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Thursday 23 July 2020

Geological Notes on the Altar Stone paper

An EH photo of the surface of the Altar Stone, visible between fallen sarsens 156 and 55b (trilithon 55 is broken into two portions of almost equal size.)  There are some doubts about its dimensions -- most references quote approx 4.9m x 1.5m x .5m -- but one EH publication refers to it as being 3m wide, which means it should be referred to as a slab and not a pillar.

An extract from Anthony Johnson's excellent plan of the Stonehenge stones.  The Altar Stone is stone 80, with two sarsens lying across it.  

Geological Notes on the Altar Stone paper

It’s good to get some comments from another geologist who has actually studied the Altar Stone and the Devonian sandstones of West Wales. I’ve been in correspondence with Richard Thomas, who now lives in Newfoundland, for some years. Last time he visited Wales, he managed to fit in a visit to Rhosyfelin as reported here:

Key stratigraphic and lithological information is here:

This is the paper being discussed:

Constraining the provenance of the Stonehenge ‘Altar Stone’: Evidence from automated mineralogy and U–Pb zircon age dating.
Richard E. Bevins, Duncan Pirrie, Rob A. Ixer, Hugh O’Brien, Mike Parker Pearson, Matthew R. Power, Robin K. Shail
Journal of Archaeological Science 120 (2020), 105188

Here are some of Richard’s points:

I found the paper very interesting, and we should commend Rob Ixer and Richard Bevins for all the hard work they have put into the provenancing of the Altar Stone.

I have suggested to the authors that there are two fundamental provenancing issues:

(1) the epic task of unequivocally identifying the parent formation and source location of the Altar Stone (Stonehenge Stone 80);

(2) proving whether or not the sandstones comprising the Altar Stone and the so-called "Altar Stone debitage" are indeed one and the same -- i.e., were derived from a single, lithostratigraphic source unit. Neither issue can be completely resolved unless fresh samples are obtained for analysis from the Altar Stone itself.

I'm under the impression that no indisputably genuine thin sections of the Altar Stone are known to exist. As summarised by Ixer et al. (2019, p.1): "It is not certain that the Altar Stone has been directly sampled, certainly not in the last two centuries, and all samples labelled Altar Stone in museum collections appear to be from loose fragments/debitage and designated as 'Altar Stone' by 19th- and 20th-century excavators." However, in an earlier paper, Ixer and Turner (2006, pp. 2-3) state that: "...the thin section labelled '277 Altar Stone Stonehenge' is ....the only piece of the monolith available for investigation." With respect to issue (2), the various Altar Stone-related papers (including Bevins et al., 2020), contain references to the parentage of the samples described that I find somewhat confusing and, at times, contradictory. For example, on p.1: " .... mineralogical data from proposed Stonehenge Altar Stone debris..."; p.3: "..three debris samples, also thought to be derived from the Altar Stone."; p.3: "..the second type of sandstone, found as rare debris at Stonehenge as well as comprising the Altar Stone..."; p.3: "...six of which have been identified petrographically as being derived from the Altar Stone..."; p.5 (Table 2 caption): ".....samples analysed in this study from the Stonehenge Altar Stone.."; p.7: "...the samples interpreted as derived from the Altar Stone..."; p.7: "...the three Altar Stone samples..."; p.11 "...the so-called Altar Stone sandstone..."; p.12: " of the clay mineralogy of the Altar Stone sandstone...". In a similar vein, from Ixer and Bevins (2013, p.14): "...potential Altar Stone debitage ..."; "Three typical Altar Stone debitage lithics.."; " is now possible to provide a 'standard' petrographical description for the Altar Stone...”.

This seems to me to be a case of mixed messaging. Ixer and Bevins are trying to prove that sandstone samples such as SH 08, HM 13 and FN 573 are identical to, and/or derived from, the Altar Stone, but how is that possible without having an absolutely bona fide Altar Stone sample for comparison? To eliminate any ambiguity, I think all supposed Altar Stone sandstone samples should have qualifiers such as: 'suspected', 'potential', 'purported', 'possible', 'so-called', etc. added to their descriptions. Unless, of course, 277 is the real McCoy (see below).

One technical point: it's the Mill Bay Formation that is the bio- and lithostratigraphic equivalent of the Senni Formation of the Brecon Beacons area, and not the entire Cosheston Subgroup.

The key point of the paper: Ixer and Bevins have confirmed in the new paper that the analysed micaceous sandstones from the Stonehenge Landscape were not derived from the Mill Bay Fm. section at Mill Bay. The underlying, 435-550m-thick Llanstadwell Fm. also contains numerous green, highly micaceous sandstone units although personally, I don't think they're a match for the Altar Stone either.

From the location information in Ixer et al., 2020 (p.10), it's clear that sample MB3 collected by Brian John actually came from the lower Lawrenny Cliff Fm. -- which explains its compositional differences from the other Mill Bay samples. According to my old field notes, the junction between the two formations (underlying Mill Bay Fm and overlying Lawrenny Cliff Fm) crosses the HWM about 60m south-west of the Whalecwm slipway.

Regarding Ixer et al., 2019 (p.4), my macroscopic description of the Altar Stone (included in Thorpe et al., 1991) was based on me crawling over it and taking photographs (back in January, 1973). I never looked at the samples described by Huggett (1993).

QEMSCAN is a great tool -- especially when used to complement thin section optical mineralogy. The latter will still be essential for determining the Altar Stone's provenance. Using it you can quickly rule out potential source rocks. For example, it only took me a minute to realise that OU9 (444) was not a Cosheston sandstone. Likewise, as you know, verifying whether (p.7) "...quartz and albite cementation, or grain boundary dissolution may also have occurred..." would be simple in thin section. In my view, the fact that QEMSCAN 'reports' rock fragments as their constituent minerals is a major shortcoming, particularly since intraformational and extraformational lithoclasts are such a common component of most Anglo-Welsh ORS sandstones. I suspect that thin section analysis of the diagenetic histories of possible source sandstones will play an important role in unravelling the Altar Stone's provenance. Carbonate cements (including poikilotopic calcites) are common in the Senni Fm. and tend to preferentially attack certain minerals or rock fragments (e.g., K-feldspars), meaning that modal analyses can't give you a true picture of the original composition of the framework-grain fraction. Thin section work, in tandem with QEMSCAN, will be the key to identifying any given sandstone's diagnostic, compositional and textural 'signature' for comparison with those of potential source units.

Quote from P.7: "Clay minerals in sandstones may occur as a detrital matrix or as ......diagenetic cements." They also, of course, occur as claystone or fine siltstone intraclasts, and as alteration products.

Could there be any other cryptic "species" of sandstones among the Stonehenge Landscape debitage?

Is the famous slide 277 really a thin section of the Altar Stone? When I looked at this thin section, I felt that it didn't match what I'd expect a genuine Altar Stone specimen to show. This sandstone is finer grained (very fine vs. fine sand grade) and a lot less micaceous than the Altar Stone, and only contained a couple of garnets. I think it unlikely that it is a genuine Altar Stone sample.

I have no problem agreeing that the "Altar Stone" samples (Table 2; Figs. 2-5 and 8) -- especially FN 573, HM 13 and SH 08 -- share the same parentage. Again, the key question is whether they share that parentage with the Altar Stone. From an inspection of Table 2 and Fig. 2, I'm sceptical that the six numbered samples were derived either from the Altar Stone or its parent unit. Two compositional issues bother me:

(1) Thomas (1923, p.244) describes the Altar Stone's heavy mineral suite as being "exceedingly rich in garnet" which I interpret as being on a par with garnetiferous Llanstadwell Fm. and (especially) Mill Bay Fm. sandstones. However, the analyses indicate that the garnet content of the two Mill Bay Fm. sandstones is an order of magnitude greater than that of the supposed "Altar Stone" samples;

(2) the Altar Stone (from my field inspection) contains abundant muscovite -- certainly equal to the muscovite content of most 'lower' Cosheston Subgroup sandstones. From Table 2, Mill Bay 1a and 1b (average = 4.88%) contain close to double the amount of muscovite present in the six "Altar Stone" samples (average = 2.48%). I consider these differences to be significant grounds for suspecting that the latter are not of genuine Altar Stone derivation. However, this question can only be resolved when/if English Heritage allows new Altar Stone samples to be collected.

Burl (2007, p.281) gives the length, width and thickness of the Altar Stone as 5.0m, 1.1m and 0.53m, respectively. If I remember correctly, the latter's small-scale, trough cross-lamination indicates it is lying right way up stratigraphically. I'd suggest that two samples of the Altar Stone be taken: one from near the stratigraphic top of the unit and one near its base. Both sample sites would be hidden from public view (beneath the turf) because the top sample would have to be taken below the upper portion of the Stone that has been damaged/mineralogically altered by the fires lit upon its exposed surface.

Most Senni Fm. in-channel sandstone bodies that I have observed, generally fine upward. When sampling potential Altar Stone source units, it will be extremely important to ensure that the grain-size distributions of the sandstones collected match that of the Altar Stone as closely as possible, so that compositional differences related to hydraulic equivalence are minimised.

Senni Fm. sandstones vary in colour, provenance, distance from sediment source(s) and diagenetic history from West to East across the Formation's outcrop. I agree with Ixer and Bevins that the eastern portions of its outcrop are the most promising in terms of potential Altar Stone source units. It may be worthwhile checking for any fracture pattern differences across the outcrop, in order to locate joint systems capable of "releasing" sandstone blocks up to 6m long and up to 0.6m in thickness.

The zircon images and date ranges published in the new paper are fascinating! P.8: "A simple interpretation is that the Altar Stone grains reflect a more mature sedimentary environment.....". I suspect that provenance differences are a more likely explanation for the observed 'disparity' in zircon morphologies. The range in zircon morphology and their ages furnishes additional evidence of the prevalence of multicycle detritus within the Cosheston Subgroup sequence (cf. Thomas et al., 2006). The greater range of ages represented by the"Altar Stone" micaceous sandstone zircons is very interesting and suggests a geologically complex source area for these rocks and/or the availability of an abundance of second- and multi-cycle detritus.

I am a strong proponent of the glacial transport of the bluestones to a 'greater Stonehenge' catchment area rather than human transport, and consider the archaeologists' 'new' land route proposal (apparently signed up to by Ixer and Bevins) to be completely lacking in credibility. Also, during my visit to Craig Rhos-y-felin I could find no evidence in support of quarrying at the site.


Most of Richard's observations are of a technical nature,  but it's interesting that, like me, he takes issue with the assumption that all of the supposed "Altar Stone samples" have actually come from the Altar Stone,  and like me he feels that the authors of the paper should have been much more circumspect in their wording.   We also share concerns about the promotion (yet again!) of the human transport hypothesis, and about the presumption that there is a man-made Neolithic quarry at Rhosyfelin.  On the other hand we are both happy that the "Altar Stone" samples probably do come from a common source; that the sampled debris did not come from the Mill Bay Formation;  and that the real provenance of the "Altar Stone" fragments probably lies somewhere in the eastern part of the Senni Beds outcrop, far away from Preseli.  As I have said before, ice from that area flowed on more than one occasion southwards and south-eastwards towards the Severn Estuary,  so this new work does nothing at all to dent the glacial transport hypothesis.

1 comment:

Tony Hinchliffe said...

We need the tiniest chip off the altar stone so that geologists like Rob Ixer can identify the rock more precisely. Surely English Heritage owes it to the natural human curiosity to agree to this? If not, why not? Questions should be asked in Parliament. Write to your MP. If the Chinese can send a probe to Mars, this is surely not too big a request.