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Sunday, 2 May 2010

Bluestone fragments -- the plot thickens



The edge of the Irish Sea Glacier on Salisbury Plain, maybe 250,000 year ago..... (date very uncertain)

With reference to this earlier post:

http://brian-mountainman.blogspot.com/2010/02/bluestone-quarry-consigned-to-scrap.html

Rob Ixer has kindly confirmed that after studying around 7,000 fragments from the Stonehenge neighbourhood, he can say with some certainty that some of the bluestone monoliths are of rock types that are NOT represented in the "Stonehenge layer" or in the litter of fragments in the soil horizons; and that some frequently occurring non-dolerite rock fragments (ie rhyolites, ashes and sandstones) found in the "litter" are not represented in the 43 known bluestone monoliths / stumps known in the stone monument.

He also repeats this question: "Why are the geological origins of the non-dolerite bluestone so diverse and often from ‘insignificant’ outcrops?" He also says: "Detailed rock and mineral geochemistry plus statistical analysis of the ‘debitage’ may answer these and the more straightforward questions."

No doubt Rob and Richard Bevins have further publications in mind, and I look forward to seeing these. And I gather that further petrographic / geochemical work is under way on all those rhyolite samples collected during the OU project about 20 years ago.

At the risk of repeating myself, I suggest that what we are looking at here is an assemblage of glacial erratics, large and small. Exactly where they were emplaced or dumped by the glacier is another question.....

9 comments:

Kostas said...

Another great picture Brian showing a relatively 'tame' glacier edge! Where do you get such great pictures? Thanks for putting them on your blog to share with others.

Rob Ixer's work is one more piece to the puzzle that proves glacier transport! But why 250,000 years ago? Why not 20,000 years ago? I know it's controversial, but this does solve many of the problems associated with Stonehenge! If an assumption helps answer all our questions, then it must be true. Isn't this how science works?

You write,

“... what we are looking at here is an assemblage of glacial erratics, large and small. Exactly where they were emplaced or dumped by the glacier is another question..... “

Brian, certainly these small stone erratics (which Rob Ixer is now showing were not chipped away from the monument stones) were not carried by local people to Stonehenge and buried evenly in the soil. So why we can't assume that the same natural forces that carried the small stones to Stonehenge also carried the big heavy stones to Stonehenge? It will make life so much easier for us as well as for the Neolithic people!

Constantinos

Brian said...

The photo is of the edge of Crusoe Glacier, on Axel Heiberg Island, Arctic Canada. It's a clean and cold-based glacier in an area of thick permafrost.

On dating -- there is a lot of evidence from the Quaternary stratigraphy of Somerset that the ice of the Irish Sea Glacier didn't reach this far east in the Last Glaciation (Devensian) 20,000 years ago. I'm more and more coming to the view that the glaciation was much older, bearing in mind that the evidence has to be internally consistent and stacking up properly across lots of different disciplines. Have a look at the book called "The Quaternary of Somerset" (2006) and you'll see what I mean.

I have no problem with all of the erratic materials -- big and small -- arriving in the vicinity of Stonehenge at the same time. That's what I have suggested from the beginning.

Ed said...

So you are saying the 43 dolerite bluestones are not part of the Stonehenge layer and therefore not part of the glacial litter?

Brian said...

There aren't 43 dolerites, Ed. There are 43 monoliths, or bits of them -- as far as we know. Just some of them are dolerites, and just some of those are spotted, with varying degrees of spottiness.

Of course all of the stones, of all shapes and sizes, are part of the same assemblage. I'm simply homing in here on the standard story that the "litter" of the Stonehenge Layer and the soil horizon is derived from the shaping or dressing of the monoliths on the site. Isn't that what the archaeologists have been telling us for years? Now things look rather more complicated than that.

Aubrey Burl, for one, was quite exercised by the question: "If the stones were man-hauled from Wales to Stonehenge, why did they bother to haul them in their rough or original states, and then chop lots of bits off them so as to get them to the desired dimensions, once they had reached their destination? He argued that that would not have been very smart. Any smart Neolithic stone hauliers would surely have whittled them down before going to all the trouble of hauling masses of extra weight over land and sea... Economy of effort, and all that.

Ed said...

Brian, you seem to miss (or avoid) my point that [quote]:

"Rob Ixer....can say with some certainty that some of the bluestone monoliths are of rock types that are NOT represented in the 'Stonehenge layer' or in the litter of fragments in the soil horizons".

So how can they be part of the same assemblage?

Brian said...

Thanks Ed -- slight misunderstanding here -- my fault for not explaining better......

The stones (of all shapes and sizes) can easily be part of the same assemblage. In glacial deposits you often find that in the groupings of different stone sizes you have different lithologies. For example, in Pembrokeshire I have seen many glacial deposits where there are gabbro boulders but no smaller fragments of that particular rock type. I think this might have something to do with the manner of entrainment, and the tendency for attrition crushing / diminution during transport. So because gabbro boulders, once entrained, are incredibly resistant to further erosion, they stay more or less intact, whereas softer rocks like ashes, shales, sandstones and maybe even some schists etc tend to crumble and break down -- providing the eventual matrix of cobbles, gravel, sand, silt and clay in the moraine as it is deposited.

I wouldn't like to speculate much further on this. Somebody -- maybe Rob and his colleagues -- needs to do a proper numerical and lithological analysis of what is in this litter. I would really like to see some decent studies made of the soil horizons AWAY from Stonehenge and the other areas of disturbed / man-influenced surface materials.

Ed said...

Thanks Brian, but this matter needs further clarification because Ixer's statement as it stands quoted on your blog that the "bluestone monoliths are of rock types that are NOT represented in the 'Stonehenge layer' or in the litter of fragments in the soil horizons" would seem to imply that they are possibly not part of what you claim to be the glacial assemblage on Salisbury Plain.

Brian said...

OK -- we need clarification here. Can I invite Rob, if he reads this, to clarify the situation? I'm reporting second-hand here -- maybe when Rob delivers his talk in Devizes he can let us have sight of his text. The we will avoid any further confusion!

Anonymous said...

Yes when I have thought it through.
I have recognised no altar stone material in the Stonhenge layer -even if I have mis-identified some of the rare non-Lower Palaeozoic sst and they ARE altar stone then their numbers are very very small.
I have the numbers-of course most of the Stonehenge layer comprises sarsens and preselite plus the other types of bluestones it is these that are mis-matched.
BUT THERE ARE ONLY 4 VOLCANIC ORTHOSTATS WITH PROOPER PETROGRAPHY BASED ON VERY SMALL SAMPLES.cOMPARED WITH 1000S OF VOLCANIC DEBITAGE.
I am still thinking it through.