Ogallala Group (SDTo;0)

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
Original map label	To
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 > Carbonate > Limestone (Bed) Sedimentary > Clastic > Mudstone > Claystone (Bed) Sedimentary > Clastic > Siltstone (Bed) Sedimentary > Clastic > Sandstone (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
Geographic coverage	Bennett - Bon Homme - Brule - Charles Mix - Corson - Gregory - Jackson - Lyman - Mellette - Shannon - Todd - Tripp - Yankton