- [Narrator] The investigation to pinpoint the precise source of the bluestones in Wales gets underway.
But the Preseli Hills are vast and peppered with countless outcrops of volcanic rock, any one of which could be where the Stonehenge bluestones were mined.
By comparing samples from the Stonehenge bluestones with rocks in the Preseli Hills, 20th century geologists made a crude match with a couple of southern outcrops.
But Mike isn't convinced.
He wants to find evidence of quarrying activity and to ensure he digs in the right place, he needs the most up-to-date geological analysis of the area.
Geologist Richard Bevins has mapped and sampled hundreds of Preseli outcrops.
Each one is unique.
- Examining the rocks of this area, there are subtle differences.
You have to experience a lot of exposure to spot the minor differences, which become critical.
- [Narrator] Richard's research is critical when comparing the Stonehenge bluestones with the Welsh outcroppings, but it alone is not enough.
With the help of Jane Evans, Mike and Richard also use the latest tools of geochemistry to learn more.
- The idea that we might be able to use these methods to pin down the absolute origin possibly of some of these rocks was a fascinating challenge.
I don't think that it's ever been used in the archeological context before, so I think this was a first.
- [Narrator] Bluestone samples from both Stonehenge and the Preseli outcrops are pulverized, transformed into the finest sand, then panned like gold to separate out the minerals.
- We can look at the chemical compositions of small crystals for lots of different elements and then we can do a matching of those elements, almost like a fingerprint.