Geologic units in Addison county, Vermont

Stony Point Formation (Upper Ordovician) at surface, covers 9 % of this area

Dark-gray calcareous shale with beds of bluish-gray limestone. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Cheshire Quartzite (Lower Cambrian) at surface, covers 7 % of this area

Light-gray- to tannish-gray-weathering, massive to poorly bedded vitreous quartzite. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Dunham Dolostone (Lower Cambrian) at surface, covers 5 % of this area

Buff- and pink-mottled and massive, or light-gray, pinkish-gray-weathering, and massive to poorly bedded dolostone. Contains distinctive small pebbles and grains of well-rounded quartz; minor beds of dolostone-breccia and conglomerate occur near Rutland. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Monkton Quartzite (Middle Cambrian) at surface, covers 5 % of this area

Reddish-brown, pebbly, thin- to thick-bedded sandstone, orangey-gray- and buff-weathering well-bedded dolostone, and reddish-brown-weathering dolomitic quartzite. Unit discontinuous in southern Vermont. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Glens Falls Limestone (Upper Ordovician) at surface, covers 5 % of this area

Dark-bluish-gray-weathering, thinly bedded dark-gray to black granular limestone; locally grades upward into sooty- weathering shaly limestone beds rich in fragments of the trilobite Cryptolithus. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Pinnacle Formation - metawacke member (Cambrian and Neoproterozoic) at surface, covers 4 % of this area

Gray- to light-brownish-gray-weathering, massive to bedded muscovite-biotite-chlorite metawacke, conglomerate, and blue-quartz pebbly phyllite, wacke and feldspathic quartzite. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Middlebury Limestone (Middle Ordovician) at surface, covers 4 % of this area

Buff-streaked, dark-bluish-gray, thinly bedded and well-foliated dolomitic limestone, shown above the Champlain thrust south of Middlebury. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Chipman Formation - Bascom Formation (Middle and Lower Ordovician) at surface, covers 3 % of this area

Interbedded orangey-tan- to buff-weathering dolostone and bluish-gray to gray mottled dolomitic limestone or calcite marble and calcareous sandstone. In southern Vermont east of the Taconic Range, rocks mapped as Bascom Formation may include unmapped members of the Chipman Formation. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Pinney Hollow Formation - phyllite member (Cambrian and Neoproterozoic) at surface, covers 3 % of this area

Light-greenish-gray to lustrous pale-green chlorite-muscovite-quartz (±chloritoid±garnet±magnetite) phyllite. Unit is locally albitic and contains minor beds of quartzite. Cover rocks of Green Mountain anticlinorium.

Hortonville Formation - shale and phyllite (Upper Ordovician) at surface, covers 3 % of this area

Dark-gray siliceous shale and phyllite containing thin beds of bluish-gray argillaceous limestone and minor beds of gray to tan quartzite. Grades into limy shales of the Stony Point Formation. Unit mapped west of the Taconic allochthon, and northwest of the Sudbury slice. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Granitic gneiss of Lincoln Mountain massif (Middle Mesoproterozoic) at surface, covers 3 % of this area

Hetero-geneous unit consisting of medium-grained biotite-microcline-plagioclase gneiss and pinkish-gray, medium- to coarse-grained microcline-perthite granitic gneiss. Interpreted as intrusive granitic rock older than Y3 rocks of Chittenden Intrusive Suite. Stratton Mountain Intrusive Suite (Middle Mesoproterozoic) (1,244±8 Ma to 1,221±4 Ma). Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Cutting Dolostone (Lower Ordovician) at surface, covers 3 % of this area

Gray, thinly bedded dolomitic sandstone, grading upward into dolomitic limestone or mottled dolomitic marble. In southern Vermont east of the Taconic Range, unit is not recognized and may appear as sandy zones within rocks mapped as the Shelburne Marble. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Pinnacle Formation - metawacke and phyllite member (Cambrian and Neoproterozoic) at surface, covers 3 % of this area

Light-gray, medium-grained, massive quartz-sericite-chlorite-albite metawacke, gray to grayish-green magnetite-chlorite-muscovite-quartz schist, and phyllite or pebbly phyllite, locally rich in magnetite. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Chipman Formation - Beldens Member (Middle Ordovician) at surface, covers 2 % of this area

Light-gray to creamy-white-weathering fine-grained limestone, orangey-buff-weathering dolostone, and reddish-streaked (hematite) calcite marble. Chipman Formation (mapped south of Wings Point). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Chipman Formation - Bridport Member (Middle Ordovician) at surface, covers 2 % of this area

Thickly bedded, “beeswax-scored,” orangey-beige-weathering, yellowish-gray to light-bluish-gray dolostone, dark-gray fine-grained to aphanitic dolostone, and minor beds of bluish-gray limestone. Transitions eastward into the Beldens Member with addition of limestone beds. Equivalent to Providence Island Dolostone (Ofcpi). Chipman Formation (mapped south of Wings Point). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Mount Abraham Formation (Cambrian and Neoproterozoic) at surface, covers 2 % of this area

Lustrous, silvery-green to bluish-gray, fine- to medium-grained, white mica-chloritoid-quartz-chlorite schist and phyllite, locally with minor garnet and magnetite porphyroblasts (CZap). Distinctive chlorite streaks and 1-cm rusty needles of altered kyanite are common. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Underhill Formation (Cambrian and Neoproterozoic) at surface, covers 2 % of this area

Silvery-green quartz-muscovite-chlorite schist and phyllite, commonly with albite and magnetite; locally contains dolomite. Cover rocks north of the Lincoln Mountain massif.

Winooski Dolostone (Middle Cambrian) at surface, covers 2 % of this area

Well-bedded dolostone weathering beige, cream, and buff, with green, red, or gray phyllite, siliceous partings, and thin beds of blue-quartz-pebble conglomerate and quartzite. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Danby Formation (Upper Cambrian) at surface, covers 2 % of this area

Thin, light-gray beds of vitreous quartzite and crossbedded sandy dolostone. Unit discontinuous in southern Vermont. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Shelburne Marble (Lower Ordovician) at surface, covers 2 % of this area

Predominantly light-gray- to white- and bluish-gray-streaked calcite marble and massive white- and green-streaked calcite marble. Locally contains intermediate dolostone and gray limestone beds. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Stowe Formation - schist and phyllite member (Cambrian and Neoproterozoic?) at surface, covers 2 % of this area

Predominantly fine-grained, lustrous, well-foliated, silvery-green, grayish-green, and bright-green, quartz-ribbed and -knotted, magnetite-chlorite (biotite)-albite (plagioclase)-sericite (muscovite)-quartz phyllite and schist. Locally richly garnetiferous and biotite-flecked schist (CZsgt) at higher grades. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Clarendon Springs Formation (Upper Cambrian) at surface, covers 2 % of this area

Steel-gray-weathering, light-gray, massive calcitic dolostone grading upward into darker, more fissile calcitic dolostone containing white quartz knots near top; unit locally brecciated. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Biotite-quartz-plagioclase gneiss (Middle and Early Mesoproterozoic) at surface, covers 2 % of this area

A widespread, heterogeneous unit of well-layered, predominantly biotite-quartz-plagioclase gneisses containing variable amounts of magnetite, hornblende, and garnet, and little potash feldspar. Plagioclase-rich layers contain epidote-crowded plagioclase and isolated igneous quartz grains and probably are metadacitic volcanics and volcaniclastic rocks. Unit varies from very dark gray biotitic gneiss to light-gray more plagioclase- and quartz-rich gneiss, contains quartz-rich layers, minor amphibolites, rusty-weathering garnetiferous quartzites, and calc-silicates and marbles which locally are mappable. Association suggests an accumulation of volcaniclastic and clastic sediments. Areas of Y1,2bg associated with 1,400- to 1,350-Ma intrusive rocks range down into the Early Mesoproterozoic, whereas the upper parts may be Middle Mesoproterozoic. Rocks mapped as Y1,2bg may not all be correlative. Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Fayston Formation - albite schist member (Lower Cambrian and Neoproterozoic) at surface, covers 2 % of this area

Silvery-green to grayish-green, medium-grained albite-chlorite-muscovite-quartz (±garnet±magnetite) schist with white albite porphyroblasts; resembles albitic schists of the Tyson Formation (CZtab) and green albitic granofels of the Hoosac Formation (CZhgab). Locally contains unmapped light-gray, thin quartzites; salt-and-pepper-colored, medium- to coarse-grained pyrite-magnetite-biotite-albite-quartz schist and gneiss; and silvery-dark-gray to rusty-weathering, medium-grained chlorite-tourmaline-albite-muscovite-quartz schist. The Fayston south of Mount Abraham is interca-lated with rocks of the Tyson Formation and with the schists of the Pinney Hollow Formation. Cover rocks north of the Lincoln Mountain massif.

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

Dark-gray shale with thin discontinuous beds of crossbedded and graded dolomitic siltstone. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

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

Gray, massively bedded fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Chipman Formation - Weybridge Member (Middle Ordovician) at surface, covers 1 % of this area

Light-gray to cream-colored fine-grained calcite marble, and thin dolomitic layers, locally crossbedded; may occur as lenses at several positions within the Beldens Member. Chipman Formation (mapped south of Wings Point). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Orwell Limestone (Upper Ordovician) at surface, covers 1 % of this area

Dove-gray-weathering, black to dark- gray, fine-grained poorly fossiliferous limestone. Distinguished from the Middlebury Limestone with difficulty, especially in the area west of the Sudbury slice. Part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts). Rocks of the Laurentian Margin. Also part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Fairfield Pond Formation (Cambrian and Neoproterozoic) at surface, covers 1 % of this area

Light-gray to light-green, quartz-sericite-chlorite (±magnetite±biotite) phyllite and foliated quartzite. Cover rocks north of the Lincoln Mountain massif.

Hazens Notch Formation - undivided (Cambrian and Neoproterozoic) at surface, covers 1.0 % of this area

Schist, gneiss, and quartzite; dark-rusty-brown graphitic biotite-muscovite-chlorite-quartz (±garnet) schist and gneiss; black albite porphyroblasts, large euhedral pyrite, and beds of dark-gray foliated quartzite are common. Unit includes rusty-weathering schist without graphite and rocks identical to Fayston Formation. Cover rocks north of the Lincoln Mountain massif.

Providence Island Dolostone (Middle and Lower Ordovician) at surface, covers 0.9 % of this area

Light-buff-tan- weathering, massive to thick-bedded, fine-grained dark-gray dolostone; has “beeswax-scored” surfaces; contains thin layers of fossiliferous bluish-gray limestone (largely equivalent to the Bridport Member of the Chipman Formation, north of Orwell). Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Monastery Formation (Cambrian and Neoproterozoic) at surface, covers 0.8 % of this area

Heterogeneous unit consisting of coarse-grained, gray- to rusty-brown-weathering garnet-biotite-muscovite-quartz schist, gray albitic biotite-quartz granofels, well-bedded light-gray to steel-gray biotite, minor epidote-magnetite-actinolite-chlorite feldspathic wacke, and, near base, grayish-green laminated chlorite-muscovite-albite granofels and phyllite. Unit mapped in Hancock and Ripton in part as lateral equivalent of the Tyson Formation to the south and the Underhill Formation to the north. Eastern flank of the Green Mountain massif and eastern domes.

Pinney Hollow Formation - amphibolite and greenstone member (Cambrian and Neoproterozoic) at surface, covers 0.8 % of this area

Dark-green to black plagioclase-biotite-hornblende (±quartz) amphibolite, epidote amphibolite, and ankeritic-chlorite-magnetite-plagioclase (albite) greenstone. Shows all gradations from massive but well-foliated metabasalt to well-bedded basaltic volcaniclastic rock and volcanic metawacke. Cover rocks of Green Mountain anticlinorium.

Tyson Formation - quartz-pebble phyllite and wacke member (Lower Cambrian and Neoproterozoic) at surface, covers 0.8 % of this area

Gray to grayish-green, biotite-chlorite-quartz-pebble phyllite; albitic metawacke is similar to metawacke member of the Pinnacle Formation but more thinly bedded and contains less metawacke. Albitic biotite-(chlorite)-quartz granofels and wacke locally present. Eastern and western flank of the Green Mountain massif.

Pinnacle Formation - undivided (Cambrian and Neoproterozoic) at surface, covers 0.7 % of this area

Gray, foliated muscovite-chlorite-biotite-feldspar-quartz schist, phyllite, and metagraywacke. Quartz is commonly blue, and local thin conglomeratic horizons are present. Feldspathic biotite phyllitic metawacke is interlayered with lenses of quartz, feldspar, and gneiss-pebble to -cobble conglomerate (CZpc). Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Ottauquechee Formation - carbonaceous phyllite member (Cambrian) at surface, covers 0.7 % of this area

Predominantly dark-gray to black, carbonaceous to highly graphitic, fine-grained sulfidic biotite-muscovite-quartz phyllite having silicic laminae. Includes black quartzites not mapped separately. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Chazy Group - undivided (Upper and Middle Ordovician) at surface, covers 0.7 % of this area

Dark- to light-gray, massively bedded fossiliferous limestone and calcareous sandstone. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Underhill Formation - quartz-laminated schist member (Cambrian and Neoproterozoic) at surface, covers 0.6 % of this area

Light-gray to tan and rusty-weathering, fine-grained, well-foliated mylonitic albite-chlorite-muscovite-quartz (±dolomite)-laminated schist. Cover rocks north of the Lincoln Mountain massif.

Hoosac Formation - undivided (Lower Cambrian and Neoproterozoic) at surface, covers 0.5 % of this area

Heterogeneous unit consisting mainly of dark-gray to medium-light-gray-weathering, white-plagioclase-studded schist, gray slabby quartz-rich muscovite (±garnet) schist, and layers of dark-gray biotitic quartzite and metawacke. Eastern flank of the Green Mountain massif and eastern domes.

Moosalamoo Formation - gray phyllite and metawacke member (Lower Cambrian and Neoproterozoic) at surface, covers 0.5 % of this area

Dark-gray, sooty-weathering, splintery sulfidic to non-sulfidic quartz phyllite and pebbly and gritty biotite metawacke. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Forestdale Formation - dolostone member (Lower Cambrian and Neoproterozoic) at surface, covers 0.5 % of this area

Upper part of CZfd consists of orangey-tan- to brown-weathering quartz-pebble dolostone and dolomitic crossbedded feldspathic metasandstone; lower part of CZfd largely cream- to beige-weathering massive dolostone. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Chipman Formation - Burchards Member (Middle Ordovician) at surface, covers 0.4 % of this area

Light-bluish-gray mottled dolomitic limestone. Chipman Formation (mapped south of Wings Point). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Microcline-magnetite augen gneiss at Brandon Gap (Late Mesoproterozoic) at surface, covers 0.4 % of this area

Pinkish-gray to medium-dark-gray, biotite-magnetite-microcline-plagioclase augen gneiss. U-Pb zircon SHRIMP age of 1,149±8 Ma, no. 13 (Aleinikoff and others, 2011). Unit is associated with minor exposures of metadiorite to tonalitic gneisses; crosscuts a gneissosity in country rocks and may extend northward along eastern limb of Lincoln Mountain massif as unit Y3Ama. Chittenden Intrusive Suite (1,149±8 Ma to 1,119±3 Ma). Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Tyson Formation - quartz phyllite member (Lower Cambrian and Neoproterozoic) at surface, covers 0.4 % of this area

Rusty-weathering, gray to grayish-green, chlorite-muscovite-quartz phyllite and minor beds of pebbly-quartz metawacke. Eastern and western flank of the Green Mountain massif.

Bull Formation - Mettawee slate facies (Neoproterozoic) at surface, covers 0.4 % of this area

Predominantly greenish-gray to pale-lustrous-green chlorite-muscovite-quartz phyllite; and green and purple, bedded and mottled phyllite. Locally contains boudins and thin beds of limestone and pods of pinkish-gray to cream-white dolostone, and minor quartzite. Unit interfingers with the West Castleton Formation above and laterally grades into the Bomoseen Graywacke Member. Also shown as CZnm (Mettawee Member of the Nassau Formation of Potter, 1972). Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Black River Group - undivided (Upper Ordovician) at surface, covers 0.4 % of this area

Thin-bedded to massive dolostone, sandy dolostone, and light-gray limestone; vertical burrows in basal beds; upper beds fossiliferous with black chert. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Valcour Formation (Upper Ordovician) at surface, covers 0.3 % of this area

Dark- to light-gray fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

West Bridgewater Formation - dark-gray and green phyllite member (Middle Ordovician) at surface, covers 0.3 % of this area

Interbedded dark-gray and grayish-green chlorite-muscovite-quartz-knotted albitic phyllite, containing pods of bluish-gray dolostone. (Correlation with West Bridgewater Formation uncertain.) Mapped only in the area west of Granville. Cover rocks north of the Lincoln Mountain massif.

Migmatitic gneiss (Middle Mesoproterozoic) at surface, covers 0.3 % of this area

Light-gray and pinkish-gray to yellowish-gray, massive, medium-grained plagioclase granitic gneiss, and migmatite-veined biotite-plagioclase gneiss. Occurs prominently in Jamaica, in Andover in the Chester dome, and in Weston where it appears to form an integral part of Y1,2bg, but also locally appears to be intrusive. A mixed rock of uncertain origin. U-Pb zircon SHRIMP age of 1,326±4 Ma, no. 8 (Aleinikoff and others, 2011), suggests affinity with metamorphic and igneous events associated with the Middle Mesoproterozoic Ludlow Mountain and Proctor Hill granodiorite gneisses of the South Londonderry Igneous Suite. Age of migmatization is younger than 1,326 Ma. Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Microcline augen gneiss of Lincoln Mountain massif (Late Mesoproterozoic) at surface, covers 0.2 % of this area

Light- to medium-gray, medium-grained, biotite-quartz-plagioclase-microcline gneiss; contains microcline augen as much as 4 cm in length. Chittenden Intrusive Suite (1,149±8 Ma to 1,119±3 Ma). Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Pinnacle Formation - cobble and boulder conglomerate member (Cambrian and Neoproterozoic) at surface, covers 0.2 % of this area

Poorly sorted, matrix-supported quartz- and gneiss-cobble to boulder conglomerate, locally containing quartzite boulders as large as 3 m in diameter, in a matrix of gray-weathering magnetite-calcite-chlorite-muscovite-biotite-feldspar schist. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Quartz schist and gneiss (Middle Mesoproterozoic) at surface, covers 0.2 % of this area

Dark-tan- to rusty-tan-weathering, garnet-muscovite-biotite-quartz-plagioclase gneiss and rusty schist, locally containing abundant chloritized garnet; lustrous yellowish-grayish-green phyllonitic retrograde varieties contain chloritoid-chlorite and relict garnet (red dot overprint). Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

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

Black to dark-gray, sulfidic to nonsulfidic, carbonaceous, fine-grained, well-foliated biotite-muscovite-quartz phyllite; contains pods and thin beds of medium-dark-gray and gray-and-white-mottled dolostone, thin beds of ankeritic quartzite, and, in upper part, rhythmically layered gray quartzite, limestone breccia and laminated silicic phyllite. Dolostone contains Middle Ordovician conodonts (Ibexian to mid-Whiterockian at West Bridgewater and Whiterockian at Buels Gore and West Bolton (Ratcliffe and others, 1999; Thompson and others, 2002). Cover rocks north of the Lincoln Mountain massif.

Amphibolite (Middle Mesoproterozoic) at surface, covers 0.1 % of this area

Dark-grayish-green, fine-grained quartz-hornblende-plagioclase amphibolite, locally garnetiferous, and medium-grained hornblende-plagioclase amphibolite. Occurs with belts of calc-silicate rocks and as lenses within biotite-quartz-plagioclase paragneiss. Unit probably includes both meta-igneous and metasedimentary rocks intercalated throughout the Early and Middle Mesoproterozoic-age rocks of the Mount Holly Complex. Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Underhill Formation - quartzite, granofels, and metawacke (Cambrian and Neoproterozoic) at surface, covers 0.1 % of this area

Local lenses of white to pale-gray quartzite, quartz-albite granofels, quartz-granule chlorite (±biotite) metawacke. Cover rocks north of the Lincoln Mountain massif.

Ottauquechee Formation - carbonaceous albite schist member (Cambrian) at surface, covers 0.1 % of this area

Tan- to dark-gray-weathering, carbonaceous, medium- to coarse-grained chlorite-plagioclase-muscovite-quartz schist and interbedded tan, gray, and bluish-gray quartzite. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Pinnacle Formation - light-gray phyllitic conglomerate member (Cambrian and Neoproterozoic) at surface, covers 0.1 % of this area

Feldspathic biotite phyllitic metawacke interlayered with lenses of quartz, feldspar, and gneiss-pebble to -cobble conglomerate. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

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

Dark-gray to black, fine-grained slate and phyllite, interbedded with thinly laminated bluish-black fine-grained limestone, limestone conglomerate. Unit is interbedded near the base with green phyllite and sooty-punky-weathering calcitic quartz wacke and limestone of the Browns Pond Formation, which is shown separately where mapped. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

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

Gray, thin-bedded to massive sandstone and fossiliferous limestone. Part of the Chazy Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts). Also part of the Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Ottauquechee Formation - black quartzite member (Cambrian) at surface, covers < 0.1 % of this area

Dark-bluish-gray to black, fine-grained vitreous quartzite. Beds are as thick as 30 m or are thin and interbedded with black phyllite. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Bull Formation - Bomoseen Graywacke member (Neoproterozoic) at surface, covers < 0.1 % of this area

Pale-reddish-brown to light-gray-weathering, medium- and fine-grained, massive to thickly bedded, olive-green to gray micaceous quartz-feldspar graywacke and siltstone, locally containing coarse detrital muscovite, biotite, and autoclastic slate chips. Resembles finer grained parts of the Rensselaer Graywacke Member of the Nassau Formation (of Potter, 1972), and the Bird Mountain Grit (of Dale, 1900). Unit interfingers with and grades laterally into the Mettawee slate facies. In the Mt. Anthony area is shown as CZnb (Bomoseen Member of the Nassau Formation of Potter, 1972). Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Whitehall Formation - dolostone (Lower Ordovician and Upper Cambrian) at surface, covers < 0.1 % of this area

Predominantly light-gray and pinkish-gray, coarse-grained dolostone and cherty dolostone. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Hoosac Formation - albite-quartz granofels member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Light-gray- to whitish-gray-weathering, massive to thickly bedded medium-grained biotite-white albite-quartz granofels; locally is a medium-gray, finer grained, more biotite-rich albitic quartz schist of gneissic aspect. Eastern flank of the Green Mountain massif and eastern domes.

Biotite-epidote-quartz gneiss and epidotic quartzite (Middle and Early Mesoproterozoic) at surface, covers < 0.1 % of this area

Light-pale-yellowish-green to gray quartzite gneiss containing abundant epidote and locally magnetite, and frosted round grains of quartz associated with diopside-bearing quartzite. Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Cumberland Head Formation (Upper Ordovician) at surface, covers < 0.1 % of this area

Interbedded graded bioclastic limestone, bluish-gray calcareous shale, and laminated shale. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Hornblende gabbro-diorite (Mesoproterozoic) at surface, covers < 0.1 % of this area

Biotite-hornblende (±pyroxene) gabbro and diorite at Robinson Hill in Shrewsbury; exhibits fine-grained chill contact that crosscuts paragneiss units. Unit also in Lincoln Mountain massif and at Brandon Gap; similar rock mapped in the Adirondacks. Chittenden Intrusive Suite (1,149±8 Ma to 1,119±3 Ma). Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Quartzite - undifferentiated (Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Tan to rusty-brown or gray, thinly layered garnet-biotite quartzite and schistose quartzite associated with aluminous schists and calc-silicate rocks or interbedded within biotite-quartz-plagioclase paragneiss. Unit probably occurs at various stratigraphic levels; may be Early Mesoproterozoic in part (Y1q). Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Hortonville Formation - limestone (Upper Ordovician) at surface, covers < 0.1 % of this area

Near base contains bluish-gray, granular, smooth-weathering fossiliferous limestone. Unit mapped west of the Taconic allochthon, and northwest of the Sudbury slice. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Calc-silicate rock (Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Heterogeneous unit consists of dark-green hornblende-diopside rock or pale-green diopside rock; hornblende-calcite-diopside knotted rock; and rusty-weathering, beige scapolite-quartz-plagioclase gneiss, tremolite-phlogopite schist, and diopside quartzite. Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Underhill Formation - greenstone and amphibolite member (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Thinly bedded carbonate-albite-epidote greenstone and massive magnetite-albite-amphibole-biotite-epidote amphibolite; light- to dark-green, fine- to medium-grained epidote-chlorite albite-amphibole schist; dark-green, coarse-grained, weakly foliated epidote-biotite-albite-amphibole amphibolite. Cover rocks north of the Lincoln Mountain massif.

Mount Abraham Formation - with minor porphyroblasts (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Lustrous, silvery-green to bluish-gray, fine- to medium-grained, white mica-chloritoid-quartz-chlorite schist and phyllite, locally with minor garnet and magnetite porphyroblasts (CZap). Distinctive chlorite streaks and 1-cm rusty needles of altered kyanite are common. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Hoosac Formation - Turkey Mountain metabasalt member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Dark-green to black, hornblende-plagioclase (±garnet±epidote) amphibolite and grayish-green epidote-plagioclase ankeritic greenstone; grades into epidote-quartz-plagioclase volcaniclastic wacke. Occurs as lenses at multiple stratigraphic levels. Lower layers are transitional and alkalic metabasalts; higher units are typical midocean ridge basalt (MORB)-type metabasalts. Eastern flank of the Green Mountain massif and eastern domes.

Forestdale Formation - phyllite and dolostone breccia member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Dark-gray to sooty-black carbonaceous phyllite, interbedded dolostone boulder conglomerate to breccia, and blue- and gray-mottled sulfidic dolostone. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Mylonitic gneiss (Age uncertain) at surface, covers < 0.1 % of this area

Highly schistose, biotite-muscovite (±chlorite) feldspathic mylonite and mylonitic gneiss mapped near Brandon Gap; in the Pine Hill slice near South Wallingford is mapped as Yur. Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Underhill Formation - garnet schist (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Local lenses of garnet schist within the Underhill Formation. Cover rocks north of the Lincoln Mountain massif.

Stowe Formation - feldspathic quartzite (Cambrian and Neoproterozoic?) at surface, covers < 0.1 % of this area

Interbedded feldspathic quartzite within unit CZsa. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Pinney Hollow Formation - black phyllite member (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Dark-gray to black, sulfidic biotite-plagioclase-quartz schist, commonly interbedded with or adjacent to amphibolite and greenstone member (CZpha); locally is a silvery-gray sulfidic biotite phyllite. Cover rocks of Green Mountain anticlinorium.

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

Light-gray, yellowish-gray- to buff- weathering quartzose dolostone, pebbly dolomitic quartzite, and interbedded quartzite. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Moosalamoo Formation - feldspathic quartzite and granofels member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Gray to dark-gray, biotite-albite-quartz granofels and flaser-bedded tan to gray feldspathic quartzite; the latter resembles similar rocks of the Dalton Formation (CZdfq), whereas the former resembles albitic rocks of the Hoosac Formation (CZhab). Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Fayston Formation - schist member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Silvery-green to rusty-tan, fine-grained chlorite-quartz-sericite (±garnet±chloritoid±allanite) schist and phyllite. Resembles green phyllites of the Pinney Hollow Formation (CZph) and Mount Abraham Forma-tion (CZa) and chloritic phyllite (CZtg) of the Tyson Formation. Cover rocks north of the Lincoln Mountain massif.

Mafic dikes (Neoproterozoic) at surface, covers < 0.1 % of this area

Medium- to coarse-grained, foliated, actinolite-chlorite-calcite-epidote retrograded metadiabasic dikes; commonly have relict diabasic texture. Cardinal Brook Intrusive Suite (965±4 Ma to 945±7 Ma). Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Great Meadows Formation - dolostone (Lower Ordovician) at surface, covers < 0.1 % of this area

Light-gray, medium-grained, medium- to thick-bedded dolostone, locally cherty. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Fayston Formation - quartzite member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

White quartzite and tan to light-gray, medium-grained muscovite quartzite locally rich in magnetite. Resembles quartzite of the Tyson Formation (CZtq). Cover rocks north of the Lincoln Mountain massif.

Ottauquechee Formation - schist and quartzite member (Cambrian) at surface, covers < 0.1 % of this area

Light-gray to tan, rusty-weathering, laminated sandy muscovite-plagioclase-quartz schist and tan quartzite. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Great Meadows Formation - Winchell Creek Member (Lower Ordovician) at surface, covers < 0.1 % of this area

Tan-weathering, gray, well-bedded and crossbedded laminated quartzite and dolostone; sandy beds weather to a woody-grained surface texture. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Eagle Bridge Quartzite (Lower Cambrian) at surface, covers < 0.1 % of this area

Dull-gray, pitted, and bluish-gray dolomitic quartz wacke and quartzite distinguished by small pebbles and grains of dark-blue to black quartz, dacitic rock fragments, and abundant plagioclase. Beds resembling the Eagle Bridge Quartzite may occur at several stratigraphic positions within the black slate and gray phyllite of the West Castleton and Hatch Hill(?) Formations, undifferentiated (Cwcu), and near the base of the Poultney Formation, and probably are not all correlative. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Monastery Formation - feldspathic quartzite (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Beds of gritty feldspathic quartzite. Unit mapped in Hancock and Ripton in part as lateral equivalent of the Tyson Formation to the south and the Underhill Formation to the north. Eastern flank of the Green Mountain massif and eastern domes.

Forestdale Formation - dolostone conglomerate (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

A boulder and cobble dolostone conglomerate. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Potsdam Sandstone (Upper Cambrian) at surface, covers < 0.1 % of this area

Light-gray, tan, and dark-gray, well- bedded pebbly quartzite, crossbedded vitreous quartzite, and local conglomerate. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Microcline-augen granite and monzogranite gneiss (Late Mesoproterozoic) at surface, covers < 0.1 % of this area

Gray to whitish-gray, coarse-grained biotite-microcline megacrystic granite and monzogranitic gneiss; passes locally into more equigranular granitic gneiss (Y3Ag) and locally into extensive areas of biotite pegmatoid granitic gneiss (Y3Apg), locally muscovite-bearing. Enclaves of metasedimentary units in these granites and associated gneisses are locally albitized and enriched in magnetite; enclaves of highly aluminous altered rocks now contain restites of chloritoid and abundant sericite. U-Pb zircon ages of 1,119±3.3 Ma (no. 15) and 1,121±1.4 Ma (no. 14) determined on samples near Sherburne Center and on Telegraph Hill east of Chittenden Reservoir by Karabinos and Aleinikoff (1990). Chittenden Intrusive Suite (1,149±8 Ma to 1,119±3 Ma). Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Austin Glen Graywacke (Upper Ordovician) at surface, covers < 0.1 % of this area

Unit is indistinguishable from beds in the Austin Glen Graywacke (after Potter, 1972) (Oag) interpreted as synorogenic autochthonous rocks. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

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

Dark-gray to black, sooty- to rusty-weathering, splintery-fractured pyritic slate and phyllite and interbedded bluish-gray dolomitic quartzite. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Fort Cassin Formation - Sciota Limestone Member (Middle and Lower Ordovician) at surface, covers < 0.1 % of this area

Light-gray, fine- grained, and dark-gray, fossiliferous, and platy bluish-gray limestone. Contains upper Ibexian to lower Whiterockian conodonts at the type locality near Sciota School House, in the Benson quadrangle (J.E. Repetski, USGS, written commun., 2004). Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Pinnacle Formation - metabasalt and volcaniclastic rocks, undifferentiated (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Largely carbonate-epidote-albite-chlorite (±actinolite) greenstones. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Fort Cassin Formation (Middle and Lower Ordovician) at surface, covers < 0.1 % of this area

Shown locally from Whitehall, N.Y., to Orwell, Vt. Laterally equivalent to Providence Island Dolostone. Part of the Beekmantown Group. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

Ottauquechee Formation - feldspathic schist and granofels member (Cambrian) at surface, covers < 0.1 % of this area

Gray and grayish-green, biotite-chlorite-muscovite-albite-quartz schist or phyllite and granofels; contains coticule, locally richly garnetiferous. Rocks of the Rowe-Hawley Zone. Allochthonous cover sequence east of the Green Mountains: rift and drift stage metasedimentary and metavolcanic rocks and tectonic inclusions of ultramafic rocks.

Cheshire Quartzite - argillaceous (Lower Cambrian) at surface, covers < 0.1 % of this area

Locally contains a mappable brown-weathering, gray to brown argillaceous quartzite member. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Meta-ultramafic rocks - undifferentiated (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Brown to white-weathering, green, massive, moderately to fully serpentinized dunite and peridotite and schistose serpentinite; rusty-weathering, medium-grained talc-carbonate rock and quartz-carbonate (magnesite) rock. Part of Ultramafic rocks: occur as tectonic slivers and olistoliths in blocks within the Hazens Notch, Ottauquechee, Stowe, Rowe, and Moretown Formations. Rocks of the Early to Late Taconic Accreted Terrane of the Rowe-Hawley Zone. Eastern allochthonous sequence, oceanic and accretionary realm, ultramafic inclusions, volcanic-arc intrusives, and volcanic rocks.

Moretown Formation - amphibolite and greenstone member (Lower Ordovician to Cambrian?) at surface, covers < 0.1 % of this area

Includes light-pale-green chloritic ankeritic greenstone; black, and fine-grained hornblende-plagioclase (±garnet±epidote) amphibolite. Part of the metasedimentary host rocks of the North River Igneous Suite. Rocks of the Early to Late Taconic Accreted Terrane of the Rowe-Hawley Zone. Eastern allochthonous sequence, oceanic and accretionary realm, ultramafic inclusions, volcanic-arc intrusives, and volcanic rocks.

Tyson Formation - dolostone member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Largely massive, gray-, beige-, and pinkish-gray-weathering dolostone, beds of pebbly quartz dolostone, and pink- to orange-tan-weathering dolostone as lenses in phyllite. Contains beds of bluish-gray and whitish-gray vitreous quartzite. Similar to dolostone of the Forestdale Formation (CZfd). Eastern and western flank of the Green Mountain massif.

Quartz schist and gneiss - quartz-biotite gneiss and quartzite (Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Unit Y2rs locally includes steel-gray-weathering, garnet (small)-quartz-biotite gneiss and quartzite (Y2bgt). Part of the aluminous schists and gneisses of the Washington Gneiss and Wilcox Formation and related rocks. Mount Holly Complex paragneiss. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Pegmatite (Late Mesoproterozoic) at surface, covers < 0.1 % of this area

Light-gray to pinkish-gray, biotite-muscovite (±garnet±tourmaline±magnetite) granite pegmatite as crosscutting pods and larger bodies. Albitic garnet-muscovite pegmatite common in metapelitic rocks of the Mount Holly Complex, and especially prominent in the northern and east-central parts of the Green Mountain massif. Assignment of individual pegmatites to Y3C is highly interpretive based on crosscutting of gneissosity and weakly deformed character. Mount Holly Complex intrusive rocks. Rocks of the Green Mountain and Lincoln Mountain massifs and eastern domes.

Bull Formation - Zion Hill Quartzite member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Light-greenish-gray to whitish-gray-weathering, massive vitreous quartzite; locally contains quartz-pebble conglomerate and wacke near the base. Unit commonly 5 to 10 m thick but is as much as 65 m thick; occurs as many lenticular quartzites within the Mettawee slate facies in the Bull Formation, not restricted to one horizon. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Hortonville Formation - conglomerate (Upper Ordovician) at surface, covers < 0.1 % of this area

Locally a polymict limestone and dolostone-clast conglomerate. Unit mapped west of the Taconic allochthon, and northwest of the Sudbury slice. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Forestdale Formation - quartzite member (Lower Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Grayish-tan to light-gray vitreous quartzite and white feldspathic gritty quartzite. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.