The sea(Middle), with modern states for comparison, and the three cretaceous N. American islands.
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 of North America into two halves during most of the mid and late Cretaceous Period.
The Seaway was created as the now inexistent Farallon and North American tectonic plates collided, causing the Rocky Mountains to form in western North America and the creation of a depression or Foreland basin in the middle North America. This depression and the high eustatic sea levels existing during the Cretaceous allowed waters from the Arctic Ocean in the north and the Gulf of Mexico in the south meet and flood the central lowlands, forming a sea that transgressed (grew) and regressed (receded) over the course of the Cretaceous.
The earliest phase of the Seaway began in the mid-Cretaceous, when an arm of the Arctic Ocean transgressed south over western North America; this formed the Mowry Sea, so named for the Mowry Shale, a characteristic rock formation that is rich in oil shale. In the south, the Gulf of Mexico was an extension of the Tethys Sea, which met with the Mowry Sea in the late Cretaceous, forming the "complete" Seaway.