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.)