North Dakota NORTH DAKOTA’S THE HELL CREEK DELTA Stratigraphy ROCK ROCK UNIT COLUMN During the Cretaceous, about 65 million years ago, a well drained PERIOD EPOCH AGES MILLIONS OF YEARS AGO Holocene Oahe .01 lowland corridor existed between the rising Rocky Mountains and the Western Interior Seaway to the east. Sediments eroded from the Rocky Coleharbor Mountains were carried to this western North Dakota lowland by rivers and Pleistocene QUATERNARY 1.8 Pliocene Unnamed streams and were deposited in a huge delta, the Hell Creek Delta. These 5 Miocene sediments, now turned into sandstone, siltstone, and mudstone, are called 25 Arikaree the Hell Creek Formation. Woodlands, ponds, and swamps that existed on Brule this subtropical, deltaic coastal plain provided habitats for many kinds of Oligocene 38 exotic plants and animals including several species of dinosaurs such as South Heart Chadron Chalky Buttes Triceratops and Tyrannosaurus rex. Freshwater fishes, salamanders, Camels Butte Eocene Golden lizards, turtles, crocodiles, birds, snails, clams and small mammals coexisted 55 Valley Bear Den with the dinosaurs. Fossils of animals, including sharks, rays, and mosasaurs (large marine lizards) that inhabited shallow marine waters adjacent to the Sentinel Butte delta are found in the Fox Hills Formation and Breien Member of the Hell TERTIARY Creek Formation. The species of dinosaurs that existed at this time were the last dinosaurs to ever live. Bullion Paleocene Creek Slope Cannonball Ludlow 65 Hell Creek Fox Hills ACEOUS Pierre CRET 84 Niobrara Carlile Carbonate Calcareous Shale Claystone/Shale Outcrop in Sioux County showing the Fox Hills Formation (Cretaceous) Siltstone Sandstone Sand & Gravel overlain by the Hell Creek Formation (Cretaceous). The marine Breien Mudstone Lignite Glacial Drift Member of the Hell Creek Formation caps the hill. Outcrop is 35 m thick. View is to the northeast. ND State Fossil Collection Prehistoric Life of ND Map North Dakota Geological Survey Home Page.