The Great Inland Seaway

By editor

Park City, Stillwater County, Montana

Imagine, if you will, the rolling grasslands and rugged badlands of eastern Montana transformed not by the slow hand of wind and fire, but by a vast expanse of warm, shallow ocean. For over sixty million years during the Cretaceous Period, much of what we now call Montana lay submerged beneath the waters of the Western Interior Seaway, an immense inland sea that cleaved North America into two landmasses. This marine realm, stretching from the present-day Gulf of Mexico to the Arctic Ocean, covered roughly 2,500 miles in length and 600 miles in width at its greatest extent. Its presence shaped the land, the life, and ultimately the geological record that we witness today.

The Rocky Mountains, rising steadily to the west during the Late Cretaceous, formed a formidable barrier that influenced the sea’s breadth and depth. As the mountains pushed skyward, erosion deposited sediments into the seaway’s gentle coastal plains, creating an environment rich in life and geological complexity. The waters were likely no deeper than a few hundred feet, warm and teeming with a variety of organisms adapted to this vast marine habitat. To stand along the Yellowstone River valley between Columbus and Billings is to gaze upon the sandstone rimrocks, those striking cliffs and plateaus sculpted from the sea’s ancient deposits. These rimrocks are not mere rock faces but pages of history -- sedimentary layers of sand, silt, and organic remains laid down beneath those ancient waters.

Among the sea’s inhabitants, oysters (genus Exogyra) formed dense colonies along the shoreline, their fossilized shells accumulating into beds that still pepper the Montana landscape. Offshore, ammonites with their coiled, chambered shells floated gracefully, their tentacles probing for prey. These mollusks, belonging to the family Scaphitidae, were voracious hunters of the seafloor, preying on giant clams (Inoceramus), bivalves that could reach lengths of over three feet. Sharks, ancestors of modern species such as Squalicorax, prowled the shallows, their serrated teeth adapted to seize any creature unfortunate enough to cross their path.

Yet the rulers of this marine realm were two formidable reptiles: the plesiosaurs and mosasaurs. Neither were true dinosaurs, for they were air-breathing reptiles that had adapted fully to aquatic life. Plesiosaurs, with their elongated necks and broad, turtle-like bodies, moved with a graceful stroke of their four paddles. Mosasaurs resembled enormous marine lizards, their snakelike bodies and powerful jaws rendering them apex predators. These creatures could reach lengths exceeding 30 feet and possessed teeth designed to crush shells and tear flesh alike.

The paleontologist Barnum Brown, who discovered the first partial Tyrannosaurus rex skeleton in Montana in 1902, once remarked, “The sea that covered this land was a world unlike our own, where creatures of monstrous size and strange design ruled the waves.” His sentiment captures the alien nature of the Cretaceous seas. Fossil evidence of plesiosaurs and mosasaurs, including vertebrae and flipper bones, have surfaced repeatedly in eastern Montana, offering glimpses into their reign.

As the Cretaceous waned, around 65 million years ago, the seaway began its retreat. Climate changes and tectonic shifts caused the waters to drain, revealing the plains that would support vast herds of dinosaurs and, later, the grasslands that sustained buffalo. The sediments left behind hardened into the sandstone formations we see today, their strata tilted and folded by the ongoing uplift of the Rockies.

For centuries, these sandstone rimrocks served as natural guideposts for human travelers moving between the mountains and the plains. Native American tribes, including the Crow and the Sioux, navigated these landmarks during seasonal migrations and hunting expeditions. The rimrocks offered vantage points and shelter amid the vastness of the Yellowstone River valley.

Fast-forward to the early 20th century, and these same rimrocks played a role in the development of modern transportation. In 1912, Good Roads advocates in Montana and neighboring states sought to improve travel routes for automobiles, which were rapidly replacing horse-drawn conveyances. They established the Yellowstone Trail, one of the first interstate highways in the country. This network of county roads, marked by distinctive chrome yellow signs with black arrows, stretched some 4,000 miles from Plymouth Rock, Massachusetts, to Seattle, Washington.

The Yellowstone Trail followed the contours of the land, often paralleling the Yellowstone River and tracing the sandstone rimrocks that had guided travelers for millennia. This route facilitated commerce, tourism, and communication across the northern United States during a time when paved roads were rare. The trail’s creation embodied a spirit of progress rooted in the very geology that shaped the region.

Eventually, the Yellowstone Trail was redesignated as U.S. Highway 10. Later, in 1971, Interstate 90 bypassed much of the historic route, representing yet another transformation in the landscape’s use. Still, the sandstone rimrocks remain, a persistent feature in Montana’s geography and history.

Standing here today, amidst the rimrocks and rolling hills, one can almost hear the whispers of ancient seas and the footsteps of those early travelers. The Great Inland Seaway may have vanished, but its legacy endures in the rocks, fossils, and routes that define this land.

See also

• The Great Inland Seaway at Park City, Stillwater County
• Paleocene Mammals and Albert Silberling at Harlowton, Wheatland County
• The Hell Creek Formation at Jordan, Garfield County