North America’s Inland Sea

North America Inland Sea
North America Inland Sea | Source

Picturing a Sea

Can you picture 19 states out of the 50 states that make up the United States being under or partially under water? About 100 million years ago, this is the exact picture you would have seen. Stretching down from the Arctic Ocean through Montana and North Dakota all the way to Texas, Florida, and the Gulf of Mexico laid the Western Interior Seaway. This ancient sea split North America into two land masses. Laramidia laid to the west where California, Nevada, Washington, and other states sit. Appalachia sat to the east where we would find Ohio, New York, Maine, and many other states. The biggest the Western Interior Seaway got was over 2,000 miles long and 600 miles wide. To get to the bottom, you only had to travel close to a 1/2 mile down or 2,500 feet.

Creation of the Ocean

What created an ocean in the middle of North America? The credit belongs to two tectonic plates known to us as the Farallon tectonic plate and the North American Plate. During the Cretaceous time, the Farallon plate subducted under the North American Plate. Subduction is the process were one tetonic plate, in this case the Farallon Plate, moves under another tetonic plate, the North American. The rate of this process is average of 2 to 8 centimeter per year to give you an idea of how long it would have taken to form the seaway. Basically, as one plate slid underneath, it pulled down an area or depression which could be filled with water, which we had high sea levels during this period.


Disappearance of a Seaway

At the end of the Cretaceous time, the sea that covered North America began to disappear during a mountain building episode call the Laramide Orogeny about 70 to 80 million years ago and didn’t end until 35 to 55 million years ago. This phenomenon is names for the Laramie Mountains of eastern Wyoming, but the effect of the Laramide Orogeny is seen from Canada to northern Mexico with the inclusion of the progenitors of the Rocky Mountains and stretching east to include the Black Hills of South Dakota. After the Laramide Orogeny, water and glacier erosion have created the Rocky Mountains valleys and peaks.

Panorama taken from Westlands Park in Greenwood Village, Colorado
Panorama taken from Westlands Park in Greenwood Village, Colorado | Source

Animal's of the Seaway

Nestled in the Panhandle of Nebraska is a farm which hosts the site of a fossilized sea turtle. It is a private farm where only those who are friends can gain access, but it leaves a person wondering. Were sea turtles found in the Western Interior Seaway? Yes. Due to the carbonate deposition, carbonate meaning the salt of carbonic acid, suggest the seaway was warm, tropical, and full of marine life. Thinking of what some consider the boring land of Kansas or the plain area of Nebraska, picturing the sharks that used to live in the area is challenging, yet among the marine life was the Squalicorax. You could also find reptiles like plesiosaurs and mosasaurs. These reptile could grow up to 18 meters or close to 58 feet long. Although much smaller then the mosasaurs, the Xiphactinus is still larger then any modern day bony fish at about 16 feet long or 5 meters. Sharks and fish were not the only animals to be found, but many different animals including birds like the Hesperornis. The Hesperornis is a flightless bird who’s body structure supported swimming rather then flying. It’s small wing-like appendages were used for steering while it’s stout legs could propel it through the water.

Xiphactinus | Source

Picturing the Sea

Can you picture the Western Interior Seaway?

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Can you picture a sea in the middle of the U.S.A now? I can’t quite either. Picturing Wyoming's famous winds, the tornadoes of the area, or even the massive amounts of snow that can fall in the Rocky Mountains turned into a tropical climate is just a little beyond my imagination. It's amazing how many changes the land under our feet has gone through. Makes many people wonder what will come of the future?

Comments 2 comments

John Delano 8 months ago

This geologic description of sub ducting plates is in error.

The North American Inland Ocean was formed by the restraining mountains , The Appalachians.

Look at this same folded mountain turning southwest at Alabama, west to the Mexican mountains.

These youngest Appalachians are called The Ouchita Mountains.

Dallas Bogan 3 months ago

Such a great site! It fits right into my individual projects. Thanks


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