The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, and the North American Inland Sea, was a huge inland sea that split the continent

Continent

A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

 of North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 into two halves, Laramidia

Laramidia

Laramidia is a name coined by J. David Archibald in 1996 to describe an island continent that existed during the Late Cretaceous period, when the Western Interior Seaway split the continent of North America in two. Two landmasses existed: the eastern one is called Appalachia; Laramidia is the name...

 and Appalachia

Appalachia (Mesozoic)

In the Mesozoic Era, Appalachia was a land area which is now an eastern part of the USA and Canada, separated from Laramidia by the Western Interior Seaway, which shrank, divided across the Dakotas, retreated south towards the Gulf of Mexico and finally dried up.-Fauna:From the Turonian age of the...

, during most of the mid- and late-Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period. It was 2500 feet (762 m) deep, 600 miles (965.6 km) wide and over 2000 miles (3,218.7 km) long.
The Seaway was created as the Farallon

Farallon Plate

The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...

 tectonic plate subducted under the North American Plate

North American Plate

The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...

 during Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 time.