The Newall Boulder scatter plots etc

 

The Newall Boulder (BGS photo), bizarrely referred to as a "broken joint block" by Ixer et al.

With relation to the "Victorian Gifts" article by Ixer et al, which I discussed in an earlier post, I have been delving a bit deeper into the things they say about the Newall Boulder.

https://brian-mountainman.blogspot.com/2022/09/the-newall-boulder-with-thee-samples.html

https://the-past.com/feature/victorian-gifts-new-insights-into-the-stonehenge-bluestones/

I have been consulting with various geology professionals, and the main points raised are as follows:

1. The persistent description of the boulder as a "broken joint block" appears to be an attempt to promote the idea that it is a lump of quarried stone.  It is heavily weathered, faceted and abraded, and there is no way that it can be interpreted as a product of bedrock quarrying operations.

2.  There is a claim that the petrography of the boulder samples shows that it is derived from a supposed "quarry" at Rhosyfelin:  "We have recently reunited and examined the joint block and all its offcuts and associated thin sections, and the rhyolitic tuff shows all the key characteristics needed to assign it to Rhyolite Group C from Craig Rhos-y-Felin in north Pembrokeshire." But no detailed petrographic evidence is provided in support of this claim, and it cannot therefore to taken seriously.

3.  With regard to the OU sample analysis, the authors say:  "....geochemical data obtained from sample RSN18 ENQ 2305 by the Open University team in the early 1990s showed that it (alongside others of their debitage samples) belonged to Rhyolite Group C."   But without any reference to the extent of the terrain across which Rhyolite Group C rocks are outcropping, this provides no support for the contention that the sample came from a quarry at Craig Rhosyfelin.

4.  Here is another quotation:  ".....very recent analysis by pXRF on all pieces of the joint block plus two other visually similar rhyolitic tuffs from the same Newell collection (RSN9 ENQ 2295 and RSN10 ENQ 2296) clearly show that these fragments are compositionally Rhyolite Group C, confirming the petrographic identifications."  The geologists point out that scatter plots of this type are notoriously difficult to interpret in the absence of raw data.  On each published plot there are well over a hundred plotted points.   They supposedly show "analyses from Craig Rhosyfelin" -- but are they all from instrument readings actually obtained at Rhosyfelin, or do they include analyses of samples collected at Stonehenge and assumed to have come from Rhosyfelin?  The sheer quantity of points plotted suggests the latter.

5.  How large is the area across which the Rhosyfelin readings were taken?  If the area was 2 sq km, for example, then any sample falling within the "plotted field" cannot automatically be assumed to have come from the proposed quarrying site.

6.   Two of the geologists suggest that rhyolites and other volcanic rocks are very difficult to provenance, even if they have come from a single magma source.  They mention welding, fragmentation, incorporation of clasts and recrystallisation as factors. (Visually, the blue-black Newall Boulder rock looks very different from the light blue Rhosyfelin rhyolite.)  The authors of the "Current Archeology" article have not demonstrated satisfactorily that the Newall Boulder cannot have come from Pencaer or elsewhere on the North Pembrokeshire coast. Nor have they demonstrated that volcanics from other areas would not have plotted in similar positions on the diagrams.

7.  One correspondent who has worked with pXRF instruments suggests that in every study there are good readings and bad ones -- the latter caused by instrumental errors and problems with rock surface conditions (such as debris or wetness).  He would like an assurance that all "dubious" readings have been eliminated rather than incorporating them in the two published scatter plots.

8. With regard to the image of STILPNOMELANE used in the article, one correspondent asked whether it is a stock image, an image from a Rhosyfelin sample, or an image from a Newall Boulder sample.  The caption is so vague that the image adds nothing to the scientific argument.

It may well be that if and when a full study is published in a scientific journal, all of these points will be addressed.  Until then, the authors of the "Current Archaeology" article will have to accept that the jury is still out on the matter of what the boulder is made of, and where it came from.