Units are listed alphabetically by map symbol. Recognized geologic names, e.g., Cheshire Quartzite, appear as small-type boldface; rock terms, e.g., schist, appear as large-type boldface, and are defined in the glossary.
| Cc | Chesire Quartzite: white, glassy quartzite |
| Cd | Dalton Formation: gray, tan-weathering feldspathic quartzite, gneiss, and schist |
| Ce | Everett Schist: gray, partly rusty-weatherlng, fine- to medium-grained schist |
| Ch | Hoosac Schist: gray, rusty-weathering, fine- to medium-grained schist |
| Cm | Manhattan Schist: dark-gray to silvery, rusly-weathering, coarse-grained schistose gneiss |
| Cma | Amphibolite-bearing unit of Manhattan Schist: like Cm with amphibolite |
| Cmcl | Lower slice of Canaan Mountain Schist: dark-gray, rusty-weathering, coarse-grained schist |
| Cmcu | Upper slice of Canaan Mountain Schist: dark-gray to silvery, rusty-weathering, medium-grained schist |
| Cmcub | Basal member of upper slice of Canaan Mountain Schist: gray, rusty-weathering gneiss |
| Csa | Unit a of Stockbridge Marble: white to gray dolomite marble |
| Csb | Unit b of Stockbridge Marble: white to light-gray dolomite marble and schist |
| Csc | Unit c of Stockbridge Marble: gray dolomite marble |
| Cwb | Waterbury Gneiss: gray to dark-gray, fine- to medium-grained schist and gneiss |
| Dbl | Littleton Formation: gray to silvery, medium-grained schist and micaceous quartzite |
| Dblm | Mount Pisgah Member of Littleton Formation: gray, medium-grained, micaceous quartzite and schist |
| Dc | Canterbury Gneiss: light-gray, medium-grained, locally strongly lineated gneiss |
| Dce | "Eastford gneiss phase" of Canterbury Gneiss: light-gray, medium-grained, locally strongly lineated gneiss |
| D?d | Foliated quartz diorite: dark-gray, medium-grained, well-foliated dioritic gneiss |
| De | Erving Formation: gray, medium-grained granofels and schist |
| Dgg | Foliated granitic gneiss: light-gray, coarse-grained granitic gneiss |
| Dl | Lebanon Gabbro: dark, coarse-grained massive gabbro |
| Dld | Dioritic phase of Lehanon Gabhro: white to black, streaked, medium-grained dioritic gneiss |
| Dlp | Lamprophyre: dark-gray, fine-grained dike rock - lamprophyre |
| Dm | Maromas Granite Gneiss: light-gray to buff, medium- to fine-grained granitic gneiss |
| Dn | Hornblende norite: dark, coarse-grained massive norite |
| Dng | Nonewaug Granite: white to pink, fine- to very coarse-grained granite; parts are pegmatitic |
| DSs | Scotland Schist: gray to silvery, locally rusty, fine- to medium-grained schist |
| DSsq | Quartzite unit in Scotland Schist: micaceous quartzite and mica schist |
| DSt | The Straits Schist: silvery to gray, coarse-grained schist |
| DSts | Southington Mountain Member of The Straits Schist: gray to silvery, medium-grained schist and granofels |
| DSw | Wepawaug Schist: medium- to dark-gray, medium- to fine-grained schist or phyllite |
| Jb | Buttress Dolerite: dark-gray, brown- to gray-weathering dolerite (traprock), compositionally similar to basalt |
| Jeb | East Berlin Formation: reddish-brown silty shale |
| Jha | Hampden Basalt: dark-gray, orange- to brown-weathering basalt (traprock) |
| Jho | Holyoke Basalt: dark-gray, orange- to brown-weathering basalt (traprock) |
| Jp | Portland Arkose: reddish-brown arkose (brownstone) |
| Jsi | Silicified rock and mylonite along Mesozoic faults: sheared rock with abundant quartz veins |
| Jsm | Shuttle Meadow Formation: reddish-brown silty shale |
| Jta | Talcott Basalt: dark-gray, orange- to brown-weathering basalt (traprock) |
| Jwr | West Rock Dolerite: dark-gray, orange- to brown-weathering dolerite (traprock), compositionally similar to basalt |
| Oa | Allingtown Metavolcanics: green, fine-grained massive greenstone |
| Ob | Brookfield Gneiss: dark and light, medium- to coarse-grained dioritic gneiss |
| Obr | Brimfield Schist: gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss |
| Obrg | Gneiss (metavolcanic) member of Brimfield Schist: gray, medium-grained, layered gneiss and schist |
| Obs | Bristol Gneiss: light-gray, medium-grained gneiss |
| Oc | Collinsville Formation: gray and silvery, medium- to coarse-grained schist and dark, fine- to medium-grained amphibolite and hornblende gneiss |
| Ocg | Hornblende gneiss member of Collinsville Formation: dark, fine- to medium-grained amphibolite and hornblende gneiss |
| Och | Collins Hill Formation: gray, rusty-weathering, medium- to coarse-grained schist |
| Ochv | Metavolcanic member of Collins Hill Formation: dark amphibolite and hornblende schist, and light-gray gneiss |
| Ocm | Cobble Mountain Formation: gray to silvery, medium- to coarse-grained schist and granofels |
| OCr | Rowe Schist: light-gray to silvery, fine- to medium-grained schist |
| OCra | Amphibolite unit in Rowe Schist: black or mottled, massive amphibolite and hornblende gneiss |
| OCs | Stockbridge Marble: white to gray dolomitic marble |
| Ocs | Sweetheart Mountain Member of Collinsvllle Formation: gray and silvery, medium- to coarse-grained schist |
| Og | Ordovician? granitic gneiss: light-colored, foliated granitic gneiss |
| Ogh | Golden Hill Schist: gray to silvery, medium- to coarse-grained schist and granofels |
| Ogl | Glastonbury Gneiss: gray, medium- to coarse-grained, massive to well-foliated granitic gneiss |
| Oh | Harrison Gneiss: interlayered dark- and light-gray, medium-grained, foliated gneiss |
| Ohb | Beardsley Member of Harrison Gneiss: gray to dark-gray, medium-grained, lineated gneiss |
| Ohc | Hawley Formation (carbonaceous schist facies): gray, rusty-weathering, fine- to medium-grained schist and granofels |
| Ohn | Nodular member of Harrison Gneiss: similar to Oh with prominent quartz-sillimanite nodules |
| Ohp | Pumpkin Ground Member of Harrison Gneiss: gray to spotted, medium- to coarse-grained, foliated gneiss |
| Ol | Litchfield Norite: dark, medium- to coarse-grained, massive norite |
| Om | Middletown Formation: dark- to light-gray gneiss and granofels; hornblende gneiss and amphibolite |
| Oma | Maltby Lakes Metavolcanics: green to gray-green, fine-grained greenstone and schist |
| Omal | Lower part of Maltby Lakes Metavolcanics: gray-green to green, fine-grained greenschist and schist or phyllite |
| Omau | Upper part of Maltby Lakes Metavolcanics: green to gray-green, fine-grained greenstone and schist |
| Oml | Lower member of Middletown Formation: dark- to medium-gray amphibolite and hornblende gneiss |
| Omm | Massive mafic rock in Middletown Formation: dark, coarse-grained, massive amphibolite and mafic rock |
| Omo | Monson Gneiss: light to dark, medium- to coarse-grained gneiss |
| Omu | Upper member of Middletown Formation: light-gray, rusty-weathering gneiss and granofels |
| Oo | Oronoque Schist: gray to silver, medium- to fine-grained schist and granofels |
| Op | Preston Gabbro: dark, medium- to coarse-grained, massive gabbro |
| Opd | Dioritic phase of Preston Gabbro: medium- to dark-gray, streaked, medium-grained diorite |
| Oq | Quinebaug Formation: gray to dark-gray, medium-grained, well-layered gneiss |
| Oqb | Black Hill Member of Quinebaug Formation: gray, fine-grained, well-layered schist and granofels |
| Oqf | Felsic gneiss member of Quinebaug Formation: light- to medium-gray, fine- to medium-grained gneiss |
| Or | Ratlum Mountain Schist: gray, medium-grained schist and granofels |
| Ora | Amphibolite unit in Ratlum Mountain Schist: black or mottled, massive amphibolite and hornblende gneiss |
| Ose | Units e and d of Stockbridge Marble: white to gray calcite marble |
| Osg | Units g and f of Stockbridge Marble: white to gray calcite marble |
| Ot | Taine Mountain Formation: gray, medium-grained, well-laminated granofels |
| Ota | Tatnic Hill Formation: gray to dark-gray, medium-grained gneiss or schist |
| Otaf | Fly Pond (calc-silicale) Member of Tatnic Hill Formation: light-gray, medium-grained calc-silicate gneiss |
| Otay | Yantic Member of Tatnic Hill Formation: gray to dark-gray, fine- to medium-grained schist |
| Otb | Basal member of Taine Mountain Formation around Waterbury dome: well-layered, gray granofels |
| Otf | Trap Falls Formation: gray to silvery, partly rusty-weathering, medium-grained schist |
| Otfc | Caringtons Pond Member of Trap Falls Formation: interlayered gray, rusty-weathering schist and light-gray gneiss |
| Otfg | Schist and granulite member of Trap Falls Formation: interlayered gray to silvery, medium- to coarse-grained schist and fine-grained granofels |
| Otfs | Shelton (white gneiss) Member of Trap Falls Formation: white to light-gray granitic gneiss |
| Ot + Oc | Taine MountainandCollinsville Formations undivided: See Ot and Oc |
| Ots | Scranton Mountain Member of Taine Mountain Formation: gray, rusty-weathering, medium-grained schist |
| Otw | Wildcat Member of Taine Mountain Formation: gray, medium-grained, well-laminated granofels |
| Otwv | Whigville Member of Taine Mountain Formation: gray, medium-grained, well-laminated granofels |
| Ow | Walloomsac Schist: dark, fine-grained schist |
| Owm | Basal marble member of Walloomsac Schist: dark- to light-colored schistose marble |
| Pn | Narragansett Pier Granite: pink to red, medium- to coarse-grained, massive granite |
| Pnm | Mafic phase of Narragansett Pier Granite: gray to reddish, medium-grained, massive granite |
| Pp | Porphyry: light-colored porphyry |
| Ppa | Pinewood Adamellite: light-gray, mcdium-graincd granite |
| Ps | Syenite: gray, massive syenite |
| Pw | Westerly Granite: light-gray, pink-weathering, fine-grained, massive granite |
| Pzmy | Mylonite along Paleozoic faults: sheared, recrystallized rock - mylonite |
| Sbc | Clongh Quartzite: white, medium-grained, well-layered quartzite |
| Sbf | Fitch Formation: gray, fine- to medium-grained, calc-silicate gneiss |
| SObl | Lower member of Bigelow Brook Formation: gray, medium-grained granofels |
| SObm | Middle member of Bigelow Brook Formation: greenish-gray, medium-grained, calc-silicate rock with schist and granofels |
| SObu | Upper member of Bigelow Brook Formation: gray, rusty-weathering, medium-grained schist |
| SOh | Hebron Gneiss: interlayered dark-gray schist and greenish-gray, fine- to medium-grained calc-silicate gneiss |
| SOs | Southbridge Formation: dark- to light-gray, locally rusty, fine- to medium-grained granofels and schist |
| SOsp | Porphyritic member of Southbridge Formation: light- to medium-gray, spotted, fine-grained gneiss |
| Stb | Basal member of The Straits Schist: gray schist wilh amphibolite, marble, and quartzite |
| Trnh | New Haven Arkose: reddish, poorly sorted arkose |
| u | Ultramafic rock: dark, medium- to coarse-grained ultramafic rock |
| Yg | Gneiss of Highlands massifs: granitic gneiss, gneiss, and schist |
| Yga | Augen gneiss: gray to spotted, fine- to mediurn-grained, lineated granitic gneiss |
| Ygh | Hornblende gneiss and amphibolite: dark-gray, fine- to medium-grained amphibolite and gneiss |
| Ygn | Layered gneiss: gray, medium-grained, well-Iayered gneiss |
| Ygr | Pink granitic gneiss: light-pink to gray granitic gneiss |
| Ygs | Rusty mica schist and gneiss: dark-gray;rusty-weathering schist and gneiss |
| Zb | Branford Gneiss: gray to white, well-foliated granitic gneiss |
| Zl | Light House Gneiss: light-pink or gray to red, medium-grained, well-foliated granitic gneiss |
| Zp | Plainfield Formation: interlayered thinly bedded quartzite, mica schist, and dark-gray gneiss |
| Zpq | Quartzite unit in Plainfield Formation: light-gray, glassy, generally thin bedded quartzite |
| Zsc | Stony Creek Granite Gneiss: red to pink, medium- to very coarse-grained granitic gneiss |
| Zsh | Hope Valley Alaskite Gneiss: light-pink to gray, medium- to coarse-grained granitic gneiss |
| Zsp | Ponaganset Gneiss: dark-gray to spotted, coarse-grained, well-foliated gneiss |
| Zsph | Potter Hill Granite Gneiss: light-pink to gray, tan-weathering, fine- to medium-grained, well-foliated granitic gneiss |
| Zspp | Porphyritic phase of Potter Hill Granite Gneiss: light-to medium-gray, spotted, fine- to medium-grained, well-foliated gneiss |
| Zss | "Scituate" Granite Gneiss: light-pink to gray, medium- to coarse-grained, lineated granitic gneiss |
| Zw | Waterford Group: light to dark, medium-grained gneiss |
| Zwm | Mamacoke Formation: interlayered light- to dark-gray, medium-grained gneiss |
| Zwn | New London Gneiss: massive, gray granitic gneiss |
| Zwnj | Joshua Rock Member of New London Gneiss: medium-gray, medium-grained gneiss |
| Zwr | Rope Ferry Gneiss: interlayered light to dark, fine- to medium-grained gneiss |
Simplified descriptions are given for rock terms used above. Descriptions include general and distinguishing characteristics of rocks as they commonly occur in Connecticut.
| alaskite gneiss: | Light-colored, fine- to coarse-grained, banded metamorphic rock of granite-like composition with potassium feldspar as the major feldspar, and only a few percent dark minerals. |
| amphibolite: | Dark-colored, fine- to coarse-grained, massive to poorly layered metamorphic rock containing amphibole and plagioclase with little or no quartz. |
| arkose: | Red to brown, medium- to coarse-grained, sandstone-like, sedimentary rock containing quartz, feldspar, and rock fragments. It is the most common sedimentary rock of the Central Lowlands; locally known as brownstone. Brownstone was quarried for use as building stone. |
| basalt: | Dark-gray, orange- to brown-weathering, fine-grained, extrusive igneous rock, commonly fractured (jointed), containing calcium-plagioclase and pyroxene; occurs as lava flows in the Central Lowlands; locally known as traprock. Basalt is extensively quarried for use as crushed stone. |
| diorite: | Light and dark (salt and pepper), medium- to coarse-grained intrusive igneous rock, commonly massive-appearing, generally lacking layering or banding; composed primarily of plagioclase feldspar, quartz and dark minerals. |
| dioritic gneiss: | Light and dark, medium- to coarse-grained, compositionally banded metamorphic rock of dioritic composition. |
| dolerite: | Dark-gray, orange- to brown- to gray-weathering, medium- to fine-grained intrusive igneous rock, commonly fractured into well-developed columnar joints, compositionally similar to basalt; occurs as dikes and sills in both the lowlands and uplands; locally known as traprock. Dolerite is quarried for use as crushed stone. |
| gabbro: | Dark, medium- to coarse-grained, intrusive mafic igneous rock, commonly massive-appearing, generally lacking layering or banding; composed primarily of clinopyroxene and plagioclase; often altered. |
| gneiss: | Light and dark, medium- to coarse-grained metamorphic rock characterized by compositional banding of light and dark minerals, typically composed of quartz, feldspar, and various amounts of dark minerals: occurs with a variety of compositions and is a characteristic rock of the uplands. |
| granite: | Light-colorod, medium- to very coarse-grained intrusive igneous rock, commonly massive-appearing, lacking layering or banding; composed of quartz, feldspar, and commonly muscovite mica with minor amounts of dark minerals. Granite and granitic gneiss were quarried for use as dimension stone. Very limited quarrying continues. |
| granitic gneiss: | Light-colored, medium- to coarse-grained, compositionally banded metamorphic rock of granitic composition. Quarried for use as dimension stone (see granite). |
| granofels: | Light to dark, medium- to coarse-grained, massive to poorly layered metamorphic rock composed primarily of quartz and feldspar; lacking the compositional banding of a gneiss. |
| greenschist: | Light to dark, green, medium- to coarse-grained schist; typically consisting of the green minerals: chlorite, epidote, or actinolite. |
| greenstone: | Green to gray-green, fine- to coarse-grained, massive to poorly layered metamorphic rock composed of chlorite, hornblende, and epidote. |
| lamprophyre: | Dark-colored, medium-grained intrusive igneous rock occurring as dikes; composed of biotite, hornblende, pyroxene, and feldspars or feldspathoids. |
| mafic rock: | Dark-colored rock consisting of one or more dark (iron and magnesian) minerals as its major constituent. |
| marble: | Conspicuously white, or gray, medium- to coarse-grained, massive to layered metamorphic rock composed of calcite and/or dolomite. It is a metamorphosed limestone and underlies several major valleys in the Western Uplands. Marble is quarried for use as agricultural lime and for industrial uses. |
| mylonite: | Compact rock with streaky or banded structure, found in fault zones, and produced by extreme crushing and shearing of rocks during movement of a fault. It is particularly well developed along the Lake Char - Honey Hill fault zones of the Eastern Uplands. |
| norite: | Dark-colored, medium- to coarse-grained intrusive mafic igneous rock commonly massive-appearing, lacking layering or banding, composed of plagioclase and orthopyroxene; often altered. |
| pegmatite: | Light-colored, very coarse-grained intrusive igneous rock, composed of feldspar, quartz, and micas; frequently can be seen in highway roadcuts as light-colored parallel and crosscutting veins and lenses in the metamorphic rock of the uplands; occasionally is a host for gem minerals. Pegmatite is a common and economically important rock type in Connecticut. It occurs in bodies too small to show on a state-scale geologic map. The Branchville area of western Connecticut and the Middletown area of eastern Connecticut have pegmatites of particular significance. |
| phyllite: | Silvery, gray to dark-gray, fine- to very fine-grained, schist-Iike metamorphic rock. |
| porphyry: | Light-colored, fine-grained intrusive igneous rock with some conspicuously large crystals of quartz, feldspar, or biotite. |
| quartzite: | Light-colored to gray, massive to layered, medium-grained metamorphic rock. Very hard and resistant; a metamorphosed sandstone composed primarily of quartz. |
| shale: | Red to brown, locally gray to green, extremely fine-grained, strongly layered sedimentary rock composed of clay minerals; occurs in the Central Lowlands. |
| schist: | Light, silvery to dark, coarse- to very coarse-grained, strongly to very strongly layered metamorphic rock whose layering is typically defined by parallel alignment of micas. Primarily composed of mica, quartz, and feldspar; occasionally spotted with conspicuous garnets. |
| schistose marble: | Light-colored, fine- to coarse-grained, marble inlerlayered with schist or phyllite. |
| syenite: | Medium-gray, fine- to coarse-grained intrusive igneous rock, commonly massive-appearing, generally lacking layering or banding, composed primarily of potassium feldspar with minor dark minerals but little quartz. |
| ultramafic rock: | Dark-colored, medium- to coarse-grained intrusive igneous rock, commonly massive-appearing, generally lacking layering or banding and originally composed chiefly of olivine and pyroxene; commonly altered to serpentine and talc. Occurs as small intrusions in the uplands. |