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Thursday, 7 December 2017

More on the Carn Goedog dolerite sill

This rather splendid oblique aerial photo has just been published in Pembrokeshire Life magazine -- it was taken in 2009 by Toby Driver of the Royal Commission.  It shows Carn Alw in the foreground and Carn Goedog in the distance. Because of the light snow cover there is tremendous detail in the image -- I have added a few pointers for those who do not know the area. Click to enlarge the image.

The photo shows just how extensive the outcrops of the Carn Goedog sill actually are.  As I have explained in previous posts, the assumption that there is a Neolithic bluestone quarry at Carn Goedog is very dodgy indeed, and is based in turn on the assumption that the spotted dolerites ar Stonehenge have been precisely provenanced to the Carn Goedog tor.  But these spotted dolerites outcrop all the way along the sill to the vicinity of Carn Alw, and I have seen nothing in the geological papers that demonstrates that any of the spotted dolerite monoliths actually come from the tor rather than from outcrops further to the east or indeed towards the west.  The sampling point density is just not great enough for any definitive conclusions to be drawn.  The geologists (Richard Bevins and Rob Ixer) sampled the tor because it is a prominent feature  and therefore "attracts" the sampling process.

Confirmation bias yet again? I'm not sure I would put it as strongly as that, since I suppose many of the geological samples were taken years ago, long before Neolithic quarries were being thought of in this area.  But you must always be careful in this work not to attach "artificial significance" to places simply because they happen to be the places you have sampled........


Myris of Alexandria said...

Prescient comments, oh learned one.

Yes RB collected many/most of the samples 30 years ago when, for the Pet Rock Boys, Sh was just a pile of stones in a field -been there, been given the tee-shirts.

One could sample to extinction outcrops but diminishing returns is an exponential curve.

It is pretty clear that macroscopical/microscopical methods are not very good at discriminating between theses outcrops, correct geochemistry is.

Read the B et al paper in JAS dealing with the discrimination between outcrops. There was no wanting a particular result, it is how the number fell.
Carn Menyn nowhere, Carn Alw (not even worth thinking about)and Carn Goedog was the best match.
MPP and quarries or his wanting them have no bearing on the Pet Rock Boys see Dr I's forthcoming book review in CA of Richard 2017SH annual. It cannot be said more clearly without resorting to Old English.

No bias just clever stats.

Now when the Pet Rock Boys paper on Thomas comes out next year you may have a second bite of that forbidden fruit.

"We are stardust we are golden"

Jim Lovatt said...

Carn Goedog may have been the best match, but, unfortunately or fortunately depending on your point of view, it's the wrong place at the wrong time, by about 5000 years.

Dave Maynard said...

There's a lot going on in that image.


BRIAN JOHN said...

Sure is, Dave. It's wonderful what a light dusting of snow can do......... there's another one up there right now. Just soggy down here in Cilgwyn......