Colorado’s Ancient Ocean

By Katie Sheahan | April 8, 2026

In the year 2026, there are no living oceans in Colorado. This is unfortunate for ocean lovers, like me, that can’t live away from the mountains. I’ve tried it before, and I always come back to Colorado. While we’ll need to vacation for beach and scuba trips, the ocean isn’t actually as far away as we might think. Colorado’s ancient seas still tell hidden stories of marine life long ago.

Zoomed-in details of an ammonite fossil

The Sea Where Mountains Stand 

The land that is now Colorado has been covered by oceans many times in Earth’s history, most recently by the Western Interior Seaway between 100 million and 66 million years ago. This sea split North America in two, covering most of the state and depositing fossil-rich layers, now revealing ancient inhabitants of this land like ammonites, sharks, and plesiosaurs (Kauffman & Westermann 1984, Brenner & Peterson 1994). Rock formations like the Dakota Hogback, Pierre Shale, and Niobrara Formation, visible near Denver and Colorado Springs, are remnants of this period. Older shallow seas covered portions of central Colorado even earlier, around 300-530 million years ago. You can see evidence of this prehistoric coastline at the Dinosaur Ridge near Morrison and the Pueblo Reservoir State Park. 

Life in Colorado’s Lost Ocean 

Imagine standing in Denver or Colorado Springs, but instead of cities and mountains, you’re waist-deep in a warm, shallow sea. Giant marine reptiles glide beneath the surface, schools of fish flash below, and coiled ammonites drift past like living sculptures. Colorado’s ancient ocean was anything but empty, and the creatures that called it home might surprise us. Sharks once cruised the warm waters, still looking about the same as they do now in 2026 and 450 million years ago when they appeared in the Late Ordovician Period, long before dinosaurs. You might even glimpse a

, a long-necked marine reptile that cruised inland seas, or a Mosasaur, a massive, swimming reptile that evolution graced to look like a mashup of a shark, an alligator, and a dinosaur (Landman & Klofak 2012, Everhart 2017).

The fossilized skeleton of a plesiosaur in stone. Geology and Mineralogy

When the Sea Receded 

Fortunately for humans, the Western Interior Seaway disappeared towards the end of the Cretaceous period (roughly 70–66 million years ago) due to the uplift of the Rocky Mountains and regional tectonic forces. This tectonic activity raised the sea floor above sea level, causing the water to drain into the Gulf of Mexico and Arctic Ocean, while the area transformed into coastal plains and swamps, and were eventually developed into cities (Kauffman & Caldwell 1993, Blakey 2014).

Map of North America with the Western Interior Seaway during the Campanian (Upper Cretaceous). Paleogeography of North America during the late Campanian Stage of the Late Cretaceous (-75 Ma). Modified after Blakey.

Hiking Through History 

Today, you can head out into the Colorado wilderness to catch glimpses of this ancient ocean ecosystem, even millions of years after it disappeared. The next time you hike along the Front Range, remember that you’re walking across an ancient ocean floor. You can view marine fossils and dinosaur footprints, and walk the interpretive trails at Dinosaur Ridge. Visualize the passing of time in the seabed layers of Dakota Hogback’s dramatic uplifted ridges. Search for fossils at Pueblo Reservoir State Park, but be aware that removing fossils, rocks, or minerals from most state parks is prohibited. Bring your camera and field notebook and set out on an ancient adventure! 

The Ocean Beneath Our Feet 

Even here in a landlocked state, the ocean is not as distant as it seems. The rocks beneath our feet were shaped by ancient seas, and the water that flows through Colorado today will eventually make its way back to the ocean through connected watersheds. Oceans at Elevation is rooted in this idea, that no matter where we live, we are connected to the ocean’s past, present, and future. The same ocean that once covered Colorado now faces unprecedented challenges, from warming waters to pollution and biodiversity loss. Understanding that connection makes ocean conservation feel less abstract and more personal. Protecting the ocean isn’t just for coastal communities, it’s something we can all take part in, even from thousands of feet above sea level.

Via The Pueblo Chieftain. View of the colorfully layered strata of the Dakota Hogback illustrates different paleoenvironments such as freshwater marshes and beaches.

Blakey, R. C. (2014). Paleogeography and paleotectonics of the western interior seaway, Jurassic-Cretaceous of North America. Search and Discovery, 30392, 72.

Brenner, R. L., & Peterson, J. A. (1994). Jurassic sedimentary history of the northern portion of the Western Interior Seaway, USA. Rocky Mountain Section (SEPM).

Everhart, M. J. (2017). Oceans of Kansas: a natural history of the Western Interior Sea. Indiana University Press.

Kauffman, E. G., & Caldwell, W. G. E. (1993). The Western Interior Basin in space and time. In Evolution of the western interior basin (Vol. 39, pp. 1-30). Special Paper 39: Geological Association of Canada.

Kauffman, E. G., & Westermann, G. E. G. (1984). Paleobiogeography and evolutionary response dynamic in the Cretaceous Western Interior Seaway of North America. In Jurassic-Cretaceous biochronology and paleogeography of North America (Vol. 27, pp. 273-306). Geological Association of Canada Special Paper.

Landman, N. H., & Klofak, S. M. (2012). Anatomy of a concretion: life, death, and burial in the Western Interior Seaway. Palaios, 27(10), 671-692.