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Tuesday, 15 September 2015

Rhyolite groups A-C



On reading through "Chips off the Old Block" by Rob Ixer and Richard Bevins, I was reminded of this statement, in the Conclusions part of the text:

"If the parent orthostat for the rhyolite groups A-C were sampled and it proved to be either SH32d or SH32e then it would demonstrate that widespread and abundant debitage is directly associated with an orthostat, is the partial result of the destruction of that orthostat and has not been introduced into the Stonehenge Landscape for other reasons or by other processes including glacial ones."

A number of things.  I'm still confused by the numbering of these stones.  On the Atkinson photos, 32e (closest to stone 33) looks like a dolerite stump, whereas stones 32c and 32d do indeed look like  rather flaky rhyolites.  Is there a definitive ruling somewhere as to which stone is which?

Second, can somebody please explain what the differences are between the foliated rhyolites in Groups A, B and C?  They are mentioned in several of the key papers, but I am still confused. Presumably they are all foliated rhyolites ......... but if they are different enough to be in different petrographic categories, how can lots of fragments in all 3 groups have come from the same stone?

Third, I don't follow the logic in the argument that if the fragments in the debitage can be related to the destruction of a big stone, that somehow rules out the introduction of debris into the landscape by glacial or other processes.  Surely it is perfectly feasible for debris from a large stone or super-erratic to have been scattered about in the landscape prior to the selection and introduction of a small part of it into a stone hole? At that point it became an orthostat or a standing stone.   You have no way of knowing how big or small that stone might have been prior to its selection and use......  and of course, if it was destroyed later on, there would have been the introduction of even more flakes and debris into the debitage.
  
Update

I found this classification in the Craig Rhosyfelin paper:

A. Dark/black, sharp, flinty rhyolite ± joint planes. Rare, pale-coloured, flinty rhyolite is probably weathered dark rhyolite.

B. Rhyolite with a planar fabric. Rare, extreme examples of this group initially were classed as ‘slate/phyllite’.


C. Rhyolite with a pronounced planar and lensoidal fabric ± joint planes.

I'm still confused.  Are these three categories now deemed to be redundant, and have they been rolled into one?  But they sound rather different from one another...... and that being the case, how can they all have come from one stone?


1 comment:

Myris of lexandria said...

There is confusion in the lit as what the stones are. The pet rock boys initially though 32e was like the CRyf before the quarry face was exposed and the rhyolite could be seen in three dimensions. Post the opening of the quarry face 32d looked a better candidate and is currently the preferred.
32c looks like a possible parent for the Volcanic Group A debitage aka volcanics with subplanar texture.

Rhyolite A-C are all Rhyolite C. The original macroscopical description had about 30 different rhyolite sub-groups, using thin sections these were whittled down to 5.
The present correct? thinking is A-C CRyf, no doubt, Rhyolite D may be a red herring -it is one of Stones' stones and E which is SH48 knock off.
Simply put it is all from CRyf bar the very rare exceptions it s in their paper but need to be read in sequence.
Publishing annually means you have to follow in the footsteps of the pet rock boys as they stride towards the TRUTH correcting heir mistakes as they go. Sadly they have no Egyptian ports to enter and wise men to consult but have to take their own lights with them.
M