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Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Sunday, 25 October 2009

In reply to Ed

Hi Ed

I agree with you that the term "bluestone" is the cause of endless trouble. Some archaeologists seem to think it is a petrographic term. It is not -- it is used loosely to refer to anything, in the context of Stonehenge, that happens not to be sarsen. Most of the stones are not even blue. We would be better off without it, but sadly we are stuck with it.

Re the curved line used on the map, I'm not sure what you mean by "exaggerate".......... of course it is a guess -- we really have no idea how the junction between Irish Sea ice and Welsh ice may have run from Pembrokeshire towards the coasts of Devon and Somerset. It may have been smooth, or it may have had "kinks" in it, caused by streams out ice coming out of the South Wales Valleys -- the Towy, Taf, the smaller valleys coming off the coalfield, and then the Usk. They all held glaciers. I hope that some new modelling from Dr Alun Hubbard and his colleagues in Aberystwyth might give some more clues on this.

Why don't we find bluestones all over SW England? That's not how glaciers work. Entrainment, transport and deposition of erratics is very spasmodic and difficult to predict -- it depends to a large degree on the thickness of the ice, how it streams, and what the bed conditions were like. But I'm coming gradually to the view that dear old Geoffrey Kellaway wasn't far wrong when he suggested that the ice stream travelling up the Bristol Channel might have had three components -- one to the south comprising Scottish and Bristol Channel erratics, one in the middle with erratics from Pembrokeshire, and one in the north with erratics from the Welsh uplands and the Midlands.

1 comment:

Anonymous said...

Hi Brian,

Thank you for your detailed reply, but please bare with me while I get my head around this; what I think you are saying is that the ice flow was direct from Mynydd Preseli to Salisbury Plain (Stonehenge) and only deposited bluestones.

What I meant by "exaggerated" with regard to the ice flow direction from Mynydd Preseli (MP) is that it seems a very tight curve when compared to the direction of travel of the other ice flows on your diagram. Is there geological evidence to substantiate this?

As you rightly say, entrainment, transport and deposition of erratics is very difficult to predict, however, if the South Wales ice moved in a similar direction and possibly joined the Irish Sea Glacier would it not be reasonable to expect to see South Wales boulder types in addition to the Preseli bluestones amongst the deposits.

Best wishes,
Ed Watson