The Flatirons are steeply tilted sandstone formations on Green Mountain above Boulder, Colorado, rising over Chautauqua Park and forming the city’s most recognizable backdrop and a lasting civic emblem. Part of the Fountain Formation, the rocks were laid down roughly 290 to 296 million years ago, then uplifted during the Laramide Orogeny and later exposed by erosion. Early settlers named them for their resemblance to household irons, though earlier names included the Chautauqua Slabs and the Crags. The area is also one of Colorado’s best-known climbing destinations, with extensive routes, recreation within Boulder’s open space system, and habitat for a variety of wildlife.