Stonehenge, ruins of a megalithic monument, dating from the Stone Age and early Bronze Age, on Salisbury Plain, in southern England. Its most noticeable features are concentric rings of stones surrounding a horseshoe of upright stones, and a solitary vertical stone, the Heel Stone, to the northeast. Stonehenge was built between 1900 B.C. and 1400 B.C. in 3 distinct phases. It appears to have been both a religious center and an astronomical observatory.