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Friday, 27 February 2015

Weathering of granite lying stones and standing stones



My thanks for Geo for bringing us back to this one.  Just a reminder of his earlier post on the blog:


"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 then 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 and thus postdates a barrow."

I haven't been able to get at these two papers,  although they are referred to by Chris Scarre and others.  But I have been able to find this one:





MÉGALITHES ET MÉTÉORISATION DES GRANITES EN BRETAGNE LITTORALE, FRANCE DU NORD-OUEST

Yannick LAGEAT, Dominique SELLIER et Charles R. TWIDALE, respectivement: URA 1562 du CNRS, Université Biaise- Pascal, 29, boulevard Gergovia, 63037 Clermont-Ferrand, France; URA 1562 et UPR 403 du CNRS, Université de Nantes, chemin de la Sensive du Tertre, B.P. 1025,44036 Nantes, France; The University of Adelaide, GPO Box 498, Adelaide, South Australia 5001.

Géographie Physique et Quaternaire, 1994, vol. 48, n° 1, p. 107-113, 3 fig., 1 table.

ABSTRACT Megaliths and granite weathering in coastal Brittany, northwestern France. Menhirs are elongate granite blocks placed upright, i.e. with the long axis in the vertical, in Neolithic times. Granite menhirs are prominent in the Morbihan and Trégor districts of coastal Brittany. Two minor forms, rock basins (also known as gnammas) and flutings (grooves, Rillen, Karren), are developed on menhirs. Two distinct generations of forms can be distinguished : those that predate the menhirs being placed upright, and those that postdate erection. Several flat-floored basins (or pans) that must have originated on flattish surfaces are now found on steeply inclined surfaces. On the other hand, smaller basins have developed on the summits of the monuments. Several flutings score the steep upper slopes of the blocks. They are deepest where they cut into outwardly convex inclined rock faces. They also diverge over such protuber- ances and terminate well above ground level. Clearly both the younger generation of basins and the flutings have formed after the monuments had been placed in their present upright positions and by processes active under subaerial or epigene conditions. In this last respect they stand in contrast with similar forms reported from other parts of the world. In Brittany the estimated age of menhirs is about 5000 years. Thus the flutings have deepened at a rate of a few tens mm/1000 years. The implied rate of basin development varies between 4 and 30 mm/1000 years.

It seems to me that Dominique's work is essentially related to weathering rates,  and it's interesting to see that she and her colleagues have been able to identify flutings on sloping or vertical surfaces and small pans on flattish or horizontal surfaces which appear to have formed as a result of weathering and erosion (as a result of rainfall above all else) subsequent to the erection of the stones.  It's also interesting that many larger pans or weathering pits are found on the flanks of these menhirs, which must have been created over many thousands of years when the stone slabs were lying flat on the ground -- ie before collection and erection by the builders of the Carnac stone alignments. 

In my previous post on Carnac, here:

http://brian-mountainman.blogspot.co.uk/2011/05/carry-big-stones-long-way-non-merci-mon.html

I noted that according to Chris Scarre, "........ 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." 


If we want to, we can refer to the gathering and erection of the Carnac stones as "quarrying" -- but the weathering information is self-evident:  granite surfaces exposed to the atmosphere for a very long time (maybe millions of years) will be deeply weathered and even crumbly (the Breton granite is notoriously "crumbly" and edges and corners tend to be rather well worn where exposed to the atmosphere), whereas the buried surfaces of slabs will of course be much fresher (with sharper corners and edges).  These differences will of course be readily visible when a block or slab is lifted to a vertical position.

I'm not sure if Dominique and her colleagues have done detailed weathering depth measurements on the various faces of the standing stones, but that could be interesting too.  Most interesting of all would be cosmogenic dating on stone surfaces -- at Carnac, Stonehenge, Callanish and many other places as well.  That may tell us a lot, including the exposure ages of the various faces of the standing stones and it might also help to fix a date for the dressing of those faces which have obviously been interfered with by the megalith builders.  But as I have said before, there are so many variables involved in this sort of work (rock surface shading, orientation, vegetation interference, time spent recumbent and time spent vertical, inherited cosmic "damage" prior to stone movement etc etc) that it may take many years and literally hundreds of cosmogenic dates to sort it all out........


Some of the pans or solution / weathering pits on the surface of one of the Carnac menhirs.  As the caption says, this surface was once almost horizontal, either at or maybe beneath the ground surface.  (Weathering like this could be effective beneath a layer of turf or loose vegetation.)

24 comments:

Geo Cur said...



The micromodeles are a result , not only of rain , but other environmental factors ,heat ,ice ,salt and vegetation .
Many of the Carnac stones were quarried from outcrops .
At a nearby passage grave at Kerbourg ,Emmanuelle Mens , like the prof , a male , has shown that all of the megaliths in the monumnet had the flat quarried sides facing inwards .

Constantinos Ragazas said...

Brian,

If you can clarify this for me ... Are the solution pans found on the menhirs on just one side or on both sides? And if these are found on both sides, how can that be? Since when lying horizontal only ONE side will be up!

Kostas

BRIAN JOHN said...

Yes, many factors influence surface weathering -- in the case of the pans collected pools of rainwater, and associated wetting and drying and freezing and thawing,are probably the most important factors. My impression is that the "pans" are only on one side of those stones which were lying more or less horizontally, whereas the fluted features occur on all sides of the standing stones. I would assume they would be deepest and most pronounced on those sides facing west, since that is the flank where the fiercest rain would hit the stones and would run off with the greatest force. But that's just speculation on my part.....

Geo, if you choose to use the word "quarried face" for any face of any stone that was embedded in the ground or in a rock outcrop, and was thus protected from surface weathering, that's your privilege.

Personally, I will prefer to use the term "protected face" since that makes no assumption about the intentions or technical capacities of the people who lifted a stone up and embedded it into the ground.

Constantinos Ragazas said...

Brian,

I know you think the "pans" are only on one side of the menhirs. But is that explicitly stated in these studies? Or assumed.

Also, if on just one side, is that side the same for all menhirs in an alignment?

You are right in arguing a stone from an outcrop would have a "protected side". Geo takes that as evidence of human quarrying. Whereas this would be a natural occurrence.

Kostas

Constantinos Ragazas said...

Brian,

To the "quarry" point Geo is making here, this evidence of solution "pans" formed when these stones laid flat horizontally proves these stones were not quarried by people! Since such "pans" would take thousands of years to form. While a stone quarried by people would take a few months to set upright.

Another one bites the Neolithic dust!

Kostas

BRIAN JOHN said...

Kostas -- I have no idea whether any menhirs have these weathering basins on two or more sides. Somebody will have to read the original papers to find out.

And the fact that pans occur on faces that were horizontal does NOT preclude "quarrying" by people. Flat-lying rocks can be loose or parts of bedrock outcrops. In the one case the rock would have to be lifted or levered up off the ground surface -- in the other case it would be necessary to lever a block of stone out of the solid rock, taking advantage of natural joint patterns.

Constantinos Ragazas said...

Brian,

Certainly stones lying flat horizontally at an outcrop would have such weathering basins on the top surface. But if these are loose, would these be said to be quarried by humans?

The key here is whether weathering basins also exist on two or more sides. If they do, that would definitely rule out human quarrying.

Kostas

Geo Cur said...



If some the stones came from outcrops and were subsequently erected then "quarrying" is a reasonable description . Lifting recumbent stones that are not part of the bedrock is not quarrying . Quarrying outcrops doesn't require a great deal of capabilty for HSS and in the case of granite greatly facilitated by the natural cleavage planes .
Sellier defines two categories of micromodeles ,erosion that ocurred before extraction and erosion that ocurred after erection .

Geo Cur said...

"The key here is whether weathering basins also exist on two or more sides. If they do, that would definitely rule out human quarrying."

It is clear that when the older generation of basins /gnammas are being referred to they are nor to be found on both sides of the rock ,it they were this would be immediately behighlighted as being extraordinary .
If it is the post erection weathering it would have no bearing on quarrying .
Why should it be necessary to point out something so obvious ?

Myris of Alexandria said...

Granite does not have cleavage planes.
Jointing is the correct term
Fractures too are different from jointing.
M

Geo Cur said...



"Granite does not have cleavage planes."


Ooops ,sorry . I was quoting Mens who in turn was quoting Eric Gaume (with an acute )naybe something got lost in the translation .

Constantinos Ragazas said...

Brian,

So there are two types of weathering features on stones: "pans" formed when the stones are set horizontally and "flutes" formed when the stones are set vertically.

So how does this prove anything about human quarrying? I think GeoCur is a little confused here!

Kostas

BRIAN JOHN said...

These weathering features prove nothing about quarrying. The stones don't have to be "set" in order to be subject to weathering processes -- they just have to be stones lying on the ground or standing more or less erect, with or without human interference. All the research tells us is that surfaces exposed for a very long time to the elements will have more weathering effects on them than freshly exposed surfaces. To that extent, this is all just a statement of the obvious.

Constantinos Ragazas said...

Brian,

We agree! But I go one step further and argue if weathering basins exist on two or more sides of menhirs that would prove these stones were NOT quarried by people from an outcrop (whether lying or vertical).

I really don't see Geo's argument here! Why did he even reference these papers? Other than to broadcast to others he knows French?

Kostas

Geo Cur said...



The confusion is all yours .
Read what was written .

I never said the pans suggested or "proved " quarrying or had anything to do with quarrying .

Geo Cur said...

The original point in mentioning the Sellier papers was in response to the suggestion that the Carnac menhirs had been glaciated , the analysis on erosion and the fact that they overlay a dated barrow argues against this , whilst excluding the more obvious lack of evidence for glaciation in the area . Quarrying wasn’t mentioned .
The second reason for mentioning Sellier’s early findings was in relation to this comment ;” One day I'd like to do a detailed analysis. Some of the stones used are quite interesting -- for example, the capstone at Pentre Ifan has a glaciated upper surface and a fresh or broken underside “ Repeating the Sellier post provided an example of a geomorphologist who had been doing the same type of analysis in the past ,although these days it appears to be a bit more techie . Again there was no suggestion from the author or myself that the analysis was evidence for quarrying , it just wasn’t the point .

Constantinos Ragazas said...

Quoting GeoCur: [ under "Erratic boulder shapes and megalithic settings" post]
-----------
Geo Cur said...
" One day I'd like to do a detailed analysis. " [Brian]
See http://brian-mountainman.blogspot.co.uk/2012/10/perched-residual-brittany.html
Scroll to last post .
For an example of analysis of quarried megaliths . [bold face mine]

26 February 2015 at 14:19
------------
Geo, what were you saying about not saying anything about "quarried megaliths"?

Kostas

Geo Cur said...


When you read the post about the analysis of quarried megaliths which you linked to you will note that there is no mention of the weathering "proving" they were quarried .
As you suggested in "how does this prove anything about human quarrying? " .

Constantinos Ragazas said...

GeoCur,

Of course the referenced paper did not make that association to "quarrying". But you did! Misleading others to think so too.

My comments were directed to you.

Kostas

Geo Cur said...



You haven't read the paper so you don't what it contains .

By not reading what was actually written and imagining something different , (a regular occurrence that is tiresome to continually correct ) , you mislead yourself .

chris johnson said...

Sorry Kostas,
There is nothing misleading about Geo's first comment. He tells something interesting and for me new.

Constantinos Ragazas said...

GeoCur/Chris,

I wrote, "... the referenced paper did not make that association to "quarrying". "

You wrote, "You haven't read the paper so you don't what it contains . "

So are you now saying the referenced paper did in fact "make that association to "quarrying""?

And while you are at it, perhaps you may also tell us if the paper states if the "weathering basins" are just on one side or several sides. A direct quote would be great!

Too obvious to mention? Too obvious to be overlooked!

Kostas

Geo Cur said...


Read what is written .
You did not read the paper therefore you don't know what it says in relation to quarrying .

“So are you now saying the referenced paper did in fact "make that association to "quarrying""? “

If I didn’t say that , then I didn’t say that and you would be imagining it if you said I did , so save me the trouble of correcting you yet again , don’t say I did .

BRIAN JOHN said...

Kostas -- comment rejected. I'm fed up with this endless speculation about what the papers did or did not say. You can read the paper I referred to on the web. Go off and read it. Let's move on.