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Mineral Resources > Online Spatial Data > Geology > by state > Minnesota Geology

Geologic units in Rock county, Minnesota

Clayely Till, Ground Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Pre-Illinoian]) at surface, covers < 0.1 % of this area

Heterogeneous, clay to boulder-size clasts of glacial orgin. Exhibits a distinctive weathered, dissected surface. Contains prominent oxidized joints and fractures with gypsum or calcite. Typically overlain by up to 45 ft (14 m) of loess. Thickness of uppermost Pre-Illinoian till may be up to 120 ft (37 m). Composite thickness of all Pre-Illinoian till may be up to 1,000 ft (305 m).

Lithology: clay or mud; silt; sand; gravel

Migmatitic gneiss, amphibolite, and granite (Middle to Early Archean) at surface, covers 6 % 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

Till, Ground Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Illinoian(?)]) at surface, covers < 0.1 % of this area

Heterogeneous, clay with silt to boulder-size clasts. Exhibits a distinctive weathered, dissected surface. Contains prominent oxidized joints and fractures with gypsum or calcite. Typically overlain by up to 25 ft (8 m) of loess. Thickness up to 120 ft (37 m).

Lithology: clay or mud; silt; sand; gravel

Sioux Quartzite (Early Proterozoic) at surface, covers 48 % of this area

Sioux Quartzite - Red quartzite of fluvial to possibly marginally marine origin. Includes quartz-pebble conglomerate, claystone (catlinite, also called pipestone), a basal (rhyolite) pebble conglomerate in Pipestone County, and a basal (granite, quartz, chert, iron-formation) conglomerate in Nicollet County on the Minnesota River

Lithology: quartzite; conglomerate; claystone

Cretaceous rocks, undivided (Cretaceous) at surface, covers 46 % 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

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers < 0.1 % of this area

Clay to boulder-size clasts with locally abundant organic material. Thickness up to 75 ft (23m).

Lithology: clay or mud; silt; sand; gravel