STONEHENGE (Reuters) - Archaeologists set out on Monday to unlock one of the secrets of Stonehenge, the majestic monument in southern England -- when were the first standing stones placed at the ancient religious site?

File photo shows Stonehenge on Salisbury Plain in southern England December 22, 2006. Archaeologists set out on Monday to unlock one of the secrets of Stonehenge, the majestic monument in southern England -- when were the first standing stones placed at the ancient religious site? REUTERS/Kieran Dohert

The concentric stone circles that make up Stonehenge, 80 miles southwest of London on the sweep of Salisbury Plain, consist of giant sandstone blocks or sarsens and smaller bluestones -- volcanic rock of a blueish tint with white flecks.

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

“If you want to find out why Stonehenge was built, you need to look 250 kilometers away to the Presili Hills in north Pembrokeshire, where the first bluestones that built Stonehenge come from,” Wainwright told reporters as the two-week dig began.

The two archaeologists, who have worked extensively in the Presili Hills in recent years, believe the bluestones, which made up the first stone circles at Stonehenge, were thought to have magical curative powers.

The massive standing stones, set up as long as 5,000 years ago, dominate the even older religious site, marked by numerous burial mounds or barrows.

“If you want to find out when the first stone was placed at Stonehenge, you need to dig a small trench round one of the stone’s sockets and date what you find. That is what we are doing,” Wainwright said.

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, president of the Society of Antiquaries, and Darvill, archaeology professor at Bournemouth University, hope their findings will support their theory that Stonehenge was the ancient equivalent of a health spa.

“This was a place of healing, for the soul and the body,” said Darvill. “The Presili Hills is a magical place. The stones from there are regarded as having healing properties.”

Some 80 bluestones, weighing between one and four tonnes each, were transported by land and sea from South Wales to the Salisbury Plain site between 4,500 and 5,000 years ago.

Only about one-third of them remain on the site, the rest having been stolen or broken up over the millennia.

“In the early 1900s there were signs in Amesbury (the nearest town to the site) offering the hire of a hammer so that people could come up here to chip off their own bit of bluestone,” Darvill said.

They are distinct from the massive sandstone sarsens that make the monument instantly recognizable.

The excavation has even been blessed by Druids, spiritual descendants of the learned priests of pre-Christian Celtic Europe, who had links with the site in ancient times.