Boulder is the municipality of home rule, which is the county seat and the most populous municipality in Boulder County, Colorado, United States. It is the 11th most populated municipality in the state; Boulder is located at the foot of the Rocky Mountains foothills at an elevation of 5,430 feet above sea level. The city of Boulder is in Boulder Valley where the Great Plains intersect with the Rocky Mountains. West of town are sedimentary stone slabs tipped up on the foothills, known as the Flatirons. The Flatirons represent a commonly known Boulder sign.