North Dakota NORTH DAKOTA SAVANNA Stratigraphy ROCK ROCK UNIT COLUMN About 50 million years ago, the climate became cooler and drier PERIOD EPOCH AGES MILLIONS OF YEARS AGO Holocene Oahe .01 compared to earlier in the Paleocene. The swampy environments and the plants and animals that lived during the early Paleocene began to disappear. Coleharbor A mostly treeless plain, or savanna, became established by about 40 million Pleistocene QUATERNARY 1.8 Pliocene Unnamed 5 years ago. Rivers flowing across the savanna deposited sands and gravels in Miocene 25 Arikaree the river channels and silts and muds on the floodplains. Gallery woodlands grew along the stream margins. and lakes occupied some areas. The rich Brule assortment of fossils that are found in the Chadron, Brule, and Arikaree Oligocene 38 Formations indicate the kinds of animals that inhabited the savanna. Life South Heart Chadron Chalky Buttes during the Eocene, Oligocene, and Miocene was much different than in the Camels Butte Eocene Golden Paleocene. A diverse group of mammals resided on the savanna, many of Valley 55 Bear Den which were members of families that still exist today including ancestral dogs, cats, camels, deer, squirrels, beavers, horses, rabbits, rhinoceroses, Sentinel Butte and mice. Large tortoises, some as large as the modern Galapagos turtles, TERTIARY lived near the rivers. Several species of fish, amphibians, turtles, lizards, and birds lived in and near the lakes and rivers. Bullion Paleocene Creek Slope Cannonball Ludlow 65 Hell Creek Fox Hills ACEOUS Pierre CRET 84 Niobrara Carlile Carbonate Calcareous Shale Claystone/Shale Outcrop of the Brule Formation (Oligocene), Little Badlands, near Dickinson, Siltstone Sandstone Sand & Gravel Stark County. The Brule Formation consists of mudstones, siltstones, and Mudstone Lignite Glacial Drift sandstones. Butte is 65 m high. View is to the north. ND State Fossil Collection Prehistoric Life of ND Map North Dakota Geological Survey Home Page.