Geologic units in Chittenden county, Vermont

Pinnacle Formation - undivided (Cambrian and Neoproterozoic) at surface, covers 20 % 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.

Fairfield Pond Formation (Cambrian and Neoproterozoic) at surface, covers 12 % 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.

Underhill Formation (Cambrian and Neoproterozoic) at surface, covers 11 % 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.

Dunham Dolostone (Lower Cambrian) at surface, covers 6 % 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).

Hazens Notch Formation - undivided (Cambrian and Neoproterozoic) at surface, covers 5 % 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.

Fayston Formation - albite schist member (Lower Cambrian and Neoproterozoic) at surface, covers 5 % 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.

Winooski Dolostone (Middle Cambrian) at surface, covers 5 % 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).

Monkton Quartzite (Middle Cambrian) at surface, covers 4 % 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).

Skeels Corners Slate (Lower Ordovician and Cambrian) at surface, covers 4 % of this area

Laminated black slate with thin orange dolostone beds; includes massive dolostones mapped as Saxe Brook Formation by others in the Highgate area. Cover rocks of the Saint Albans area.

Stony Point Formation (Upper Ordovician) at surface, covers 3 % 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).

Clarendon Springs Formation (Upper Cambrian) at surface, covers 3 % 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).

Shelburne Marble (Lower Ordovician) at surface, covers 3 % 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).

Cheshire Quartzite - argillaceous (Lower Cambrian) at surface, covers 3 % 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).

Iberville Formation (Upper Ordovician) at surface, covers 3 % 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).

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).

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).

Cutting Dolostone (Lower Ordovician) at surface, covers 2 % 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 member (Cambrian and Neoproterozoic) at surface, covers 1 % 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.

Cheshire Quartzite (Lower Cambrian) at surface, covers 1 % 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).

Hazens Notch Formation - schist member (Cambrian and Neoproterozoic) at surface, covers 0.5 % of this area

Rusty-weathering, non-graphitic, sulfidic chlorite-muscovite-quartz schist. Cover rocks north of the Lincoln Mountain massif.

Monastery Formation (Cambrian and Neoproterozoic) at surface, covers 0.5 % 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.

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

Parker Slate - calcareous sandy conglomerate member (Cambrian) at surface, covers 0.3 % of this area

Conglomerate with dolostone and sandstone clasts in a calcareous sandy matrix. Cover rocks of the Saint Albans area.

Parker Slate - dolostone member (Cambrian) at surface, covers 0.3 % of this area

Massive beds of white to buff dolostone. Cover rocks of the Saint Albans area.

Hazens Notch Formation - greenstone and amphibolite member (Cambrian and Neoproterozoic) at surface, covers 0.3 % of this area

Dark-green, pitted-weathering, foliated mafic schist to massive greenstone containing varying amounts of chlorite, albite, carbonate, epidote, amphibole, and sphene; and amphibolite containing varying amounts of hornblende, actinolite, albite, chlorite, epidote, biotite, magnetite, and sphene. Cover rocks north of the Lincoln Mountain massif.

Underhill Formation - greenstone and amphibolite member (Cambrian and Neoproterozoic) at surface, covers 0.2 % 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.

Glens Falls Limestone (Upper Ordovician) at surface, covers 0.2 % 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).

Chipman Formation - Bridport Member (Middle Ordovician) at surface, covers 0.2 % 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).

Fayston Formation - schist member (Lower Cambrian and Neoproterozoic) at surface, covers 0.2 % 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.

Pinnacle Formation - calcareous greenstone member (Cambrian and Neoproterozoic) at surface, covers 0.2 % of this area

Rusty-weathering and pitted, dark-green greenstone with laminae and splotches of calcareous material; locally interbedded with calcareous metagraywacke. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Pinnacle Formation - feldspathic greenstone member (Cambrian and Neoproterozoic) at surface, covers 0.2 % of this area

Olive-drab-weathering, dark-green, fine-grained greenstone with remnant plagioclase feldspar phenocrysts up to 1.5 cm in diameter. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Parker Slate (Cambrian) at surface, covers 0.2 % of this area

Black arenaceous slate with dolomite horizons; interbedded dolostone and sandstone breccia in a dolostone matrix; and massive dolostone with thin sandstone beds. Cover rocks of the Saint Albans area.

Mount Abraham Formation (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.

Crown Point Formation (Middle Ordovician) at surface, covers < 0.1 % 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).

Pinnacle Formation - quartzite and quartz-pebble conglomerate member (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Light-gray quartz-feldspar conglomerate and quartzite. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

Beekmantown Group, undivided (Middle Ordovician to Upper Cambrian) at surface, covers < 0.1 % of this area

Gray limestones, fossiliferous limestones, and dolostones. Rocks of the Laurentian Margin. Part of the Adirondack lowlands and Lake Champlain lowlands (west of the Orwell and Champlain thrusts).

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).

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

Salt-and-pepper-colored, medium- to coarse-grained quartz-albite-biotite gneiss. Cover rocks north of the Lincoln Mountain massif.

Skeels Corners Slate - limestone matrix conglomerate member (Lower Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Limestone and sandstone clasts in a limestone matrix, interbedded with sandstone. Cover rocks of the Saint Albans area.

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.

Orwell Limestone (Upper Ordovician) at surface, covers < 0.1 % 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).

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).

Skeels Corners Slate - sandy matrix conglomerate member (Lower Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Limestone and sandstone clasts in a quartz sand matrix. Cover rocks of the Saint Albans area.

Parker Slate - sandstone member (Cambrian) at surface, covers < 0.1 % of this area

Thin- to thick-bedded calcareous coarse-grained sandstone. Cover rocks of the Saint Albans area.

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

Calcite-biotite-sphene-albite-actinolite-epidote-chlorite greenstone with dark-green porphyroblasts of actinolitic hornblende. Cover rocks of the Lincoln Mountain massif and northwestern flank of the Green Mountain massif.

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).

Parker Slate - dark conglomerate member (Cambrian) at surface, covers < 0.1 % of this area

Conglomerate consisting of round clasts of orange dolostone in a black arenaceous matrix. Cover rocks of the Saint Albans area.

Barber Hill stock - syenite (Early Cretaceous) at surface, covers < 0.1 % of this area

A small, 0.5-km-long, northwest-trending stock consisting of a coarse-grained alkali syenite core, a fine-grained quartz syenite border, and associated trachyte dikes (Kfd). K-Ar biotite age of 114±2 Ma (Armstrong and Stump, 1971). Part of the White Mountain Igneous Suite.

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).

Sweetsburg Formation (Lower Ordovician and Cambrian?) at surface, covers < 0.1 % of this area

Black to gray, graphitic, quartzose phyllite and schist, with tan-weathering layers and pods of gray dolostone and black quartzite. Cover rocks of the Saint Albans area.

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

Dark-green, pitted-weathering, foliated carbonate-albite-chlorite (±magnetite±epidote) greenstone or amphibolite. Cover rocks north of the Lincoln Mountain massif.

Ottauquechee Formation - carbonaceous phyllite member (Cambrian) at surface, covers < 0.1 % 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.