This looks like a wonderful new book by Chris Scarre -- just published by OUP, at some outrageous price. The cheapest I can find it is £65 -- so I'll give it a miss for now, while giving it some free promotion......
On trawling through the pages available on Amazon, I was struck by this:
Click on the above to make them larger. What he is saying (and he is not alone in this) is that the stones in the Carnac alignments are very closely related to the local geology -- and in particular to the spacing of fissures in the local granite bedrock. In turn, this influences the size and dimensions of the stones that litter (or used to, in the past) the ground surface and which are then used by the groups responsible for the alignments. His little diagram, and the plot of stone heights, are fascinating and convincing.
The message seems to be this: that the builders of Carnac, over quite a long period of time, used stones more or less where they found them. Indeed, it could be argued that Carnac is where it is not because of some astronomical freak or even any great ritual or ceremonial obsession -- but simply because the stones were relatively easy to gather up and easy to erect. Minimisation of effort, energy conservation, opportunism, rock scavenging -- call it what you will........ but the nice simple utilitarian message rather appeals to me.
9 comments:
The term that is required here is 'jointing' not heaven forfend 'bedding' in granite.
Many of the bluestones are obviously joint blocks.
GCU In two minds
Quite right you are -- but this fellow is not a geologist, so maybe we can forgive him....
I agree that the bluestones, both in the Preseli Hills and at Stonehenge, are blocks whose dimensions are determined by jointing patterns in the native rock.
Brian,
Why the granite fissures of the bedrock at higher elevations are wider than the fissures close to sea level? I find that very fascinating and very relevant. Do you have an explanation for this?
Kostas
Good morning Kostas. My first instinct is to say that the spacing and arrangement of fissures in granite is random, and that the Carnac situation is something determined by chance. After all, the fissures are probably many millions of years old, and bear little relationship to present-day topography. But then I thought that the arrangement of tors in Cornwall does seem to have something to do with topography -- with most tors on or close to hilltops and tors associated with areas of widely-spaced fissures in particular. The traditional geomorphological explanation is that lower downslope, where fissures are closer, bedrock rotting has been accelerated and block survival reduced, leading to greater erosion and land surface lowering.
So maybe this does have something to do with loading, or pressure release, or groundwater?
Maybe I can throw this one out to the geologists..... Rob, or anyone else out there, do you have a view on this?
I was taught is was to do with the
release of overburden pressure.
As you get closer to the erosion surface/land surface the joints open up.
But I am no expert on this. I am sure some structural geologist will have investigated the phenomenon.
GCU In two minds
There is a big discussion on granite landscapes and tors in the Geol Cons Review called "Quaternary of SW England" (Campbell et al, 1998). The authors suggest that the old Linton hypothesis is not far wide of the mark -- so we could flip the reasoning process on its head and say this:
You get a variable fracture pattern in any granite mass, depending on its cooling history, its shape and its relation with overburden etc. When the overburden is stripped off, there may be further fracturing responses (pressure release fracturing) to stresses in the granite mass or in parts of it. You then have a process in which erosion is more effective in areas of tight fissuring, and less effective in areas of widely-spaced fissures. So the former areas are turned into lowlands, and the latter areas remain as uplands.
Over-simplified, but maybe that is the best we can do.......
I'm sure there must be some learned papers in France on the geology of the Carnac area. Maybe it's all in there somewhere...
Brian,
Thanks for your reflections on the 'bedrock fissures' question.
Thinking some more on this, it now does make sense that the spacing between fissures would get smaller the closer to the 'continental separation' -- which would naturally be along the sea coast. This marks where the greatest stress of separation took place and so the fissures would reflect that. Somewhat analogous to the compression lines in a compression fracture upon impact.
I also think a combination of 'heat and ice' with extreme geothermal cooling during an ice age may have some similar effects. As the distribution of 'hot/cold' thermal stress in the bedrock and the resulting cracks will be wider where the bedrock is thicker.
Do we agree that generally the matching dimensions and shapes of the sarsens and of the bluestones at Stonehenge can be explained by such natural processes, rather than these stones being quarried and shaped to these dimensions by men?
Kostas
Not sure about bringing continental drift into this, Kostas. And you seem to be mixing up compression and tension.....
But yes -- by and large, I think most people will agree that the sarsens and the bluestones are for the most part natural shapes, determined by fractures in the ground or in rock outcrops. The inner horseshoe of bluestones (all spotted dolerite) do appear to have been shaped and bashed about a bit by the builders-- but all the others seem to be unmodified, except maybe for some rounding off of edges during glacial transport.
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