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Thomaston Dam pegmatite, Thomaston Dam, Thomaston, Litchfield Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 41' 35'' North , 73° 3' 38'' West
Latitude & Longitude (decimal): 41.69333,-73.06083
GeoHash:G#: drkj1qn9r
Locality type:Pegmatite
Köppen climate type:Dfb : Warm-summer humid continental climate


A multi-zoned pegmatite with common mineralogy is well exposed in a rock cut where the road across the top of the dam crosses the spillway at the east end of the dam. Most of the pegmatite consists of a white, albite-rich zone with obvious, feathery, slightly radiating, muscovite-annite aggregates around 0.25 meters long that point toward the center of the pegmatite. The other major zone is a core consisting of white massive quartz with tan, very coarse-grained microcline and accessory muscovite. No other accessories are visible. Note that collecting is not allowed at the dam site per US Army Corps of Engineers site rules, and their office is right around the corner from this pegmatite. Plus the walls of the cut are a bit physically unstable.

The specific age of this pegmatite is unknown but it does intrude the Ordovician Ratlum Mountain Schist. It may be related to the Devonian Nonewaug Granite, which is widespread just to the north and west of the dam site.


Mineral List


5 valid minerals.

Rock Types Recorded

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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 Macrostrat.org

Devonian - Silurian
358.9 - 443.8 Ma



ID: 2989044
The Straits Schist (Goshen Formation of Massachusetts)

Age: Paleozoic (358.9 - 443.8 Ma)

Stratigraphic Name: Goshen Formation; The Straits Schist

Description: Silvery to gray, non-rusty, coarse- to very coarse grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, garnet, and commonly with staurolite and kyanite or sillimanite; graphitic almost throughout. Distinctive sequence of metamorphic strata overlying the Collinsville Formation in the Bristol quad., is here correlated with the Straits Schist of Fritts (1963) in the Southington quad. Consists mainly of rusty-weathering, medium- to coarse-grained, commonly crenulated, garnet-bearing, graphitic, muscovite-rich, plagioclase-quartz schist with graded bedding. Unit is called The Straits Schist following the original usage of Rodgers and others (1959). In the southern part of the Bristol quad., unit includes the Southington Mountain Member (here reduced in rank) in its upper part. Age of The Straits is inferred to be Silurian or Devonian. Unit is tentatively correlated with the Goshen Formation based on lithologic similarities and a possible unconformity at the base (Simpson, 1990).

Comments: Part of Central Lowlands; Iapetus (Oceanic) Terrane - Connecticut Valley Synclinorium; Hartland Belt Secondary unit description per CT008.

Lithology: Major:{schist}

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]

Ordovician - Neoproterozoic
443.8 - 1000 Ma



ID: 3190671
Precambrian-Phanerozoic sedimentary rocks

Age: Neoproterozoic to Ordovician (443.8 - 1000 Ma)

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

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



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