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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): 41.72333,-72.26917
Köppen climate type:Dfb : Warm-summer humid continental climate

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 geological map and associated 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 does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

541 - 1000 Ma

ID: 2770775
Tatnic Hill Formation

Age: Neoproterozoic (541 - 1000 Ma)

Stratigraphic Name: Tatnic Hill Formation

Description: 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.

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.

Lithology: Major:{gneiss,schist}, Incidental:{amphibolite, calc silicate rock}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License

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