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Mount Carroll Quarry, Mount Carroll, Carroll Co., Illinois, USA

This page kindly sponsored by Ed Clopton
Latitude & Longitude (WGS84): 42° 5' 3'' North , 90° 1' 47'' West
Latitude & Longitude (decimal): 42.08444,-90.03000
Köppen climate type:Dfa : Hot-summer humid continental climate

Dolostone quarry west of Mount Carroll, Illinois, located immediately north of Illinois Route 64/U.S. Route 52, 2 1/2 miles west of the intersection with Illinois Route 78.

The quarry exploits the Wise Lake and overlying Dubuque formation of the Ordovician-age Galena Group. The Wise Lake consists of hard, dense, light grey dolostone with scattered small mineralized vugs (mostly mm-size marcasite and pyrite on minute sparkling white rhombs of dolomite; sphalerite, galena, and barite are less common). The Dubuque consists of softer, darker, yellower dolostone with more, larger mineralized vugs (cm-size marcasite, pyrite, sphalerite, yellow barite, and galena; calcite, smithsonite, and cerussite? are rare) and occasional petroluem-filled cavities. The Ordovician-age Maquoketa Shale is present as overburden.

Marcasite from Mt. Carroll appears to be relatively stable, with no deterioration noted as yet (2015) in dozens of specimens collected by E. L. Clopton in the 1980s and 90s and subjected to seasonal extremes of humidity and dryness since then. Several specimens of Mt. Carroll pyrite, however, have deteriorated.

Mineral List

8 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

252.17 - 541 Ma

ID: 3187973
Paleozoic sedimentary rocks

Age: Paleozoic (252.17 - 541 Ma)

Lithology: Sedimentary rocks

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]

Katian - Sandbian
445.2 - 458.4 Ma

ID: 2724341
Galena Group (Trenton Limestone)

Age: Late Ordovician (445.2 - 458.4 Ma)

Stratigraphic Name: Spechts Ferry Shale; Millbrig Kbentonite Bed; Guttenberg Dolomite

Description: Galena Group (Trenton Limestone), includes Decorah, Dunleith, Wise Lake and Dubuque Formations in northern Illinois.

Comments: Thickness from stratigraphic column for northern Illinois (north of 40 degrees north latitude), from oldest to youngest: Decorah Fm (0-25’), Dunleith Fm (0-135’), Wise Lake Fm (0-80’), and Dubuque Fm (0-40’). Unit is listed as the Galena - "Trenton" Group on the stratigraphic column for northern Illinois (north of 40 degrees north latitude). The Decorah Formation is comprised, from oldest to youngest, of the Spechts Ferry Shale, Millbrig K-bentonite Bed, and Guttenberg Dolomite according to the stratigraphic column on the back of the geologic map.

Lithology: Major:{dolostone,limestone}, Minor:{bentonite}

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

This page contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


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Economic Geology (1989): 84: 2139-2154.
Richards, R. P., Clopton, E. L., Jaszczak, J. A. (1995). Pyrite and marcasite intergrowths from northern Illinois. Mineralogical Record: 26(2):129-138.
Willman, H. B., Kolata, D. R. (1978). The Platteville and Galena groups in northern Illinois. Illinois State Geological Survey Circular 502. Urbana, Illinois: Illinois State Geological Survey.

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