Geologic units in Essex county, Vermont

Albee Formation (Ordovician and Cambrian) at surface, covers 21 % of this area

Light-gray to greenish-gray, white-weathering, fine-grained feldspathic metasandstone and metasiltstone, and light-gray to greenish-gray to dark-gray phyllite. Lesser amounts of quartzite. Rare calc-silicate nodules. Generally sharply bedded, but graded beds as well as slump structures are locally obvious. Tourmaline is a sporadic accessory mineral. May be sulfidic (either pyrite or pyrrhotite) and rusty weathering. “Pinstriping” is common. U-Pb zircon SHRIMP age of 492.5±7.8 Ma from a porphyritic tonalite sill about 2 km east of West Bath, N.H. (D.W. Rankin, USGS, unpub. data, 2011). Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Gile Mountain Formation - quartzite and metapelite member (Lower Devonian) at surface, covers 20 % of this area

Gray to light-gray, fine-grained micaceous quartzite a few centimeters to tens of centimeters thick, interbedded with dark-gray graphitic slate, phyllite, or schist. Part of the Connecticut Valley Trough.

Averill pluton (Devonian) at surface, covers 13 % of this area

Gray to pink, medium- to coarse-grained garnet-muscovite-biotite granite and pegmatite, undifferentiated. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Frontenac Formation (Devonian and Silurian) at surface, covers 6 % of this area

Thick-bedded, ankeritic, micaceous, and feldspathic metasandstones interlayered with subordinate dark-gray metapelite. Metasandstone beds commonly are rusty weathering and up to 4 m thick; calc-silicate lenses locally present. Part of the Connecticut Valley Trough.

Maidstone pluton (Devonian) at surface, covers 6 % of this area

Almost-white to dark-gray, locally pink, medium- to coarse-grained hypidiomorphic, granular, biotite-muscovite-microcline-plagioclase granite; accessories include apatite, sphene, pyrite, and magnetite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Nulhegan pluton - inner zone (Devonian) at surface, covers 6 % of this area

Light-gray to dark-gray, medium- to coarse-grained, massive, biotite-clotted, hornblende-quartz monzodiorite, minor granodiorite, and granite. Rb/Sr whole-rock age of 390±14 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Ironbound Mountain Formation (Lower Devonian) at surface, covers 5 % of this area

Medium-dark-gray to grayish-black lustrous slate, phyllite, and schist containing sparse to moderately abundant 1-mm to- 5-cm-thick beds of light-gray, fine-grained metasandstone and metasiltstone, commonly pyritiferous and calcareous. Some graded beds. Gradational contact with Dco above and Dir below. Interpreted to be correlative with the Meetinghouse Slate Member of the Gile Mountain Formation. Part of the Connecticut Valley Trough.

Victory pluton (Devonian) at surface, covers 4 % of this area

A western zone of medium- to fine-grained biotite-muscovite granite and minor hornblende-biotite tonalite and an eastern, more melanocratic zone of biotite-hornblende quartz monzonite like that in the Nulhegan pluton, as well as biotite granodiorite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Compton Formation - metasandstone member (Lower Devonian) at surface, covers 4 % of this area

Light-gray to tan, micaceous, locally calcareous metasandstone and slate or metamudstone in beds a few centimeters to tens of centimeters thick. Graded bedding common. Interpreted to be correlative with the Gile Mountain Formation. Part of the Connecticut Valley Trough.

Nulhegan pluton - core (Devonian) at surface, covers 3 % of this area

Quartz monzonite. Rb/Sr whole-rock age of 390±14 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Lost Nation granite (Early Silurian to Middle Ordovician) at surface, covers 2 % of this area

Foliated biotite and (or) hornblende granite; locally diorite and lesser amounts of gabbro. Where present, potassium feldspar is microcline. Contact aureole is in the Albee Formation. U-Pb zircon ages of 442±4 Ma, no. 30 (Moench and Aleinikoff, 2003), and 444.1±2.1 Ma, no. 29 (Rankin and Tucker, 2009); and U-Pb sphene age of 443±3 Ma, no. 31 (Moench and Aleinikoff, 2003). Part of the Highlandcroft Plutonic Suite: Epizonal to mesozonal, foliated and metamorphosed (greenschist facies) plutons exposed northwest of the Ammonoosuc fault. Compositions range from granite to diorite to lesser amounts of gabbro. Part of the Bronson Hill arch intrusive rocks.

Nulhegan pluton - western zone (Devonian) at surface, covers 2 % of this area

Light-gray to dark-gray, medium- to coarse-grained, massive, biotite-clotted, hornblende-quartz monzodiorite, minor granodiorite, and granite. Rb/Sr whole-rock age of 390±14 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Albee Formation - Scarritt member (Ordovician and Cambrian) at surface, covers 1 % of this area

Dark-gray slate interlayered with thin beds of light-gray, fine-grained micaceous and feldspathic metasandstone (typically ribby weathering). Abruptly graded beds <1 cm to 30 cm thick are locally common as is channeling and, in places, soft-sediment deformation. Commonly sulfidic and rusty weathering. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Gile Mountain Formation - Meetinghouse Slate member (Lower Devonian) at surface, covers 0.8 % of this area

Dark-gray slate and phyllite containing sparse to moderately abundant beds of light-gray, fine-grained metasandstone and metasiltstone, 1 mm to 1 cm thick. Part of the Connecticut Valley Trough.

Echo Pond pluton - gabbro and diorite (Devonian) at surface, covers 0.8 % of this area

Outer zone of quartz gabbro and quartz diorite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Monadnock Mountain pluton - quartz syenite (Middle Jurassic) at surface, covers 0.8 % of this area

Greenish-gray to pink hornblende-biotite quartz syenite. Monadnock Mountain pluton: Composite stock of quartz syenite, syenite granite, and essexite. K-Ar age of 175±4 Ma (Foland and Faul, 1977). Part of the White Mountain Igneous Suite.

Ironbound Mountain Formation - rhythmically graded member (Lower Devonian) at surface, covers 0.7 % of this area

Light- to medium-gray, fine-grained micaceous metasandstones that grade upward into subordinate dark-gray slate or phyllite; some rocks are calcareous. Graded sets range in thickness from a few centimeters to about a meter; typically they are 10 to 30 cm thick. Contact with Di gradational. Part of the Connecticut Valley Trough.

Waits River Formation - carbonaceous phyllite and limestone member (Lower Devonian and Upper Silurian) at surface, covers 0.7 % of this area

Dark-gray to silvery-gray, lustrous, carbonaceous muscovite-biotite-quartz (±garnet) phyllite containing abundant beds of punky-brown-weathering, dark-bluish-gray micaceous quartz-rich limestone in beds ranging from 10 cm to 10 m thick. Part of the Connecticut Valley Trough.

Newark pluton (Devonian) at surface, covers 0.6 % of this area

Pinkish-gray to cream-colored, medium- to coarse-grained magnetite-microcline-perthite-biotite granite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Washburn Brook Formation - siltstone, quartzite, volcanogenic chert, and ironstone (Upper and Middle Ordovician) at surface, covers 0.4 % of this area

Metamorphosed gray siltstone, quartzite, volcanogenic chert, and ironstone, all typically containing coticule and magnetite. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Echo Pond pluton - granodiorite and granite (Devonian) at surface, covers 0.3 % of this area

Light-gray to pink, medium- to coarse-grained, massive, seriate to porphyritic granodiorite and granite. Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Washburn Brook Formation - metamorphosed sedimentary breccia (Upper and Middle Ordovician) at surface, covers 0.3 % of this area

Metamorphosed sedimentary breccia interlayered with dark-gray slate and micaceous siltstone. Clasts include light-colored, fine-grained metasandstone and metasiltstone of the Albee Formation, dark-gray or greenish-gray slate, and coticule-bearing metasiltstone and chert as well as sparse quartz pebbles. Matrix consists of fine-grained metasandstone, metasiltstone, or dark-gray or greenish-gray slate. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Devonian intrusive rocks (Devonian) at surface, covers 0.3 % of this area

Binary and biotite granite and granodiorite, undifferentiated. Includes small dikes labeled Dg. Part of the New Hampshire Plutonic Suite.

Ammonoosuc Volcanics of Billings (1935) - metarhyolite tuff, lapilli tuff, tuff breccia, and lava (Upper and Middle Ordovician) at surface, covers 0.2 % of this area

Greenish-gray, light-bluish-gray, or medium-bluish-gray metarhyolite tuff, lapilli tuff, tuff breccia, and lava. Generally porphyritic with 5 to 20 percent plagioclase and, in some places, quartz phenocrysts. Generally strongly foliated with waxy sheen on foliation surfaces. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Comerford Intrusive Complex - metatholeiitic mafic dikes (Late Silurian) at surface, covers 0.2 % of this area

Abundant, foliated to weakly foliated, metatholeiitic mafic dikes; some sheeted. Shown as overprint. Part of the Lake Memphremagog Intrusive Suite (425±3 Ma to 418.5±2 Ma).

Partridge Formation (Upper Ordovician) at surface, covers 0.2 % of this area

Dark-gray to grayish-black, rusty-weathering sulfidic slate and phyllite interlayered with felsic volcanic rocks and tuffs, and amphibolite (Opa). Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Albee Formation - magnetite-rich areas (Ordovician and Cambrian) at surface, covers 0.1 % of this area

Magnetite-rich areas of the Albee Formation (OCal) - Shown as an overprint. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Gile Mountain Formation - thick-bedded micaceous feldspathic quartzite member (Lower Devonian) at surface, covers 0.1 % of this area

Brown to gray, noncarbonaceous quartz-mica schist and feldspathic quartzite in beds 50 cm to 5 m thick; gradational to Dgqs through interbedding of phyllite beds and decrease in thickness of quartzite beds. Part of the Connecticut Valley Trough.

Gile Mountain Formation - amphibolite member (Lower Devonian) at surface, covers < 0.1 % of this area

Hornblende amphibolite and hornblende-plagioclase-quartz granofels; interpreted as metabasaltic and volcaniclastic rocks. Part of the Connecticut Valley Trough.

Monadnock Mountain pluton - granite (Middle Jurassic) at surface, covers < 0.1 % of this area

Hornblende-biotite-microperthite granite, gradational into quartz syenite. Monadnock Mountain pluton: Composite stock of quartz syenite, syenite granite, and essexite. K-Ar age of 175±4 Ma (Foland and Faul, 1977). Part of the White Mountain Igneous Suite.

Ammonoosuc Volcanics of Billings (1935) - metamorphosed andesitic and basaltic tuff, crystal tuff, and tuff breccia (Upper and Middle Ordovician) at surface, covers < 0.1 % of this area

Dark-greenish-gray to medium-bluish-gray metamorphosed andesitic and basaltic tuff, crystal tuff, and tuff breccia; minor pillow lava. Commonly contains plagioclase and (or) altered mafic phenocrysts. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Joslin Turn Tonalite (Early Silurian to Middle Ordovician) at surface, covers < 0.1 % of this area

Greenish-gray to light-brownish-gray, medium-grained, weakly foliated metamorphosed tonalite. Primary minerals include quartz, plagioclase, biotite, magnetite, pyrite, and apatite; secondary minerals include chlorite, epidote, sericite, and calcite. Granophyric intergrowths of quartz and plagioclase. U-Pb zircon age of 469±1.5 Ma, no. 27 (Moench and Aleinikoff, 2003). Part of the Highlandcroft Plutonic Suite: Epizonal to mesozonal, foliated and metamorphosed (greenschist facies) plutons exposed northwest of the Ammonoosuc fault. Compositions range from granite to diorite to lesser amounts of gabbro. Part of the Bronson Hill arch intrusive rocks.

Ironbound Mountain Formation - amphibolite member (Lower Devonian) at surface, covers < 0.1 % of this area

Hornblende amphibolite and hornblende-plagioclase-quartz granofels; interpreted as metabasalt and mafic volcaniclastic rock. Part of the Connecticut Valley Trough.

Willoughby pluton - main zone (Devonian) at surface, covers < 0.1 % of this area

Light-gray to pink, medium- to very coarse grained, massive, miarolitic garnet-muscovite peraluminous leucogranite. Pegmatitic main zone. Rb/Sr whole-rock age of 376±9 Ma (Ayuso and Arth, 1992). Igneous rocks of the Northeast Kingdom batholith of Ayuso and Arth (1992). Part of the New Hampshire Plutonic Suite.

Monadnock Mountain pluton - essexite (Middle Jurassic) at surface, covers < 0.1 % of this area

Pyroxene-biotite-hornblende andesine gabbroic rock grading into quartz syenite. Monadnock Mountain pluton: Composite stock of quartz syenite, syenite granite, and essexite. K-Ar age of 175±4 Ma (Foland and Faul, 1977). Part of the White Mountain Igneous Suite.

Ironbound Mountain Formation - Halls Stream Grit member (of Myers, 1964) (Lower Devonian) at surface, covers < 0.1 % of this area

Lenticular masses of coarse-grained quartzose volcaniclastic grit and cobble metaconglomerate commonly with abundant dark-gray metapelitic matrix (diamictite) interlayered with metasandstone, metapelite, and porphyritic metarhyolite. Grit contains subangular clasts of plagioclase and potassic feldspar as large as 2.5 cm across and larger clasts of dark-gray slate. Conglomerate contains rounded clasts of metarhyolite, fine-grained granitoid, and rare marble, and angular clasts of dark-gray slate. Part of the Connecticut Valley Trough.

Compton Formation - amphibolite member (Lower Devonian) at surface, covers < 0.1 % of this area

Garnetiferous hornblende schist and minor hornblende amphibolite. Part of the Connecticut Valley Trough.

Ammonoosuc Volcanics of Billings (1935) - metasiltstone and phyllite (Upper and Middle Ordovician) at surface, covers < 0.1 % of this area

Medium-light-bluish-gray, medium-bluish-gray, medium-dark-gray, to medium-dark-greenish-gray metasiltstone and phyllite, and medium-gray feldspathic metawacke. Purple tinge common; coticule and magnetite locally abundant. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Frontenac Formation, Graded-bedded metagraywacke and subordinate gray phyllite (Silurian?) at surface, covers < 0.1 % of this area

Frontenac Formation, Graded-bedded metagraywacke and subordinate gray phyllite.

Albee Formation - dark-gray slate and phyllite member (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Commonly sulfidic and rusty weathering. Indistinguishable from the Scarritt Member, but crops out in small areas. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Gile Mountain Formation - gri (Lower Devonian) at surface, covers < 0.1 % of this area

Lenticular masses of metamorphosed quartzose volcaniclastic grit and conglomerate, commonly having abundant dark-gray pelitic matrix interlayered with sandstone, pelite, and porphyritic rhyolite (Dgmr). Conglomerate contains rounded clasts of rhyolite, fine-grained granitoid, and angular clasts of dark-gray slate. Correlative with Halls Stream Grit Member of the Ironbound Mountain Formation (of Myers, 1964) to the north. Part of the Connecticut Valley Trough.

Granite and Tonalite (Devonian) at surface, covers < 0.1 % of this area

Porphyritic, light-gray, hypabyssal biotite granite and tonalite in sill-like bodies - Contains phenocrysts of euhedral and embayed quartz, euhedral micro-cline with Carlsbad twinning (in granite), plagioclase, and euhedral biotite (±muscovite). Part of other plutons of the New Hampshire Plutonic Suite.

Waits River Formation - mafic member (Lower Devonian and Upper Silurian) at surface, covers < 0.1 % of this area

Massive, coarse-grained hornblende-plagioclase gneiss and granofels; finely foliated hornblende-plagioclase amphibolite; actinolite-epidote-chlorite greenstone. Part of the Connecticut Valley Trough.

Granite and trondhjemite dikes of Chester and Athens domes and west of the Connecticut Valley trough (Devonian) at surface, covers < 0.1 % of this area

Light-gray to white, garnet-muscovite-biotite granodiorite at Gassetts quarry, having a U-Pb zircon SHRIMP age of 392±6 Ma, no. 46 (Aleinikoff and others, 2011), from rims on Proterozoic-age cores (J.N. Aleinikoff, USGS, written commun., 2002); and whitish-gray muscovite-rich quartz monzonite and granodiorite, granite pegmatite, and aplite which occur as crosscutting nonfoliated dikes within the core of the domes and as folded and well-foliated dikes on the east and west flanks of the domes; also granite dikes and small stocks of very light gray to white, muscovite-rich, locally garnet-bearing, fine- to medium-grained biotite-muscovite quartz monzonite and granodiorite, locally orbicular, that intrude cover rocks east of the Green Mountain massif from Jamaica to Northfield, including the Liberty Hill locality and the granodiorite stock east of Plymouth. The latter contains inherited zircon with rims having an imprecise SHRIMP age of about 380 to 390 Ma (Aleinikoff and others, 2011); a similar dike south of Plymouth has a U-Pb zircon SHRIMP age of 365±5 Ma, no. 49 (Aleinikoff and others, 2011). These closely resemble undated white granodiorites of the Bethel area and dikes in the Chester and Athens domes; also resemble small dikes of granite, trondhjemite, and quartz monzonite largely west of the Connecti-cut Valley trough. Devonian granitic rocks of southern Vermont. Part of the New Hampshire Plutonic Suite.

Frontenac Formation, undivided (Silurian?) at surface, covers < 0.1 % of this area

Interbedded thick feldspathic wackes, tan and green slates, and minor calcareous lenses.

Ironbound Mountain Formation, undivided (Lower Devonian) at surface, covers < 0.1 % of this area

Interbedded gray phyllite, in places containing feldspathic clasts, and feldspathic metasandstone, variably graded.

Highlandcroft Granodiorite of Billings (1935, 1937) (Early Silurian to Middle Ordovician) at surface, covers < 0.1 % of this area

Medium-greenish-gray to dark-greenish-gray, medium-grained, foliated metamorphosed granite, granodiorite, and tonalite containing quartz, microcline, saussuritized plagioclase, hornblende, biotite (chlorite alteration), and secondary calcite and sericite. Nonconformably overlain by the Clough Quartzite and Fitch Formation. U-Pb zircon age of 450±5 Ma, no. 28 (Lyons and others, 1986). Part of the Highlandcroft Plutonic Suite: Epizonal to mesozonal, foliated and metamorphosed (greenschist facies) plutons exposed northwest of the Ammonoosuc fault. Compositions range from granite to diorite to lesser amounts of gabbro. Part of the Bronson Hill arch intrusive rocks.

Ironbound Mountain Formation, Grits at Halls Stream in northern New Hampshire (Lower Devonian) at surface, covers < 0.1 % of this area

Thickly bedded feldspathic volcaniclastic grit and interbedded gray slate. Equivalent to Grenier Ponds Member of the Ironbound Mountain Formation in western Maine.

Frontenac Formation - amphibolite member (Devonian and Silurian) at surface, covers < 0.1 % of this area

Garnetiferous hornblende schist and minor hornblende amphibolite. Part of the Connecticut Valley Trough.

Gile Mountain Formation - felsic metavolcanic member (Lower Devonian) at surface, covers < 0.1 % of this area

Very light gray, fine-grained porphyritic metafelsite schist or granofels near Maidstone Lake. Groundmass recrystallized to an aggregate of quartz, microcline, plagioclase, biotite, muscovite, and apatite; grain size about 0.05 mm. Relict phenocrysts of embayed quartz, microcline (some in granophyric intergrowths with quartz), and saussuritized plagioclase. U-Pb zircon age of 407.0±3.3 Ma, no. 45 (Rankin and Tucker, 2009). Part of the Connecticut Valley Trough.

Albee Formation - iron-formation member (Ordovician and Cambrian) at surface, covers < 0.1 % of this area

Ironstone, magnetite-rich rock and coticule. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.

Ammonoosuc Volcanics of Billings (1935) - undivided (Upper and Middle Ordovician) at surface, covers < 0.1 % of this area

A heterogeneous unit of interlayered and interfingering metamorphosed volcanic, volcaniclastic, and sedimentary rocks. Compositions range from basalt to sodic rhyolite. Fragmental rocks dominate (tuff to tuff breccia), but include sparse mafic pillow lava and felsic lava. Sedimentary protoliths include dark-gray sulfidic shale, ironstone, siltstone, graywacke, volcanic conglomerate, and rare limestone. Stratified rocks of the Bronson Hill arch and Sawyer Mountain belt.