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Hancock Brook Dam, Plymouth, Litchfield Co., Connecticut, USAi
Regional Level Types
Hancock Brook DamDam
Plymouth- not defined -
Litchfield Co.County

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Latitude & Longitude (WGS84): 41° 37' 19'' North , 73° 2' 16'' West
Latitude & Longitude (decimal): 41.62194,-73.03778
GeoHash:G#: drkhccp92
Locality type:Dam
Köppen climate type:Dfb : Warm-summer humid continental climate
Nearest Settlements:
Wolcott16,639 (2017)4.8km
Oakville9,047 (2017)5.1km
Plymouth12,284 (2017)5.7km
Thomaston1,910 (2017)6.5km
Terryville5,387 (2017)6.6km

One of several flood control dams built in northwest Connecticut by the U. S. Army Corps of Engineers after the disastrous flood of 1955. Rock (Ordovician Collinsville Formation gneiss) blasted from the southern flank of Mount Toby in the Hancock Brook valley was used in the adjacent dam.

Regions containing this locality

North America PlateTectonic Plate

Select Mineral List Type

Standard Detailed Strunz Dana Chemical Elements

Mineral List

6 valid minerals.

Detailed Mineral List:

Formula: Na(AlSi3O8)
Formula: Fe2+3Al2(SiO4)3
Formula: KFe2+3(AlSi3O10)(OH)2
Formula: (Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Formula: Al2(SiO4)O
Formula: NaCa4[Al9Si27O72] · nH2O

List of minerals arranged by Strunz 10th Edition classification

Group 9 - Silicates
Chabazite-Ca9.GD.10(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Stilbite-Ca9.GE.10NaCa4[Al9Si27O72] · nH2O

List of minerals arranged by Dana 8th Edition classification

Group 51 - NESOSILICATES Insular SiO4 Groups Only
Insular SiO4 Groups Only with cations in [6] and >[6] coordination
Group 52 - NESOSILICATES Insular SiO4 Groups and O,OH,F,H2O
Insular SiO4 Groups and O, OH, F, and H2O with cations in [4] and >[4] coordination
Group 71 - PHYLLOSILICATES Sheets of Six-Membered Rings
Sheets of 6-membered rings with 2:1 layers
Group 76 - TECTOSILICATES Al-Si Framework
Al-Si Framework with Al-Si frameworks
Group 77 - TECTOSILICATES Zeolites
Zeolite group - True zeolites
Chabazite-Ca77.1.2.1(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Stilbite-Ca77.1.4.3NaCa4[Al9Si27O72] · nH2O

List of minerals for each chemical element

H Chabazite-Ca(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
H Stilbite-CaNaCa4[Al9Si27O72] · nH2O
H AnniteKFe32+(AlSi3O10)(OH)2
O Chabazite-Ca(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
O KyaniteAl2(SiO4)O
O Stilbite-CaNaCa4[Al9Si27O72] · nH2O
O AlmandineFe32+Al2(SiO4)3
O AnniteKFe32+(AlSi3O10)(OH)2
O AlbiteNa(AlSi3O8)
Na Stilbite-CaNaCa4[Al9Si27O72] · nH2O
Na AlbiteNa(AlSi3O8)
Al Chabazite-Ca(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Al KyaniteAl2(SiO4)O
Al Stilbite-CaNaCa4[Al9Si27O72] · nH2O
Al AlmandineFe32+Al2(SiO4)3
Al AnniteKFe32+(AlSi3O10)(OH)2
Al AlbiteNa(AlSi3O8)
Si Chabazite-Ca(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Si KyaniteAl2(SiO4)O
Si Stilbite-CaNaCa4[Al9Si27O72] · nH2O
Si AlmandineFe32+Al2(SiO4)3
Si AnniteKFe32+(AlSi3O10)(OH)2
Si AlbiteNa(AlSi3O8)
K AnniteKFe32+(AlSi3O10)(OH)2
Ca Chabazite-Ca(Ca,K2,Na2)2[Al2Si4O12]2 · 12H2O
Ca Stilbite-CaNaCa4[Al9Si27O72] · nH2O
Fe AlmandineFe32+Al2(SiO4)3
Fe AnniteKFe32+(AlSi3O10)(OH)2

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

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]

Middle Ordovician
458.4 - 470 Ma

ID: 2973029
Collinsville Formation

Age: Middle Ordovician (458.4 - 470 Ma)

Stratigraphic Name: Collinsville Formation

Description: Mixture of rock types as described for the two members; in many areas felsic and mafic striped metavolcanic rocks predominate.

Comments: Part of Central Lowlands; Iapetus (Oceanic) Terrane - Connecticut Valley Synclinorium; Gneiss Dome Belt. Hawley Formation and equivalent formations (includes Collinsville Formation) (Middle Ordovician). Two members referred to in primary description are units: Ocs and Ocg. Secondary unit description per CT008. Secondary unit description from USGS Geologic Names lexicon (ref. CT017): Metamorphic strata in the report area that lie above the Bristol Gneiss (as it is here revised) and below The Straits Schist are assigned to the Collinsville Formation. Unit is divided into a lower unnamed hornblende gneiss member (was uppermost part of Stanley's (1964) Bristol Member) approximately 400 ft thick, an unnamed metaquartzite member that averages 100 ft, and the upper Sweetheart Mountain Member adopted here as defined by Stanley. Inferred age is Middle Ordovician. Correlates, at least in part, with the Hawley Formation of MA (Simpson, 1990). Also from ref. CT017: Part of Bronson Hill anticlinorium, not part of Rowe-Hawley zone, but discussed in this report to emphasize importance of junction between Rowe-Hawley zone and Bronson Hill anticlinorium. Collinsville is exposed in Granville and Shelburne Falls domes, and to a limited extent in Goshen dome (Hatch and Warren, 1982). Consists of various light-colored, plagioclase-rich gneisses and interlayered amphibolite and hornblende gneiss. Upper part is amphibolite-rich; lower part is predominantly feldspathic gneiss. In places, thin aluminous feldspathic schist, locally containing coticule and amphibolite, forms an upper unnamed member of formation in Granville dome, or is Sweetheart Mountain Member of Stanley (1964) in Collinsville and Bristol domes in CT. Most complete sequence is in Shelburne Falls dome where L.M. Hall (1977, written commun.) has recognized seven mappable subdivisions. With minor modification, they are (ascending) 1) amphibolite with thin felsic gneiss layers; 2) rusty-weathering massive granulites; 3) very homogeneous garnetiferous biotite gneiss; 4) interlayered amphibolite and felsic gneiss; 5) felsic gneiss with scattered biotite +/-magnetite, garnet, and hornblende; 6) interbedded amphibolite and white felsic gneiss; and 7) gray, tan-weathering granulites containing thin coticule layers. Members 4, 5 and 6 are approximately equivalent to Ammonoosuc Volcanics. Member 3 is lithically similar to Monson and Fourmile Gneisses. Age in report is Ordovician. [Papers presented as chapters in U.S. Geological Survey Professional Paper 1366 are intended as explanations and (or) revisions to MA State bedrock geologic map of Zen and others (1983) at scale of 1:250,000.] Original map source: Connecticut Geological and Natural History Survey, DEP, in cooperation with the U.S. Geological Survey, 2000, Bedrock Geology of Connecticut, shapefile, scale 1:50,000

Lithology: Major:{gneiss,amphibolite}, Minor:{metavolcanic}, Incidental:{granulite}

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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Force, Edward. (1990s), Unpublished sketch map.

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