Presence of rounded boulders and large cobbles at base of White River Group (Oligocene) strata in Southwest North Dakota and Northwest South Dakota

Cenozoic clastic sediments Brule Formation lower Oligocene Harding County South Dakota clastic rocks North Dakota Chadron Formation Oligocene cobbles Stark County North Dakota Slope County North Dakota stratigraphy Paleogene White River Group boulders Dunn County North Dakota South Dakota United States Tertiary sedimentary rocks sediments