The Ferris Formation in southern Wyoming preserves a thick fluvial-deltaic record from about 66 to 63 million years ago, spanning the end of the Cretaceous and start of the Paleocene. In the Hanna Basin, its sediments were laid down by shifting rivers, channels, floodplains, lagoons, bays, and delta plains near the western shoreline of the Western Interior Sea. Tidal facies, shark teeth, and shrimp burrows show lingering marine influence. Especially significant is its record across the Cretaceous-Paleogene boundary, where reworked Cretaceous fossils below and Paleocene mammal fossils above capture the transition from dinosaur-dominated ecosystems to the early age of mammals.