibolite

Mount Holly Complex (Precambrian) at surface, covers 24 % of this area

Mount Holly Complex - Mainly fine- to medium-grained biotitic gneiss, locally muscovitic, and in western areas chloritic; massive and granitoid in some localities, fine-grained or schistose and compositionally layered in others; also abundant amphibolite and hornblende gneiss, and minor beds of mica schist, quartzite, and calc-silicate granulite; includes numerous small bodies of pegmatite and gneissoid granitic rock. Includes a suite of metatonalites, metatrondhjemite, and possible metadacite with chemical characteristics of a calc-alkaline volcanic-plutonic suite. Mappable units are College Hill Granite Gneiss and 10 unnamed subdivisions including several varieties of gneiss as well as schist, amphibolite, and quartzite. U-Pb zircon upper intercept ages of 1.35 to 1.30 Ga have been determined and interpreted as age of crystallization (Ratcliffe and others, unpub. data). Cores of abraded zircon obtained from College Hill Granite Gneiss of Mount Holly Complex have a U-Pb upper intercept age of 1245 +/-14 Ma, interpreted as crystallization age for that granite (Aleinikoff and others, 1990). Dust collected by abrasion of zircons, thought to represent migmatitic overgrowth, has a Pb-Pb age of approx 1100 Ma. These data suggest that College Hill Granite Gneiss was intruded at 1245 Ma and migmatized at 1100 Ma. On north and south slopes of College Hill, College Hill Granite Gneiss grades outward into migmatitic biotite granite gneiss of Mount Holly Complex. College Hill is discordant to contacts and folds in paragneiss units of Mount Holly Complex. Dacitic metavolcanics are found within Washington Gneiss of Berkshire massif of MA (Ratcliffe and Zartman, 1968). They are interbedded with thick succession of rusty-weathering, quartz-pebble gneisses, calc-silicate rocks and garnet-sillimanite schist similar to, but much thicker than, the rusty-weathering gneiss and schist unit of Mount Holly Complex exposed in Green Mountains of VT. It is possible that the metadacitic and metatrondhjemitic suite of VT constitutes a lateral, south-to-north facies of the Washington Gneiss of MA (Ratcliffe, in press).

Lithology: biotite gneiss; amphibolite; mafic gneiss; mica schist; quartzite; granulite; pegmatite; granite

Ottauquechee Formation (Cambrian) at surface, covers 1 % of this area

Ottauquechee Formation - Black carbonaceous phyllite or schist containing interbeds of massive quartzite commonly criss-crossed by veins of white quartz; quartzite is dark gray and carbonaceous, light gray, or white; also includes light green quartz-sericite-chlorite phyllite or schist and sercitic quartzite; beds of phyllitic graywacke and feldspar granule conglomerate are north of Lamoille River. Schist contains abundant porphyroblasts of garnet and biotite from Ludlow south. The Ottauquechee contains two major units: A black phyllite and the Thatcher Brook Member. The black phyllite contains a previously unreported sub-unit of gray carbonate schist. The Thatcher Brook Member (named in an abstract by Armstrong and others, 1988) is a carbonaceous albitic schist with greenstones and ultramafics. These rocks have previously been included in the Ottauquechee but have never been differentiated from the black phyllite. Member is in fault contact with the silvery green schist of the Pinney Hollow Formation to the west. Age is Cambrian (Ratcliff, in press).

Lithology: phyllite; schist; quartzite; graywacke; conglomerate

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).

Lithology: amphibolite; amphibole schist

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

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

Lithology: granofels; schist

Waits River formation, Standing Pond Volcanic Member (Devonian) at surface, covers 2 % of this area

Waits River formation, Standing Pond Volcanic Member - Amphibolite, garnet amphibolite, coarse garnet schist with fasciculitic hornblende, and hornblende maculite; contains pillow lavas near St. Johnsbury and passes eastward into actinolitic greenstone and greenstone south of Windsor.

Lithology: amphibolite; schist; greenstone

Hoosac Formation, Turkey Mountain Member (Cambrian) at surface, covers 0.5 % of this area

Hoosac Formation, Turkey Mountain Member - Amphibolite and actinolitic greenstone characterized by oval, 1/8 to 3/8 inch spots, chiefly of epidote. (Southern and Central Vermont). Discontinuous lenses of metabasalt, informally referred to as Turkey Mountain metabasalt member of Hoosac Formation, actually occur at different stratigraphic positions extending through a stratigraphic distance of 100 to 400 m above base of Hoosac Formation. From type area on Turkey Mountain in Saxtons River quad south to Massachusetts State line, basalts form at least three relatively persistent units (Ratcliffe, 1991). Basalt mapped in northeast corner of this map, above Wilmington thrust system, correlates with type Turkey Mountain. As used here, Turkey Mountain metabasalt member consists of several laterally and vertically discontinuous, nonidentical flows and volcaniclastic deposits including, but not restricted to, type Turkey Mountain Member of Hoosac Formation as used by Doll and others (1961) and by Skehan (1961). They mapped the lower basalts as unnamed greenstones in Hoosac Formation. Turkey Mountain metabasalt member consists of light-green to dark-green epidote-amphibole greenstones and amphibolite metabasalts. Metabasalt varies from massive to very well layered. Finely laminated, quartzose and epidotitic volcaniclastic beds several centimeters thick are interlayered with more massive, strongly foliated, black amphibolite. Where in contact with surrounding metasedimentary rocks, layering within metabasalt and volcaniclastic beds is concordant and gradational with enclosing metasediment. Light-gray or yellowish-greenish-gray, well-laminated quartzite or, less commonly, gritty, pebbly conglomerate 0.5 to 5 m thick marks upper contact with unnamed granofels member of Hoosac Formation. Base of Turkey Mountain metabasalt member in contact with rusty muscovite-albite-biotite schist. Metabasalts probably originated as thin composite basalt lava flows that contained intercalated basaltic volcaniclastic rocks. Age is Late Proterozoic and Early Cambrian (Ratcliffe, 1993).

Lithology: amphibolite; greenstone

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

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

Lithology: amphibolite; greenstone; schist; granofels

Biotite Quartz Diorite Gneiss (Devonian) at surface, covers 0.7 % of this area

Biotite quartz diorite gneiss.

Lithology: granitic gneiss

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

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

Lithology: schist; phyllite

Undifferentiated Granitic Gneiss (Devonian) at surface, covers 0.4 % of this area

Undifferentiated Granitic Gneiss - Small dikes and sills do not show.

Lithology: granitic gneiss

Fitch Formation (Upper Silurian; Pridolian and Ludlovian) (Upper Silurian - (Pridolian and Ludlovian)) at surface, covers < 0.1 % of this area

Fitch Formation - Metamorphosed limestone, calcareous sandstone, siltstone, and dark pelitic schist; lower contact is disconformable on the Clough Quartzite. Fossiliferous.

Lithology: marble; quartzite; phyllite; pelitic schist

Missisquoi Formation (Ordovician) at surface, covers 2 % of this area

Missisquoi Formation - Rusty weathering carbonaceous mica schist, quartzite and micaceous quartzite.

Lithology: mica schist; quartzite

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

Bethlehem Gneiss - Two-mica granodiorite gneiss. Revised as the Bethlehem Granodiorite to emphasize its average granodiorite composition. Mapped as the Haverhill, Mount Clough, Indian Pond, and Fairlee plutons. Consists of strongly metamorphosed, foliated, gray, medium-grained biotite granodiorite and local tonalite and granite. Age is Devonian based on isotopic age of 410+/-5 Ma (J.N. Aleinikoff, this report, from Fairlee pluton), which is also consistent with age of 409+/-5 Ma reported by Kohn and others (1992) from Indian Pond pluton and Bellows Falls pluton (the latter is outside of map area in southwestern NH) (Moench and others, 1995).

Lithology: granitic gneiss

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

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

Shaw Mountain Formation - Quartzite, quartz conglomerate, cummingtonite schist, amphibolite, and quartz-sericite schist with porphyroblasts of biotite and garnet.

Lithology: quartzite; conglomerate; schist; amphibolite; mica schist

Gile Mountain Formation (Lower Devonian) at surface, covers < 0.1 % 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.

Lithology: phyllite; schist; quartzite; granofels; marble

Littleton Formation (Devonian) at surface, covers 6 % 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.

Lithology: slate; phyllite; quartzite; schist; mica schist

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

Cavendish Formation, Dolomite and Marble (Cambrian?) at surface, covers 0.2 % of this area

Cavendish Formation, Dolomite and Marble - Buff dolomite; minor white to pink calcite marble; actinolitic and diopsidic marbles and beds of actinolite diopside granulite common in Chester dome. The Cavendish Formation is reinstated and considered part of the Mount Holly Complex in VT. Usage follows Thompson (1950), but is extended to include some rocks on Star Hill, including inner and outer cover rocks assigned by Downie (1982) to Hoosac and Pinney Hollow Formations. Formation is divided into four map units: calc-silicate rock and gneiss, marble, feldspathic schist or granofels, and the Gassetts Schist Member. The Cavendish correlates with the Wilcox Formation of the Mount Holly Complex in the Green Mountain massif, and therefore, is of Middle Proterozoic age (Ratcliffe, in press).

Lithology: dolostone (dolomite); marble; granulite

Gile Mountain Formation (Devonian) at surface, covers 3 % of this area

Gile Mountain Formation - Gray quartz-muscovite phyllite or schist, interbedded and intergradational with gray micaceous quartzite (graywacke northeast of Nulhegan River), calcareous mica schist, and, locally, quartzose and micaceous crystalline limestone like that of the Waits River formation. The phyllite and schist commonly contain porphyroblasts of biotite, garnet, or staurolite, and locally kyanite, andalusite, or sillimanite. Used as Early Devonian Gile Mountain Formation. Generally consists of gray to tan metawacke and schist or phyllite, gradational into its Meetinghouse Slate Member, but much more thickly bedded and less pelitic. Contains minor metavolcanic lentils. Unnamed metavolcanic member is possibly equivalent to Putney Volcanics of southeastern VT. Separately mapped interbedded gray slate or phyllite and brown-weathering calcite-ankerite metasiltstone, and minor marble and quartzite, resembles Waits River Formation of VT. Meetinghouse Slate Member consists of gray to black phyllite and silty metasandstone turbidite. Report includes geologic map, cross sections, correlation chart, and four 1:500,000-scale derivative maps (Lyons and others, 1997).

Lithology: phyllite; mica schist; quartzite; limestone; graywacke

Missisquoi Formation, Barnard Volcanic Member (Ordovician) at surface, covers 5 % of this area

Missisquoi Formation, Barnard Volcanic Member - Fine- to medium-grained biotite gneiss, hornblende gneiss, and amphibolite.

Lithology: biotite gneiss; mafic gneiss; amphibolite

Cavendish Formation, Bull Hill Gneiss (Cambrian?) at surface, covers 1 % of this area

Cavendish Formation, Bull Hill Gneiss - Quartz-plagioclase-microcline-biotite gneiss characterized in many areas by augen of microcline as much as 2 inches long; fine- to medium-grained quartz-plagioclase-biotite or biotite-muscovite gneiss. Cardinal Brook Intrusive Suite is here named in the cores of the Chester-Athens dome and Rayponda-Sadawga dome in the eastern and southern Green Mountains, VT, and the northern part of the Berkshire massif, MA. Includes the Stamford Granite of Hitchcock (1861), the Somerset Reservoir Granite (new name), the Harriman Reservoir Granite (new name), and the Bull Hill Gneiss of Richardson (1929-30). Because of uncertainty regarding the geologic position of the Bull Hill, it is restricted to the Chester and Athens domes and the original definition of Richardson is retained. Rocks mapped as Bull Hill in the Jamaica area are assigned to the Somerset Reservoir Granite and those in the Rayponda-Sadawga dome are assigned to the Harriman Reservoir Granite. Structural position is unclear. U-Pb zircon age is Middle Proterozoic (960-950 Ma). . [GNU Staff note--This report mistakenly uses the phrasing "Bull Hill Gneiss of Richardson (1929-30)" which would normally imply that the unit has not been adopted for use by the U.S. Geological Survey (USGS) because of inadequate definition by Richardson or successive workers in the area. However, in this report, the phrasing simply means that Richardson's definition and use are preferred over the usage on the VT State Geologic Map of Doll and others (1961) and that the unit meets the requirements for formal usage by the USGS.] (Ratcliffe, 1991).

Lithology: gneiss; biotite gneiss

Pinney Hollow Formation, Chester Amphibolite Member (Cambrian) at surface, covers 1 % of this area

Pinney Hollow Formation, Chester Amphibolite Member - Thin-layered, ligniform amphibolite and hornblende schist; includes actinolitic greenstone and greenstone north of Windham. (Southern and Central Vermont).

Lithology: amphibolite; amphibole schist; greenstone

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

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

Ultramafic Rocks (Ordovician) at surface, covers 0.2 % of this area

Ultramafic Rocks - Dunite, peridotite, and serpentinite

Lithology: dunite; peridotite; serpentinite

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

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

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

Stowe Formation (Cambrian-Ordovician) at surface, covers 1 % of this area

Stowe Formation - Quartz-sericite (muscovite-paragonite)-chlorite phyllite and schist; porphyroblasts of albite, garnet, chloritoid, or kyanite common locally; includes phyllitic graywacke north of Lamoille River. Schist contains abundant segregations of granular white quartz. The Stowe Formation in the study are contains two unnamed members: a silvery green schist and a greenstone. The schist is a fine-grained, silvery to dark green quartz-muscovite-albite-chlorite schist. It is in fault contact with the black phyllite of the Ottauquechee Formation. The greenstone is a homogenous, fine-grained, light green actinolite-albite-epidote-calcite-chlorite schist. Large outcrops of the resistant greenstone are common. Age according to map symbols is Proterozoic and Cambrian. Unit is correlated with the Rowe Schist (of Zen, 1983). [Rowe Schist on 1983 MA map is Cambrian and Ordovician. No explanation here for older age.] (Walsh, 1992).

Lithology: phyllite; mica schist; graywacke

Undifferentiated Gneiss (Precambrian) at surface, covers 0.4 % of this area

Undifferentiated Gneiss - Undifferentiated gneissic biotite granite, quartz monzonite, and granodiorite.

Lithology: granitic gneiss; quartz monzonite; granodiorite

Ammonoosuc Volcanics (Ordovician) at surface, covers 0.4 % of this area

Ammonoosuc Volcanics - Fine-grained chloritic and biotitic gneiss and greenstone in areas north of Bellows Falls; biotite gneiss and amphibolite south of Bellows Falls. (Southeastern Vermont).

Lithology: biotite gneiss; greenstone; amphibolite