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Sunday, 15 January 2012

More geology on the way......



Spotted on Rob Ixer's academic blog site -- from December.  More on the rhyolites on the way........ not sure if the paper is fully published yet. It is available online for subscribers.

Provenancing the rhyolitic and dacitic components of the stonehenge landscape bluestone lithology: New petrographical and geochemical evidence. Bevins, Richard E. / Ixer, Rob A. / Webb, Peter C. / Watson, John S., Journal of Archaeological Science, In Press, Accepted Manuscript, Dec 2011 

Abstract

The source of the bluestone component found in the Stonehenge landscape has long been the subject of great interest and considerable debate. The bluestones are a mix of lithologies, the standing orthostats being predominantly dolerites, variably ‘spotted’, with only four of them being of dacitic and rhyolitic composition and the Altar Stone being sandstone. However in the 1920s the spotted dolerites were sourced to outcrops which comprise tors in the summit regions of the Mynydd Preseli in north Pembrokeshire, west Wales. There were also speculations about the possible sources of the dacitic and rhyolitic components, ideas which were elaborated on in the early 1990s when the original petrological provenancing was supplemented by whole-rock geochemical analysis. Most recently, new petrographical investigations have been combined with zircon geochemical data to determine the possible source of one type of rhyolite, the so-called ‘rhyolite with fabric’, found abundantly as débitage in the Stonehenge landscape (but not composing the four orthostats) to outcrops in the vicinity of Pont Saeson, especially a large craggy outcrop called Craig Rhos-y-felin, located in low ground to the north of the Mynydd Preseli. In order to test this provenance whole-rock geochemical analysis has been undertaken on samples of débitage from the Stonehenge landscape and from the Pont Saeson area, including Craig Rhos-y-felin. These data are then compared with other new and existing geochemical data for dacitic and rhyolitic lithologies recovered from the Stonehenge landscape, including the four orthostats, as well as geochemical data from outcrops of the same lithologies from the two main volcanic horizons exposed across north Pembrokeshire, namely the Fishguard Volcanic Group and the Sealyham Volcanic Formation, both of Ordovician age. This study concludes that previous, 20th century, attributions of provenance to a number of dacitic and rhyolitic outcrops in the north Pembrokeshire have been in error whilst the new data for the Pont Saeson rhyolite accords well with elemental contents recorded in the ‘rhyolite with fabric’ lithology from the Stonehenge landscape débitage. This study therefore endorses the proposal that the Pont Saeson area is indeed the source of the ‘rhyolite with fabric’ lithology recovered from numerous sites in the Stonehenge landscape, and is the only reliable provenance for any of the dacitic and rhyolitic bluestone material collected to date. It also serves to endorse the use of zircon chemistry as a provenancing tool in archaeopetrological investigations.

List of the recent articles:
"Provenancing the rhyolitic and dacitic components of the stonehenge landscape bluestone lithology: New petrographical and geochemical evidence". Bevins, Richard E. , Ixer, Rob A. , Webb, Peter C. , and Watson, John S., Journal of Archaeological Science, 2012, pp

“Craig Rhos-Y-Felin, Pont Saeson is the dominant source of the Stonehenge rhyolitic ‘debitage’”, by RA Ixer & RE Bevins, Archaeology in Wales 50 (2011), 21–31

“Stonehenge rhyolitic bluestone sources & the application of zircon chemistry as a new tool for provenancing rhyolitic lithics”, by RE Bevins, NJP Pearce, & RA Ixer, Journal of Archaeological Sciences 38 (2011), 605–22

“The petrography, affinity and provenance of lithics from the Cursus Field, Stonehenge”, by RA Ixer & RE Bevins, Wiltshire Archaeological & Natural History Magazine 103 (2010) 1–15

“The detailed petrography of six orthostats from the bluestone circle, Stonehenge”, by RA Ixer & RE Bevins, Wiltshire Archaeological & Natural History Magazine 104 (2010), 1–14

34 comments:

Anonymous said...

Thanks for posting this Brian.

One thing stands out for me about these 'rhyolites with fabric' in Dr Ixer's work. These came from the 'Stonehenge landscape' and not exclusively from Stonehenge proper. So my earlier question stands. What human purpose brought these fragments to the 'Stonehenge landscape'? Especially if these 'rhyolites with fabric' fragments cannot be sourced to any orthostats at Stonehenge or any other megaliths used in any monuments nearby?

Look for more made up stories from the archeologists explaining this!

Kostas

BRIAN JOHN said...

I don't think there is anything mysterious or sinister here -- have a look at this, from Mike Pitts's Digging Deeper blog:

Mike Pitts blog -- Digging Deeper

http://mikepitts.wordpress.com/

"As I was driving down thinking about it, it struck me that one of the really interesting aspect of this research is the fact that all the samples of rock matched to Pont Saeson come from chips and flakes (debitage), and not from megaliths. What does this mean?

One of the distinctive features of the rhyolitic rocks is that they are flinty – they have a good conchoidal fracture. That makes them relatively easy to break up, if they are standing as monoliths at Stonehenge. But it also makes them pretty good for making tools, or portable artefacts of some kind. There are plenty of flaked bluestone “tools” in museum collections from Stonehenge (some of them from my own dig, as illustrated above, from my PPS report). Which of these are made from debris created when stones were dressed on site? Which are made from broken up megaliths? And which were made in Wales and brought to Stonehenge by people visiting, perhaps on a pilgrimage of some kind? Clearly the distinction has important implications for how we understand Stonehenge.

These are questions that future research can answer, through excavation in Wales and at Stonehenge and study of the debris – that we can do this is a reflection of the quality and utility of the new research. Ixer and Bevins identified five groups of rock amongst the rhyolitic pieces they studied, of which three (by far the bulk of all they saw) they have matched to the Pont Saeson outcrops. There is one buried stump at Stonehenge (stone 32e) that they say could well be from Pont Saeson (to be confirmed), but the four standing rhyolitic stones are different. One of the latter (stone 48) belongs to one of the two very rare classes that Ixer and Bevins identified, which have yet to be matched to a source. One way excavation at Stonehenge would help us, is in allowing modern identification of the stumps and other bits of megaliths at the site."

Hope this helps.

Anonymous said...

Brian,

Quoting from your Mike Pitts quote,

“... it struck me that one of the really interesting aspect of this research is the fact that all the samples of rock matched to Pont Saeson come from chips and flakes (debitage), and not from megaliths...”


And also,

“That makes them relatively easy to break up, if they are standing as monoliths at Stonehenge. But it also makes them pretty good for making tools, or portable artefacts of some kind.”


From earlier comments you made Brian, you don't think a flaky easy to break rock type is very suitable for tools or orthostats. I agree with that! In my previous post, I warned of the new stories and explanations put forth by archeologists to justify these curious findings. I wasn't off the mark, was I! Now we are told the Stonehenge landscape was a prehistoric 'stone tool factory'. But why bring fragments from Wales to make into tools? Why not make them on site in Wales?

Looking at the geological map in your post of North Pembrokeshire, it does appear there was considerable land erosion at Mynydd Preseli and southwards to Bristol Channel through what appears to be a narrow valley between two hills with dolerite outcrops.

If so, my earlier comments made months ago stand. Is it possible Craig Rhosynfelin was part of a higher ridge level with the hill top and was eroded down to present size? If so, and if the foliated rhyolite fragments came (by ice or meltwater) from the top part of the ridge, then this would belie the 'bluestone quarry' idea at Craig Rhosyfelin.

Kostas

BRIAN JOHN said...

I agree, Kostas, that there is room for debate here. Mike Pitts is, in my view, a little premature in assuming that these foliated rhyolites would make good stone tools. They look pretty flaky to me, and liable to break along flattish planes. I have always thought of these stons as "rubbish stones" as far as megalithic monuments are concerned -- and I suppose they would also be rubbish for tool making. Maybe the geologists can give us some guidance on this point?

BRIAN JOHN said...

As far as the evolution of the landscape is concerned, the evidence from all over Pembrokeshire is that the land surfaces as we see them today are pretty old. In cliff sections we can see old shattered rock surfaces underlying Devensian till, with slope deposit accumulations between rock and till. That argues for a landscape in the Devensian which was not much different from that of today -- Craig Rhosyfelin has been where it is today, in relative terms, for tens if not hundreds of thousands of years.

Anonymous said...

Brian,

Are you and I the only ones troubled by the significance attached to these foliated rhyolite fragments traced to the Rhosyfelin outcrop?

Here we have stone fragments of a type of stone unsuitable as orthostat, not traced to any known orthostats at Stonehenge and found in several locations in the Stonehenge landscape. Yet we are led to believe the provenance of such fragments to the Rhosyfelin outcrop proves these stones were quarried by prehistoric people and carried to Stonehenge to built Stonehenge!

No matter how this is spinned it just does not make sense human agency is responsible for any of this.

Kostas

Tony H said...

Mike Pitts used to be the Curator of Avebury Museum. You would have thought he would know whether 'rhyolites with fabric' or 'foliated rhyolites' are capable of making decent tools or not, or am I missing the point?

Anonymous said...

Tony H

Good point! Mike Pitts can perhaps show us some decent tools made from 'foliated rhyolites'.

Even so, it still does not answer the question why bring the stones to Stonehenge to make tools? Why not just make such tools where the stones are?

Such ''human agency' explanation for the presence of these foliated rhyolites in the Stonehenge landscape is a far fetch in my view!

Kostas

BRIAN JOHN said...

Kostas -- this has been a point debated for many years. Aubrey Burl (amongst others) argued that if you were going to break up or shape bluestones in any way, it would make more sense to do whatever you had to do at the collection point rather than at the delivery point -- economy of effort and all that. The same would hold true for the manufacturing of implements -- where there is an incredible proportion of waste for every useful artifact you create.

The only 2 situations where I see the cost / benefit ratio changing are as follows:

1. If you found all the bluestones more or less on the spot, at Stonehenge, you could do whatever you liked with them -- use some maybe as orthostats and maybe use other stones (maybe the ones that were the wrong shape, or were crumbly or broken to start with) for making tools.

2. If you used the stones in stone settings, ran out of stones or ran out of patience, and decided "Oh, to hell with it! We can't be bothered with this any more. But now that we have the stones here anyway, we might as well use them for making tools....."

Anonymous said...

Brian,

Points well taken. We agree there just is no sensible reason for prehistoric people to bring these stones to Stonehenge. That leaves Nature doing the moving. In some manner yet to be fully understood.

But if these foliated rhyolite fragments are found only in certain areas in the Stonehenge landscape (like the Stonehenge Layer, the Avenue and the Cursus) that raises questions about glacier transport being so specific.

Kostas

chris johnson said...

In my area the vast majority of stone tools (flint) were made on the spot from material the hunter/gatherers had brought with them. We can be very sure of this because there is no natural flint within 30 miles as far as we know.

I think it would be rather impractical to make a collection of stone tools in advance and expect them to survive the trip. Much more practical to make what you needed when you needed it.

As for "decent" tools we should be careful of applying modern notions. For every carefully crafted arrowhead there are thousands of scrapers, cutting knives, and borers which seem to have made quickly and discarded casually. They litter the camping sites.

Stimulated by these discussions I need to do some more fieldwork in the Spring. While there is hardly any stone of any type here, there are a few big erratics. I had not noticed before joining this blog, but none of them seem to have been used for making tools. I wonder why? Was there something taboo about breaking up an erratic....

BRIAN JOHN said...

I assume that flint nodules would have been traded items -- but with respect to erratics and large blocks of stone in situ, the cost / benefit principle kicks in. Not worth the effort of carrying the large stone to where you want to use it -- better to travel to the stone and do whatever it is there.... except, of course, if it is being protected by some fierce warriors and you are desperate to have it because it is deemed to be magic. (Now we are straying into fantasy, and the scenarios proposed by Profs GW, TD and MPP.....!!)

chris johnson said...

I agree. I don't see any point to carrying large stones for use in tool making. It would have been nodules or smaller chunks that could be put into a leather bag or somesuch.

Another thing, I said in an earlier thread that taking stone to Stonehenge for casual tool making seems like carrying coals to Newcastle. My presumption is that flint nodules are readily available there, and even had flint been traded the price would have been low. How true is this?

Then again, why use Bluestone for tool making when flint is available..

Anonymous said...

Chris you write,

“why use Bluestone for tool making when flint is available..”


Good question. Let's wait to hear from the experts!

Kostas

Percy Vere said...

Kostas,

“why use Bluestone for tool making when flint is available..”

"Good question. Let's wait to hear from the experts!"

Similarly,
Why use gold for wrist-watches when plastic is available.

Anonymous said...

Percy Vere,

So…, bluestones make beautiful wrist-watches! Or is it that wrist-watches make good hammers?

Thank you for providing us such rare insight into the primitive mind!

Are you an expert?

Kostas

BRIAN JOHN said...

Let's not generalise about bluestone, because there are so many different types. With regard to foliated rhyolite -- the real question is: "Was foliated rhyolite easier to work, and did it make better tools, than flint?" If not, do we have any evidence of it being used for ceremonial or ornamental objects?

BRIAN JOHN said...

Kostas -- your point is absurd. Percy was asking a perfectly valid question.

Anonymous said...

Brian,

I suggest you hold judgment on my comment until you answer your own question.

“Does foliated rhyolite make for better tools? Was foliated rhyolite used for ornamental objects? Or is foliated rhyolite 'rubbish stone', as you have often claimed in the past?”

Of course you can always make the argument that one man's rubbish is another man's ornament and call it art! Archeologists will be very pleased with you!

Kostas

Percy Vere said...

Dear Brian,
Thanks for the support, I was simply showing that when making artifacts we sometimes use gold purely for decorative purposes, even when cheaper or harder-wearing materials are available; materials that would do the job equally as well.

My primitive mind obviously functions on a much lower, more easily understood, level than that of the Kostas; a level at which you need not be an expert, the only requirement being a little common sense.

I suggest that 'bluestone' was used for appearance, while flint would have been used for its strength and cutting abilities.

BRIAN JOHN said...

I'm asking the archaeologists. Percy's question is perfectly valid -- I think I'm right in saying that in the Bronze Age many axes were made out of materials which were quite useless in a utilitarian sense, and it is reasonable to conclude that they were ceremonial or ornamental -- or maybe created just as showpieces, to enable axe makers to show off their skill. These items may have been high-value, high-status objects. The question is this -- were such things also going on at the time when Stonehenge was built? I assume so....

BRIAN JOHN said...

Agree. Some of the "preselite" axes in the museums look pretty useless to me for cutting or hacking anything........

Anonymous said...

Percy Vere,

Sorry if you misunderstood. My reference to 'primitive mind' was not you. But the thinking of primitive people you say would use bluestones for ornamental purposes. I value 'common sense' and often praise it in my comments.

New wrinkles in the 'primitive mind' introduced 'after the fact' of an unexpected finding are not convincing. A little like a new twist in a story told by someone caught in a lie.

Kostas

Percy Vere said...

Kostas,
I must confess to not fully following your reasoning. Many examples of grave goods have no purpose other than ornamental or decorative.
Some of these items must have involved long distance travel, such as amber beads from the Baltic and jet, a hard black variety of lignite from Whitby on the North Yorkshire coast of England, found in Bronze Age tombs.
The grave goods of the 'Amesbury Archer' included two gold hair trusses, and he is thought to have travelled from Switzerland. Neither amber nor jet would be much use as tools and the use of gold hair tresses is self evident.
The flint used to construct the mace-head discovered in the Knowth passage-mound, Boyne Valley, Ireland, came from the Orkney Islands and it is highly doubtful if the mace-head was made for any purpose other than ornamental.
Why should the same not apply to bluestones and rhyolites?

Perhaps Brian can confirm that in Ixer/Bevins 2011, the description of the 'A' class rhyolitic rocks from the Stonehenge area uses the term "flinty rhyolite", which perhaps indicates a 'tool' application?

Anonymous said...

Percy Vere,

We're confused talking about Stonehenge when we're not talking about Stonehenge but something else. Your last post is an example of this.

The foliated rhyolite fragments found at Stonehenge were initially assumed to be 'debitage' from the dressing of some orthostats at Stonehenge. When none of the known orthostats matched with any of the fragments, the story changed to ...

1) some orthostats disintegrated into a pile of foliated rhyolite fragments.
2) Stonehenge culture experienced a 'stone destruction stage' breaking up orthostats into fragments.
3) The foliated rhyolite fragments were brought to Stonehenge from Preseli by pilgrims to Preseli.
4) The foliated rhyolites have healing powers and fragments were used to make people well in a Neolithic SPA at Stonehenge.
5) The foliated rhyolites were brought as 'raw material' to 'tool factories' at Stonehenge.
6) The foliated rhyolites make beautiful ornaments and ceremonial artifacts used by the priestly/ruling class to display their wealth and promote their power.

I can go on! But these are enough to make my point. All these are made up stories! There are no historical records to support any of these scenarios.

The only 'common sense' position consistent with what is known is that natural processes, possibly glaciers or meltwater streams, brought these foliated rhyolite fragments (what Brian has in the past described as 'rubbish stones') to the Stonehenge landscape.

If these were found in any other place in the world except Stonehenge that would have been a forgone conclusion!

Kostas

chris johnson said...

Brian, as an experienced bender down and picker up of odd stones I have yet to find one on Prescelly that has obvious signs of having being used as a tool. You have huge experience in this geography and I wonder whether you have had more luck?

BRIAN JOHN said...

No Chris -- in all my years of looking at the ground and gathering up interesting stones, I have never found one that I would consider to be a hammerstone, or an unfinished axe, or any other sort of artefact. There HAVE been finds -- the Museum collections are proof of that -- but stones shaped by the hand of man are indeed very rare -- even in the eastern part of Preseli!

chris johnson said...

Brian, thanks for responding.

I have assumed that Dyfed is eroding fast - a bit like the Rocky Mountains as you said recently. Could it be that these signs exist but are now buried deeply? This has been my feeling, as the evidence of the stone circles like Gors Fawr and others seem to show considerable activity in the neolithic.

Anonymous said...

Chris you write,

“ I have assumed that Dyfed is eroding fast”

Interesting! What do you mean?

Kostas

BRIAN JOHN said...

Most of the Pembrokeshire landscape is fundamentally unchanged over hundreds of thousands of years, except around the coast, and in North Pembrokeshire where glacial activity of several types has made quite a profound impact. All of this in spite of what Kostas may fervently desire....

Percy said...

Kostas,
When you say in response:
"The foliated rhyolite fragments found at Stonehenge were initially assumed to be 'debitage' from the dressing of some orthostats at Stonehenge. When none of the known orthostats matched with any of the fragments, the story changed to ..." etc.

Are you sure that every bluestone at Stonehenge has been sampled and petrographically analysed, because if they have not then the above paragraph must invite the question --- Can you say that the fragments did not originally come from the possible dressing/destruction of the now absent bluestones?
The second question is ---- "Why am I wasting my time"?

Brian, he's all yours.

Anonymous said...

Brian you write,
“in North Pembrokeshire where glacial activity of several types has made quite a profound impact”

Isn't this the general area where the bluestones came from?

Hmm … “what Kostas may fervently desire.... “ ?

Truth is ice cold, Brian!

Kostas

BRIAN JOHN said...

No, Kostas. Please pay more attention. See this post:

http://brian-mountainman.blogspot.com/2011/11/glacial-lake-brynberian.html

The biggest landscape changes are in the north, between Cardigan and Newport, where there are substantial quantities of sands and gravels dumped by the retreating Irish Sea Glacier in the Devensian, and south of Fishguard, where there are very dramatic meltwater channels probably cut during several glacial episodes. Elsewhere, landscape changes have been very subtle.

Anonymous said...

Percy,

The statement,“none of the known orthostats matched with any of the fragments” stands. The emphasis is on 'known'. The question some have is whether the buried stump may provide a match.

Perhaps.Is anybody checking? And shouldn't they have checked before releasing any scientific papers and making statements to the media? The fact that these foliated rhyolite fragments were sourced to the Rhosyfelin outcrop pales in comparison!

But this still does not explain why such foliated rhyolite fragments are also found at the Avenue by the Heel Stone and at the Cursus and perhaps at other locations? More buried stumps matching there too?

As to whether these rhyolite fragments formed “from the possible dressing/destruction of the now absent bluestones” that would be 1) and 2) in my list of 'made up stories' in my previous post.

Why are you wasting your time? Why am I wasting my time. Because we want to know the truth of Stonehenge!

Kostas