The Hell Creek Formation stretches across parts of Montana, North Dakota, South Dakota, and Wyoming and preserves rocks from the latest Cretaceous into the earliest Paleocene. Deposited by river channels, deltas, and occasional swamps along the margin of the Western Interior Seaway, it records a mild, moist, forested coastal plain where crocodylians and palm trees lived. It is especially significant for preserving life and environments just before and around the Cretaceous-Paleogene extinction event, including an iridium-rich boundary layer. Famous for abundant fossils, Hell Creek has yielded Tyrannosaurus, Triceratops, and many other animals and plants, making it one of North America’s most important fossil-bearing formations.