The Great Inland Seaway

By editor

Columbus, Stillwater County, Montana

The rimrocks that rise abruptly along the Yellowstone River valley between Columbus and Billings form a rugged and ancient floor, one that once lay beneath a vast, shallow sea. For more than sixty million years during the Cretaceous Period, this broad expanse of eastern Montana rested underwater, submerged beneath what geologists call the Western Interior Seaway. This immense body of water stretched from the Gulf of Mexico northward to the Arctic Ocean, dividing the North American continent into two distinct landmasses. It is difficult to imagine now, as the prairie grass sways and the river flows quietly below, but this landscape was once a warm, shallow ocean teeming with life.

Before the great uplift of the Rocky Mountains, which would begin roughly 70 million years ago, the continent’s elevation was considerably lower, allowing the sea to invade deep inland. The Western Interior Seaway was seldom more than a few hundred feet deep. Its waters were rich and warm, providing habitat for an astonishing diversity of marine creatures. Thick banks of oysters (genus Exogyra) clung to the shallow shorelines, filtering the waters as they grew in dense colonies. Among the more fascinating denizens were the ammonites, relatives of the modern nautilus, with their coiled shells and tentacles. These creatures fed upon clams and other shelled organisms in the shallows, their spiral shells now found in abundance throughout Montana’s sedimentary layers.

Sharks prowled these waters, their teeth often found in the fossil record, revealing a variety of predatory species. At the summit of this marine food chain were the plesiosaurs and mosasaurs, colossal reptiles adapted to ocean life. The plesiosaurs, with their long, serpentine necks and paddle-like limbs, could grow to lengths of forty feet, their bodies agile and swift beneath the waves. Mosasaurs, often described as giant marine lizards, bore sleek, snakelike forms and powerful jaws filled with sharp teeth. Both breathed air but spent their lives hunting in the water, preying on fish, ammonites, and smaller marine reptiles. Fossils of these creatures have been uncovered in numerous sites throughout eastern Montana, including the famed Hell Creek Formation.

The story of the Great Inland Seaway is not one of sudden inundation but of gradual change over millions of years. As the Cretaceous Period progressed, the sea rose and fell repeatedly, leaving layers of sandstone, shale, and limestone. These sediments accumulated on the seafloor, preserving the remains of organisms and the subtle features of ancient shores. Eventually, about 65 million years ago, the seaway began to retreat as the Laramide orogeny commenced. This period of mountain building thrust the Rocky Mountains upward, tilting the ancient seafloor and exposing it to erosion and weathering.

The Yellowstone River, cutting its valley through these sedimentary layers, carved deeply into the soft rocks, leaving behind the rimrocks that now tower above the floodplain. These cliffs, composed primarily of sandstone and shale, bear the imprint of ancient currents and tides. Their stratified surfaces tell the story of shifting shorelines and sedimentation in a prehistoric sea that existed long before the towering peaks of the Rockies rose above them.

The rimrocks have served as landmarks for centuries. Native American tribes, including the Crow and Sioux, navigated these heights and valleys as they followed the buffalo across the plains. When European explorers Lewis and Clark passed near this region in 1805, they noted the striking rock formations that bordered the river. William Clark wrote in his journal, “The cliffs rise abruptly from the river’s edge, their layered faces telling of ages when the land was drowned beneath the sea.” This observation underscores the continuity of natural history visible to the keen eye.

The sandstone bluffs also played a practical role in more recent history. By 1912, as automobiles began to appear on Montana’s roads, local Good Roads enthusiasts and county officials recognized the importance of these natural corridors. They organized one of the first interstate road projects in the state, following the Yellowstone River valley and the rimrocks that guided travelers between the mountains and the eastern plains. The route allowed easier movement of goods and people, linking the communities of Columbus, Billings, and beyond.

The fossils embedded in these rocks offer windows into a world that seems utterly foreign to our own. The great sea’s retreat coincided with the end of the Cretaceous Period and the extinction of the dinosaurs--an event that reshaped life on Earth. The sedimentary record preserves not only marine creatures but also the footprints of terrestrial dinosaurs that roamed the coastal plains adjacent to the sea. Species such as Triceratops horridus and Tyrannosaurus rex have been uncovered in formations laid down as the sea withdrew, revealing a landscape transitioning from ocean to land.

Geologists have studied the Western Interior Seaway extensively, piecing together its dimensions and ecology. Dr. Charles D. Walcott, the renowned paleontologist who discovered the Burgess Shale, once remarked, “The Western Interior Seaway was a great artery of life, a nursery and hunting ground for creatures both strange and magnificent, whose remains now lie hidden beneath the plains and hills of Montana.” His words remind us that this sea was not a barren expanse but a dynamic ecosystem.

Today, standing upon the rimrocks, one can observe layers of sediment that formed in ancient tides, examine fossilized oyster shells, and imagine the plesiosaurs gliding through the water below. The Yellowstone River continues to shape the landscape, but the story of the Great Inland Seaway remains etched in stone, inviting us to consider the profound transformations that have sculpted this land.

See also

• The Great Inland Seaway at Columbus, Stillwater County
• The Great Inland Seaway at Park City, Stillwater County
• Paleocene Mammals and Albert Silberling at Harlowton, Wheatland County