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Thursday, 18 October 2012

Perched residual, Brittany


Since we have been talking about perched blocks lately, here is one from the north coast of Brittany, west of Roscoff.  Click to enlarge -- see the person for scale.  we see a cluster of perched blocks here -- the biggest is the one on the right.  Glaciation has nothing to do with any of this -- over many millions of years the granite has been weathered away in those areas where jointing is closest, and where rotting has been able to make most progress -- and the residuals are simply the bits of resistant or coherent granite that have been left behind.  They have been "let down" onto the current land surface bit by bit, probably in some sort of "jerky" motion, involving long periods of stability and then short episodes of sliding or settling.

I wonder how many of the big menhirs of Brittany have simply been "let down" in this way?  This is the mechanism most frequently cited in the case of the stones at Carnac.

This is the sort of mechanism also assumed to have operated on Salisbury Plain with respect to the sarsen stones -- which are essentially concreted residuals derived from an overlying sandstone layer that has been for the most part removed over millions of years.  As I have argued many times on this blog, they didn't need to come from Marlborough Downs -- the current thinking seems to be that they were all scattered about in the Stonehenge area, and did not need to be collected up from any great distance away.

10 comments:

Anonymous said...

Brian,

Are glaciers ruled out because glaciers did not exist in these areas? Isn't this exactly the same argument also used to disprove your 'glacier transport' theory?

Were we to just momentarily assume glaciers did exist, would that explain these perched blocks? If you notice, the surface of the rock these are resting on is very smooth, polished and rounded. Evidence perhaps of glaciation?

And the big menhirs? How does 'granite rotting' explain these? And the 'stone alignments' of Brittany? Or that was due to 'human agency'. Another vast 'no purpose' exertion of scarce human resources. Explaining Prehistory through 'no purpose' explanations does not explain Prehistory!

More than a year ago you reported on parallel stone 'ridges' found deep in the English Channel. It was 'big news' at the time. Want to do a follow-up post to this?

Kostas

TonyH said...

Slippin' and a slidin'
Peepin' and a hidin'
Been told a long time ago

LITTLE RICHARD/BUDDY HOLLY/ JOHN LENNON

Clearly the above-mentioned performers, and whoever/whichever wrote the lyrics, must have had Geological knowledge and the capacity for allusion for the benefit of the unwashed, winkle-picker masses of the late 1950's.

TonyH said...

I will send your Post directly to Dave Field of English Heritage, who as you know is a sensible dissident to the prevailing Conventional Wisdom (which is of course in no way true, deeply- considered wisdom!).

I am referring to your last paragraph, where you explain that the sarsen stones now part of Stonehenge may well have been ALWAYS nearby, through geological time, and there was no need for them to have any human agency involved in them reaching Stonehenge.

Let's hope the new, state-of-the-art, dispays in the next Stonehenge Visitor Centre will include a proper, respectful consideration of this probable geological scenario over millions of years. That's what we pay our taxes for!! It may not be Boy's Own, Ripping Yarns class, but it is a probability that ought to be made known to that man/woman in the street, who's often brighter than he/she's given credit for.

BRIAN JOHN said...

Kostas -- having walked 120 km along this piece of coast a few weeks ago (and wonderful it was, too) I looked very carefully for traces of glaciation - ice smoothed surfaces, erratics, till and other glacially derived deposits -- and found none. I have to conclude that wherever the ice did get to, if it ever did stray into the English Channel, it did not get this far.

The situation in Brittany is not analogous to that of Salisbury Plain -- in that case glacier modelling suggests that glacier ice MIGHT have encroached onto the chalklands, and we do have evidence of glacial deposits to the W, NW and N -- and not all that far away. So it is by no means a silly idea to think of glacial effects on the plain.....

BRIAN JOHN said...

Tony -- this is not new. Suggested by Andrew Goudie and many others over very many years. If the sarsens and the bluestones were all available within striking distance (as I think) it does not deny either the ambitions or the technical skills of the builders of Stonehenge. All that deserves our admiration -- and gives EH plenty of scope for the effective marketing of our wondrous ancestors.........
But I still think that they ran out of stones. In other words -- eventually, when the cost of fetching them from further and further away became greater than any benefits to be accrued, they just packed up and abandoned the project.

Anonymous said...

Brian,

Thanks for your response. And thanks for affirming “it is by no means a silly idea to think of glacial effects on the [Salisbury] plain “. I'll accept your observations on the glaciation of the Brittany coast for now. But I wont completely abandon the idea. Why? Because there is no other way of explaining the stone alignments in Brittany and elsewhere. It just does not make sense to me these senseless rows of stones were put there by prehistoric people.

Anything new about the parallel stone ridges submerged in the English Channel? I am very intrigued by these!

Kostas

BRIAN JOHN said...

Now here's a nice thought for you, Kostas. Been reading a book about the "Magelithic Empire" in which the authors, in all seriousness, refer to Carnac as a gigantic showroom / builders merchants in which the sturdy quarrymen of that area have simply set out their wares (ie those nice standing stones) in straight lines, for the delectation of their potential customers, who used to wander up and down the rows choosing the stones which they liked the look of. Presumably they were graded according to size and other characteristics, and priced accordingly. Not sure whether the owners of Carnac Stones Inc charged an extra fee for delivery and erection -- but maybe the prices were all-inclusive. A nice idea -- I like it!!

BRIAN JOHN said...

I know nothing more of those stone ridges in the English Channel -- I have already reported on all the info currently to hand.

Anonymous said...

Brian,

From what I recall of the Carnac stone alignments, they were not all that straight. More like parallel contour lines. What you would expect of a receding glacier. And except for a few gaps between stones, most seem to have existed undisturbed. Carnac Stones Inc. could not have survived long. Since apparently they sold very few stones! Or each stone was only a 'sample display' capable of being duplicated at the customer's wishes.

Some serious questions:

1)Do sea glaciers carry “erratics, till and other glacially derived deposits”? Makes sense land glaciers do, but what about entirely sea glaciers?

2)Could such glacier evidence be found submerged in the English Channel? Could the mysterious 'stone ridges' found submerged in the English Channel be such evidence? As well as the Carnac alignments? (sorry, I don't believe in the Carnac Stones Inc. explanation of these!)

Kostas

Geocur said...

Dominque Sellier ,Prof of physical geography at Nantes Uni and usually described as a geomorphologist has shown in two papers 1) that the landscape around Kerlescan consisted of grantitic outcrops “”Analyse morphologique des marques de la meteorisation des granites a partir de megalithes morbihannais “1991 . And , following on from other studies relating to cleavage planes and the tendency of granite to fracture along orthogonal planes , which when quarried displays two main faces , one displaying the fresh face corresponding to the surface where the stone was broken away and the weathered face , a convex side of the stone that was initially exposed to the open air . A close inspection of the exposed face reveals traces of erosion called micromodeles and the in 2) Elements de reconstitution du paysage premegalthique sur les sites des alignemnets de kerlescan a partir des criteres geomorphologiques “(1995 ) defines two specific categories of micromodeles which can be used to distinguish between weathering before extraction and erosion after erection . Of course as has been mentioned before the Kerlescan alignment overlies a thus postdates a barrow .