Western Interior Seaway

A long time ago, a giant sea lived here.

It split the land into two parts. The water was warm and tropical.

Many creatures lived in the sea. Big sharks and fish swam in the water. Large reptiles lived there too. Some birds even swam in the waves.

The sea was a very busy place. It is amazing to think about this old ocean.

A long time ago, a giant sea split North America in two.

This was the Western Interior Seaway. It stretched from the Rockies to the Appalachian Mountains. The water was warm and tropical.

Tiny life forms made white chalk on the sea floor. Today, we can see this chalk in places like Monument Rocks.

The sea was full of life. Large reptiles like mosasaurs swam in the deep water. Many kinds of sharks lived there too. Huge fish called Xiphactinus were even bigger than modern fish.

Some birds lived near the waves. One bird, Hesperornis, had strong legs for swimming. It could not fly. Other birds had teeth in their beaks. Giant flying reptiles called pterosaurs also filled the sky.

Sea levels rose and fell over millions of years. When the water rose, the sea grew wider. When the water fell, new land appeared. This changed where animals could live and move.

A long time ago, a massive sea split North America into two separate pieces of land.

This was called the Western Interior Seaway. It sat right over the area we now call the Great Plains. The sea divided the continent into Laramidia to the west and Appalachia to the east. It existed for about 34 million years. This time went from the early Late Cretaceous to the earliest Paleocene. The water connected the Arctic Ocean to the Gulf of Mexico.

At its largest, the sea was very deep and wide.

This sea formed because the Earth's crust began to sink. As the Rocky Mountains started to grow, the land ahead of them dipped down. This created a path for ocean water to flow in. An arm of the Arctic Ocean moved south to form the Mowry Sea. At the same time, the Gulf of Mexico grew as an extension of the Tethys Ocean. Eventually, these two parts merged to make one complete seaway. The sea level rose and fell many times over the years. When the water rose, it was called a transgression. When the water fell, it was called a regression.

Scientists use fossils to learn about this ancient world. They look at tiny shells called foraminifera to study the water. These records show that the sea was very warm and tropical. Some studies suggest temperatures reached as high as 35 degrees Celsius. We know this because of the way carbonate was left behind. This happened because of tiny algae called coccolithophores. These small living things left behind white chalky layers.

Today, we can see these layers in places like Monument Rocks in Kansas. These are huge chalk formations that tower over the land.

Many amazing creatures lived in these warm waters.

Large marine reptiles like mosasaurs and plesiosaurs were top predators. There were also many kinds of sharks, like the giant Ptychodus mortoni. Some fish, such as Xiphactinus, were even larger than any fish alive today. Many different types of mollusks lived on the sea floor. Some oyster-like creatures called Inoceramids grew to be over one meter wide. Even birds lived near the water. The Hesperornis was a flightless bird with strong legs for swimming. Other birds, like Ichthyornis, had teeth in their beaks.

Understanding this sea helps us see how our world changes. The way the land moved shaped the mountains we see now. The shifting sea levels changed how animals moved between landmasses. When the water fell, different species could mix together on land. When the water rose again, they were separated once more. This cycle helped create the variety of life we study today. We can still find the remains of this era in rocks like the Pierre Shale. These fossils tell the story of a world that looked very different from our own.

The Western Interior Seaway was a massive inland sea that once split North America in two.

This body of water covered much of what we now call the Great Plains. It divided the continent into two distinct landmasses: Laramidia to the west and Appalachia to the east. The seaway existed for approximately 34 million years. This period stretched from the early Late Cretaceous to the earliest Paleocene. During its existence, the sea provided a water connection between the Arctic Ocean and the Gulf of Mexico.

Geological shifts caused this sea to form. As the Laramide orogeny began, the crust started to grow into the modern Rocky Mountains. This mountain growth caused the continental crust ahead of it to downwarp or sink. This sinking created a space for ocean water to fill. Initially, an arm of the Arctic Ocean moved south to create the Mowry Sea. In the south, the Gulf of Mexico was an extension of the Tethys Ocean. Eventually, these two water bodies merged to form the complete Western Interior Seaway.

The seaway was a dynamic environment with changing water levels. It experienced many cycles of transgression, where the sea level rises, and regression, where it falls. One major rise created the Niobrara Sea. Another rise, known as the Claggett transgression, was named after the Claggett Shale. Later, the Bearpaw transgression created a shallow sea that covered much of Wyoming and Montana. These cycles meant that land animals were often separated by water. However, when sea levels fell, land populations could rejoin and mix different species.

Climate played a huge role in the seaway's character. Scientists use biostratigraphy, which is the study of fossils to date rocks, to understand ancient temperatures. Records from foraminifera, which are tiny single-celled organisms, show the water was quite warm. During the Turonian age, average sea surface temperatures were high. Some studies suggest temperatures might have reached 35 degrees Celsius. The presence of widespread carbonate deposition shows the sea was tropical. This happened because of abundant calcareous planktonic algae.

This tropical environment left behind incredible geological features. Tiny organisms called coccolithophores secreted chalky platelets. Over millions of years, these deposits formed thick layers of chalk. In northwest Kansas, these deposits remain visible today. A famous example is Monument Rocks, which are large chalk formations rising above the land.

The seaway also functioned like a giant estuary. This means it acted like a place where river water meets the sea. Rivers from the east and west brought in silt, which created shifting delta systems along the coasts.

The seaway was home to a vast array of marine life. Large predatory reptiles like mosasaurs and plesiosaurs swam in these waters.

There were also many types of sharks, including the giant Ptychodus mortoni. Some bony fish, like Xiphactinus, grew larger than any fish alive today. On the seafloor, invertebrates like ammonites and squid-like belemnites were common. Inoceramids, which are oyster-like mollusks, were also very successful. Some species, such as Inoceramus grandis, grew to be over one meter in diameter.

Even animals that lived in the air were part of this ecosystem. Early birds lived near the water's edge. The Hesperornis was a flightless bird with strong legs for swimming. Another bird, Ichthyornis, had a toothy beak. Large pterosaurs, such as Pteranodon, also shared the skies. These flying reptiles were major participants in the surface ecosystem. The diversity of life in the seaway shows how interconnected the land, sea, and air can be. The remains of these creatures are still found today in rock formations like the Pierre Shale.