The Western Interior Seaway, a vast shallow sea that once split North America in two, drained away over roughly 20 million years as sea levels dropped and the Rocky Mountains rose. At its peak around 85 million years ago, this body of water stretched from the Gulf of Mexico to the Arctic Ocean, turning the continent into two separate landmasses. By about 70 million years ago it was shrinking fast, and it was effectively gone before the asteroid impact that ended the age of dinosaurs 66 million years ago.
What the Seaway Looked Like at Its Peak
At maximum extent during the late Cretaceous, the seaway was roughly 2,000 miles long and up to 600 miles wide, covering what is now the Great Plains, much of the Midwest, and large parts of central Canada. Water depths were shallow compared to modern oceans, generally a few hundred feet at most, which made the seaway warm, sunlit, and biologically productive.
The climate along the seaway was dramatically warmer than modern North America. Surface water temperatures along the western shore averaged around 30°C (86°F), based on chemical analysis of fossil shells. Even at higher latitudes in what is now Alberta and Saskatchewan, temperatures only dropped about 7°C across 14 degrees of latitude, a much flatter temperature gradient than exists today. There were no polar ice caps, and the entire planet was running hotter than it does now.
Why It Disappeared
Two forces worked together to drain the seaway. The first was a global drop in sea level. During the late Cretaceous, ocean levels fell significantly, with estimated drops ranging from about 20 meters to over 100 meters depending on the time interval. These weren’t sudden events but slow shifts driven by changes in ocean floor spreading rates and the cooling of tectonic plates. As mid-ocean ridges became less active, they displaced less water, and global sea levels fell.
The second force was the Laramide orogeny, the mountain-building event that created the Rocky Mountains. As tectonic plates collided along the western edge of North America, the land rose. Sediment eroded off the new mountains and poured eastward, gradually filling in the shallow sea basin. The combination of rising land, falling sea levels, and massive sediment deposits squeezed the seaway from both sides. By around 70 to 66 million years ago, the continuous waterway had broken into disconnected remnants and swampy lowlands.
Life in the Seaway
The Western Interior Seaway supported a rich and often terrifying ecosystem. Mosasaurs, a group of marine lizards, were the apex predators for the final stretch of the seaway’s existence, living from about 75 to 69 million years ago. Some species reached lengths of up to 50 feet, roughly the length of a school bus. They shared the water with large plesiosaurs like Dolichorhynchops, long-necked marine reptiles that hunted fish and squid.
The fish were enormous too. Xiphactinus audax, one of the most common large predators throughout the seaway’s history, was a bony fish that grew up to 20 feet long. Fossils of Xiphactinus have been found with entire smaller fish preserved inside their rib cages, swallowed whole. One of the very first fossils ever scientifically collected from the seaway was an upper jaw fragment of a predatory fish called Saurocephalus, found by the Lewis and Clark Expedition in 1803 in what is now northeastern Nebraska.
The seaway was also home to some of the earliest birds adapted to a fully aquatic lifestyle. Hesperornis regalis was a flightless, foot-propelled diving bird, somewhat like a modern loon but with teeth and vestigial wings. So many Hesperornis specimens were found in one layer of rock that paleontologists named it the “Hesperornis beds.” Giant flying reptiles like Pteranodon soared over the water, hunting fish hundreds of miles from the nearest shoreline.
What It Left Behind
The seaway’s seafloor didn’t vanish. It became the bedrock beneath the Great Plains. The Pierre Shale, a thick deposit of dark marine mudstone laid down on the seaway floor, crops out in a belt up to 20 miles wide along the Front Range in Colorado. It extends across South Dakota, Wyoming, Montana, Nebraska, Kansas, and into the Canadian prairies. The Niobrara Formation, a chalky limestone layer deposited in the seaway’s warm, shallow waters, is one of the richest fossil beds in North America, particularly in western Kansas.
Marine vertebrate fossils are extremely common in these deposits. The Smoky Hill Chalk member of the Niobrara Formation has produced thousands of specimens: mosasaurs, plesiosaurs, giant fish, pterosaurs, and toothed birds. If you’ve seen a mounted mosasaur skeleton in a natural history museum, there’s a good chance it came from rocks that formed on the bottom of the Western Interior Seaway.
The seaway also left behind economically important resources. Much of the oil and natural gas extracted from the Great Plains and western Canada formed in organic-rich sediments deposited in the seaway’s oxygen-poor bottom waters. The chalk and shale formations serve as both source rocks and reservoir rocks for petroleum. The flat, fertile farmland of the Great Plains owes its character in part to the fine-grained marine sediments that the seaway deposited over millions of years, later covered by glacial soils.
Could It Come Back?
In theory, a significant rise in global sea levels could flood low-lying portions of the Great Plains again, but the geography has changed too much. The Rocky Mountains now form a high western barrier, and the land itself sits hundreds of feet higher than the old seaway floor did relative to sea level. Even the most extreme projections of sea level rise from climate change (on the order of tens of meters over centuries) would flood coastlines long before water reached the interior of the continent. The Western Interior Seaway was a product of a specific combination of tectonic conditions, ocean spreading rates, and a greenhouse climate that no longer exists.