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BTChronostratigraphic Units
 .Phanerozoic
 ..Cenozoic
 ...Paleogene
 ....Oligocene
 .....Brule Formation
RTWyoming
 Colorado
 Nebraska
 South Dakota
 White River Group
 lower Oligocene
 middle Oligocene
SNIn White River Group. Lower and middle Oligocene. Previously considered lower Oligocene. In NE Colorado, W Nebraska, W South Dakota, and E Wyoming. Indexed with lower Oligocene and/or middle Oligocene as applicable.