Stonehenge is the ruins of a megalithic monument (see megalith), dating from the Stone Age and early Bronze Age, on Salisbury Plain, 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, some 100 meters to the north-east. Stonehenge was built between c. 1900 BC and c. 1400 BC in three distinct phases. It appears to have been both a religious center and an observatory from which predictions of astronomical events could be made.