Laramie Formation, Geological formation in Colorado, United States

The Laramie Formation is a geological sequence in northeastern Colorado composed of alternating layers of sandstone, shale, and coal beds. These deposits formed during the Late Cretaceous period and provide important evidence of ancient environments that once covered this region.

Clarence King named the Laramie Formation in 1876 for exposures in northeastern Colorado. In 1873 Edward D. Cope, accompanying Ferdinand V. Hayden's survey, collected dinosaur specimens along Bijou Creek on the east side of the Denver Basin. Cope named Cionodon arctatus, Polyonax mortuarius and Agathaumas milo without descriptions; these scrappy specimens are held at the American Museum of Natural History and the names are no longer valid.

Scientists have discovered multiple Triceratops fossils and ten different mammal species within the formation, enriching our understanding of Late Cretaceous life.

Exposures ring the edges of the Denver Basin, from the Golden area on the west side, where the formation is 120–150 m thick, to Bijou Creek on the east side, where it is 60–90 m thick. Clay was mined around Golden from 1877 and coal between 1884 and 1979, so old mine workings and steeply inclined seams occur in places. Much of the land is private; check access before entering.

The formation holds some of North America's oldest grass fossils, revealing how early plants evolved during the Cretaceous period. These rare plant discoveries make the site significant for paleontologists around the world.