North Horn Formation

The North Horn Formation is a widespread non-marine rock unit in central and eastern Utah that preserves rivers, lakes, and floodplains from the latest Cretaceous into the early Paleocene. Thickest near North Horn Mountain and much thinner farther east, it lies above older Late Cretaceous formations and below the Flagstaff Formation. Its layers record shifting terrestrial environments, from coarse river deposits to coal-bearing and floodplain sediments. The formation is especially important because it contains the difficult-to-locate Cretaceous–Paleogene extinction interval, marked mainly by a change from dinosaur-dominated fossils to mammal-rich faunas. Its fossils make it one of Utah’s key records of the transition from the last age of dinosaurs to the early age of mammals.

Utah · United States