The Western Interior Seaway was a large inland sea that existed from the early Late Cretaceous (100.5–66 Ma) to earliest Paleocene, splitting the continent of North America into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea stretched from the Gulf of Mexico and through the middle of the modern-day countries of the United States and Canada, meeting with the Arctic Ocean to the north. At its largest, it was 2,500 feet deep, 600 miles wide and over 2,000 miles long.