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Geologic units in Big Stone county, Minnesota

Cretaceous rocks, undivided (Cretaceous) at surface, covers 74 % of this area
Cretaceous rocks, undivided - Dakota, Graneros, Greenhorn, Carlie, Niobara, and Pierre formations and their nonmarine equivalents in northwestern, southwestern, and southeastern Minnesota
Lithology: shale; sandstone; limestone
Felsic to intermediate volcanic and volcaniclastic rocks, mica schist, phyllite, and granitoid rocks (Late Archean) at surface, covers < 0.1 % of this area
Felsic to intermediate volcanic and volcaniclastic rocks, mica schist, phyllite, and granitoid rocks - Variably and cataclastically deformed. Unit forms aeromagnetic "quiet zone" and probably contains some rocks of Paleoproterozoic age.
Lithology: felsic volcanic rock; intermediate volcanic rock; schist; phyllite; granitoid
Migmatitic gneiss, amphibolite, and granite (Middle to Early Archean) at surface, covers 7 % of this area
Migmatitic gneiss, amphibolite, and granite - Montevideo and Morton Gneisses (3600-3000 m.y.) in the Minnesota River Valley, southwestern Minnesota; McGrath Gneiss (2750 m.y.) east of Mille Lacs Lake; components of Hillman Migmatite southwest of Mille Lacs Lake; and Sartell Gneiss in Stearns County. Inferred to include various younger rocks, including granitoid intrusions in the Hillman Migmatite and pillowed basalt in poorly exposed areas of southwestern Minnesota.
Lithology: gneiss; amphibolite; granite; basalt
Syntectonic to pretectonic granitoid rocks (Late Archean) at surface, covers 19 % of this area
Syntectonic to pretectonic granitoid rocks - Granite and granodiorite of the Vermilion Granitic Complex, the Giants Range and Bemidji batholiths, as well as smaller intrusions of tonalite and monzonite of the Algoman orogen in northern Minnesota. Also includes the Odessa, Sacred Heart, and Fort Ridgely Granites exposed along the Minnesota River Valley in southwestern Minnesota.
Lithology: granite; granodiorite; tonalite; monzonite

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