Geologic units in Washington county, New York

Canajoharie Shale (Middle Ordovician) at surface, covers 10 % of this area

Canajoharie Shale

Poultney Formation (Middle and Lower Ordovician) at surface, covers 10 % of this area

Dull-white and whitish-gray-weathering, and pale-green and gray, thinly bedded to laminated slate and phyllite. Has distinctive beds, 1 cm to several centimeters thick, of siliceous argillite and metasiltstone and locally abundant thin beds of micritic black limestone near the base, interbedded with dark slate. Contains graptolites ranging from Ibexian to Whiterockian (Berry, 1961). Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Bull Formation - Mettawee slate facies (Neoproterozoic) at surface, covers 6 % 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.

Netop Formation - chlorite phyllite member (Cambrian and Neoproterozoic) at surface, covers 6 % of this area

Light-green to gray, lustrous, chlorite±chloritoid-muscovite-quartz phyllite and greenish-gray metasiltstone. Rocks of the Dorset Mountain slice (includes Dorset Mountain proper and Mount Equinox, southward to West Mountain near Bennington). Part of the Taconic Allochthon.

West Castleton Formation (Middle and Lower Cambrian) at surface, covers 5 % 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.

Poultney Formation (""A"" Member) (Cambrian) at surface, covers 4 % of this area

North of Troy: shale, limestone; Hatch Hill Formation-shale, dolostone; West Castleton Formation-shale, limestone, conglomerate.

Charnockite, granitic and quartz syenite gneiss (Middle Proterozoic) at surface, covers 4 % of this area

Variably leucocratic, containing varying amounts of hornblende, pyroxenes, biotite; may contain interlayered amphibolite, metasedimentary gneiss, migmatite. Overprint signifies inequigranular texture or phacoidal structure.

Hortonville Formation - shale and phyllite (Upper Ordovician) at surface, covers 4 % 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).

Bull Formation - Bomoseen Graywacke member (Neoproterozoic) at surface, covers 3 % 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.

Undivided metasedimentary rock and related migmatite (Middle Proterozoic) at surface, covers 3 % of this area

Undivided metasedimentary rock and related migmatite.

Hybrid rock: mangeritic to charnockitic gneiss (Middle Proterozoic) at surface, covers 3 % of this area

With xenocrysts of calcic andesine and, locally, xenoliths of anorthosite; with increasing percentage of anorthosite component, passes gradationally into anorthositic rocks.

Beekmantown Group (in part) (Lower Ordovician) at surface, covers 3 % of this area

In St. Lawrence Valley: Ogdensburg Dolostone (Beauharnois Dolostone in Canada); In Champlain Valley: Providence Island Dolostone; Fort Cassin Formation-limestone, dolostone; Fort Ann Formation (Spellman of Clinton and Essex Counties)-limestone, dolostone; Cutting Formation-dolostone (locally cherty), limestone, siltstone. In Vermont: includes Bridport, Bascom, Cutting, and Shelburne carbonates.

Mount Merino Formation (Upper Ordovician) at surface, covers 3 % of this area

Light-gray, powdery-weathering, and red, green, and dark-gray, thinly bedded siliceous argillite and mudstone distinguished from the Indian River Slate by abundance of cherty siliceous layers. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Biotite-quartz-plagioclase paragneiss, amphibolite, and related migmatite (Middle Proterozoic) at surface, covers 2 % of this area

Locally sillimanitic; commonly garnetiferous in and adjacent to Adirondack Highlands.

Beekmantown Group (in part) (Cambrian) at surface, covers 2 % of this area

In Champlain Valley: Whitehall Formation-dolostone, limestone (with Cryptozoon reefs); Ticonderoga Formation-dolostone (locally cherty), sandstone. In Vermont: Clarendon Springs Dolostone; Danby Formation-sandstone, quartzite, dolostone.

Mettawee Slate (Cambrian?) at surface, covers 2 % of this area

North of 43 degrees includes Castleton (North Brittain) Conglomerate. Mudd Pond Quartzite, Zion Hill Quartzite, and Bomoseen Graywacke Members.

Hatch Hill Formation (Upper Cambrian) at surface, covers 2 % 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.

Glacial and Alluvial Deposits (Quaternary) at surface, covers 2 % of this area

Underlying bedrock geology unknown.

Austin Glen Graywacke (Upper Ordovician) at surface, covers 2 % 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.

Undifferentiated Middle Ordovician thru Lower Cambrian allochthonous rocks (Cambrian - Ordovician) at surface, covers 2 % of this area

Principally pelite; lesser quartzite, limestone, conglomerate, graywacke.

Mount Merino and Indian River Formations (Ordovician) at surface, covers 1 % of this area

Shale, slate, cherts.

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

Graywacke, shale

Potsdam Sandstone (Cambrian) at surface, covers 1 % of this area

Potsdam Sandstone (Covey Hill in Quebec)

Poultney Formation (""B"" and ""C"" Members) (Ordovician) at surface, covers 1 % of this area

North of Troy; shale, slate, siltstone.

Amphibolite (Middle Proterozoic) at surface, covers 1 % of this area

Commonly biotitic; garnetiferous, pyroxenic, in and adjacent to central massif of Adirondacks.

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

Same description as Hortonville Formation - 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. Tectonically shredded, silty varieties near and on Whipstock Hill constitute the Whipstock Breccia Member of Potter (1972) (Oww). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Indian River Slate (Upper Ordovician) at surface, covers 1.0 % of this area

Deep-maroon and bluish-green-weathering, well-bedded and variegated slate; contains minor centimeter-thick, white-weathering, red and bluish-black cherty layers characteristic of the Mount Merino Formation. Contains graptolites of the C. bicornis Biozone (Berry, 1961). Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Biotite and/or hornblende granite gneiss (Middle Proterozoic) at surface, covers 1.0 % of this area

Locally pyroxenic; commonly with subordinate leucogranitic gneiss, biotite-quartz-plagioclase gneiss, other metasedimentary rocks, amphibolite, migmatite. Amphibolite with porphyroblasts of K-feldspar locally prominent in northwest Adirondacks. In northwest Adirondacks, grades into Yphg.

Fort Ann Formation (Lower Ordovician) at surface, covers 0.9 % of this area

(Shown only in New York State). Medium- to dark-gray and mottled, medium- to thick-bedded dolomitic limestone and buff-weathering 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).

Biotite-quartz-plagioclase gneiss, amphibolite, and related migmatite (Middle Proterozoic) at surface, covers 0.8 % of this area

Locally sillimanitic; commonly garnetiferous in and adjacent to Adirondack Highlands.

Taconic Melange (Middle Ordovician) at surface, covers 0.6 % of this area

Chaotic mixture of Early Cambrian thru Middle Ordovician pebble to block-size angular to rounded clasts in a pelitic matrix of Middle Ordovician (Barneveld) age. Rims and floors earlier submarine gravity slides of Taconian Orogeny.

Glens Falls Limestone (Upper Ordovician) at surface, covers 0.6 % 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).

Browns Pond Formation of Rowley and others (1979) (Lower Cambrian) at surface, covers 0.6 % of this area

Gray to black slate, punky-weathering calcitic wacke and mudstone, and thin limestone breccia in part equivalent to the West Castleton Formation. Shown only in the Granville, N.Y., area. Locally purple and green slate above black slate of the Browns Pond is interpreted as a lens of the Mettawee slate facies in the Bull Formation. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Pawlet Formation (Upper Ordovician) at surface, covers 0.5 % of this area

Light-gray, tan-weathering, mica-speckled, massive to thin-bedded quartz-plagioclase wacke interbedded with dark-gray carbonaceous slate. Contains distinctive autoclastic chips of gray slate, fragments of dacitic to andesitic volcanics, and subangular clasts of dark-gray quartz and oligoclase. Interbedded black slates contain graptolites of the C. bicornis Biozone (see Webby and others, 2004, fig. 2.1) (lower to middle Mohawkian). Interpreted as uncomformable on rocks as old as the Hatch Hill Formation and possibly the West Castleton Formation of the allochthon. 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.

Glacial and Alluvial Deposits (Quaternary) at surface, covers 0.5 % of this area

Underlying bedrock geology unknown.

Metagabbro (Middle Proterozoic) at surface, covers 0.5 % of this area

Metagabbro, olivine metagabbro, derived amphibolite.

Metagabbro (Middle Proterozoic) at surface, covers 0.5 % of this area

Metagabbro, olivine metagabbro, derived amphibolite.

Fort Cassin Formation (Middle and Lower Ordovician) at surface, covers 0.5 % 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).

Biotite-quartz-plagioclase paragneiss (Middle Proterozoic) at surface, covers 0.4 % of this area

Commonly very low in biotite content, with interbedded feldspathic and biotitic quartzite and amphibolite; sillimanite and garnet common, graphite sporadic.

Fort Cassin Formation - Sciota Limestone Member (Middle and Lower Ordovician) at surface, covers 0.4 % 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).

Pulaski and Whetstone Gulf Formations (Upper Ordovician) at surface, covers 0.4 % of this area

Shale, siltstone.

Whitehall Formation - dolostone (Lower Ordovician and Upper Cambrian) at surface, covers 0.4 % 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).

Potsdam Sandstone (Cambrian) at surface, covers 0.4 % of this area

Potsdam Sandstone

Trenton and Black River Groups, undivided (Middle Ordovician) at surface, covers 0.4 % of this area

Mohawk Valley: Dolgeville, Denley, Sugar River, Kings Falls, Glens Falls, Rockland, Amsterdam, and Lowville Limestones. Washington County: Glens Falls and Orwell Limestones.

Providence Island Dolostone (Middle and Lower Ordovician) at surface, covers 0.4 % 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).

Leucogranitic gneiss (Middle Proterozoic) at surface, covers 0.4 % of this area

Sodic plagioclase ranges from generally subordinate to locally dominant; locally with biotite, hornblende, pyroxene, garnet, sillimanite, disseminated magnetite; commonly contains metasedimentary layers, amphibolite, migmatite; plagioclase-rich variety is host to magnetite ore bodies in eastern Adirondacks.

Winooski Dolostone (Middle Cambrian) at surface, covers 0.3 % 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).

Calcitic and dolomitic marble (Middle Proterozoic) at surface, covers 0.3 % of this area

Predominantly; variably siliceous; in part with calcsilicate rock and amphibolite.

Potsdam Sandstone (Upper Cambrian) at surface, covers 0.3 % 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).

Middlebury Limestone (Middle Ordovician) at surface, covers 0.3 % 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).

Calcitic and dolomitic marble (Middle Proterozoic) at surface, covers 0.3 % of this area

Variably siliceous; in part with calcsilicate rock and amphibolite.

Biotite-quartz-plagioclase gneiss (Middle to Early Mesoproterozoic) at surface, covers 0.2 % of this area

Light-greenish-gray to pinkish-gray, well-foliated and well-layered quartz-rich gneiss and more mafic biotite-hornblende-pyroxene-quartz-plagioclase gneiss irregularly distributed within unit. Unit may be in part older than the tonalitic gneiss (Y2bt). Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Great Meadows Formation - Winchell Creek Member (Lower Ordovician) at surface, covers 0.2 % 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).

Great Meadows Formation - dolostone (Lower Ordovician) at surface, covers 0.2 % 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).

Quartzite, quartz schist and graphitic schist (Middle Proterozoic) at surface, covers 0.2 % of this area

In part feldspathic, micaceous, garnetiferous, sillimanitic.

Fort Cassin Formation - Ward Siltstone Member (Lower Ordovician) at surface, covers 0.2 % of this area

Light-tan to gray, thinly bedded and crossbedded, calcareous and dolomitic siltstone and quartzite in beds similar to the quartzite of the Root Pond Quartzite Member (Ofcrp). 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).

Nassau Formation (Cambrian?) at surface, covers 0.2 % of this area

South of 43 degrees; slate, shale, thin quartzite, includes Stuyvesant Conglomerate, Diamond Rock Quartzite, Curtis Mountain Quartzite, and Bomoseen Graywacke Members.

Quartz-feldspar gneiss (Middle Proterozoic) at surface, covers 0.2 % of this area

With variable amounts of garnet, sillimanite, biotite.

Carbonaceous phyllite and siltstone (Lower Cambrian and Neoproterozoic) at surface, covers 0.1 % of this area

Medium- to dark-gray carbonaceous phyllite, gray slate, and metasiltstone, locally containing light-gray, medium- to thick-bedded quartzite and dolomitic quartzite (CZbq). Unit resembles rocks of the Netop Formation but lacks the distinctive lenses and pods of bluish-gray dolostone of the Netop on Dorset Mountain, although lenses of whitish quartzite are present. Rocks of the Dorset Mountain slice (includes Dorset Mountain proper and Mount Equinox, southward to West Mountain near Bennington). Part of the Taconic Allochthon.

Hague Gneiss of Alling (1918) (Middle Mesoproterozoic) at surface, covers 0.1 % of this area

Yellowish-grayish- green to rusty-sulfidic-weathering, garnet (large)-sillimanite-biotite-orthoclase-plagioclase-quartz schist or gneiss, locally containing mappable and discontinuous lenses, layers or pods of quartzite, marble, or calc-silicate rock. Contains a coarse sillimanite-feldspar gneissosity older than the regional Ottawan foliation. Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

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

Boudins of whitish-gray-weathering, bluish-gray quartzite. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

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).

Chipman Formation - Bascom Formation (Middle and Lower Ordovician) at surface, covers 0.1 % 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).

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

Layers of gray limestone. 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).

Chipman Formation - Bridport Member (Middle Ordovician) at surface, covers < 0.1 % 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).

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).

Granodioritic augen gneiss and quartz monzonite gneiss (Late Mesoproterozoic) at surface, covers < 0.1 % of this area

Light-gray to pinkish-gray, garnet-biotite-plagioclase-micro-perthite augen granodioritic gneiss, locally massive but well foliated. Contains deformed phenocrysts (augen) aligned in old relict flow (?) foliation. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Quartzite, quartz-biotite schist and graphitic schist (Middle Proterozoic) at surface, covers < 0.1 % of this area

In part feldspathic, micaceous, garnetiferous, sillimanitic.

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

Occuring as slivers caught along thrusts of later allochthones, or carbonate blocks in Taconic Melange. Also mapped as horses along normal faults.

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

Contains basal limestone, locally referred to as the Whipple Marble Member. Unit mapped along east side of the Taconic allochthon south to Bennington. Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

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

Light-gray to tannish-white, well-bedded and vitreous steel-gray quartzite interbedded with orangey-tan- and beige-weathering dolostone and thin beds of fossiliferous limestone. Named for occurrences near Root Pond in the Benson quadrangle, but herein extended to include thin lenses of quartzite that occur interbedded at several positions in the Fort Cassin Formation. Interbedded limestones contain upper Ibexian conodonts southwest of Root Pond (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).

Leucogranitic (alaskitic) gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Sodic plagioclase ranges from generally subordinate to locally dominant; locally with biotite, hornblende, pyroxene, garnet, sillimanite, disseminated magnetite; commonly contains metasedimentary layers, amphibolite, migmatite; plagioclase-rich variety is host to magnetite ore bodies in eastern Adirondacks.

Biotite and/or hornblende granite gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Locally pyroxenic; commonly with subordinate leucogranitic gneiss, biotite-quartz-plagioclase gneiss, other metasedimentary rocks, amphibolite, migmatite. Amphibolite with porphyroblasts of K-feldspar locally prominent in northwest Adirondacks. Overprint signifies inequigranular texture or phacoidal structure. In northwest Adirondacks, grades into Yphg.

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.

Netop Formation - wacke member (Cambrian and Neoproterozoic) at surface, covers < 0.1 % of this area

Bluish-gray, fine-grained metawacke and metasiltstone, perhaps equivalent to the Bomoseen Graywacke Member of the Bull Formation. Rocks of the Dorset Mountain slice (includes Dorset Mountain proper and Mount Equinox, southward to West Mountain near Bennington). Part of the Taconic Allochthon.

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

Light-tan to yellowish-gray, massive to well-layered magnetite-garnet quartzite and magnetite-biotite-garnet quartzite in beds as much as 10 m thick. Occurs in two layers, one within or attached to the Hague Gneiss and one within biotite-quartz-plagioclase gneiss (Y2bpg) beneath the Hague Gneiss. The latter is rich in microcline and passes through interbedding into a quartzose facies of Y2bpg. Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Pharaoh Mountain Gneiss of Wiener and others (1984) (Late to Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Rusty-grayish-brown- to brown-weathering, knubbly coarse-grained magnetite-garnet-hornblende and pyroxene-bearing charnockitic gneiss and garnet-hornblende granite gneiss. Has large polycrystalline aggregates of plagioclase as much as 1.5 cm long, recrystallized from pre-Ottawan phenocrysts (?) of plagioclase. Contains mappable and folded screens of marble, calc-silicate rock (Y2cs), mafic diopside-hypersthene gneiss, and interbedded sillimanite-garnet quartzite (Y2qz). Interpreted as largely intrusive but may contain some charnockitic gneiss of uncertain origin. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Whipstock Breccia in the Walloomsac Formation (Upper Ordovician) at surface, covers < 0.1 % of this area

Largely a tectonic breccia formed in situ; contains abundant pseudo-pebbles. Part of the Taconic Allochthon. Wildflysch-like conglomerates within the Hortonville, Ira, and Walloomsac Formations occur as local areas of black slate rich in inclusions of quartzite, greenish-gray slate, wacke, and punky-weathering bluish-gray limestone, interpreted as sedimentary breccias, deposited in front of the advancing Taconic allochthon (Upper Ordovician) (Zen, 1961; Potter, 1972; Fisher, 1985). Exposed near the western and northern margin of the allochthon and in the Bennington area at the type Whipstock. Here and at many localities the Forbes Hill and Whipstock breccias are tectonic breccias formed in situ by disruption of thin to thick beds, laminae, and carbonate-quartz-sulfide veins rather than clastic sedimentary rocks. The cleavage and related folding commonly is a late strain-slip cleavage characterized by a strong down-plunge lineation parallel to reclined hingelines of minor folds of foliation and compositional layering. Units are retained although interpretation as sedimentary wildflysch deposits is in part questionable.

Chazy, Black River, and Trenton Limestones - undifferentiated (Upper and Middle Ordovician) at surface, covers < 0.1 % of this area

See individual formation descriptions. Shown locally west of the Taconic allochthon. Vermont Valley sequence and Middlebury synclinorium (above 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.

Carbonate - limestone (Cambrian and Neoproterozoic?) at surface, covers < 0.1 % of this area

Pods, lenses, or zones of thinly bedded limestone (ls), dolostone (d), and limestone conglomerate in the Mettawee slate facies in the Bull Formation, West Castleton Formation, and Hatch Hill Formation. These rocks locally contain Lower Cambrian fossils, but may range in age from Neoproterozoic to Late Cambrian. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

Biotite tonalite gneiss (Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Predominantly chalky-white-weathering, massive biotite tonalite gneiss. Passes laterally into white, fine-grained trondhjemitic aplite near contacts with larger screens of paragneiss. U-Pb zircon Thermal Ionization Mass Spectrometry (TIMS) age of 1329±37 Ma (McLelland and Chiarenzelli, 1990) obtained from exposure 1.5 km west of Austin Hill on the west side of South Bay, west of Whitehall, N.Y. Contains lenses and screens of rusty sulfidic garnet-biotite-feldspar-quartz schist, dark-gray garnet-feldspar quartzite, and calc-silicate gneiss on south end of Austin Hill. Unit interpreted as intrusive into some paragneiss units that are here older than 1328±32 Ma (McLelland and Chiarenzelli, 1990). Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Marble and calc-silicate gneiss (Middle Mesoproterozoic) at surface, covers < 0.1 % of this area

Medium-dark-gray to light-gray diopside-phlogopite-scapolite-calcite marble, phlogopite-tremolite-talc schist, and dark-gray diopside-hornblende (actinolite)-plagioclase calc-silicate gneiss. Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Biotite and/or hornblende granite gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Locally pyroxenic; commonly with subordinate leucogranitic gneiss, biotite-quartz-plagioclase gneiss, other metasedimentary rocks, amphibolite, migmatite. Amphibolite with porphyroblasts of K-feldspar locally prominent in northwest Adirondacks. Overprint signifies inequigranular texture or phacoidal structure. In northwest Adirondacks, grades into Yphg.

Bull Formation - Mud Pond Quartzite member (Lower Cambrian) at surface, covers < 0.1 % of this area

Buff- to gray-weathering vitreous quartzite as much as 6 m thick, containing deeply weathered ovoidal areas of carbonate-cemented quartzite. Rocks of the Giddings Brook, Sunset Lake, and Bird Mountain slices. Part of the Taconic Allochthon.

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

A black carbonaceous highly graptoliferous phyllite. Tectonically shredded, silty varieties near and on Whipstock Hill constitute the Whipstock Breccia Member of Potter (1972) (Oww). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).

Shelburne Marble (Lower Ordovician) at surface, covers < 0.1 % 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).

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

Garnet-hornblende (±olivine) gabbro and dioritic gneiss; similar rock mapped in the Chittenden Intrusive Suite in Vermon. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

Mafic gneiss (Middle to Early Mesoproterozoic) at surface, covers < 0.1 % of this area

Dark-gray to black, fine-grained, magnetite-garnet-hornblende-biotite-diopside-plagioclase gneiss, commonly containing beds of dark-gray vitreous magnetite-garnet quartzite, 2 to 5 cm thick, tremolite-pyroxene gneiss, and biotite-rich, rusty-weathering garnet-quartz schist and gray sulfidic sillimanite quartzite. Occurs as screens within tonalitic gneiss and the Pharaoh Mountain Gneiss and is interpreted as paragneisses older than the tonalitic gneiss (Y2bt). Paragneiss. Rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

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

South of Troy; shale, conglomerate, limestone.

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

Pinkish-gray to light-gray, unfoliated, magnetite-biotite (±tourmaline) granite pegmatite, <1 m to 10 m thick. Crosscuts Ottawan foliation and locally fills narrow, northeast-trending, steeply dipping normal ductile shear zones. Intrusive rocks of the Adirondacks in Vermont and in the Whitehall, N.Y., area.

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

Basal limestone member. Tectonically shredded, silty varieties near and on Whipstock Hill constitute the Whipstock Breccia Member of Potter (1972) (Oww). Vermont Valley sequence and Middlebury synclinorium (above the Orwell and Champlain thrusts).