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Ogallala Group

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).
State South Dakota
Name Ogallala Group
Geologic age Phanerozoic | Cenozoic | Tertiary Quaternary | Miocene Pliocene(?) Pleistocene-Early
Comments Also includes the Thin Elk, Bon Homme, Herrick, Medicine Root, and "western derived" gravels.
Primary rock type sandstone
Secondary rock type siltstone
Other rock types claystone; limestone; gravel; clay or mud; conglomerate
Lithologic constituents
Major
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Siltstone (Bed)
Sedimentary > Clastic > Mudstone > Claystone (Bed)
Sedimentary > Carbonate > Limestone (Bed)
Minor
Sedimentary > Clastic > Conglomerate (Bed)
Map references
Martin, J. E., Sawyer, J. F., Fahrenbach, M. D., Tomhave, D. W., and Schulz, L. D., 2004, Geologic Map of South Dakota: South Dakota Geological Survey
Unit references
Martin, J. E., Sawyer, J. F., Fahrenbach, M. D., Tomhave, D. W., and Schulz, L. D., 2004, Geologic Map of South Dakota: South Dakota Geological Survey
Counties Bennett - Bon Homme - Brule - Charles Mix - Corson - Gregory - Jackson - Lyman - Mellette - Shannon - Todd - Tripp - Yankton