The Fox Hills Formation is a geologic formation in the western United States, primarily recognized in regions such as Montana, North Dakota, South Dakota, Wyoming, and Colorado. It dates to the late Cretaceous period, specifically the Maastrichtian age, and is stratigraphically situated above the Pierre Shale and below the Hell Creek Formation. The formation is composed predominantly of sandstone, siltstone, and shale, reflecting deposition in a nearshore marine to coastal plain environment during the retreat of the Western Interior Seaway.
Fossils found within the Fox Hills Formation include marine invertebrates such as mollusks (notably oysters and ammonites), as well as vertebrate remains, including marine reptiles and early mammals. Dinosaur fossils are less common but have been documented, indicating proximity to terrestrial ecosystems. The formation is significant for understanding the paleoenvironmental conditions and faunal transitions near the end of the Cretaceous.
The Fox Hills Formation is named after exposures in the Fox Hills of southwestern North Dakota and has been studied extensively in the context of stratigraphy and sedimentology of the Great Plains. It serves as an important unit for correlating geological and biological changes leading up to the Cretaceous–Paleogene extinction event.