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Monday, 19 December 2011

Indy report on foliated rhyolite research

 Shattered rockfall / scree debris beneath the rock outcrop at Craig Rhosyfelin.  The "foliated rhyolites" here are claimed to match much of the rhyolite debitage at Stonehenge -- and Ixer and Bevins claim to be able to fix the source to within a few metres.  Time will tell whether that claim is reliable or not.

Now the Independent has also reported on the Ixer / Bevins research, in a much more measured way.  All in all, a pretty balanced report, apart from the headline, which should have read "Scientists discover one of many sources of rock used in Stonehenge's first circle."  But I suppose that such a headline would have been deemed too boring...........

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Scientists discover source of rock used in Stonehenge's first circle

Discovery reignites debate over transportation of smaller standing stones

David Keys
http://www.independent.co.uk/news/science/archaeology/news/scientists-discover-source-of-rock-used-in-stonehenges-first-circle-6278894.html
Sunday 18 December 2011

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.

17 comments:

Anonymous said...

Now this article both Richard and I saw before publication (unlike The Times or any other).
It is but a pale shadow of the PRIMARY literature which is required reading-make the effort boys-but is correctish in the part that concerns provenancing of the stones. Sacred Springs and Welsh Circles are another matter and thankfully not geological
speedy

BRIAN JOHN said...

Thanks Speedy -- yes, this one is well constructed and quite nuanced. much more like it!!

Anonymous said...

Brian
The picture that is posted in the discussion from October
http://brian-mountainman.blogspot.com/2011/10/on-transport-of-pillars.html
shows a stone being replaced into position.This stone appears to have been worked on near the bottom and may help explain why there are fragments in the area.
Maybe more of these stones were also worked on in this manner?

BRIAN JOHN said...

Yes, there is evidence that a number of the bluestone orthostats have been modified in some way -- although the stones cited most often are the spotted dolerites and dolerites. The Rhyolite debitage which is currently attracting attention seems to have come from other stones that may have been partially or wholly destroyed.

Sure, there is a lot of this debris about -- of many different rock types. I have no problem at all in accepting that many bluestones have been modified -- although some may indeed be in their original state.

Alex Gee said...

How does one ACCESS the primary literature?

BRIAN JOHN said...

A few articles are accessible on Rob's personal web site. Otherwise, I suggest you write to the authors and ask for PDFs.......

Anonymous said...

I wish they'd stop implying that archaeologists, as a group, are in favour of the human transport idea. A lot of us simply aren't.

Anonymous said...

I have a terrible fear that no rock outcrop for miles around Brynberian will escape the attention of pith-helmeted antiquarians searching for their Holy Bluestone Grail. Preseli could be laid to waste. I hope the relevant authorities will not encourage them with half-baked projects such as the ill-fated attempt to drag a harmless rock from Carn Meini to Milford Haven over plastic mats back in the 1990s.

BRIAN JOHN said...

Apologies, Anon --

I hope I haven't implied that ALL archaeologists subscribe to the idea of human transport all the way from A to B. In fact, at various meetings I have been to, a number of professional archaeologists have come up to me and said "Bravo! About time somebody made those points and probed some of those fanciful ideas......"

But saying things under their breath after a meeting is over is not the same as standing up and being counted when it matters. Their timidity is surprising -- and a little distressing. What are they scared of?

BRIAN JOHN said...

Other Anon -- well, they are gearing up for the Great Preseli Quarry Hunt. Funding applications are probably on the desks of the funding committees as we speak. All very sad -- and it's also sad, I think, that the manner in which the Rhosyfelin rhyolite work has been presented by the geologists will encourage that sort of mania.......

Geo Cur said...

They are not scared , due to lack of evidence ,direct or circumstantial they , quite reasonably ,are unwilling to take up sides .Just because you don't support Rangers doesn't make you a Celtic fan .

BRIAN JOHN said...

You may be right -- but I wouldn't want to assume to know what is inside their heads. Whatever they REALLY think (and I suppose there are many shades of agnosticism) all this fence-sitting gets a bit boring...

Anonymous said...

Brian,

Isn't it likely the Rhosyfelin outcrop was much higher 10K years ago than it is today, perhaps even reaching the hilltop? Glacier advances and meltwater streams may have worn it down to what we have today? There does seem to be some open depression on the Preseli Hills near where Rhosyfelin is located. At least it seems so from the maps and photos I've seen.

Kostas

BRIAN JOHN said...

No chance of any of that.

Tony H said...

Thank God for The Independent. A newspaper that dares to reach parts of the brain other newspapers have never heard of, e.g. those parts reponsible for reasoning and intellectual thoght.

Chris johnson said...

Interesting thought kostas. I am sure Brian knows the real answer but I would have thought rhosyfelin would have been rising since the ice age

BRIAN JOHN said...

Chris -- there might have been some isostatic depression and uplift in North Pembrokeshire (as in all other glaciated areas) in response to glaciation and deglaciation, but there is no reason to think that Rhosyfelin would have been out of step with everywhere else. The landscape relations of one district vis a vis another would have been unchanged.