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Tuesday 20 June 2017

The Lower Palaeozoic sandstones at Stonehenge

 Lump of Lower Palaeozoic sandstone found during the Darvill / Wainwright excavations at Stonehenge in 2008.  This is the biggest sandstone fragment found thus far -- 
it weighs more than 8500 g.

This paper has been floating around for a long time, and is finally published.  Details:


"The petrography, geological age and distribution of the Lower Palaeozoic Sandstone debitage from
the Stonehenge Landscape."
by Rob Ixer, Peter Turner, Stewart Molyneux, and Richard Bevins.

Wiltshire Archaeological & Natural History Magazine, vol. 110 (2017), pp. 1–16


The three major groups of debitage found in the Stonehenge Landscape are dolerites, rhyolitic tuffs (almost exclusively from Craig Rhosyfelin, now designated as Rhyolite Group A–C) and ‘volcanics with sub-planar texture’ now designated as Volcanic Group A and Volcanic Group B. The only other significant debitage group, but only accounting for about 5% by number, is an indurated sandstone now called the Lower Palaeozoic Sandstone.
The Lower Palaeozoic Sandstone is a coherent lithological group with a slight metamorphic fabric and is a fine-grained feldspathic sandstone with characteristic dark, mudstone intraclasts. Palynological (acritarch) dating of the sandstone suggests that it is Late Ordovician or younger whilst the petrography suggests that it is older and more deformed than the Devonian (ORS) sandstones exposed in South Wales.
Spatially, as with all the major debitage groups, the Lower Palaeozoic Sandstone is widely and randomly distributed throughout the Stonehenge Landscape; temporally, almost none of the debitage has a secure Neolithic context but some may have later Roman associations. The debitage cannot be matched to any above-ground Stonehenge orthostat but may be from one or two buried and, as yet, unsampled stumps. (These are stumps 40g and 42c)
The lithology is believed to be from an unrecognised Ordovician (or less likely Silurian) source to the north or northeast of the Preseli Hills.
Although there has been confusion within the archaeological literature between the ‘Devonian’ Altar Stone, Lower Old Red Sandstone (Devonian) Cosheston Group sandstone and the Lower Palaeozoic Sandstone, all three are very different lithologies with separate geographical origins.


 This is a detailed and useful paper which carries forward the work on the Altar Stone (by Ixer and Turner in 2006) and examines it in the context of other sandstone fragments in the Stonehenge collections.  It is still the case that the Altar Stone is inadequately sampled and hence inadequately understood;  but the balance of probabilities at the moment is that it came from the Senni Beds near Laugharne and Llansteffan rather than from the Cosheston Sandstone of Mill Bay on Milford Haven.  The authors here seek to sort out some of the confusion of earlier researchers who have often assumed that assorted sandstone fragments (and those two stumps 40g and 42c) are related to the Altar Stone and therefore have the same provenance.  Rob Ixer and his colleagues demonstrate that nearly all of the "sandstone debitage" in the Stonehenge area is of Lower Palaeozoic age, not Devonian.  The use of palynology as well as detailed petrography (mostly thin section work) showing a "slight metamorphic fabric" is fascinating and quite convincing.

The authors argue that the samples examined from the Stonehenge collections have come from the northern or north-eastern corner of Pembrokeshire, to the north of the Preseli Hills.  The samples are not all identical, and seem to show two sandstone types.  Quote:  Although this might suggest two separate sandstone sources, the petrography of the two lithic samples (and indeed all of the other debitage samples) suggests that they are part of a single sourced lithology and the apparent age discrepancy is a sampling issue. It is hoped that further sampling of the Lower Palaeozoic sandstone will help to determine this.

I'm not sure what is meant here -- if the samples have come from "a single sourced lithology" they could of course have come from widely separated localities or provenances wherever that lithology outcrops -- and how can the "apparent age discrepancy" be a sampling issue?  If there is an age difference between one sample and another, does that not mean that one sample is older than the other, given that there are always some statistical / confidence issues?  Whatever the truth of the matter, it is clear that the debitage needs to be matched with sandstone stumps 40g and 42c -- if those two stones really have provided the debitage fragments, more progress will have been made.  And if they don't match, things get even more interesting.........

At the end of the article:
Wainwright et al. at the conclusion of the Cleal et al. (1995) monograph stated in their ‘future
directions’ for research at Stonehenge that ‘A detailed petrological description of each stone
and the identification of its source’ was needed (Cleal et al. 1995, 492). This paper completes the
initial petrographical descriptions of the major types of non-dolerite bluestone debitage from the
Stonehenge landscape begun in 2006. Now that there is enough petrography to enable a convincing match between buried Stonehenge orthostats, their surface debitage and their potential geographical sources to be realised, it only needs the buried stumps to be sampled.

I think we would all agree with that.......


I'm intrigued by the mention of this unpublished paper, which precedes a lot of the detailed geological work undertake by Ixer and Bevins.  Given that it is now very easy to upload previously unpublished papers as PDF files on ResearchGate, might that not be a good option for Rob to pursue?  Better for it to be read than cited when nobody can see it!

IXER, R.A. 2007. Waiting by the river: Stonehenge and the Severn Estuary. Abstract. Stone artefacts as material and symbolic markers in cultural landscapes. An international perspective. Implement Petrology Group Meeting. York. September 2007

Citing things that are inaccessible is to take the route to notoriety........


Myris of Alexandria said...

The paper is far weightier than any 'floating' iceberg.

'Waiting by the River' is an abstract that I think can be accessed. Dr Ixer does not appear to have a copy anywhere but much of the point (and it was written in the dark, pre-Pet Rock Boys, ages) will be restated in the up-coming 'round up paper' (not the Antiquity paper THAT will be far more fun).
Incidentally there have been a few bodies floating past.
I do recommend that you read the Comrade Inspector Chen Cao novels but you need a strong stomach and a soothing bowl of Dragon Well Tea (see prices of said tea on Amazon!!!!)Do, do read them ALL.
You stagger along the fine line between disingenuousness and determined ignorance.
The apparent age differences are not because they are two different rocks with separate ages but purely a sampling issue (three more samples have been submitted for dating. The rocks are the same- and the age 'difference' probably is the related to the amount of mudstone clasts. This is mad clear in much detail and I am certain you fully understood that.
You know as well as anyone that age dates will range and one takes the optimum.

Were Dr Ixer to find the abstract I am certain that self publicist that he is, he will display it. Its main use now is to show how long he has dismissed the floating raft idea.

Don't be tiresome there are people who take this academic fun at apparent face value, stop short changing them.

DO SOOOOOOO agree about the call to resample buried orthostats. Obvious but .....

BRIAN JOHN said...

Here we are, giving you a glowing plug, Myris, and all you do is complain. Maybe I should just ignore your new papers in the future......

Anyway, for as long as I discuss the things that are in the literature, I will apply the same principles as I would if I was reviewing something as a referee -- that means careful scrutiny, and it means asking questions where things are not adequately explained, and flagging up things that are in my view not soundly supported by the evidence cited.

Who's talking about rafts and icebergs? Not me.....