The Kirtland Formation is a Late Cretaceous sedimentary rock unit in the San Juan Basin of northwestern New Mexico and southwestern Colorado, formed from coastal plain, channel, and shoreline deposits along North America’s inland seaway. Lying above the Fruitland Formation, it preserves a narrow interval near the end of the Campanian age, with dated members marking its sequence through roughly 75.02 to 73.49 million years ago. Named in 1916 for exposures near the Kirtland Post Office, it became a key reference for Upper Cretaceous stratigraphy and paleontology in the Southwest. Its rocks preserve important local faunas and notable vertebrate fossils, helping trace faunal change shortly before the end-Cretaceous extinction.