The Fort Union Formation is a Paleocene geologic formation extending across Wyoming, Montana, and nearby parts of the northwestern United States. Composed of sandstones, shales, and coal beds, it is especially significant in the Powder River Basin, where the Tongue River Member holds major coal, uranium, and coalbed methane resources, including thick subbituminous coal seams such as the Wyodak Coal. The formation preserves evidence of broad swamps, rivers, and lakes that developed after the end of the Cretaceous, with sandier rocks in the southwest and more coal-rich deposits in the northeast. It also has an important fossil record tied to life’s recovery after the Cretaceous–Paleogene extinction event.