Sea level was high in the Cretaceous, and the middle part of North America sank down. A shallowly-subducting plate pulled on the bottom of the lithosphere, causing it to move down. The sea entered both north and south, and came and went during the course of the Cretaceous. Widespread carbonate deposition suggests that the Seaway was warm and tropical, with abundant calcareous algae. The shrunken, regressive phase of the Western Interior Seaway is sometimes called the Pierre Seaway.