The prehistoric builders of Stonehenge are renowned for hauling the megalithic bluestones 140 miles from the Preseli Mountains in north Pembrokeshire, west Wales, to their final home on Salisbury Plain, Wiltshire.
However, perhaps the mammoth journey, roughly 4,500 years ago, proved too much for them – for when it came to later sourcing the larger sarsen stones, they looked closer to home.
Archaeologists from English Heritage and the University of Brighton have solved a centuries-old mystery and concluded that the sarsens originated in the West Woods of the Marlborough Downs, just 15 miles north of Stonehenge.
It is not far from the Neolithic monument site of Avebury, and the area is likely to have been a significant or ritual landscape. The monument at Avebury is a Neolithic henge containing three stone circles, around the village of Avebury, Wiltshire.
The breakthrough came when a piece of core – drilled from Stonehenge's "Stone 58" during repair work in the 1950s – was returned to English Heritage from the US last year, which gave archaeologists the chance to analyse the chemical make-up without damaging the monument.
English Heritage historian Susan Greaney said: "The bluestones we've known come from mid-west Wales from the 1920s, and they were easy to source because there are inclusions and quartz which you can match to the outcrop.
"The large stones are pretty uniform and just look like a big block, so it's difficult to interrogate them and find where they are from."
The new research used X-ray spectrometry to look at the very small trace elements, and it was found the Stonehenge sarsens matched those in the Marlborough Downs. The sarsens average approximately 20 tonnes but can be as heavy as 30, and the largest is 30 feet long.
"There are no sarsens of this size left in the downs, that's why it's been such a mystery and some people have suggested they come from elsewhere as you get outcrops in other places where you have chalkland, like Decon and Kent,” Ms Greaney added.
"The West Wood was overlooked because its under ancient woodland, and a lot of sarsens were removed in the 19th century for roadstones, particularly in Swindon, but there are still sarsens buried in amongst the trees.
"To be able to pinpoint the area that Stonehenge’s builders used to source their materials around 2,500 BC is a real thrill."
While the Preseli stones were probably chosen for ritual or location reasons, the sarsens were likely to have been picked for their size, archaeologists believe.
But although 10 times shorter than the journey from Wales, moving 30 tonne stones would still have proven a tricky prospect and the sarsens may have moved on the River Avon for part of the way.
"We can now say, when sourcing the sarsens, the overriding objective was size – they wanted the biggest, most substantial stones they could find and it made sense to get them from as nearby as possible," said Ms Greaney.
"This is in stark contrast to the source of the bluestones, where something quite different – a sacred connection to these mountains perhaps – was at play. Yet again, this evidence highlights just how carefully considered and deliberate the building of this phase of Stonehenge was."
Professor David Nash, a geomorphologist from the University of Brighton, said: "It has been really exciting to harness 21st-century science to understand the Neolithic past, and finally answer a question that archaeologists have been debating for centuries.”
The team is now hoping to carry out more survey work on the Marlborough Downs, looking for sarsen chips and wedges which might indicate where the original location of the stones.
The research was published in the journal Science Advances.
In 2008, the X-ray spectrometry technique was used to visualise the paint below Vincent van Gogh's Patch of Grass and pick out the individual brush strokes as well as the flesh tones of the hidden head of a woman.
Cornell University has also used it to analyse inscriptions from Greek and Roman pottery. Carvings that have been entirely worn away on the surface can still be visible through the tiny amount of iron left by a chisel, allowing words to emerge from seemingly smooth stone.
In 2011, an unfinished self-portrait of Rembrandt was discovered under another painting using X-ray fluorescence.
Conservation experts used the technique to understand the origins of the 1,400-year-old Staffordshire Hoard, a collection of more than 3,500 Anglo-Saxon artefacts discovered by a metal detector enthusiast in a field in Staffordshire in 2009.