Ask people how Stonehenge was built and you'll hear stories of sledges, ropes, boats and sheer human determination to haul stones from across Britain to Salisbury Plain, in south-west England. Others might mention giants, wizards, or alien assistance to explain the transport of Stonehenge's stones, which come from as far as Wales and Scotland .
Authors
- Anthony Clarke
Research Associate, School of Earth and Planetary Sciences, Curtin University
- Chris Kirkland
Professor of Geochronology, Curtin University
But what if nature itself did the heavy lifting in transporting Stonehenge's megaliths? In this scenario, vast glaciers that once covered Britain carried the bluestones and the Altar Stone to southern England as "glacial erratics", or rocks moved by ice, leaving them conveniently behind on Salisbury Plain for the builders of Stonehenge.
This idea, known as the glacial transport theory, often appears in documentaries and online discussions . But it has never been tested with modern geological techniques.
Our new study , published today in Communications Earth and Environment, provides the first clear evidence glacial material never reached the area. This demonstrates the stones did not arrive through natural ice movement.
While previous research had cast doubt on the glacial transport theory, our study goes further and applies cutting-edge mineral fingerprinting to trace the stones' true origins.
A clear mineral fingerprint
Giant ice sheets are messy, leaving behind piles of rock, scratched bedrock and carved landforms .
However, near Stonehenge, these tell-tale clues are either missing or ambiguous. And because the southern reach of ice sheets remains unclear, the glacial transport idea is open to debate.
So, if no big and obvious clues are present, could we look for tiny ones instead?
If glaciers had carried the stones all the way from Wales or Scotland, they would also have left behind millions of microscopic mineral grains, such as zircon and apatite , from those regions.
When both minerals form, they trap small amounts of radioactive uranium - which, at a known rate, will decay into lead. By measuring the ratios of both elements using a technique called U-Pb dating , we can measure the age of each zircon and apatite grain.
Because Britain's rocks have very different ages from place to place, a mineral's age can indicate its source. This means that if glaciers had carried stones to Stonehenge, the rivers of Salisbury Plain, which gather zircon and apatite from across a wide area, should still contain a clear mineral fingerprint of that journey.
Searching for tiny clues
To find out, we got our feet wet and collected sand from the rivers surrounding Stonehenge. What we discovered was striking.
Despite analysing more than seven hundred zircon and apatite grains, we found virtually no mineral ages that matched the bluestone sources in Wales or the Altar Stone's Scottish source.
Zircon is exceptionally tough: grains can survive being weathered, washed into a river, buried in rocks, and recycled again millions of years later. As such, zircon crystals from Salisbury Plain rivers span an enormous stretch of geological time, covering half the age of the Earth, from around 2.8 billion years ago to 300 million years ago.
However, the vast majority fell within a tight band, spanning between 1.7 and 1.1 billion years old. Intriguingly, Salisbury River zircon ages match those from the Thanet Formation , a blanket of loosely compacted sand that covered much of southern England millions of years ago before being eroded.
This means zircon in river sand today is the leftovers from ancient blankets of sedimentary rocks, not freshly delivered sand from glaciers during the last Ice Age 26,000 to 20,000 years ago.
Apatite tells a different story. All grains are about 60 million years old , at a time when southern England was a shallow, subtropical sea. This age doesn't match any potential source rocks in Britain.
Instead, apatite ages reflect the squeezing and uplifting caused by distant mountain-building in the European Alps , causing fluids to move through the chalk and "reset" apatite's uranium-lead clock. In other words, the heating and chemical changes erased the mineral's previous radioactive signature and started the clock ticking again.
Much like zircon, apatite isn't a visitor brought in by glaciers but is local and has been sitting on Salisbury Plain for tens of millions of years.
A new piece of the Stonehenge story
Stonehenge sits at the crossroads of myth, ancient engineering and deep-time geology.
The ages of microscopic grains in river sand have now added a new piece to its story. This gives us further evidence the monument's most exotic stones did not arrive by chance but were instead deliberately selected and transported.
Anthony Clarke receives funding from the Australian Research Council.
Chris Kirkland does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
