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.
State Connecticut
Name Upper member [of Bigelow Brook Formation]
Geologic age Silurian and perhaps Ordovician
Lithologic constituents
Major
Metamorphic > SchistChiefly 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.
Minor
Metamorphic > Granoblastic > Granofelsinterlayered with quartzose granofels with less biotite but with calc-silicate minerals
Comments Part of Eastern Uplands; Iapetus (Oceanic) Terrane - Merrimack Synclinorium; Hebron Gneiss and equivalent formations (including equivalents of Paxton and Oakdale Formations of Massachusetts) (including Bigelow Brook and Southbridge Formations) (Silurian and Ordovician). The age of the Brimfield Group and its Bigelow Brook, Hamilton Reservoir, and Mount Pisgah Formations in this area is considered to be Cambrian(?) based on the intrusion of 440 m.y. Hedgehog Hill gneiss emplaced near the top of the Hamilton Reservoir Formation in the upper part of the Brimfield, although the top of the Brimfield, at least, may be Ordovician or older Paleozoic (Pease, 1989) per CT013.
References

Rodgers, John, compiler, 1985, Bedrock Geological Map of Connecticut: Connecticut Geological and Natural History Survey, Hartford, Connecticut, 2 sheets, scale 1:125,000.

Pease, M.H., Jr., 1989, Correlation of the Oakdale Formation and Paxton Group of central Massachusetts with strata in northeastern Connecticut: U.S. Geological Survey Bulletin, 1796, 26 p.

https://pubs.er.usgs.gov/publication/b1796

NGMDB product
Counties Tolland - Windham