The concentric stone circles that make up Stonehenge, 130 km southwest of London, consist of giant sandstone blocks and smaller bluestones－volcanic rock of a bluish tint with white flecks.

In the first archaeological dig at the World Heritage site since 1964, Stonehenge experts Tim Darvill and Geoff Wainwright will use modern carbon dating techniques and analysis of soil pollen and seashells to work out when the stones were set up.

"If you want to find out why Stonehenge was built, you need to look 250 km away to the Presili Hills, where the first bluestones that built Stonehenge come from," Wainwright said.

The two archaeologists believe the bluestones were thought to have magical curative powers. Theories of the role of Stonehenge range from the supernatural－one says the legendary wizard Merlin built it－to sacrifices linked to sun worship.

Wainwright and Darvill hope their findings will support their theory that Stonehenge was the ancient equivalent of a health spa.

Some 80 bluestones, weighing between one and four tons each, were transported to the site between 4,500 and 5,000 years ago. Only about one-third of them remain, the rest having been stolen or broken up over the millennia.