The sediments of Dinosaur Provincial Park span 2.8 million years and three formations. The terrestrial Oldman Formation is at the base of the strata; the terrestrial Dinosaur Park Formation above, and the marine Bearpaw at the top. The Dinosaur Park Formation, which contains most of the fossils with articulated skeletons, was mostly laid down by large rivers in very warm temperate coastal lowlands. It was on the western margin of the Western Interior Seaway. The formation dates to about 75 million years ago. The Dinosaur Park Formation lasted about a million years.