STONEHENGE'S bluestones are under renewed investigation in the first major dig at the site since 1964.

The bluestones are the smaller of the two types of stone that make up the monument, and were the first to be erected there.

Professors Tim Darvill and Geoffrey Wainwright hope to find out why the stones were considered so powerful and important that our ancestors brought them all the way from the Preseli Hills in Pembrokeshire, the only place where they occur naturally.

This is crucial to understanding the purpose and meaning of Stonehenge itself.

The two men have been working on the theory that it was a centre for healing.

The two-week dig is being filmed for a BBC Timewatch programme, to be broadcast in the autumn, and there is a CCTV link inside a marquee for visitors to view the work on site.

Prof Darvill, from the University of Bournemouth, and Prof Wainwright, of the Society of Antiquaries, hope to use radiocarbon dating to arrive at a more precise dating of the Double Bluestone Circle, which was the first stone structure at the site, and is thought to have been built around 2550BC, although there is no visible trace of its original setting.

What visitors see now are freestanding bluestones re-erected later, and the larger sarsen stones.

By digging a trench across the south-eastern part of the original circle the experts hope to retrieve fragments of the early bluestone pillars.

The dig will also investigate the Stonehenge Layer, a layer of debris and stone chippings across the whole area of the stone circle.

Prof Wainwright said: "This is the culmination of six years of research which Tim and I conducted in the Preseli Hills.

"The excavation will date the arrival of the bluestones and contribute to our definition of the society which undertook such an ambitious project."

The Preseli Hills were a centre for ceremonial and burial, and the springs below them were regarded as medicinal until relatively recent times. Some of the burials which have been excavated around Stonehenge have been of people who had problems such as broken bones which might have made them seek supernatural help.

Folklore accounts from the 14th century claim that a magician called Merlin brought stones from the West of the British Isles to Salisbury Plain, claiming that these possessed healing powers when used in conjunction with water.

This is why the two professors think Stonehenge may have been a centre for healing.

English Heritage's chief executive Dr Simon Thurley said: "The bluestones' arrival marked a turning point in the history of Stonehenge, changing the site from being a fairly standard henge with timber structures and occasional use for burial, to the complex stone structure whose remains dominate the site today."