Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Saturday 7 August 2021

Stonehenge sarsens from here, there and everywhere?

Sarsen stones in West Woods.  Much speculation, and not enough geology......

Yet another paper from the incorrigible geologists Ixer and Bevins -- this time on the Stonehenge sarsens.  David Nash will not be best pleased, because it suggests that matters at West Woods are rather more complex than previously suggested, and that the sarsen debris at Stonehenge might well have come from all over the place.  At last, and after many pleas from me (and I dare say others as well), the hammerstones and knockoffs are being examined, and are contributing something to our knowledge of the old ruin.

Petrography of sarsen debitage from the Stonehenge Landscape – a broad and perhaps scattered church by Rob A. Ixer and Richard E. Bevins
Wiltshire Arch & bNat Hist Mag, vol 114, 2021, pp 18-33

The petrography of a small selection of sarsen debris from the Stonehenge Landscape, the core of upright 58 from the central trilithon within the circle and sarsens from West Woods is described. Most conform with the few earlier descriptions of Stonehenge sarsens and include both ‘saccharoid(al)’ and ‘hard/quartzite’ examples but extend the range of lithologies to include puddingstone, heavily ferruginised and silicified material. Lithologically there is nothing special about them; their essential mineralogy and textures are typical of sarsens from southern England.

Although recent provenance studies of the saccharoid/monumental sarsens have suggested their exact provenances to West Woods there has been no equivalent work or suggestion for the hard sarsen hammerstone/maul cobbles that occur within the Stonehenge Landscape. Their reported detailed petrography, here, is the beginning of that work.



"The number of samples in this study is small and (other than sarsen SH58) it is impossible to say how representative they are of the sarsen debris found in the Stonehenge Landscape. However, collectively they indicate that sarsens associated with Stonehenge show a wider range of fabrics than previously described, albeit within an overall limited sandstone lithological repertoire. The work clearly indicates that further petrography on other samples is needed."

Several samples of sarsen stone are examined, including one from Sarsen 58 (this was the stone intensively studied in the recent paper by Nash et al).  They included samples from the Heelstone, Aubrey Holes, Amesbury and Fargo Plantation -- and they were found to conform to two main and well-known sarsen classes -- saccharoidal and hard.  Quote:  The long-recognised two main classes of sarsen at Stonehenge, namely saccharoidal (the orthostats) and hard (mauls/hammerstones), may have different (or even multiple) geographical origins.  In other words, they have probably not all come from West Woods...........  Furthermore, because they are not unusual, they could have come from many different locations.  This, of course, argues against the very tight provenancing apromoted very forcefully by David Nash.
One of the interesting features of this study is the mention of two samples (Sarsen 1A and 1B) from West Woods, collected by Tim Daw and now subjected to analysis.  After detailed analysis, one is labelled as hard / floating sarsen and the other as "a very silicified saccharoidal sarsen."  Quote:  "The variability in petrography within such a small and randomly selected sample gives much credence to Howard’s suggestion that the Stonehenge sarsens can only be properly studied and their provenance(s) suggested after a large number of samples have been rigorously examined. This is echoed by the marked petrographical variability between the two sarsens from West Woods, albeit collected half a kilometre apart from each other and from Nash and his team’s sampling site (1A being a typical ‘hard’ floating sarsen whilst 1B is an extremely silicified saccharoidal sarsen, an excellent orthoquartzite), suggesting that multiple ‘in situ’ sampling of sarsens from potential collection sites would be needed before any strong petrographical match can be attempted."

Quote:  "Although recent geochemical work on the monumental saccharoidal sarsens from Stonehenge has suggested that the boulders came from West Woods (Nash et al. 2020), the origin of the hard sarsen Stonehenge cobbles is still totally unknown and requires investigating. They may be unsampled material from the grey wether sites investigated by Nash et al. (2020) as their sampling was restricted to large saccharoidal boulders similar to those from Stonehenge. This possibility is tentatively suggested by the 0.51% titania in one of the sarsens from Monkton Down (ibid.) and the petrography of one of the West Wood samples. Or they may have an unknown geographical origin or multiple origins, away from the large stones; an origin including those close to the circle since ‘smaller sized sarsen cobbles are widespread across the area local to Stonehenge’ (Simon Banton pers. comm. 2020)."

This all throws the provenancing of the Stonehenge sarsens and the sarsen cobbles / hammerstones / packing stones wide open, and suggests that Nash et al need much more sampling before they can feel at all confident that their enthusiastic conclusions from last year are correct.  West Woods may be a lovely spot, but until the sarsen geology of that particular location is better understood, based on multiple sampling points, nobody can be confident that most of the Stonehenge sarsens really did come from there.
I agree with Howard that what we want is "a large number of samples, rigorously examined."

This is what we argued about a year ago, in more than 50 comments!


David Nash said...

Thank you for your concern about my well-being, Brian. Rest assured I am sleeping well in light of Rob Ixer’s latest paper. He was kind enough to let me see the manuscript before it was published. Rob has done a very good job describing the petrography of a number of samples of sarsen, bringing together hard and saccharoid sarsens from disparate contexts. What his study shows is that the petrography of sarsens in the Stonehenge landscape is variable. We already suspected this but seeing that variability published is a positive step forward. He also shows that the petrography of sarsens in West Woods is likely to be variable and that there are both hard and saccharoid sarsens present there. This is also good news. It is – to my knowledge – the first study in the UK to positively identify both types of sarsen in the same local area (something well-documented for silcretes in other parts of the world) and might finally put to rest the long-standing (and baseless) idea that hard sarsens and saccharoid sarsens have to come from different places.

What Rob’s work doesn’t do, however, is throw “the provenancing of the Stonehenge sarsens and the sarsen cobbles / hammerstones / packing stones wide open”. Our work last year focussed on the immobile trace element chemistry of saccharoid sarsens. As you will recall, it considered the chemistry of the host sands within which the sarsen formed. It did not discuss the petrography of the sarsen, i.e. the organisation of minerals and types of cement within the stone. This was intentional. The petrography of a sarsen is shaped by the post-depositional history of the sediment, and can vary within an area and even within an individual boulder. As such, it is perfectly possible for two sarsen stones developed in the same sediment in the same place to exhibit the same immobile trace element chemistry but appear different under the microscope because their cements are slightly different.

Personally, I would not use qualitative petrography alone to provenance a sarsen (or any other rock type for that matter). You need to remember that what you see under a microscope is affected by the very limited 2D area intersected by the thin-section. This becomes even more problematic if the petrography of the rock in question is spatially variable. This is what Rob is inferring when he states “multiple ‘in situ’ sampling of sarsens from potential collection sites would be needed before any strong petrographical match can be attempted”. Rather, I would look at properties of the rock that are unlikely to be changed as a result of post-depositional diagenesis or affected by the vagaries of where a thin-section is cut – hence our focus on immobile trace elements from crushed samples reflecting a volume of stone. These properties can then be supplemented by other techniques as a second/third/fourth level check on the chemical data. The good news is that our latest paper in PLoS ONE equips us with a large dataset from Stone 58 that could be used to complement geochemical analyses and refine potential source areas.

Glad to see you like West Woods. I was up there only on Tuesday - beautiful place. If you're interested, I'd be more than happy to take you on a sarsen tour some time and view the variability of the remaining stones.

BRIAN JOHN said...

Good to hear from you, David. Look forward to seeing further studies. Sadly, trips to Salisbury Plain are off the agenda for the foreseeable future, but i will bear in mind your kind offer!