North America was not always a single continent.  For a period of time from the Late Cretaceous (when T-Rex walked the earth) to the early Paleocene there was once a large inland sea called the Western Interior Seaway that split the continent in two. Today people visiting the Great Plains can still see the remains of this prehistoric sea.

In much more recent history at the end of the last ice age, North America once had a massive inland lake that was larger than the combined Great Lakes today (called Lake Agassiz). Another prehistoric lake was Lake Lahontan in Nevada - that one can the remains of ancient settlements along its ancient shores today.

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What To Know of the Western Interior Seaway

The Western Interior Seaway split the North American continent. The eastern half of the continent is known as Appalachia and the western half as Laramidia. This ancient sea stretched from the Gulf of Mexico all the way up through what is today the USA and Canada to the Arctic Ocean.

  • When: Late Cretaceous to Earliest Paleocene
  • North America: Split Into Appalachia and Laramidia

At its largest extent it was 2,500 feet (760 m) deep, 600 miles (970 km) wide, and over 2,000 miles (3,200 km) long - this varied considerably throughout its history. It was a particularly shallow sea and was created by the downwarping of the continental crust during the Cretaceous.

Max Size:

  • Depth: 2,500 feet (760 m) Deep
  • Width: 600 miles (970 km) Wide
  • Length: 2,000 miles (3,200 km) Long
  • Extended: From The Gulf of Mexico To the Arctic Ocean

The Seaway is believed to have been warm and tropical. It covered parts (or all) of what is now Montana, North Datoka, South Datoka, Wyoming, Iowa, Nebraska, Colorado, Utah, Arkansas, Oklahoma, New Mexico, and Texas - as well as much of Canada including Alberta, Saskatchewan, Manitoba, Ontario, and the Northwest Territories.

Related: Did You Ever Wonder What It Would Be Like To Walk With Dinosaurs? You Can At These Parks

Prehistoric Marine Life of The Seaway

The Western Interior Seaway is known to have been a shallow sea and one that teemed with remarkable diversity and abundance of marine life.

"The waters of the Western Interior Seaway were warm, shallow, and inhabited by a plethora of marine animals."

Cretaceous Atlas of Ancient Life

Its inhabitants included bony fish, sharks, mosasaurs, plesiosaurs, birds, mollusks, ammonites. Overhead flew winged pterosaurs. Fortunately for us today, the Seaway's ocean floor was periodically anoxic (with little or no oxygen). That meant that dead animals which sank to the bottom would decay slowly aiding in their preservation and fossilization.

Some of the marine animals that lived here included:

  • Marine Reptiles: Plesiosaurs and Mosasaurs (The Monster "Dinosaur" that Ate The Indominus Rex In The 2015 Movie Jurassic World) - It Grew Up To 18 Meters or 59 Feet Long
  • Sharks: Many Prehistoric Sharks Like Squalicorax, Cretoxyrhina, and the Giant Shellfish-Eating Ptychodus mortoni (A Monster Thought To Grow Up to 10 Meters or 33 feet Long)
  • Giant Fish: Xiphactinus - A Fish That Grew To 5 meters or 16 feet (Larger than Any Modern Bony Fish)

Related: Get Your Dino On: This Family-Friendly, Prehistoric-Themed Park Can Be Found In Three States

Where To See The Legacy Of The Seaway

Traces of the ancient Seaway remain visible in marine deposits and fossils throughout the central United States. Tell-tale signs of the Seaway are the Pierre Shale and the Austin Chalk. Here are some of the most notable outcroppings and deposits.

Monument Rocks:

Also called Chalk Pyramids, these huge chalk formations in Gove County in Kansas are the remains of carbonate deposition in the Seaway. They reach up to 70 ft (21 m) high and include stunning formations like buttes and arches. They are located 25 miles (40 km) south of Oakley, Kansas.

These rock layers were formed through the accumulation of the remains of untold billions of small, photosynthetic algae. Some of the most amazing paleontological discoveries have been found near the Monument Rocks (including mosasaurs, pterosaurs, and fossil fish).

  • Age: Around 80 Million Years Ago
  • First: First Landmark In Kansas To Be A National Natural Landmark

Smoky Hills Chalk:

The Smoky Hills Chalk (also called Castle Rock) outcrop is east of Monument Rocks. This is like the Monument Rock and has some particularly well-preserved specimens that even include soft tissues and skin impressions of mosasaurs.

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Laramie Formation: 

The Laramie Formation in northeastern Colorado is the result of deposits on a coastal plain and in coastal swamps that flanked the Western Interior Seaway. Today the geologic formation contains coal, uranium, and clay deposits but are also famous for their plant and animals fossils.

It was the sight of some of the first dinosaurs to be discovered in the American West. One of the most remarkable finds includes a nearly complete skull of Triceratops.

  • Fossils: Dinosaur Fossils Are Found Here

Next: The Biggest Dinosaur Fossil Ever Found Belonged To A Long-Reining Canadian T-Rex