Wyoming’s Bighorn Basin: A Landscape Written in Deep Time

Wyoming’s Bighorn Basin is one of North America’s most extraordinary geological regions, preserving nearly 2.5 billion years of Earth’s history across its mountains, canyons, badlands, and fossil beds. According to the Wyoming State Geological Survey, the basin is a large structural depression surrounded by the Bighorn Mountains, Owl Creek Mountains, Bridger Range, and Absaroka Mountains, formed primarily during the Laramide Orogeny approximately 70 to 40 million years ago. This mountain building event uplifted the surrounding ranges while the basin itself sank, creating a vast sedimentary bowl that would preserve layer upon layer of ancient environments.

Long before the basin took its current shape, much of the region was covered by shallow inland seas during the Paleozoic Era (roughly 540 to 250 million years ago). Marine deposits from these ancient oceans formed limestone, dolomite, sandstone, and shale layers that are still visible today in formations such as the Madison Limestone and Bighorn Dolomite. The U.S. Geological Survey notes that these rock layers preserve marine fossils including brachiopods, trilobites, and corals, revealing that Wyoming was once submerged beneath warm tropical waters.

During the Mesozoic Era (252 to 66 million years ago), the landscape shifted dramatically as rivers, coastal plains, and the massive Western Interior Seaway repeatedly advanced and retreated across the region. These environments created rich sedimentary deposits such as the Morrison and Cloverly formations, which contain dinosaur fossils, tracks, and prehistoric plant remains. The Bighorn Basin was important in preserving evidence of both dinosaurs and changing ecosystems during this era.

Following the extinction of the dinosaurs, the Bighorn Basin became one of the world’s most significant fossil archives during the Paleocene and Eocene epochs (66 to 34 million years ago). Subtropical forests, rivers, and floodplains covered the basin, depositing the Fort Union and Willwood formations. Research from institutions including the Smithsonian and regional geologists has shown these formations preserve one of the clearest records of early mammal evolution, including primitive horses, primates, and other mammals adapting after the dinosaur extinction. These rocks also record the Paleocene-Eocene Thermal Maximum, a major ancient climate warming event.

Later volcanic activity from the nearby Absaroka volcanic field added ash and volcanic sediments to the basin, while millions of years of erosion sculpted the dramatic landscapes visible today in places like Shell Canyon, Red Gulch Dinosaur Tracksite, and Bighorn Canyon. The National Park Service describes this erosion as a key force exposing the basin’s colorful rock layers and fossil resources.

Today, the Bighorn Basin stands as a natural geologic archive where visitors can witness the story of ancient oceans, dinosaur habitats, mammal evolution, volcanic influence, and tectonic upheaval all in one region. From billion year old basement rocks to Ice Age shaped landscapes, the basin offers one of the most complete windows into Earth’s changing history anywhere in the American West.