The Interior Plains were once part of an ancient ocean.

The Interior Plains were once covered by an ancient body of water known as the Western Interior Seaway, which existed during the Late Cretaceous period. This vast inland ocean split North America into two landmasses, stretching from the Gulf of Mexico to the Arctic Ocean. Over millions of years, sediments from marine organisms accumulated on the ocean floor, forming the rich soils and sedimentary rock layers characteristic of the region today. As the ocean receded, it left behind fossils of marine life, providing valuable geological and paleontological insights. This ancient ocean's influence continues to shape the ecology and landscape of the Interior Plains.