I have been looking again at the new paper called "Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H. Thomas" by Richard Bevins and Rob Ixer ----- and I have to say I am a bit less convinced than I was the other day.....
The authors tell us that the 4 rhyolite orthostats at Stonehenge (numbered 38, 40, 46 and 48) are ash-flow tuffs with pumice, lithic and crystal fragments and in one case well-preserved glass shards. In contrast, the Carn Alw and Stonehenge samples described by HH Thomas and now re-investigated are "recrystallized, typically spherulitic rhyolitic lavas which might have originally been part of a thick lava flow or dome." If the orthostats are classified correctly, they do not match any of the five slides examined in this new paper -- or indeed the slides originally examined by Thomas in his 1923 paper. We have to take that on trust, because the authors do not reproduce photomicrographs from samples from any of the 4 Stonehenge orthostats -- so we do not know how different they actually are.
So what we are talking about here is this terrible stuff called the "debitage". Was Thomas at fault in saying that the two thin sections of rocks found at Stonehenge actually came from Carn Alw? Richard Bevins and Rob Ixer say he was at fault, and that these two samples (Thomas's Figs 1 and 3) are different from those which they have examined from Carn Alw. When we look closely, they do not seem to me to be all that different, but the reader does have a few problems in sorting this out, because both HHT and the new authors reproduce bits and pieces of their thin section slides, sometime this way up and other times that way up, sometimes under crossed polars and other times under plane-polarised light, and at different scales. So real comparisons are notoriously difficult for Joe Public to make.
We have to take it on trust that the Carn Alw rhyolites are actually very different from anything found at Stonehenge, but on looking through my references and my own field notes, I see that some of the Carn Alw rhyolites are spherulitic and others are not, and that there is great variation in rock textures from one part of this large craggy outcrop to another. We see from the paper that 6 samples from Carn Alw have been analysed, but we do not see the photomicrographs reproduced in the paper. So here is a request to the authors. Please can we see them? I promise to publish them on this site as an aid to discussion.
A lot of the discussion in the paper relates to the supposed origin of different spherulite structures. The authors suggest that on Carn Alw the rhyolites are genuine volcanic rocks which have not been redeposited -- whereas other rhyolites may have been deposited in water, as shown by the transitions between spherulite structures and argillacious transitions to flaky or platy structures -- typical of fine clay particles settling out in waterlain sediments.
Quote: "We consider that the spherules in sample E1993 (Figure 3 of Thomas and Figs 8a and 8b of this paper) and in sample SH08-390, collected during the 2008 Stonehenge excavations (see Fig 9) might be accretionary lapilli rather than spherules generated as a result of recrystallization of rhyolitic volcanic (sic). Accretionary lapilli develop in volcanic ash clouds as a result of collision of liquid-coated particles with binding of the ash particles as a result of surface tension. The variable supply of ash particles of different sizes results in a concentric growth pattern in the lapilli. Accretionary lapilli falling into the sea will accumulate in the sedimentary sequence, forming an horizon related to the supply of the lapilli. The end of the supply of the lapilli will see a switch back to the accumulation of normal sediment which in the case of an environment offshore would typically be mud-dominated (ie argillitic). However, accretionary lapilli typically range in diameter between 3-4 mm but reaching up to 10 mm, considerably larger than the spherules examined here, although interestingly Lowman and Bloxam (1981) described air fall tuffs containing accretionary lapilli from the Fishguard Volcanic Group west of Ty Canol Wood, some 3 km WNW of Craig Rhos-y-felin."
So the question is this: have all rock samples showing "accretionary lapilli" come from unknown areas (like the area to the west of Tycanol Wood) or could some of them have come from Carn Alw?
Another question is this: how unique are the banded rhyolites and rhyolites with spherules described by Thomas and reexamined in this paper? The authors assign one of Thomas's slides to Rhosyfelin on the basis of similarities, but how secure is that assignation? In other words, are there any other outcrops within or outside the mapped outcrops of the Fishguard Volcanic Group where similar banded rhyolites occur? Or are they using Rhosyfelin as a probable source simply because they know it well -- in other words, could the assignation be down to sampling bias?
And again: how certain are the authors that they know about all of the variations that occur within the crags of Carn Alw?
And again: how certain are the authors that there are not other areas where rhyolites with spherules can be found? Indeed, to their credit they suggest that there might be, and suggest that at least three source areas for the rhyolitic debitage at Stonehenge are still to be found. That's in addition to the source areas that still have to be found for the four Stonehenge rhyolite orthostats.
There will be many more twists in this story before it is done....... and thank goodness that we have two very experienced geologists on the case.