USGS Home
Contact USGS
Search USGS

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

Geologic units in Tolland county, Connecticut

Waterford Group (Proterozoic Z?) at surface, covers 2 % of this area

Waterford Group (may be equivalent in part to Monson Gneiss) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite. Usage of Waterford Group follows Goldsmith (1980; 1985). Described as a sequence of metavolcanic and metaplutonic plagioclase gneisses and amphibolites that unconformably overlies the Plainfield Formation in the Hope Valley terrane. (Hope Valley and Esmond-Dedham terranes compose the Avalon superterrane of this report.) Thickness is variable; ranges to 3,100 m. Subdivided (ascending) into Mamacoke Formation with its upper Cohanzie Member (first used?), New London Gneiss, and Rope Ferry Gneiss. Age is Late Proterozoic based on U-Pb analyses of zircon and sphene in the Rope Ferry Gneiss (620+/-3 Ma, Wintsch and Aleinikoff, 1987) (Skehan and Rast, 1990).

Lithology: gneiss; amphibolite

Portland Formation (Lower Jurassic) at surface, covers < 0.1 % of this area

Portland Formation - Reddish-brown to pale red conglomerate and arkose.

Lithology: conglomerate; arkose

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

Fitch Formation - Gray, fine- to medium-grained calc-silicate rock, composed of quartz, biotite, calcite, actinolite, diopside, microcline, and locally garnet, scapolite, or epidote, interlayered with two-mica schist.

Lithology: calc-silicate rock; mica schist

Glastonbury Gneiss (Ordovician) at surface, covers < 0.1 % of this area

Glastonbury Gneiss - Massive granitic gneiss in core of Glastonbury dome and in adjacent areas.

Lithology: granitic gneiss

Silicified rock and mylonite along Mesozoic faults (probably mainly Jurassic) at surface, covers 0.2 % of this area

Silicified rock and mylonite along Mesozoic faults - Close network of quartz veins and veinlets cutting each other and older rock, which is mostly replaced by very fine grained quartz. In places, incompletely replaced rock shows strongly mylonitic texture.

Lithology: mylonite

Gneiss (metavolcanic) member [of Brimfield Schist] (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Gneiss (metavolcanic) member [of Brimfield Schist] - Medium-gray, medium-grained, layered gneiss and schist, composed of oligoclase, quartz, and biotite; some gneiss and most schist layers contain garnet and sillimanite; some gneiss layers contain garnet, hornblende or pyroxene or grade into amphibolite or calc-silicate rock. Probably includes metavolcanic rocks.

Lithology: gneiss; schist; amphibolite; calc-silicate rock; metavolcanic rock

Brimfield Schist (Upper? and Middle Ordovician) at surface, covers 19 % of this area

Brimfield Schist (includes Hamilton Resevoir Formation) - Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Lithology: schist; gneiss; amphibolite; calc-silicate rock

Southbridge Formation (Silurian or Ordovician or both) at surface, covers 0.4 % of this area

Southbridge Formation - Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist, and amphibole, calc-silicate minerals, K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist.

Lithology: granofels; schist; calc-silicate rock; amphibolite

Biotite granitic gneiss (Devonian) at surface, covers < 0.1 % of this area

Biotite granitic gneiss.

Lithology: granitic gneiss

Littleton Formation (uncertain) (Devonian) at surface, covers 0.2 % of this area

Littleton Formation (uncertain)- Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist.

Lithology: schist; quartzite

Monson Gneiss (Ordovician, Cambrian, or Proterozoic Z) at surface, covers < 0.1 % of this area

Monson Gneiss - Layered to massive biotite-plagioclase gneiss, amphibolite, microcline augen gneiss.

Lithology: biotite gneiss; amphibolite; augen gneiss

Hornblende norite (Devonian?) at surface, covers < 0.1 % of this area

Hornblende norite - Dark, coarse-grained, massive rock, composed of bytownite, hornblende, and hypersthene.

Lithology: norite

Collins Hill Formation (Upper? and Middle Ordovician) at surface, covers 0.9 % of this area

Collins Hill Formation ( = Partridge Formation of New Hampshire) - Gray, rusty-weathering, medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, muscovite, biotite, and garnet, and commonly staurolite, kyanite, or sillimanite, generally graphitic, interlayered with fine-grained two-mica gneiss, especially to the west, and with calc-silicate and amphibolite layers, also rare quartz-spessartine (coticule) layers.

Lithology: schist; gneiss; amphibolite; calc-silicate rock

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

Littleton Formation - Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist.

Lithology: schist; quartzite

Brimfield Schist (uncertain) (Upper? and Middle Ordovician) at surface, covers < 0.1 % of this area

Brimfield Schist (uncertain) (includes Hamilton Resevoir Formation) - Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende- and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock.

Lithology: schist; gneiss; amphibolite; calc-silicate rock

Buttress Dolerite (Middle? Jurassic) at surface, covers 0.4 % of this area

Buttress Dolerite - Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine.

Lithology: diabase; basalt; gabbro

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

Erving Formation - Granofels and schist where mapped separately.

Lithology: granulite; schist

Monson Gneiss (Middle or Lower Ordovician?) at surface, covers 5 % of this area

Monson Gneiss (may be equivalent to part of Waterford Group) - Interlayered light to dark, mostly medium to coarse-grained gneiss and amphibolite; gneiss composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; traces of garnet, epidote, and magnetite.

Lithology: gneiss; amphibolite

Clough Quartzite (Silurian) at surface, covers 0.5 % of this area

Clough Quartzite - White, medium-grained, glassy to granular, well-layered quartzite and muscovitic quartzite, locally with garnet; conglomeratic (commonly with tourmaline) in lower part.

Lithology: quartzite; conglomerate

Lebanon Gabbro (Devonian) at surface, covers < 0.1 % of this area

Lebanon Gabbro - Dark, speckled, coarse-grained, massive but locally sheared gabbro, composed of hornblende, labradorite, and opaques. Some bodies contain biotite, and quartz; some smaller ones are nearly pure hornblende with local augite.

Lithology: gabbro

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

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

Lithology: schist; phyllite

Upper member [of Bigelow Brook Formation] (Silurian and perhaps Ordovician) at surface, covers 2 % of this area

Upper member [of Bigelow Brook Formation] - Chiefly gray, rusty-weathering, medium-grained, generally well layered and locally fissile schist, composed of plagioclase, quartz, biotite, garnet, and sillimanite, locally with K-feldspar or cordierite, fissile layers commonly with graphite and pyrrhotite, interlayered with quartzose granofels with less biotite but with calc-silicate minerals.

Lithology: schist; granofels

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

Paxton Formation - Undifferentiated biotite granofels, calc-silicate granofels, and sulfidic schist. The Paxton, here of group rank, includes strata formerly mapped in CT as the Hebron Formation and in MA as the Paxton Formation. It conformably overlies the Oakdale Formation and structurally and conformably underlies the Brimfield Group. It is undivided in central MA; in northeast CT and adjacent MA it is divided into the Dudley and Southbridge Formations. Age is Late Proterozoic(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss into the overlying Brimfield Group and an age of 1188 m.y. for detrital zircons from the Paxton (Pease, 1989).

Lithology: granofels; schist

Glastonbury Gneiss (Middle? Ordovician) at surface, covers 12 % of this area

Glastonbury Gneiss - Gray, medium- to coarse-grained, massive to well-foliated granitoid gneiss composed of oligoclase, quartz, microcline, and biotite (as patches), also epidote and hornblende in many areas, commonly associated with layers of amphibolite; elsewhere minor muscovite and garnet.

Lithology: granitic gneiss; amphibolite

Foliated quartz diorite (Devonian in part, probably Ordovician in part) at surface, covers 0.7 % of this area

Foliated quartz diorite - Mainly dark-gray, medium-grained, well-foliated gneiss (locally strongly sheared, especially near contacts), composed of plagioclase, quartz, biotite, and hornblende, locally also pyroxene.

Lithology: mafic gneiss

Middletown Formation (Middle Ordovician) at surface, covers 4 % of this area

Middletown Formation ( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules.

Lithology: gneiss; granofels; amphibolite; mafic gneiss; biotite gneiss; calc-silicate rock

Erving Formation (Devonian) at surface, covers 0.3 % of this area

Erving Formation - Gray, medium-grained, well-foliated and generally well layered granofels and schist, composed of quartz, plagioclase, and biotite, also muscovite in schist, and accessory garnet and kyanite.

Lithology: granofels; schist

Yantic Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Yantic Member [of Tatnic Hill Formation] - Medium- to dark-gray, fine- to medium-grained schist, composed of quartz, oligoclase, biotite, and muscovite, some layers with garnet, staurolite, and kyanite or garnet and sillimanite, local epidote, or K-feldspar; some layers of rusty-weathering graphitic, pyrrhotitic, two-mica schist.

Lithology: schist; mica schist

Foliated granitic gneiss (Devonian?) at surface, covers 0.1 % of this area

Foliated granitic gneiss - Light-gray, coarse-grained, strongly to weakly foliated gneiss, composed of phenocrysts of K-feldspar in a groundmass of plagioclase, quartz, K-feldspar, and biotite, with accessory sillimanite and garnet.

Lithology: granitic gneiss

Massive mafic rock (in Middletown Formation) (Middle Ordovician) at surface, covers 0.2 % of this area

Massive mafic rock (in Middletown Formation) - Dark, coarse-grained, massive amphibolite and metagabbro, composed of hornblende and plagioclase; in places with quartz and epidote, in others with patches of actinolite or anthophyllite, chlorite, and epidote or garnet. May be intrusive.

Lithology: amphibolite

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Mafic and felsic gneisses of volcanic derivation with calc-silicate granofels.

Lithology: mafic gneiss; felsic gneiss; granofels

Tatnic Hill Formation (Upper? and Middle Ordovician) at surface, covers 3 % of this area

Tatnic Hill Formation - Medium- to dark-gray, medium-grained gneiss or schist composed of quartz, andesine, biotite, garnet, and sillimanite, locally kyanite, muscovite, or K-feldspar, interlayered with locally mappable units and thinner layers of rusty-weathering graphitic pyrrhotitic two-mica schist, amphibolite, and calc-silicate rock.

Lithology: gneiss; schist; mica schist; amphibolite; calc-silicate rock

Hope Valley Alaskite Gneiss (Proterozoic Z?) at surface, covers 1 % of this area

Hope Valley Alaskite Gneiss - Light-pink to gray, medium- to coarse-grained, locally porphyritic, variably lineated and foliated alaskitic gneiss, composed of microcline, quartz, albite or oligoclase, and minor magnetite, and locally biotite and muscovite. Lineation formed by rods of quartz. Locally contains quartz-sillimanite nodules.

Lithology: granitic gneiss

Middle member [of Bigelow Brook Formation] (Silurian or perhaps Ordovician) at surface, covers < 0.1 % of this area

Middle member [of Bigelow Brook Formation] - Greenish-gray, medium-grained calc-silicate rock, composed of plagioclase, quartz, and diopside (locally hornblende and scapolite), interbedded with schist and granofels composed of plagioclase, quartz, biotite, and commonly garnet and sillimanite.

Lithology: calc-silicate rock; schist; granofels

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) at surface, covers 1 % of this area

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] - Light-gray, medium-grained, layered to massive calc-silicate gneiss, composed of andesine, quartz, hornblende or actinolite, epidote, and commonly diopside, biotite, and scapolite; some layers are calcitic.

Lithology: gneiss

Canterbury Gneiss (Devonian) at surface, covers 3 % of this area

Canterbury Gneiss (may be equivalent to Ayer Granite of Massachusetts) - Light-gray, medium-grained, variably foliated, locally strongly lineated gneiss, composed of quartz, oligoclase, microcline, and biotite, locally also muscovite, or epidote, and generally with megacrysts 1 to 2 cm long of either or both feldspars.

Lithology: gneiss

Southbridge Formation (Silurian or Ordovician or both) at surface, covers 3 % of this area

Southbridge Formation - Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist, and amphibole, calc-silicate minerals, K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist.

Lithology: granofels; schist

Biotite-hornblende diorite and quartz-bearing diorite (Devonian) at surface, covers < 0.1 % of this area

Biotite-hornblende diorite and quartz-bearing diorite - Mostly foliated; intrudes Dl.

Lithology: diorite; quartz diorite

Mount Pisgah Member of Littleton Formation (Devonian) at surface, covers 0.8 % of this area

Mount Pisgah Member of Littleton Formation - Gray, medium-grained, well-layered (locally graded) granofels or micaceous quartzite with some schist, composed of quartz, oligoclase, biotite, garnet, and sillimanite.

Lithology: granofels; quartzite; schist

Eastford gneiss phase [of Canterbury Gneiss] (Devonian) at surface, covers 0.6 % of this area

Eastford gneiss phase [of Canterbury Gneiss] - Mainly light-gray, medium-grained, foliated to strongly lineated gneiss, composed of quartz, microcline, oligoclase, or albite, biotite, and muscovite.

Lithology: gneiss

Lower member [of Bigelow Brook Formation] (Silurian or Ordovician or both) at surface, covers 2 % of this area

Lower member [of Bigelow Brook Formation] - Chiefly gray, medium-grained, well-layered granofels, composed of quartz, oligoclase, and biotite, commonly with garnet and sillimanite, interlayered with thinly fissile sillimanitic, graphitic, pyrrhotite schist.

Lithology: granofels; schist

Ammonoosuc Volcanics (Middle Ordovician) at surface, covers < 0.1 % of this area

Ammonoosuc Volcanics - Amphibolite, felsic gneiss, garnet-amphibole quartzite, and marble too thin to show separately at map scale. Gedrite, anthophyllite, cummingtonite locally abundant in amphibolite layers.

Lithology: amphibolite; felsic gneiss; quartzite; marble

Portland Arkose (Lower Jurassic) at surface, covers 10 % of this area

Portland Arkose - Reddish-brown to maroon micaceous arkose and siltstone and red to black fissile silty shale. Grades eastward into coarse conglomerate (fanglomerate).

Lithology: arkose; siltstone; shale; conglomerate

Hebron Gneiss (Silurian and Ordovician) at surface, covers 18 % of this area

Hebron Gneiss - Interlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar, and greenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite. Local lenses of graphitic two-mica schist.

Lithology: schist; calc-silicate rock; mica schist

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

Partridge Formation (includes Brimfield Schist of Emerson, 1917) - Sulfidic mica schist and subordinate amphibolite.

Lithology: mica schist; amphibolite