Scientists have succeeded in locating the exact source of some of the rock believed to have been used 5000 years ago to create Stonehenge's first stone circle.
By comparing fragments of stone found at and around Stonehenge with rocks in south-west Wales, they have been able to identify the original rock outcrop that some of the Stonehenge material came from.
The work - carried out by geologists Robert Ixer of the University of Leicester and Richard Bevins of the National Museum of Wales - has pinpointed the source as a 70 metre long rock outcrop called Craig Rhos-y-Felin, near Pont Saeson in north Pembrokeshire. It's the first time that an exact source has been found for any of the stones thought to have been used to build Stonehenge.
The discovery has re-invigorated one of academia's longest running debates - whether the smaller standing stones of Stonehenge were quarried and brought all the way there from Pembrokeshire by prehistoric humans or whether they had already been plucked out of ancient rock outcrops and carried all or part of the way to Wiltshire by glaciers hundreds of thousands of years earlier.
Archaeologists tend to subscribe to the 'human transport' theory, while some geomorphologists favour the glacial one. The debate is solely about Stonehenge's early/smaller standing stones (often known collectively as 'bluestones') - not about the larger ones (most of the so-called 'sarsens') which were incorporated into the monument several centuries later.
The Leicester University and National Museum of Wales scientists' discovery - reported in the journal, Archaeology in Wales - does not solve the mystery of how Stonehenge's Welsh-originating stones ended up in England, but it does potentially open up the possibility of finding archaeological evidence of quarrying activity that could indicate a human rather than a glacial explanation (indeed that archaeological search has already been launched by archaeologists from Sheffield and other universities). Conversely, any lack of such evidence would help those scholars arguing in the opposite direction. As the geological research continues, it's likely that numerous other rock outcrops in various parts of Pembrokeshire will be positively identified as sources of other stones used to build early versions of Stonehenge. Over past decades, the approximate area they came from has been identified - and the ongoing research will almost certainly succeed in pinpointing additional exact sources.
But although the stone fragments from Stonehenge will allow the scientists to track down where the material originally came from, those same fragments represent an altogether different mystery.
Literally thousands of fragments of rock - almost certainly from monoliths used at or around Stonehenge - have, over the years, been found in or near the world famous monument.
These fragments (mostly less than 50 grams each) appear to have been deliberately chipped off ancient monoliths at some stage in antiquity - many of them probably in the Neolithic.
However, most of the fragments examined so far are from particular types of rock which were used for less than 10% of the early (i.e. Welsh originating) Stonehenge monoliths. The fragments - found not just at Stonehenge itself but also elsewhere in the Stonehenge landscape - tend to be of a different geological character to the vast majority of early Stonehenge standing stones (which are mostly made of a different type of Pembrokeshire-originating rock). Indeed the rock type from Craig Rhos-y-Felin (just pinpointed by the new scientific research) was probably used for just one of the Stonehenge monoliths (a now buried stone, last seen in the 1950s).
This suggests that there may have been other stone circles or other 'standing stone' monuments in the landscape which have now vanished, but could in the future be found by other scientists (from Birmingham and other universities) who are carrying out an ongoing program of geophysical survey work throughout that landscape.
A further unsolved mystery is why prehistoric people were chipping fragments off probable monoliths. It's possible that they were chipped off in order to give monoliths a better shape. Alternatively, some monoliths or other rock material may have been broken up and re-cycled as stone axes - potentially imbued with particularly high status or conceivably perceived as having magical powers.
The detective work, that the University of Leicester and the National Museum of Wales scientists had to carry out to pinpoint the precise Pembrokeshire source of many of these fragments, was extremely complex.
First of all the geologists needed to sort through thousands of tiny fragments of Pembrokeshire-originating rock found by archaeologists at and around Stonehenge over the past 70 years.
Then the two scientists began to look particularly closely at around 700 of them which were made of a specific type of volcanically-originating rock (geologically, dating back some 460 million years) known as 'foliated rhyolite'.
They then succeeded in tentatively locating the approximate area of north Pembrokeshire which those 700 fragments originated from.
This was subsequently confirmed by comparing the chemical signature of tiny crystals (each one-five-hundredths of a millimetre in diameter) in the Stonehenge fragments with similar rocks in north Pembrokeshire.
Finally, by examining the detailed inter-relationships between minerals in samples from Stonehenge and north Pembrokeshire, they succeeded in pinpointing the precise rock outcrop.
If the stones were brought to Stonehenge from Pembrokeshire by human effort, the location of the newly discovered source (Craig Rhos-y-Felin) has interesting cultural implications.
For the newly discovered source is around five miles away from a wider area already known to have been the source for some of Stonehenge's other monoliths.
If humans were responsible for quarrying and transporting the stones from Pembrokeshire, then it would suggest that Stonehenge's Neolithic designers were extremely choosy and very specific as to where they got their stones from.
Research over recent years by Tim Darvill of Bournemouth University and Geoffrey Wainwright, a former chief archaeologist at English Heritage, suggests that the Pembrokeshire stones may have had a particular ideological or magical significance.
The outcrops where some of the stones come from are thought to have been associated with sacred springs and local Welsh stone circles.
It's argued that, by importing those particular rocks the 160 miles from Pembrokeshire to Wiltshire, the builders of Stonehenge thought they were taking possession of more than just plain rock. They may have regarded them as extremely important - and could even have seen them as possessing supernatural powers.
The newly discovered source is also significant because of its location. It lies on low ground to the north of the Preseli Mountains. This would have made transport to Wiltshire much more difficult than it would have been for other Pembrokeshire rocks used in Stonehenge and, known to have come from the High Preseli several miles to the south.
Transporting the north Pembrokeshire stones by sea would have required sailing round St. David's Head, a particularly difficult and dangerous route for a Neolithic boat. Alternatively the prehistoric quarrymen and their colleagues would have had to haul the stones over the top of the nearby Preseli Mountains. However, if humans took the stones to Stonehenge, it is also possible that the stones had already been used to construct circles in Pembrokeshire - and were therefore moved from those locations to Stonehenge, rather than from the original sources themselves.