Round Lake National Natural Landmark preserves an unusual meromictic lake and fifty-nine acres of adjoining old-growth forest within Green Lakes State Park near Fayetteville, New York. Recognized in 1973, the site is significant for deep waters that do not fully mix, leaving oxygen-poor lower layers and preserving sediment records of environmental change over thousands of years. Mineral-rich groundwater also contributes to the lakes’ changing greenish color and shoreline deposits. The surrounding woodland survived extensive regional clearing that had transformed much of Upstate New York by 1855, remaining largely uncut until its incorporation into the state park in 1928 and preserving a rare remnant of central New York’s original landscape.