Montana’s Blackleaf Formation preserves fossils from the Albian to Cenomanian stages of the Cretaceous and is notable for both its fossil record and its distinctive sedimentary structures. A described section includes an isolated, sinuous sandstone body cutting across three mudrock units, with a bone-bearing lower sandstone rich in volcanogenic grains and bordered by a claystone rim marked by calcite veins. The formation has yielded reptiles, fish, and plants, including Oryctodromeus cubicularis and other vertebrates and flora that illuminate mid-Cretaceous environments in the Northern Rockies. By combining fossiliferous beds with clearly recognizable field features, it has become important to both paleontology and geology.