We’ve finally figured out where Stonehenge’s giant boulders came from

The origins of the giant boulders at Stonehenge have long been a mystery – but now we have uncovered where they came from.

David Nash at the University of Brighton in the UK and his colleagues have identified the source of 50 of the 52 large boulders, known as sarsens, that make up the monument’s iconic stone circle.

By analysing the stones’ chemical composition, the team has traced their origins to 25 kilometres away from the monument, in the West Woods in Wiltshire.

The sarsens comprise Stonehenge’s outer circle as well as a horseshoe-shaped inner ring. Many are in trilithons: two vertical stones topped with a horizontal lintel.

Stonehenge also contains smaller rocks, known as bluestones, near its centre, the origins of which have previously been traced to Wales.

The researchers analysed the chemistry of the sarsens via a technique called portable X-ray fluorescence spectrometry, essentially a handheld X-ray gun. With this, they took five readings at different positions for each stone.

This revealed that 50 sarsens shared a common chemistry, containing more than 99 per cent silica, with trace elements including aluminium, calcium and iron.

“Two, much to our surprise, were different to that main cluster, but also different to each other,” says Nash. This suggests they have two separate origins.

Next, the researchers analysed a fragment of stone, taken from a collapsed sarsen when it was re-erected in 1958, to obtain a geochemical breakdown of the rock. They used this to sample areas of similar stone across southern Britain.

The site in the West Woods, one of six the team sampled in the Marlborough Downs, turned up with a match. “We didn’t expect we would ever find the original source area,” says Nash.

Identifying the origins of the sarsens opens up the possibility of future archaeological research into the routes they may have been transported to Stonehenge, says Nash.

The origins of the other two sarsens have yet to be identified.