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.
State Connecticut
Name Yantic Member [of Tatnic Hill Formation]
Geologic age Upper? and Middle Ordovician
Lithologic constituents
Major
Metamorphic > SchistMedium- 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.
Comments Part of Eastern Uplands; Iapetus (Oceanic) Terrane - Merrimack Synclinorium; Brimfield Schist and equivalent formations (includes Tatnic Hill Formation) (Upper? and Middle Ordovician). The age of the Putnam Group, including its Tatnic Hill Formation, is considered to be Late Proterozoic(?) based on the 730 my age of the Fishbrook Gneiss in MA and the 440 my age of the Hedgehog Hill gneiss intruded into the upper part of the younger Brimfield Group (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 New London - Tolland - Windham