The stone circles known as tipi rings at this site mark the location of a prehistoric Native American campsite. The stones probably anchored the skins of conical tents known by the Sioux word tipi, placed around the base to hold down the skins and provide additional support in high winds. After the introduction of the metal ax, wooden pegs gradually replaced the stones for holding down the skins. The tipi was used for shelter and sleeping, while most daily activities occurred outside. A hearth in the center provided heat and cooking in poor weather. In prehorse times, tipis averaged approximately 12 feet in diameter, with poles up to 15 feet long, and required eight to 12 buffalo hides, preferably from animals killed during the summer because the hides were thinner and lighter. A smudge fire inside a new tipi allowed smoke to permeate the leather, waterproofing it and aiding its preservation. More than 1 million tipi rings have been estimated in the western United States, making them one of the most common archaeological features in that part of the country.