 Mesozoic deposition in the Rockies occurred in a mix of marine, transitional, and continental environments as local relative sea levels changed.By the close of the Mesozoic, 10,000 to 15,000 feet (3000 to 4500 m) of sediment accumulated in 15 recognized formations.The most extensive non-marine formations were deposited in the Cretaceous period when the western part of the Western Interior Seaway covered the region.