Archaeologists carrying out an excavation at Stonehenge say they have broken through to a layer that may finally explain why the site was built.

The team has reached sockets that once held bluestones – smaller stones, most now missing or uprooted, which formed the site’s original structure.

The researchers believe that the bluestones could reveal that Stonehenge was once a place of healing.

The dig is the first to take place at Stonehenge for more than 40 years.

The team now needs to extract organic material from these holes to date when the stones first arrived.

Professor Tim Darvill, of Bournemouth University, who is leading the work with Professor Geoff Wainwright, president of the Society of Antiquaries, said: “The first week has gone really well. We have broken through to these key features.

“It is a slow process but at the moment everything is going exactly to plan.”

The two-week excavation is being funded by the BBC and filmed for a special Timewatch programme to be broadcast in the autumn.

Professors Darvill and Wainwright say that finding out more about the history of the bluestones could be key to solving the mystery of why the 4,500-year-old landmark was erected.

They believe that the bluestones, which were transported 250km (150 miles) from the Preseli Hills in Wales to the Salisbury Plain in Wiltshire, were brought to the site because the ancient people believed they had healing properties.

Professor Geoffrey Wainwright said the site could have been a “Neolithic Lourdes”.

The giant sarsen “goal posts”, which came from about 20km (12 miles) away, were thought to have arrived much later.

As well as reaching the bluestone sockets, the archaeologists have also unearthed a whole host of other finds as they have peeled back the layers of the 2.5m-by-3.5m (8.2ft-by-11.5ft) trench.

These include a beaker pottery fragment, Roman ceramics and ancient stone hammers.

Daily text and video reports on the Stonehenge dig are published at the BBC Timewatch website. A BBC Two documentary will be broadcast in the autumn and will detail the findings of the investigation

Yvette Staelens reveals some of the unearthed treasures