Part of an erratic boulder found in a wall on the Parrog, Newport. Not "rhyolite with fabric" but "rhyolite with at least six fabrics"............ Origin unknown......
It's a funny old life. There we were a few days ago, taking a stroll down on the Parrog in Newport, when I noticed that one of the glacial erratics in a wall that I have walked past hundreds of times before appeared to have a patch of foliated rhyolite on it. As I will no doubt be reminded by certain geologists who have axes to grind, I know nothing at all about geology, but I do know a similarity when I see one --and I think I know a rhyolite from a gneiss.
I was a bit suspicious at first, since this rock looks similar in some ways to the "laminated mudstone" in the N corner of Traeth Mawr, near the waterfall. But on closer examination that rock is seen to have a uniform grey-blue colour and concentrations of whitish sand grains in very thin layers. It belongs to the Ordovician Dinas Island Formation, and is described by BGS as a "laminated hemipelagic mudstone with subordinate turbidite sandstones"......... so it's a deep-water sedimentary rock similar to the rocks exposed along much of the coast between Newport and Fishguard.
Back to the boulder. It's made of a much harder rock, with richer colours, immense internal fabric variations and a typical rhyolite sheen. I await more specialist geological advice, but I do not think the boulder has a local origin within Newport Bay. But just inland of Newport there is a great expanse of the tuffs and lavas of the Fishguard Volcanic Series, and within this band there are scores of outcrops of rhyolite. This boulder could have come from any one of these outcrops.
Extract from the BGS Geology Viewer, annotated. There are frequent rhyolite outcrops within the marked FVG area.
Very colourful banded rhyolite exposed on the side of the Parrog boulder
Foliated rhyolite (?) on the boulder, very similar in appearance to some of the foliated rhyolite "with Jovian fabric" seen at Rhosyfelin. Click to enlarge.
It's interesting to see how the terms "rhyolite with fabric" and "Jovian fabric" have been used in the marketing of the "monolith quarry" at Rhosyfelin. Over and again the terms are used (mostly by archaeologists) with a pretence that they are scientific or diagnostic terms that are crucial in the arguments about the precise provenancing of Stonehenge rhyolite fragments to Rhosyfelin and to nowhere else. These extracts are typical:
From: Parker Pearson et al (2015) Craig Rhos-y-felin: a Welsh bluestone megalith quarry for Stonehenge — p 1333
More than 1200 chippings from the 2008 Stonehenge Riverside Project (Parker Pearson 2012) and the SPACES project (Darvill & Wainwright 2009), and from excavations at Stonehenge in 1980 (Pitts 1982), have been characterised as ‘rhyolite with fabric’ (Ixer & Bevins 2010; Bevins et al. 2011). Most of these have been found in the centre of Stonehenge, but they also occur in its environs, almost as extensively as the spotted dolerite chippings. Six ‘rhyolite with fabric’ chippings were recovered from Aubrey Hole 7 in 2008. Of the 27 from the Stonehenge Avenue, one was found in a layer beneath the Avenue’s banks, and was thus deposited before 2480–2280 cal BC (see Darvill et al. 2012), indicating that ‘rhyolite with fabric’ was present at Stonehenge before the Early Bronze Age. The fabric of this particular type of rhyolite is macroscopically typically planar, with a prominent foliation developed on the millimetre scale. In thin section, the foliation is seen to be slightly lensoidal, and contains flattened, ovoid lithic clasts (2–5cm) of microtonalite. Locally, the fabric is extremely well developed and described as ‘Jovian’ because it resembles the swirling weather patterns on Jupiter (Ixer & Bevins 2011). The main rock is commonly traversed by thin quartz veins that are tightly folded, with their folds being axial planar to the foliation, suggesting that the rock fabric is most probably not a primary texture but a later (tectonic) flattening fabric. A strong petrographic match for these rhyolite fragments has been found with outcrops in the Pont Saeson area just north of Mynydd Preseli, specifically the outcrop of Craig Rhos-y-felin, belonging to the Fishguard Volcanic Group, of Ordovician age (Ixer & Bevins 2010; Bevins et al. 2011). This match is closest for samples from the north end of the outcrop’s near-vertical western edge ............
From Parker Pearson et al, 2022:
It is likely that just one bluestone pillar was extracted from the outcrop at Craig Rhos-y-felin (Fig. 4). This striking outcrop is situated in the bottom of a steep-sided valley along which flows a stream that rises close to Carn Goedog and forms a tributary of the River Nevern (Parker Pearson et al., 2015; 2019). Initial geological sampling at 19 locations around this outcrop through the stratigraphy of the rhyolitic body revealed just one location, close to the northern tip of the outcrop, where the ‘Jovian’ micro-structure of spherical to lensoidal features (named after the famous ‘spots’ in Jupiter’s atmosphere) within the rhyolite matches the fabric identified within some of the flakes from Stonehenge and its environs (Ixer and Bevins, 2011).
More than 1200 chippings from the 2008 Stonehenge Riverside Project (Parker Pearson 2012) and the SPACES project (Darvill & Wainwright 2009), and from excavations at Stonehenge in 1980 (Pitts 1982), have been characterised as ‘rhyolite with fabric’ (Ixer & Bevins 2010; Bevins et al. 2011). Most of these have been found in the centre of Stonehenge, but they also occur in its environs, almost as extensively as the spotted dolerite chippings. Six ‘rhyolite with fabric’ chippings were recovered from Aubrey Hole 7 in 2008. Of the 27 from the Stonehenge Avenue, one was found in a layer beneath the Avenue’s banks, and was thus deposited before 2480–2280 cal BC (see Darvill et al. 2012), indicating that ‘rhyolite with fabric’ was present at Stonehenge before the Early Bronze Age. The fabric of this particular type of rhyolite is macroscopically typically planar, with a prominent foliation developed on the millimetre scale. In thin section, the foliation is seen to be slightly lensoidal, and contains flattened, ovoid lithic clasts (2–5cm) of microtonalite. Locally, the fabric is extremely well developed and described as ‘Jovian’ because it resembles the swirling weather patterns on Jupiter (Ixer & Bevins 2011). The main rock is commonly traversed by thin quartz veins that are tightly folded, with their folds being axial planar to the foliation, suggesting that the rock fabric is most probably not a primary texture but a later (tectonic) flattening fabric. A strong petrographic match for these rhyolite fragments has been found with outcrops in the Pont Saeson area just north of Mynydd Preseli, specifically the outcrop of Craig Rhos-y-felin, belonging to the Fishguard Volcanic Group, of Ordovician age (Ixer & Bevins 2010; Bevins et al. 2011). This match is closest for samples from the north end of the outcrop’s near-vertical western edge ............
From Parker Pearson et al, 2022:
It is likely that just one bluestone pillar was extracted from the outcrop at Craig Rhos-y-felin (Fig. 4). This striking outcrop is situated in the bottom of a steep-sided valley along which flows a stream that rises close to Carn Goedog and forms a tributary of the River Nevern (Parker Pearson et al., 2015; 2019). Initial geological sampling at 19 locations around this outcrop through the stratigraphy of the rhyolitic body revealed just one location, close to the northern tip of the outcrop, where the ‘Jovian’ micro-structure of spherical to lensoidal features (named after the famous ‘spots’ in Jupiter’s atmosphere) within the rhyolite matches the fabric identified within some of the flakes from Stonehenge and its environs (Ixer and Bevins, 2011).
But what do these terms actually mean? Are they of any use in demonstrating a really precise match between samples from widely different locations?
Rhyolite with Fabric
The term "rhyolite with fabric" is a very strange one, since all rocks have a fabric. According to Wikipedia, "fabric" describes the spatial and geometric configuration of all the elements that make it up. There are two principal varieties of oriented fabrics: primary (or depositional) and secondary (or deformational). So there is no such thing as "rhyolite without fabric", and on that basis the term "rhyolite with fabric" can be dismissed as meaningless. As for the "Jovian fabric" in Rhyolite Group C which is claimed to be so unique that certain Stonehenge fragments can be provenanced to "within a few square metres" at Rhosyfelin, I have argued over and again that this is a claim unsupported by the evidence presented.
https://brian-mountainman.blogspot.com/2014/04/on-significance-of-rhosyfelin-locality-8.html
https://brian-mountainman.blogspot.com/2015/09/rhosyfelin-some-geological-questions.html
https://brian-mountainman.blogspot.com/2015/09/rhosyfelin-and-spot-provenancing.html
https://brian-mountainman.blogspot.com/2016/08/rhosyfelin-spot-provenancing-and-few.html
The published thin sections from the Stonehenge samples and those collected from Rhosyfelin and the Pont Saeson area have produced no perfect matches. Ixer and Bevins have assured us in various publications that "rhyolite with fabric" (or "Jovian fabric") as seen in the Pont Saeson area does not occur anywhere else in North Pembrokeshire, in any of the other abundant rhyolite outcrops. They tell us that, but they have never actually demonstrated it through the presentation of evidence in an accessible journal. In several papers they discuss the geochemistry and petrography of some of the rhyolite outcrops of North Pembrokeshire, and find that they do not match any of the fragments collected from the Stonehenge debitage and referred to as Rhyolite Group C.
Quote from p 22 of the 2011 paper: "In the Pont Saeson area the Fishguard Volcanic Group comprises a strongly foliated to foliated and lensoidal rhyolitic rock the like of which is not seen elsewhere in the outcrop of the group across the 32 km of strike section from Pen Caer in the west to Crymych in the east."
Quote from p 24: "Although there are subtle but distinct differences between different rhyolitic outcrops at Pont Saeson, including those on Craig Rhos-y-felin, they share a distinctive petrography that is unrecognised from elsewhere in the Fishguard Volcanic Group."
About 20 samples are analysed and discussed (including those collected for the OU research published in 1991), but it is obvious from any examination of the published thin sections that there are no exact matches, and we are not shown any slides of samples of foliated rhyolite from other outcrops across North Pembrokeshire. One of the ancient rules of scientific writing (and writing TV drama, come to think of it) is "show, don't tell....."
Williams-Thorpe and Thorpe (1991) demonstrated many years ago that on geochemical grounds the rhyolite debris at Stonehenge could be identified as having come from multiple North Pembrokeshire sources. They favoured Carn Alw as the source of four monoliths and some Stonehenge fragments, but this was later disputed by Bevins and Ixer.
If one reads the key research articles by Bevins and Ixer (2010 and 2011), one sees that neither the "rhyolite with fabric" nor "Jovian" labels are of very great importance. The labels are probably used in order to appeal to non-geologists. In fairness to them, they explain that detailed petrology and geochemistry are far more important in differentiating Group C rhyolites from the groups which used to be called A, B, D, E and F. But they don't make it easy for the rest of us. Now Group A samples are called andesites and samples from groups B, D and G are referred to as dacites. Confused? You are not alone. We must not forget that no known monolith at Stonehenge provides a match for any of the samples taken from the Pont Saeson area. The only reason for the emphasis on Rhosyfelin is that MPP, Ixer, Bevins et al think they they have found a quarry there. Sources still have not been found for all the other rhyolite fragments and rhyolite monoliths at Stonehenge. They may have come from North Pembrokeshire, and maybe not.
Williams-Thorpe and Thorpe (1991) demonstrated many years ago that on geochemical grounds the rhyolite debris at Stonehenge could be identified as having come from multiple North Pembrokeshire sources. They favoured Carn Alw as the source of four monoliths and some Stonehenge fragments, but this was later disputed by Bevins and Ixer.
If one reads the key research articles by Bevins and Ixer (2010 and 2011), one sees that neither the "rhyolite with fabric" nor "Jovian" labels are of very great importance. The labels are probably used in order to appeal to non-geologists. In fairness to them, they explain that detailed petrology and geochemistry are far more important in differentiating Group C rhyolites from the groups which used to be called A, B, D, E and F. But they don't make it easy for the rest of us. Now Group A samples are called andesites and samples from groups B, D and G are referred to as dacites. Confused? You are not alone. We must not forget that no known monolith at Stonehenge provides a match for any of the samples taken from the Pont Saeson area. The only reason for the emphasis on Rhosyfelin is that MPP, Ixer, Bevins et al think they they have found a quarry there. Sources still have not been found for all the other rhyolite fragments and rhyolite monoliths at Stonehenge. They may have come from North Pembrokeshire, and maybe not.
To make life even more confusing, in a paper by Ixer et al (2023) on the Andesite Group A fragments they accept that fragments classified within some of the other volcanic groups might have come from "the same lithic" -- in other words, from the same boulder or monolith. This is an admission that there is huge variation in fabrics and geochemistry across the rhyolite fragments found at Stonehenge and across the rhyolite outcrops of North Pembrokeshire -- and the rhyolite boulder featured at the head of this post is a classic example of that.
Also found on the web -- highly deformed mylonite from the western Alps:
Jovian Fabric
The use of the term "Jovian fabric" as a diagnostic feature is similarly dodgy. When I asked a European petrologist about the lensoidal or "jovian structure" of some of the rhyolites at Rhosyfelin, and showed him some of the images from Ixer and Bevins, he replied: ".... they are not rare structures, in fact, they are quite common in rhyolites. In this case they are due to magmatic flow. Rhyolites are rocks deriving from very viscous lavas and form flow structures like this! Sometimes the cause is high temperature deformation during metamorphism."
Patterns in the atmosphere of the planet Jupiter -- the inspiration for the use of the term "Jovian" by Ixer and Bevins.
Also found on the web -- highly deformed mylonite from the western Alps:
Jovian textures -- courtesy Alessandro Da Mommio
Augen texture in gneiss (Photo: Marli Millar)
One of the samples showing a "Jovian fabric" from Rhosyfelin, published by Ixer and Bevins. All of the published thin sections are different, pointing to a range of textural characteristics and probably a range of provenances.
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Ixer and Bevins may be right in their claim that certain fragments at Stonehenge have come from the Pont Saeson area -- and that would be fine, as far as I am concerned. But the find of this interesting boulder down on The Parrog reminds us that there are other rhyolites in North Pembrokeshire that are foliated and light blue in colour, with a wide range of internal characteristics, and I would like to see some evidence that these outcrops have NOT contributed any of the rhyolitic debitage that has ended up at Stonehenge.
Ixer and Bevins may be right in their claim that certain fragments at Stonehenge have come from the Pont Saeson area -- and that would be fine, as far as I am concerned. But the find of this interesting boulder down on The Parrog reminds us that there are other rhyolites in North Pembrokeshire that are foliated and light blue in colour, with a wide range of internal characteristics, and I would like to see some evidence that these outcrops have NOT contributed any of the rhyolitic debitage that has ended up at Stonehenge.
I don't think that any of the rhyolite outcrops of North Pembrokeshire (such as Carn Alw, Carn Gwiber, Carn Cwn, Foel Drygarn, Carn Afr and Carn Fron) can be excluded as possible sources for the rhyolite fragments found at Stonehenge unless it can be demonstrated that the outcrop samples analysed thus far from such places really are typical or characteristic of the outcrops as a whole. The old problems of sampling density and sampling bias are there, as they always were. But where extravagant claims are made about provenancing that is "accurate to within a few square metres", those claims need to be backed up by pretty extraordinary and detailed evidence. And as I have said many times before, the quality of the "evidence" presented thus far by Ixer, Bevins et al is inadequate.
Whatever the truth of the matter, I think we should accept that the terms "rhyolite with fabric" and "Jovian fabric" are useless and even misleading. They do not point to unique characteristics, and the terms are of no use whatsoever in helping us to understand where some of the fragments in the Stonehenge debitage might have come from.
1 comment:
By Jove! Very underwhelming, then......heavens to Murgatroyd.
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