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Mineral Resources > Online Spatial Data > Geology > by state > South Dakota Geology

Geologic units in Bon Homme county, South Dakota

Alluvium (Phanerozoic | Cenozoic | Quaternary) at surface, covers 10 % 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

Outwash, Undifferentiated (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers < 0.1 % of this area

Heterogeneous sand and gravel, with minor clay and silt. Deposits of glaciofluvial orgin including outwash plains, kame terraces, and other undifferentiated deposits. Thickness up to 30 ft (9m).

Lithology: sand; gravel; clay or mud; silt

Eolian Deposits (Phanerozoic | Cenozoic | Quaternary) at surface, covers 2 % of this area

(loess and sand dune) Silt to medium-grained sand. Deposited as sand sheets and barchan, linear, and dome-like dunes and as veneer on uplands. Thickness up to 300 ft (91m).

Lithology: silt; sand

Till, Stagnation Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers 27 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by hummocky terrain with abundant sloughs resulting from stagnation of ice sheets. Composite thickness of all Upper Wisconsin till may be up to 300 ft (91 m).

Lithology: clay or mud; silt; sand; gravel

Till, Minor Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers < 0.1 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by elevated linear ridges including minor, washboard, or recessional moraines. Composite thicknessof all Upper Wisconsin till may be up to 300 ft (91 m).

Lithology: clay or mud; silt; sand; gravel

Niobrara Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.3 % of this area

White to dark gray argillaceous chalk, marl, and shale. Weathers yellow to orange. Contains thin, laterally continuous bentonite beds, chalky carbonaceous shale, minor sand, and small concretions. Thickness 160-225 ft (49-69 m).

Lithology: limestone; limestone; shale; clay or mud; sand

Till, End Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers 19 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by elevated linear ridges with hummocky terrain locally at former ice sheet margins. Composite thicknessof all Upper Wisconsin till may be up to 300 ft (91 m).

Lithology: clay or mud; silt; sand; gravel

Pierre Shale (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.6 % of this area

Blue-gray to dark-gray, fissile to blocky shale with persistent beds of bentonite, black organic shale, or light-brown chalky shale. Contains minor sandstone, conglomerate, and abundant carbonate and ferruginous concretions. Thickness 1,000-2,700 ft (305-823 m).

Lithology: shale; clay or mud; sandstone; conglomerate

Outwash, Terrace (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers 0.2 % of this area

Heterogeneous clay to gravel of glaciofluvial orgin. Thickness up to 60 ft (18m)

Lithology: clay or mud; silt; sand; gravel

Niobrara Formation (Phanerozoic | Mesozoic | Cretaceous-Late) at surface, covers 0.7 % of this area

Argillaceous chalk, limestone and shale. Chalk is medium gray to white, interbedded with thin layers of chalky shale; contains many fossil clams, oysters, and formanifera. Limestone is light gray to medium gray and yellowish gray, interbedded with medium-gray chalky shale; also contains fossil clams, oysters, and formanifera. Bedding plans commonly marked by thin layers of gypsum and locally a thin bed of red flint occurs at top of unit. Approx. max thickness 570 ft.

Lithology: limestone; shale

Ogallala Group (Phanerozoic | Cenozoic | Tertiary Quaternary | Miocene Pliocene(?) Pleistocene-Early) at surface, covers 0.2 % of this area

Includes: Ash Hollow Formation- White, tan, and gray, well-cemented, calcareous sandstone and silty limestone often referred to as "mortar beds". Thickness 90-250 ft 27-76 m) Valentine Formation- Gray, unconsolidated, fine- to coarse grained, fluvial siltstone, channel sandstone, and gravel derived from western sources. Thickness 175-225 ft (53-69 m). Fort Randall Formation- Pink and gray claystone with interbedded sandstone. Also includes green to gray orthoquartzite, bentonitic clay, and conglomerate. Thickness up to 130 ft (40 m).

Lithology: sandstone; siltstone; claystone; limestone; gravel; clay or mud; conglomerate

Till, Ground Moraine (Phanerozoic | Cenozoic | Quaternary | Pleistocene [Upper Wisconsin]) at surface, covers 38 % of this area

Heterogeneous, clay with silt to boulder-size clasts of glacial orgin. A geomorphic feature that is characterized by smooth, rolling terrain. Composite thickness of all Upper Wisconsin till may be up to 300 ft (91 m).

Lithology: clay or mud; silt; sand; gravel