The mysterious boulder 38

The strange mis-shapen boulder referred to as SH38, partly obscured by sarsen SH14. Courtesy the Stones of Stonehenge web site. Its place of origin is still a mystery.

In checking out what the score is with regard to dark blue flinty welded tuffs and so forth, I wondered whether this boulder might be related to the Newall boulder. There is a chance, but much more geology needs to be done by the experts. I checked back to a previous post about SH38 and the assorted volcanics at Stonehenge and in the surroundingt landscape:

https://brian-mountainman.blogspot.com/2015/04/new-paper-on-bluestone-38.html

Hard ‘Volcanics with sub-planar texture’ in the Stonehenge Landscape by Rob A. Ixer, Richard E. Bevins and Andy P. Giże
Wilts Arch & Nat Hist Mag 108 (2015), pp 1-14

On giving the paper a fresh read, I am impressed by how strongly biased it is towards the ruling human transport hypothesis, with -- over and again -- a "forcing" of analysis and interpretation into the following assumptions: (a) that the bulk of bluestone fragments in the debitage MUST have come from North Pembrokehire; and (b) that bluestone fragments can only exist at Stonehenge if they have been knocked off past or present monoliths transported by our heroic ancestors. So the paper is deeply unsatisfactory since it completely fails to address the possibilities that the bluestone monoliths might have been glacially transported, and that large numbers of fragments in the debitage might have nothing whatsoever to do with the known monoliths. Furthermore, many of the small boulders, pebbles and broken fragments might have nothing whatsoever to do with Pembrokeshire. They could be glacial erratics from closer to home, or maybe from further afield.

Putting all that to one side, it's tempting to suggest that there might be a link between the Newall boulder and SH38, and that the "welded vitric acid tuff" might be related to other rock samples looked at by Ixer and his colleagues and identified as Volcanic Group B.  Ixer and his colleagues referred to "small, often sub-rounded, rather than angular flaked, fragments throughout Stonehenge and its environs."

That comment has always caused me -- and others -- to wonder whether the geologists have been looking at glacially-transported or glaciofluvially-transported stones in the Stonehenge sediments without recognising what they have been looking at..........  Again, this suggests a possible link with the Newall boulder, which is faceted and almost certainly glacially transported.

Quote:
"...........
Volcanic Group B, hard rocks that are partially characterised by an unusual mineralogy including two forms of graphitising carbon. Only twelve Volcanic Group B samples have been recognised from the Stonehenge debitage."

Quote:

"Conclusions 

 
Orthostat SH38 and twelve pieces of debitage that constitute the new Volcanic Group B class of debitage are sufficiently uniform in terms of their mineralogy, grain size and textures that it seems probable that they are all from the same rock rather than just from the same outcrop.

Although this debitage is numerically rare it has a wide spatial distribution in the Stonehenge Landscape notably within the Darvill and Wainwright April 2008 excavation and Heelstone Ditch but also including within Trench 45 in The Avenue and Aubrey Hole 7 in Stonehenge. Although a lithic with graphitising carbon was found from close to the Stonehenge Greater Cursus no SH38 debitage has been recognised from there with any certainty. The SH38 debitage distribution is similar to that found for orthostat SH48 but is more extensive than that for the Altar Stone.

The temporal distribution of the SH38 debitage is very similar to that for SH48 in that most pieces are found from post Neolithic contexts but are less ‘bunched’ than that from the Altar Stone.

The newly reported SH38 debitage has extended the range of petrographical features beyond those seen in orthostat SH38, notably to include the presence of large zircons, rare earth-bearing minerals, tube pumice and a significant fine-grained siliceous component. This in turn suggests that were the single geochemical analysis for SH38 (Thorpe et al. 1991) and taken from a very small sample , to be augmented by new analyses from the present samples, a geochemistry that was closer to the bulk geochemistry for SH38 could be achieved. An enhanced petrography plus a more representative geochemistry would help to narrow the possible geographical sources for the orthostat. On present knowledge this is still expected to be found within the Ordovician Volcanic sequences, in the north Pembrokeshire area but the net is tightening."

Petrologist RK Harrison on the petrology of the Newall Boulder: “This large, dark blue-grey, hard, flinty (? partly worked artifact) shows a white weathered crust up to 5 mm thick. The thin section shows a complex structure of very finely banded welded tuff (compressed foliated shards cemented by fine silica) with composite quartz grains and strings of dusty leucoxene, separated by patches of much finer grained, finely fluxioned glassy lava with patches of granular quartz. This specimen appears to represent a complex of originally viscous glassy lava and welded vitric tuff, all presumably of rhyolitic composition.”.   

I wonder whether that is a description of one of the rock types now referred to by Rob Ixer and his colleagues as "Volcanic Group B" or "Dacite Group B" ??

Match all this with the 1971-72 descriptions of the welded acid vitric tuffs examined in 1971-72 by Kellaway, Harrison, Smith, Howells and Nutt, and we have an intriguing possibility that there is rather more glacially transported material lying around in the Stonehenge landscape than the geologists would have us believe.