The National Natural Landmarks in Colorado include 16 of the more than 600 National Natural Landmarks (NNLs) in the United States; fifteen fully within Colorado and one shared with Wyoming. They cover areas of geological, biological and historical importance, and include lakes, mountains, rock formations and numerous fossil sites. The landmarks are located in 14 of the state's 64 counties. Four counties each contain all or part of two NNLs, while two landmarks are split between two counties. The first two designations, Slumgullion Earthflow and Summit Lake, were made in 1965, while the most recent designation, Sulphur Cave and Spring, was made in 2021. Natural Landmarks in Colorado range from 60 to 380,000 acres (24.3 to 153,780.5 ha; 0.1 to 593.8 sq mi) in size. Owners include private ind