Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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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:


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:

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.)

A nearly frozen wave

A photo from Jonathan Nomerfroh, taken during the current cold spell on the coast of Nantucket, USA.  Yet another for the collection of "weird and wonderful" phenomena from the world of ice.....

This is what happens when a wave approaches the shore and has to pass through a shallow water zone where the sea is almost freezing -- with many ice crystals present.  So the wave appears to be passing through a slush zone.

This happens quite often in the polar regions, usually at the onset of winter -- but it is not often photographed........

New work at Stanton Drew

Thanks to John Oswin for sending info about new work at Stanton Drew.  The "Big Ground Mound" at Quoit Farm has been excavated (in 2014) and subjected to close scrutiny using various high-tech devices -- and the conclusion is that the mound is not a relic of a long barrow or anything else man-made, but is an entirely natural feature.

That may be disappointing for archaeologists, but speaking as a geomorphologist, I find that rather refreshing and honest -- and a sign of good science.

The 2015 report is entitled "Probing the Big Ground Mound" and is by Vince Simmonds, John Oswin and John Richards.  It's published by the Bath and Camerton Archaeological Society.
Two different versions of the report are available on the website:

The report also contains a report of an experimental laser survey of the stones that make up "The Cove" -- although I have to admit to being somewhat at a loss as to what the purpose of the survey actually was........

Wednesday, 25 February 2015

Volcano cross-section

The island of La Gomera is essentially a single very large volcanic peak which has had a complex history of eruptions.  There have not been so many huge catastrophic events here as on other islands, so there are no large calderas (as there are on Gran Canaria, for example.) 

The above photo shows part of the cliff above the beach of La Playa, Valle Gran Rey, more or less at the western tip of the island.  At the base of the cliff there are assorted basalts, grey ashes and ignimbrites, then above those we see a series of ash beds -- some of them grey but others red and buff in colour.  These beds have been disturbed / consumed by several intrusions, and we can see the basalt dykes pushed right through them, causing major disruptions in the bedding.  On the right we can see some of the huge rockfalls from slope collapses as the cliff has been eaten away by the sea.

Tuesday, 24 February 2015

Igneous dyke, La Gomera

For the geologists in our midst.  One of the most amazing images of a dyke you are ever likely to see.  This one is exposed in a road cutting on La Gomera, high up on the "cumbre" above Vallehermoso.  The dyke has been intruded into red layered volcanic ashes and as it has melted its way  towards the surface (which must have been much higher than the eroded surface we see today) it has simply gobbled up the ash layers, leaving an incredibly clean contact on either side.

The dyke is made of basalt (I think) and it is about 10m wide.  The stone wall marks the edge of the roadway.  Because the road here is relatively new, this exposure is remarkably fresh.  Within a few years it will no doubt degrade.

Thursday, 19 February 2015

Submerged forest - Tywyn Beach

All hail to the photographer Howie Mudge for this fantastic photo of the submerged forest (mostly beds of peat) following the 2014 winter storms.  Published by the Welsh Photographers.

No particular reason for this apart from the joy of sharing!  And of course it is a reminder of climate change and the Holocene sea level rise.....

Some old comments

Apologies, all -- I have just found an assortment of posts on a strange "moderation page" on Blogger which haven't been notified to me in the usual way.  Not at all sure why........ anyway, I have pasted them in now -- even though some are more than a month old.

I also notice that there are some "Anonymous" posts that are making perfectly sensible points worthy of discussion.  But because my system is primed to dump ALL anonymous posts into the Trash (it's the only way to deal with spam) I do not even look at them.  So I urge any punters who wish to make comments to use either their names or pseudonyms, and by doing that the comments will come to my attention.  Thanks!!