Time Traveling At Hell Creek State Park

By Jack Horner, Curator of Paleontology, Museum of the Rockies, Montana State University, Bozeman

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            In the area of Hell Creek State Park, north of Jordan, a careful observer can travel in time to visit the remains of a Montana ocean, a tropical beach, and a land once trodden by Tyrannosaurus rex .  

To the untrained eye it doesn’t look like much.   Gray and tan colored hillsides rising above the shores of Fort Peck Reservoir, covered mostly by sparse grass and sage, and occasionally by a windblown scrub cedar or pine.   When these hillsides are wet the mud is called gumbo. Everything from a human’s feet to a pickup truck will get stuck in it.    But combine a little imagination with a geological perspective and you see a wondrous sequence of forgotten times, and long-gone lives.   This is Montana in the rear-view mirror, some 65 million years ago.

            In the area around Hell Creek State Park the soils are mostly dark colored, and the shale is gray or black.   This material was deposited when an inland ocean divided North America into two sub-continents, some 70 million years ago.   Stretching from the Arctic Ocean to the Gulf of Mexico this tropical marine waterway was home to gigantic marine lizards called mososaurs and pliosaurs, giant sea-going turtles, fishes, and a host of giant clams, squid relatives called ammonites, and crawdad sized lobsters.   Washed out of the shale rock along the shores of Fort Peck today, these fossils of long extinct creatures testify to Montana’s last great ocean.

            Sitting atop the dark shale and forming cliffs on the hills above the park, is a yellowish colored sandstone layer formed by the beach of the inland ocean.   Burrows dug by little shrimp, and the low angling cross-bedded layers in these sandstones reveal that the rocks are beach deposits.   Chunks of petrified wood and occasional water-worn fossil bones are remnants of debris washed up on the ancient coast.

Above the sandstone beach are rocks that were deposited by large meandering rivers flowing into the ocean.   Gray and tan sandstones with high angled cross-bedded layers, and adjacent gray mudstones represent the river channels, and their flood deposits.

These sediments helped form the badland landscapes of eastern Montana.    Fossils in these sediments are the remains of plants and animals that inhabited the coastal plain between the ancestral Rockies to the west, and the inland sea to the east.  

Some plants and aquatic animals of this time were similar to subtropical plants and animals in today’s southern Louisiana and Florida.   But, others were so bizarre and different that they baffle our imaginations!   Tyrannosaurus , Triceratops , the duck-bill Edmontosaurus , the dome-headed dinosaur Pachycephalosaurus , and others including Thescelosaurus , Ornithomimus , Ankylosaurus , Torosaurus , Stigymoloch , Buganosaura , and Dromaeosaurus are some of the dinosaurs.   Primitive mammals, odd birds, aquatic lizards called champsosaurs, and a host of other creatures filled out the ecosystem.  

            The area now occupied by Hell Creek State Park was at the bottom of the inland ocean, and the western beach of the seaway was likely westward in the area of Lewistown.   The meandering rivers, lined with palm and sequoia, hosting plant-eating dinosaurs and dinosaur-eating dinosaurs existed to the west of the beach.  

As sea-level dropped 68-65 million years ago, the river systems moved east following the coast.  

What we see at Hell Creek today is the result, a stack of fossil-filled sediment affirming a receding landscape where humans couldn’t possibly have survived.   The area around Hell Creek State Park reveals an awesome time in the history of the world.   

Visit Hell Creek State Park by traveling 25 miles north of Jordan on the County Road. Inquire locally about current road conditions.