Geologic units in Carroll county, Illinois

Galena Group (Trenton Limestone) (Upper Ordovician (Stage 5 to Stage 6)) at surface, covers 48 % of this area

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

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 30 % of this area

Silurian System undivided, includes Sexton Creek Limestone, St. Clair Limestone, and Moccasin Springs Formation in southern Illinois; includes Wilhelmi Formation, Elwood Dolomite, Kankakee Dolomite, Joliet Dolomite, Sugar Run Dolomite, and Racine Dolomite in northeastern Illinois; includes Mosalem, Tete des Morts, Blanding, Sweeney, Marcus, and Racine Dolomites in northwestern Illinois.

Maquoketa Formation or Group (Upper Ordovician (Stage 6 to Hirnatian)) at surface, covers 19 % of this area

Maquoketa Formation or Group, includes Cape Limestone, Cape La Croix Shale, Thebes Sandstone, Orchard Creek Shale, Girardeau Limestone, and Leemon Formation in southern Illinois; includes Scales Shale, Fort Atkinson Limestone, Brainard Shale, and Neda Formation in northern Illinois; includes Noix Oolite in western Illinois.

Platteville Group (Upper Ordovician (Stage 5)) at surface, covers 3 % of this area

Platteville Group, includes Pecatonica, Mifflin, Grand Detour, Nachusa, and Quimbys Mill Formations; includes Plattin Limestone in southwestern Illinois.

Ancell Group (Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)) at surface, covers 0.2 % of this area

Ancell Group, includes St. Peter Sandstone, Dutchtown Limestone, Joachim Dolomite, and Glenwood Formation; includes Prairie du Chien Group in Jo Daviess County of northwestern Illinois.

Galena Group and Platteville Formation (Upper Ordovician, Turinian-basal Richmondian (upper Sandbian-upper Katian)) at surface, covers 0.1 % of this area

Galena Group includes Decorah, Dunleith, Wise Lake, Dubuque formations; Platteville Formation includes Pecatonica, McGregor members; Glenwood Shale forms thin basal shale unit. Primary lithologies: dolomite, part porous to vuggy, fossiliferous, part cherty; limestone and dolomitic limestone, fossiliferous, part cherty (note: Galena Group entirely limestone to north, entirely dolomite to south, interstratified limestone-dolomite in intervening areas). Secondary lithologies: shale, green-gray, calcareous, with thin to nodular limestone, part fossiliferous (Decorah Sh); limestone, wavy-bedded to nodular, part fossiliferous (Decorah-Platteville); dolomite and limestone, slightly argillaceous. Minor: shale partings and thin beds, green-gray to brown-gray, part fossiliferous (Dubuque Fm); shale partings and thin beds, light to dark brown, part organic-rich (Decorah-Platteville); dolomite, sandy to very sandy (Pecatonica Mbr); shale, green-gray, noncalcareous, unfossiliferous, part sandy (Glenwood Sh); thin sandstone to siltstone (Glenwood Sh); dark phosphatic grains (primarily Glenwood, Pecatonica, Decorah); phosphate-pyrite-iron oxide crusts (on some hardground surfaces); pyrite nodules; thin K-bentonite beds (Galena Gp); ironstone ooids (Decorah Sh). Maximum thicknesses total interval: 330 ft (100 m); Galena Group, 250-280 ft (76-85 m); Platteville Formation, 25-55 ft (8-17 m); Glenwood Shale, 3-10 ft (1-3 m).

Maquoketa formation (Upper Ordovician; Richmondian (upper Katian)) at surface, covers < 0.1 % of this area

Includes Elgin, Clermont, Fort Atkinson, Brainard, and Neda members. Overlain by Silurian strata in east-central Iowa, incised paleovalleys beneath Mosalem Fm.; truncated beneath Devonian strata in northern-most counties. Primary lithologies: shale, green-gray, variably dolomitic, mostly unfossiliferous; dolomite, argillaceous, part shaly, variably fossiliferous (part with common trilobites); dolomite, part argillaceous, part cherty to very cherty (northern Iowa). Secondary lithologies: shale, brown to brown-gray, organic, part graptolitic, part finely laminated (lower strata); dolomitic limestone to limestone, argillaceous, part cherty to very cherty, variably fossiliferous, part crinoidal (northern Iowa); interbedded dolomite and shale, part nodular, part fossiliferous. Minor: phosphorite, granular to massive (basal unit); dolomite, phosphatic to very phosphatic, argillaceous (Elgin Mbr.); ooidal ironstone and red shale (Neda Mbr.); pyrite, finely disseminated to nodular, pyrite cements. Maximum thicknesses: generally 200-275 ft (60-85 m) beneath Silurian, locally 100-200 ft (30-60 m) beneath Mosalem Fm.; 75-180 ft (23-55 m) where capped by Devonian strata in northern Iowa.