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