Ferris Formation

The Ferris Formation is an Upper Cretaceous (~67-66 Ma) to Paleocene (66-63 Ma), fluvial-deltaic geological formation in southern Wyoming. It contains a variety of trace and body fossils, preserved in sandy fluvial channel deposits and overbank units. Diverse vertebrate fossils have been reported from the formation.

The Ferris Formation is up to 2,000 metres (6,600 ft) thick in the Hanna Basin and represents a rapid accumulation of predominantly fine-grained sediment on a broad deltaic and fluvial environments. Some of the fluvial channels contain evidence of tidal influence and brackish water, in the form of tidal facies, shark teeth, and crustacean burrows, which demonstrates that the western shoreline of the Western Interior Sea was close in proximity to the Hanna Basin even during the latest Cretaceous. The local K-T boundary is contained within a sandy channel deposit made up of stacked bars. Reworked Cretaceous fossils are preserved at the base of the channel complex, associated with mud rip-up clasts, and Paleocene mammal fossils are preserved in the upper portion of the bar.