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US Route 6 Willimantic Bypass expressway road cuts, Coventry, Tolland Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 43' 24'' North , 72° 16' 9'' West
Latitude & Longitude (decimal): 41.7233333333, -72.2691666667

Completed in the early 1980s, the U.S. Route 6 Willimantic Bypass expressway is casually referred to as being in Willimantic, the nearby city. Actually it is not in the city's incorporated boundary and instead is situated in four towns - from west to east: Columbia, Coventry, Windham (which includes the city of Willimantic) and Mansfield. At the time of construction, it was intended to be part of Interstate I-84, but those plans were eventually abandoned. Because of this, old or incorrect labels may refer to the locality as I-84 or Route 84. Before the bypass, U.S. Route 6 used to pass through downtown Willimantic on a stretch of road that is now part of state Route 66.

The expressway begins in the town of Columbia, at a 4-way at-grade intersection with State Route 66. From here the expressway heads northeast and immediately crosses into the town of Coventry where the eastbound and westbound sides of the expressway split, with a hill in between them. The Coventry section contains by far the major road cuts on the expressway and produced specimens found in old collections. The road cuts expose gneiss of the Ordovician Tatnic Hill Formation (part of the Putnam-Nashoba Terrane), which here and in other places around the state can host sillimanite crystals in quartz-rich segregations. Fractures in the gneiss host drusy epidote, quartz, prehnite and zeolites. The locality coordinates are for this portion of the expressway.

The expressway then crosses the Willimantic River and enters the town of Windham. At the interchange with State Route 32 there are additional road cuts exposing gneiss of the Waterford Group (part of the Avalonian Terrane) and a small pegmatite dike, mostly on the ramps rather than on the highway, which include part of a small quarry. But these cuts are minor compared to those in Coventry.

After the Rt 32 interchange, the expressway enters the town of Mansfield. There are no bedrock road cuts in this section. After U. S. Route 6 Willimantic Bypass crosses the Natchaug River it once again enters Windham. 0.5 miles (0.80 km) after entering Windham, the U.S. Route 6 Willimantic Bypass ends at an interchange with State Route 66 and U. S. Route 6. There are no bedrock road cuts in this final section.

Mineral List

6 valid minerals.

Regional Geology

This information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This data will improve over time as more accurate maps and data sets are added.

Silurian-Devonian358.9 - 443.8 MaSilurian-Devonian sedimentary rocks

Sedimentary rocks

Ordovician443.8 - 485.4 MaOrdovician sedimentary


Neoproterozoic541 - 1000 MaTatnic Hill Formation

Major:: {gneiss},Minor:: {schist},Incidental:: {mica schist, amphibolite, calc-silicate rock}

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.

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 for regional geology:

Data provided by

Garrity, C.P., and Soller, D.R.,. Database of the Geologic Map of North America: adapted from the map by J.C. Reed, Jr. and others (2005). U.S. Geological Survey Data Series 424 .

USGS compilers. State geologic map data. State Maps.

Geological Survey of Canada. Generalized geological map of the world and linked databases. doi:10.4095/195142. Open File 2915d.

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