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Saturday, 7 July 2012

Rob Ixer: Rock Provenancing in Wales

Thanks to Rob for reminding us of this article published in 2003 (I think) -- and dealing with some of the problems and opportunities of geological provenancing of stones in Wales and indeed beyond the Welsh borders. 

Foundered or founded on rock- a future for Welsh Provenance Studies 

R.A.Ixer
School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham, B15 2TT

http://goodprovenance.com/provenancing.htm


Here is a brief extract:

Altered dolerites are common and widely distributed in Wales with large outcrops, but many have sufficiently distinctive petrographical and geochemical properties that they can be distinguished from each other. The best-known example of Welsh lithic provenancing is of dolerites, namely the Preseli Spotted Dolerites that comprise many of the Stonehenge Bluestones. By using some petrography and trace element geochemistry, including extending an existing geochemical database, Thorpe et al (1991) were able to match (provenance) individual bluestones to a limited number of outcrops at Preseli. Later, Ixer (1996; 1997), using very detailed petrography, in both transmitted and reflected light, moderated by the geochemical work of Thorpe et al (1991), felt he was able to refine further the match, so identifying individual bluestones with specific outcrops. In both cases the workers were able to establish the geological provenance of the stones, namely where the rocks were formed. Provenancing in this example was straightforward but time consuming and quite costly, in the region of high hundreds to low thousands of pounds. The majority of the costs were spent in extending the geological database into a geoarchaelogical one dedicated to the provenancing problem.
 

Somewhat controversially Thorpe et al (1991) argued that the Stonehenge monolithics were exploited from glacial erratics on Salisbury Plain rather than taken directly from their Preseli outcrops, so suggesting that for the bluestones their archaeological and geological provenances are different. This distinction between the two sorts of provenance is as important as it is contentious and the current inability to be able to recognise the difference is a serious problem in lithic studies. In the case of the exploitation of primary, in situ raw materials the geological and archaeological provenances are identical to each other and to the geographical location of the resource. However, for naturally transported materials (secondary sources), be they gold grains from a gold placer deposit, flint and chert from Recent river or marine gravels, or lithics from glacially transported boulder clay the geological and archaeological sources have become separated. The archaeological provenance (the site of exploitation) has been moved from the geological provenance (original outcrop), sometimes, as in the case of gold grains, huge distances. It is probable that the significance of secondary sources has been undervalued in provenance studies, for example the role of glacial erratics as the raw materials for polished stone axes.
 

Although Briggs (1989) and more recently Williams-Thorpe et al (1999a) have argued that erratics could be a viable source for some polished stone axes most workers are happier with the concept of dissemination of axes from discrete factory sites.  

The full list of references can be found on the web page.  There's a full list of Rob's publications here:

http://www.rosiehardman.com/publications.htm

5 comments:

chris johnson said...

I recommend people read the article in its entirety.

The more I learn about this broad subject the less inclined I am to believe that stone-age people broke-up erratic boulders casually. I rather think they had a special meaning in those days - dropped from the stars, thrown by the gods, or whatever. For those with a folk memory extending back an unlikely length of time they might even have been gifted by the glaciers.

I don't know whether anybody studied this systematically? In my area there is very little stone of any kind, yet there are lots of neolithic and mesolithic remains. The few genuine erratics that have been reported are untouched as far as I can see and have been told - although I will soon talk to our archaeological society about looking for cup-marks. The many stone tools (largely flint) seems to have been imported from a considerable distance.(>50kms).

Stone axes which seem to have had a non-functional purpose are a special case. Definitely not tools with a day-to-day purpose and they may indeed have been crafted from special rocks although it would have taken a powerful shaman to pull it off, I believe.

Like Brian I feel a story coming on. Powerful shamans at Stonehenge boldly destroying the erratics as the milky way slowly disappeared below the horizon

Lots to discover, and I hope Dr Ixer gets his wish for a central database before it is too late.

geocur said...

Chris ,I believe you live in Holland ,if so , there are very few examples of rock art and the only ones that come to mind are on the capstones or orthostats of hunebedden .

chris johnson said...

Thanks Geo,
I was not aware of rock art on the hunebedden. It is a few hundred kms from here and a while since I visited. Perhaps I can persuade my wife to go back - do you know which?

Here in E.Eindhoven it is mostly sand. Very rare erratics but otherwise no rock I am aware of. Should you ever drive through you may see lots of boulders but almost all have been imported in recent memory by international truck drivers. Now our favorite local industry (trucking) has moved to Poland. Maybe Polish villages are now filling up with erratics.

I think there are less than 5 genuine erratics in the wider area and despite the lack of stone they have been left untouched I think - still I am not an expert and need to talk to more people now I know my question better. I never found a tool locally that was not flint and imported likely from Maastricht area by hunter gatherers on seasonal expeditions.

Anonymous said...

Would Dr Ixer or his side kick Myris be kind enough to answer the following query?

Is it possible to determine visually (Hand lens/ Microscopy) whether a clast of rhyolitic vitric tuff originated in the Llyn /Snowdonia area or South West Wales/Pembrokeshire??

Or would a detailed geochemical analysis be required??

Regards
AMG

Geocur said...

Previous message lost .
Chris , one is D12 near Eext on a capstone . D11-14 are all within two miles .