Geologic units in Berkshire county, Massachusetts

Stockbridge Formation (Lower Cambrian) at surface, covers 10 % of this area

Massive to finely laminated steel-gray calcitic dolomite marble containing a prominent zone of white quartz nodules near top.

Washington Gneiss (Proterozoic Y) at surface, covers 8 % of this area

Rusty-weathering, muscovite-biotite-sillimanite and/or kyanite-garnet schist; blue-quartz ribbed conglomerate, interlayered garnet-plagioclase-quartz metadacite.

Walloomsac Formation (Middle Ordovician) at surface, covers 8 % of this area

Dark-gray, graphitic quartz phyllite and schist containing minor lenses of limestone.

Gray, well-layered biotite-plagioclase-quartz gneiss (Proterozoic Y) at surface, covers 8 % of this area

Containing beds of amphibolite, aluminous schist, quartzite, and calc-silicate gneiss.

Tyringham Gneiss (Proterozoic Y) at surface, covers 7 % of this area

Light pinkish-gray ferrohastingsite-biotite, quartz-rodded granodioritic to quartz monzonitic gneiss, coarsely porphyritic, locally having fine-grained aplitic border. Intrudes all Berkshire Proterozoic Y units.

Stockbridge Formation (Lower Ordovician) at surface, covers 7 % of this area

White to blue-gray and white layered calcite marble.

Stockbridge Formation (Lower Cambrian) at surface, covers 7 % of this area

Beige, tan, and dark-gray weathering quartzose dolomite marble containing interbeds of black, green and maroon phyllite and punky weathering blue quartz pebble quartzite.

Everett Formation (Lower Cambrian and Proterozoic Z) at surface, covers 4 % of this area

Light-green and greenish-gray chlorite-muscovite-albite or chloritoid-rich phyllite. Predominantly dark-gray chloritoid-rich schist in Lenox Mountain.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 3 % of this area

Tan weathering, muscovite-microcline quartzite and feldspathic quartzite rich in black tourmaline, locally includes thin beds of other rock types listed below.

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 3 % of this area

Undifferentiated Hoosac Formation.

Nassau Formation (Lower Cambrian and Proterozoic Z) at surface, covers 3 % of this area

Lustrous, soft green, yellowish-green and purple laminated chloritoid-chlorite phyllite (Mettawee Member).

Washington Gneiss (Proterozoic Y) at surface, covers 3 % of this area

Well-layered, rusty-tan weathering muscovite-biotite plagioclase-microcline-quartz granofels containing layers of rusty sulfidic calc-silicate rocks.

Stockbridge Formation (Lower Cambrian) at surface, covers 2 % of this area

White to light-powdery-blue-gray dolostone with disseminated grains of quartz and prominent sprays of tremolite in higher-grade areas.

Cheshire Quartzite (Lower Cambrian) at surface, covers 2 % of this area

White, massive vitreous quartzite.

Granitoid Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Biotite ferrohastingsite granodioritic and granitic gneiss with large schlieren of biotite, locally contains garnet and muscovite.

Nassau Formation (Lower Cambrian and Proterozoic Z) at surface, covers 2 % of this area

Gray to dark-greenish-gray, siliceous phyllite with abundant beds of quartzite, olive-gray metasiltstone and subgraywacke (includes Bomoseen Graywacke Member and Zion Hill quartzite Member).

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 2 % of this area

Light-green to light-bluish-gray schist having thin granular quartz lenses and lamellae. Kyanite and staurolite typical at higher grades.

Pinkish-gray, fine-grained, well-laminated felsic biotite-microcline-plagioclase-quartz gneiss (Proterozoic Y) at surface, covers 2 % of this area

Probably metamorphosed rhyolite.

Moretown Formation (Middle Ordovician or older) at surface, covers 2 % of this area

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

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

Blue and gray mottled limestone and calcite marble and beds of beige dolostone.

Greylock Schist (Lower Cambrian and Proterozoic Z) at surface, covers 1 % of this area

Soft, lustrous green phyllite with minor beds of green quartzite and well laminated gray dolomitic phyllite; resembles Hoosac (CAZhg) and Nassau (CAZnp).

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 1 % of this area

Lustrous greenish-gray schist characterized by 1-1.5 cm garnets; resembles Gassetts Schist of Vermont.

Lee Gneiss (Proterozoic Y) at surface, covers 1 % of this area

Hornblende-spotted plagioclase-hornblende-quartz gneiss and granofels and well-layered biotite-hornblende-plagioclase-quartz gneiss containing knots of diopside or epidote.

Calc-silicate granofels and gneiss (Proterozoic Y) at surface, covers 0.9 % of this area

Including calcitic or dolomitic chondrodite-diopside marble, coarse hornblende-plagioclase-diopside and diopside rock, locally containing beds of lustrous muscovite-kyanite sillimanite-garnet schist.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.9 % of this area

Orangish-gray, gray, and light-greenish-gray muscovite-quartz schist and interlayered feldspathic quartzite and quartz conglomerate; minor beds of rusty albitic schist.

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.7 % of this area

Green to gray-green chlorite-sericite-quartz phyllite; interbeds of chloritoid- or albite-rich schist and minor quartzite, locally rich in garnet and kyanite.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.7 % of this area

Tan to orangish-tan quartz and gneiss cobble and pebble conglomerate, rusty feldspathic schist, and lustrous greenish-gray muscovite quartz schist.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.7 % of this area

Black to dark-gray carbonaceous quartz schist.

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.6 % of this area

Greenish chlorite-albite-magnetite-sericite-quartz schist and granofels.

Dalton Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.6 % of this area

White to yellowish-gray weathering, flaggy quartzite interbedded in feldspathic quartzite.

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers 0.6 % of this area

Gray to black, fine-grained , slightly rusty, moderately carbonaceous schist; minor fine- to medium-grained dark-gray to white quartzite. Minor lenses of rock identical to OCAr.

Greylock Schist (Lower Cambrian and Proterozoic Z) at surface, covers 0.5 % of this area

Light-green to gray, white albite-spotted phyllite and interbedded blue-quartz metagraywacke, salmon-pink dolostone, and blue-quartz conglomerate. Minor interbeds of black and white albite-spotted phyllite resembles Hoosac (CAZhga).

Black and white, well-layered hornblende-biotite-plagioclase gneiss and amphibolite (Proterozoic Y) at surface, covers 0.4 % of this area

Contains irregular pods of diopside or cummingtonite-talc rock or amphibole calc-silicate, epidote-layered quartz-plagioclase gneiss near Hinsdale.

Walloomsac Formation (Middle Ordovician) at surface, covers 0.4 % of this area

Orange-brown weathering, graphite-albite-biotite calcitic marble and schistose marble and interbedded black phyllite.

Well-layered hornblende-biotite gneiss (Proterozoic Y) at surface, covers 0.4 % of this area

Well-layered hornblende-biotite gneiss .

Stockbridge Formation (Lower Ordovician) at surface, covers 0.3 % of this area

Tan-beige weathering quartzose calcite and dolomite marble; minor cross-laminated quartzite.

Walloomsac Formation (Middle Ordovician) at surface, covers 0.3 % of this area

Tan to dark-blue-gray laminated quartzite and calcareous quartzite previously mapped as Bellowspipe Quartzite.

Stamford Granite Gneiss (Proterozoic Y) at surface, covers 0.3 % of this area

White to gray biotite-oligoclase-microcline Rapakivi granite gneiss containing blue quartz. Intrudes Yb, Ybu.

Canaan Mountain Formation (Lower Ordovician and Proterozoic Z) at surface, covers 0.3 % of this area

Rusty-weathering, coarse garnet schist and feldspathic schist.

Hatch Hill Formation (Upper Cambrian) at surface, covers 0.3 % of this area

Bluish-gray weathering black sulfidic slate and chert.

Nassau Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.2 % of this area

Dark-gray to black chloritoid-rich phyllite.

Austerlitz Phyllite (Cambrian?) at surface, covers 0.2 % of this area

Austerlitz Phyllite - minor quartzite

Washington Gneiss (Proterozoic Y) at surface, covers 0.2 % of this area

Coarse- to medium-grained hornblende-garnet amphibolite, hornblende-plagioclase gneiss and phlogopite-hornblende-plagioclase amphibolite (metabasalt).

Moretown Formation (Middle Ordovician or older) at surface, covers 0.2 % of this area

Green to dark-green greenstone or amphibolite.

Hoosac Formation (Lower Cambrian and Proterozoic Z) at surface, covers 0.2 % of this area

Rusty-brown to dark-gray, albite-spotted muscovite-biotite schist or gneiss, with interlayered black garnet-biotite-albite-quartz schist near base; interfingers with Dalton Formation.

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

Slate, phyllite, schist, metagraywacke.

White to gray and black-spotted muscovite-biotite granite and granodiorite (Ordovician) at surface, covers 0.2 % of this area

Intruded near or along thrust faults. Intrudes CAZh and Proterozoic Y gneisses.

Washington Gneiss (Proterozoic Y) at surface, covers 0.1 % of this area

Rusty-weathering diopside and sulfidic -rich calcite marble and calc-silicate rock.

Tectonic breccia (Ordovician) at surface, covers 0.1 % of this area

Consisting of completely mixed phyllites of Walloomsac Formation Taconic allochthon.

Everett Schist (Cambrian?) at surface, covers < 0.1 % of this area

Minor meta-graywacke lenses. Includes Greylock Schist in Massachusetts.

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

Light-gray to tan, albite-muscovite-calcite spangled schist.

Tectonic breccia (Ordovician) at surface, covers < 0.1 % of this area

Zones of mixed inclusions of Stockbridge Formation, Walloomsac Formation, and phyllites of the Taconic allochthon.

White, magnetite-bearing alaskite and trondhjemite (Ordovician) at surface, covers < 0.1 % of this area

Associated with blastomylonite along thrust slices. Intrudes Proterozoic Y gneisses.

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

Gray conglomerate containing pebbles of albite and blue quartz, and boulders of gneiss.

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

Greenish-gray albitic phyllite.

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

Olive-gray to green, blue-quartz pebble metagraywacke and quartzite.

Nassau Formation - Rensselaer Graywacke Member (Lower Cambrian and Proterozoic Z) at surface, covers < 0.1 % of this area

Greenish-gray, plagioclase-rich, blue quartz pebble metagraywacke and minor gneiss-cobble conglomerate.

Stockbridge Formation (Cambrian - Lower Ordovician) at surface, covers < 0.1 % of this area

Calcitic and dolomitic marble.

Rowe Schist (Lower Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Fine- to medium-grained, well-layered and foliated amphibolite; epidote-rich layers locally abundant. Includes its typical Chester Amphibolite Member at Chester, Massachusetts.

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

Dark-green to greenish brown, massive to well foliated stilpnomelane-chlorite-oligoclase-epidote metabasalt and basaltic tuff.

Biotite-hornblende mafic dikes (Proterozoic Z) at surface, covers < 0.1 % of this area

Locally with relict subophitic texture. Intrudes Yw and Ygg.

Everett Schist (Cambrian?) at surface, covers < 0.1 % of this area

Grayish to greenish (some rusty-weathering), fine- to medium-grained, foliated but poorly layered schist or phyllite, composed of quartz, albite or oligoclase, muscovite, garnet, staurolite or chloritoid, and generally chlorite. Local layers are dark-gray to silvery schist or phyllite.

Serpentinite and/or talc rock (Precambrian to Phanerozoic) at surface, covers < 0.1 % of this area

Interpreted as tectonic slivers.

Unit c [of Stockbridge Marble] (Upper Cambrian) at surface, covers < 0.1 % of this area

Gray, generally massive dolomite marble, commonly contains quartz grains, locally beds of sandstone; may be calcitic near top.

Nassau Formation of Potter (1972) - Mettawee member (Neoproterozoic) at surface, covers < 0.1 % 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.

Serpentinized peridotite stocks (Ordovician to Proterozoic Z) at surface, covers < 0.1 % of this area

Intrudes Yw and Ygg.

Walloomsac Schist (Middle Ordovician) at surface, covers < 0.1 % of this area

Black to dark-or silvery-gray, rarely layered schist or phyllite, composed of quartz, albite, and commonly garnet and staurolite or sillimanite (locally strongly retrograded to chlorite and muscovite). Locally feldspathic or calcareous near the base.

Diorite at Goff Ledges (Ordovician) at surface, covers < 0.1 % of this area

Diorite at Goff Ledges (Ogd) and similar diorite (Od) in the Windsor quadrangle. Very coarse-grained to pegmatitic, slightly foliated hornblende-plagioclase diorite, minor hornblende pyroxenite. Ogd intrudes CAZh and CAZhk. Od intrudes the Rowe Schist of the Rowe-Hawley zone.

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.

Unit b [of Stockbridge Marble] (Upper and Middle? Cambrian) at surface, covers < 0.1 % of this area

White, pink, cream, and light-gray, generally well bedded dolomitic marble interlayered with phyllite and schist and with siltstone, sandstone, or quartzite, commonly dolomitic.

Units e and d [of Stockbridge Marble] (Lower Ordovician) at surface, covers < 0.1 % of this area

White to gray massive calcite marble, commonly mottled with dolomite and locally interlayered with dolomite marble and calcareous siltstone and sandstone.