Stonehenge is a prehistoric megalithic structure on Salisbury Plain in Wiltshire, England, two miles west of Amesbury. It consists of an outer ring of vertical sarsen standing stones, each around 13 feet high, seven feet wide, and weighing around 25 tons, topped by connecting horizontal lintel stones, held in place with mortise and tenon joints, a feature unique among contemporary monuments. Inside is a ring of smaller bluestones.