THE BOOK
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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Wednesday, 29 April 2015

De Geer Moraines


This is a wonderful LIDAR image of De Geer moraines in SW Finland.  These stringy little moraines are generally assumed to form parallel to the retreating ice front  -- but some way up-glacier from the ice edge -- in conditions where the bed material is saturated and where the ice is almost being lifted off the bed in a lake or marine environment. 

When is a pingo not a pingo?


 A fabulous photo of De Geer Moraines (cross-valley moraines) on Baffin Island.  It is now being suggested that many of the "rampart remnants" found in Wales were formed as morainic features like these, and are not pingo remnants at all......  I am not 100% convinced, but the evidence is persuasive.

Arising out of some of the comments of recent days, I have been digging a bot more on the matter of pingos in Wales, and how common they might be........... and it appears that they are less common than we might think.

Dave mentioned that the "pingos" near Blaenporth look more like bits and pieces of eskers than the remnants of circular pingos -- good point!  The same is true of some of the Norfolk features as well -- cf the monochrome photo which I published.

In 2004, many years after the work of Edward and Sybil Watson was published, CCW announced the following project: 

Mapping and Conservation Assessment of Pingos in Wales
This was a joint CCW/University of Cardiff (Department of Earth Sciences) project undertaken as a Ph.D. study that mapped pingos (ground ice depressions) in Wales. Pingos are arguably better developed in Wales than anywhere else in the British Isles. However, prior to this study no comprehensive review of this geomorphological resource had ever been undertaken. Moreover, these are relatively small-scale and fragile landforms and, over the years, many pingos have been drained or destroyed. Many pingo basins contain significant flora and fauna, often with a thick peat deposit that contains an important pollen record which can be radiocarbon dated, thereby providing an important history of environmental change spanning some 10,000 years. Key objectives for the project were to locate all remaining pingos and similar landforms across Wales, to map and compare them, and to examine the structure of a selected few in greater detail. The study provided a framework for conserving key parts of this important geomorphological resource (either as SSSI or RIGS).


The work involved Dr Peter Brabham and a research student called Neil Ross.  There was a brief report in "Earth Heritage" No 23,  2004/5, and in 2006 Neil was awarded his doctorate for a thesis in which he examined in great detail the evidence for and against pingos in Wales.  The complete thesis can be accessed here:

http://orca.cf.ac.uk/id/eprint/56198

Neil Ross PhD thesis 2006
Title:
A re-evaluation of the origins of Late Quaternary ramparted depressions in Wales
Neil Ross
School of Earth, Ocean and Planetary Sciences Cardiff University
2006

(See also: Ballantyne and Harris 1994)

Here are some of the excellent illustrations from Neil's thesis.


 Map of "ramparted depressions" in Southern Britain.  Note the two main clusters -- in Wales and Norfolk.  Note that the author does NOT refer these features as "pingos"........


The distribution of so-called "pingo groups" in West wales, after Watson and Watson.


Some of the sinuous ramparts showing up on a satellite image.  This group is south of Blaenporth, in the Hirwaun Valley.


Another group of sinuous features near Ponthirwaun, in the Hirwaun Valley


Neil Ross's map of the "ramparted" features in the Hirwaun Valley.  While there are many irregularities, note that there is an approximate lineation from NE - SW.  That is perpendicular to the last direction of ice flow across this area.


Contour model showing the location of the Hirwaun Valley "ramparted features" and associated depressions.  Neil estimates that these features lie approximately at the highest level of pro-glacial Lake Teifi.  The blue contour shows the approx position of the shoreline.


I haven't had a chance to look through the whole thesis yet, but it looks to me like an impressive piece of work -- and the evidence is pretty convincing that these ridges and depressions are related to sub-glacial processes which operated near the maximum of the Late Devensian glaciation of the area.  The nature of the sediments in the depressions and in the ramparts do not suggest a periglacial origin at all  -- and Neil has done a lot of excavating and a lot of sedimentological analysis. 

Neil suggests that very few of the "pingos" recorded in Wales are actually pingos -- and grave doubt can be expressed about some of the Norfolk features as well.

So we have a nice old-fashioned controversy here.  More of which anon.

Now for something completely different....



As some of you will know, when I'm not digging the garden or fooling around on this blog, I write novels and try to sell them.  One has to do a spot of promotion now and then, so this week I am doing a Kindle Freebie Promo Whammy -- all eight novels of the Angel Mountain Saga free in the Kindle editions, each one free for just 24 hours.

Right now, the ones available for free download are On Angel Mountain, House of Angels and Dark Angel (vols 1-3).    Tomorrow, there will be three more, and on Friday the last two.

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Tuesday, 28 April 2015

Pingo pictures


A melting pingo in an area of continuous permafrost, NWT, Canada.  The dome is much lower than it was, but there is still an ice core present.  As the pingo dome subsides, other patterned ground processes have taken over, allowing the polygonal pattern of the surrounding terrain to extend across the pingo as well. 



Oblique aerial shots of Norfolk "pingo landscape" -- some of them have flooded or boggy centres, and others don't.....

Sunday, 26 April 2015

Stone shapes

Now for a bit of simple sedimentology.  The slight confusion over the description of rhyolite flakes in the Stonehenge debitage as "rounded" or as "more rounded than the less rounded ones" (just joking) reminded me that actually we don't need any confusion at all.  I have done posts on this topic before -- just put in "angular stones" or some such thing into the search box, or take a look at this post:

http://brian-mountainman.blogspot.co.uk/2010/04/more-on-stone-shapes.html

The literature is full of charts and diagrams of particle shapes -- ranging in size from sand grains to mighty boulders -- and the measurement of these shapes is still a favourite field trip exercise for school children and degree students.  In a river valley filled with terrace gravels or pebble beds, for example, you would expect the degree of roundness of the particles to increase steadily from the headwaters towards the sea.  You go to your first sampling site and collect your pebbles systematically and without bias, compare them visually against a chart (or measure much more carefully, using the Cailleux technique), make your records, and then go off and do the same thing at all your other sites.  Simples!

You can make as many particle shape categories as you like.  As a research student, I used just four -- I reckon six to be good, but more than 8 to be more trouble than it's worth.....  The chart below with 9 categories is by Krumbein, and the one with 6 categories is by Power,


You can portray your results in many different ways -- bar charts, pie charts etc.  Below is a typical representation of the results of a river valley pebble survey.  You can of course make things more sophisticated by taking samples of different stone sizes, and measuring the particle shape distributions for each stone size....... that can sometimes tell you something interesting.


So there we are.  No excuse for misunderstandings in the future.....




Saturday, 25 April 2015

On pingos and circular enclosures


 The slightly raised embankment of the mysterious feature on the Preseli north slope.  Outside the circular feature there is dry heath, and inside, as we can see, is a soggy area with rushes.

Thanks to Hugh and Geo, we have homed in on a rather interesting circular embanked feature not far from Carn Goedog.  According to Dyfed Archaeological trust, its origins are mysterious, because there does not seem to be a ditch and bank arrangement (either outside in or inside out) and because the central enclosure is sunken and soggy.  When I read the site description I immediately thought "Pingo?" -- and this possibility is worth considering.

Here's the description of the Nant Cledlyn SSSI in Ceredigion:  "Nant Cledlyn Pingos SSSI is of national importance because it provides some of the best- preserved pingos in Wales. Each individual pingo comprises an elevated rampart that surrounds an internal basin that in some cases contains a significant thickness of peat. It is thought that the pingos were formed during very cold climate (permafrost) conditions that prevailed approximately 10,000 years ago, and that sediment deposition commenced at approximately the same time."

There are quite a few fossil or relict pingos in Wales, and they were properly described for the first time by Edward and Sybil Watson from Aberystwyth Univ. Geography Department -- a somewhat eccentric and wonderful couple who were inveterate field geomorphologists.  Here is their map from a 1974 paper in Geografiska Annaler:


The Ceredigion examples are highly degraded, and sometimes all we can see today are slightly curved sections of embankments or ramparts.  Here is another example from Thompson Common in Norfolk:


These are very subtle features in the landscape, dating for the most part from the Younger Dryas around 10,000 years ago, and it's very easy to confuse them with ring cairns or other man-made features.  But geomorphologists have now found them all over the British Isles, and they are indicative of very severe permafrost conditions where peculiar hydrological conditions apply.  When I was in East Greenland in 1962 some of my colleagues were studying a group of pingos in Pingodal which were in various stages of disrepair.  They are ephemeral, and may last for a few decades or centuries.  They have a core of ice -- an ice lens -- which tends to grow each year as more water arrives by subsurface flow (beneath the permafrost layer) and then freezes onto the lens.  They can grow to 30m in height, and in Siberia and Alaska they are very spectacular features of the tundra landscape,  as shown in the photos below:



They are also called "hydrolaccoliths" and "pseudo-volcanoes" -- the latter name is quite descriptive, because when the ice lens expands to the point where the surface sediment is too thin to provide effective insulation, it all starts to melt.  The surface subsides, bits of the lens are exposed, and a "crater" begins to develop. This fills with meltwater, and that accelerated the melting of the lens until it is entirely wasted away, leaving a circular depression with an irregular rampart around its edge.

We should use the term "pingo" for something which is active, as in the Arctic examples above, and the term "fossil pingo" or "relict pingo" for the sort of thing we see in Wales.

There are two types of pingo -- open system and closed system -- but we won't go into the technicalities here.  The most spectacular examples tend to be located on wide open plains or gravelly river valley floors, but they are also found in undulating or sloping terrain.

So could the Preseli example be a pingo?  Quite possibly --  there are some small relict pingos on Trerhos Common, to the west of Wolfscastle, and others on the south side of Preseli.  More to the point, the SSSI citation for Mynydd Preseli mentions relict pingos on Brynberian Moor, Gors Fawr and Waun Isaf -- and this is the type of territory we are talking about here.

Watch this space....... I suspect that the more one looks for ancient pingos, the more one will find.

Wednesday, 22 April 2015

Llangolman gravel pit -- Anglian fluvio-glacial gravels?


Prof Danny McCarroll examining one of the erratics in the Llangolman gravel pit, adjacent to St Colman's Church


It was good to take another look at this site today, in the company of farmer Huw Absalom and Prof Danny McCarroll and his wife Louise.   It was almost too hot!  And the faces of the gravel pit were so dry that it was quite difficult to pick out the stratigraphy.

But in our discussions Danny agreed that we were probably looking at Anglian fluvio-glacial materials, maybe 450,000 years old.  He agreed with me that the degree of staining by manganese and iron oxide is indicative of great age, as is the extent of pebble "rotting" which is far greater than in the Devensian gravels seen in other pits in the Cardigan - Moylgrove area.

We examined the fossil frost cracks in the upper gravel horizons, which suggested the presence of permafrost during and maybe after a long period of gravel deposition.  Another thing Danny noticed is that one of these frost cracks seems to have a sand layer running across its top edge -- suggesting that this sand layer might have been deposited in a subaerial environment following an episode of ground freezing -- making it a suitable candidate for OSL dating. That's something we need to work on.

On my previous visit I thought that the gravel spread in the vicinity of St Colman's Church was a remnant of a previously much more extensive cover.  But  now I think I agree with Danny that this is probably a mound of fluvio-glacial material which we might call a kame.  It has been eroded away in some places by spring sapping, but it is clearly in a hilltop position, and it is probable that it owes its origin to an accumulation of meltwater debris carried into an open pit in a dead-ice environment.

More Rhosyfelin geomorphology


Two developments today:

1.  I received a letter from a geologist with whom I exchange occasional correspondence -- and he tells me that he has been in touch with the National Museum of Wales on the subject of Rhosyfelin and the archaeological interpretations of the features there.  He says he was surprised, at a recent lecture by Richard Bevins, to hear such a senior geologist  "indulging in propagating the MPP meta-narrative, riddled as it is with speculative interpretations."  He also complained about the manner in which the archaeologists have "progressively destroyed" the geological / sedimentary evidence that might have given solid information to anybody with geomorphological training about the Holocene history of this site.  He made some very cogent points about the fracture plane -- which I look forward to discussing with him -- and finally expressed his great surprise that "MPP has been allowed to wreak havoc in a section of the National Park, effectively reconfiguring the topography by what can only be regarded as an engineering activity."  So there we are then.......  I am clearly not alone.

2.  I spent a pleasant couple of hours at the site today in the company of Prof Danny McCarroll of Swansea University and his wife Louise, giving the place a good going over.  They are both geomorphologists with great field experience, and suffice to say that like other geomorphologists who have visited the site they could see NOTHING indicative of any human intervention either on the rock face or in the area dug over and left in such a mess.  We looked at the so-called railway tracks, wedges and props, scratches made by heavy stones being dragged along, shattered slabs damaged by the transit of orthostats, and so on.  Their response was the same as mine:  "Is this some sort of a joke?"  Enough said.  If Danny and Louise read this and want to add anything, they will be very welcome.

One thing Danny did suggest -- and I am in agreement with him on this -- is a visit from the specialists who belong to the Quaternary Research Association if the dig site can be left open for inspection at the end of the Sept 2015 excavation season.  The Association has a very big membership of Quaternary specialists from the UK and further afield, and if 15 - 20 of them arguing things out on site cannot sort out the truth, nothing will!  So I will get a request into the system.

Wednesday, 15 April 2015

The Floss Stone at Garn Turne


This photo shows the Floss Stone in the foreground, slap in the middle of the forecourt or portal area as designated by earlier archaeologists.  The green grassy area between the Floss Stone and the "giant capstone" is where most of the digging was concentrated in 2011 and 2012.  Naturally enough, not much attention has been paid to the Floss Stone in the past, because the "giant capstone" is the one featured in all the books!  We would probably agree that the huge "capstone" has not been moved, but there has been some excavation beneath it.  As I understand it, Vicky Cummings and Colin Richards think that the Floss Stone may have originally been closer to its massive relative at the "focal point" of the forecourt, where the horns come together, but that it slid or fell over before landing in its present position.

Hope I've got that right.......

Tuesday, 14 April 2015

Back to Garn Turne



 Above: one of the natural rock outcrops of Garn Turne, about 50m from the earthfast dolmen.  Below:  View directly towards the SW, with Lion Rock (Poll Carn) in the distance, beyond Trefgarn Gorge.  The "entrance" to the burial chamber faces the camera -- away from the impressive view across the landscape.  The builders of the tomb probably did not move this gigantic stone at all -- it was used exactly as it was found.

Been to have a look at Garn Turne today.  It's about 2 km from Wolfscastle and just over 1 km from Sealyham Bridge, in the middle part of Pembrokeshire.  There is some doubt about whether the Devensian ice reached this far, but the current feeling is that the ice limit lay a little distance to the west, near Wolfscastle and the northern edge of the Trefgarn Gorge.

The site is a very beautiful one -- with a small series of crags in a rough paddock which is surrounded by green cultivated fields.  The outcrop -- or series of outcrops -- called Garn Turne is made of the volcanic rocks of the Sealyham Volcanic Formation, intruded into or resting above dark grey silty shales and mudstones of Arenig (Ordovician) age.  The rocks are older than the Fishguard Volcanic Formation -- green-grey in colour and with a rough knobbly surface texture.  The rocks are classified as andesite and dacite lava and hyaloclastite, with some interbedded tuffaceous mudstones.

The crags consist of a series of vast detached and tumbledown blocks resting on bedrock outcrops with small cliff faces.  Right across the paddock there are hundreds of other boulders and detached blocks, many of which are in approximately their "original" positions and others which have been moved downslope by slope processes over hundreds of thousands of years and possibly by Anglian ice action as well.

The famous earthfast dolmen excavated by Colin Richards and others a couple of years ago is just a huge slab of bedrock weighing more than 60 tonnes with other stones moved into position around it.  Colin thought originally that the big "capstone" (if that is the right word) was quarried from the rock outcrops 50 - 100m upslope and moved to its lower position by the cromlech builders; but he seems now to have abandoned that idea.  I agree that it was used where it was found, with a burial "chamber" simply excavated beneath it.  The forecourt faces NE -- for the simple reason that that was the natural direction in which the stone "faced" when it was embedded in the ground.

The smaller stones do not appear to have been shaped at all by the men who placed them into position-- they have sub-rounded edges but rough and irregular outlines.  Some of them have probably been moved a short distance by ice -- so they have again been used more or less where found.

Geologists Unearth Fully Intact Rock



Thanks to Geo for the heads-up on this one.  Great report!   Should be read by all archaeologists who do not already subscribe to THE ONION........


Geologists Unearth Fully Intact Rock
News in Brief • Nature • environment • Science & Technology • science • ISSUE 51•13 • Apr 3, 2015

FORT COLLINS, CO—Describing the discovery as the most flawless specimen ever unearthed, a team of geologists working in northern Colorado announced Friday they had excavated a fully intact rock. “This stunning find provides an illuminating glimpse into what rocks may have looked like in their complete form millions of years ago,” said lead geologist James Powell, adding that the extremely well-preserved rock offers unprecedented insight into the physical structure, shape, and characteristics of early rocks in ways that incomplete stone fragments and shards never could. “Previously, reconstructing a whole rock from small remnant pieces was difficult, and we never had a complete specimen. We’d invariably end up with several missing parts, and we could only speculate about what might have filled them. But now, we can safely say that these empty spaces were most likely filled with other bits of rock. It looks nothing like we could possibly imagine.” Powell added that further research was necessary to determine if a geological link existed between rocks, pebbles, and boulders.

Monday, 13 April 2015

Rock surface weathering, Broad Haven, Pembs



Interesting differences in sandstone weathering surfaces, Coal Measures sandstones, just north of Broad Haven Beach.  Both of these are gently sloping surfaces around HWM, in the salt spray zone.

Sunday, 12 April 2015

Bluestone dimensions




Above are two of the illustrations from the EH report relating to the laser scanning project (2012).

Ref:
Research Report Series    32-2012
STONEHENGE LASER SCAN: ARCHAEOLOGICAL ANALYSIS REPORT
Marcus Abbott and Hugo Anderson-Whymark, with contributions from Dave Aspden, Anna Badcock, Tudur Davies, Mags Felter, Rob Ixer, Mike Parker Pearson and Colin Richards


Free pdf download.

The table of areas, volumes and weights is really rather strange.  Knowing "above ground" volumes and weights is of no use to anybody -- why didn't they give us maximum length x max width x max depth so as to give us an idea what these stones actually look like? The Altar Stone is not on the list, and neither is stone 38.  Neither are the stumps 32c.d and e.  But as far as the standing stones are concerned, we do at least get some idea of which ones are large and which ones are small..... and the slabs obviously have smaller surface areas and therefore smaller volumes and weights.



Altar Stone surface markings (revised)


The is a really interesting photo of the surface of the Altar Stone -- with surface textures and markings freakishly revealed.  Click to enlarge.  Forget about the shadow -- but what is the explanation for these markings?

Some info kindly added by Geo -- worth adding.  Here is another pic, acknowledged as from the Modern Antiquarian site.  Its from Turrerich, near Aberfeldy in Scotland, posted by Tiompan.

 Although the Altar Stone appears to be much finer grained, both stone surfaces suggest thin-bedded or laminated sediments with some contortions.  the cup marks on the Scottish stone are rather typical, but apart from one suspicious feature in the foreground of the top pic, the Altar Stone seems to be unmodified by the hand ogf man.  Unmodified by the foot of man too, I suspect -- there might have been a small amount of "polishing" over the centuried by people walking over the stone, but my suspicion is that these features are the result of many thousands of years of surface weathering.  Acid waters in the Scottish case and calcium carbonate waters on Salisbury Plain?

Friday, 10 April 2015

Rhosyfelin: there are fairies at the bottom of my shrubbery


I'm not sure whether to laugh or cry.  They say that Stonehenge makes men mad, and I'm beginning to feel that Rhosyfelin does as well.

To start with, let's put the record straight.  For the last four years, on this blog, I have urged the archaeologists not to leap to conclusions on what went on at Rhosyfelin, and to have nothing to do with ruling hypotheses about Neolithic quarries. (They are, apparently, undeterred....)  I have similarly tried to urge caution at public meetings addressed by Mike Parker Pearson and others, starting with the public meeting in 2011.  Over and again I have pleaded with them to bring in geomorphologists who know the area, so as to avoid silly mistakes on the interpretation of sediments.  In Emails I have offered to help -- and all my offers have been ignored.  Twice I have arranged to meet the archaeologists at the dig site to assess the evidence on the ground, and twice, having turned up, I found nobody there.  One might be forgiven for thinking that geomorphology / glacial geology has no place at all in the great scheme of things........  and yet we can be quite sure that at least some of the Rhosyfelin diggers follow this blog quite carefully.

There have been no site reports or interim findings published relating to the digs of 2011, 2012, 2013 and 2014.  Nothing on the web and nothing on paper.  But after a great deal of frustration, and a great deal of urging on all sides, it seems that a paper containing a comprehensive description of the dig discoveries, with radiocarbon dates included alongside sediment and organic analyses and some more petrography from the geologists, is in the process of publication.  Those involved are sworn to secrecy.  It will be a multi-author cooperative project. It sounds as if it has been peer-reviewed and accepted for publication. 

Now we come to the surreal bit.  We are reminded by our good and mysterious friend Myris (who seems to know what is going on) that if the "news" of Pleistocene sediments at Rhosyfelin is to be discussed, let alone accepted by archaeologists, it needs to be published, preferably in a peer-reviewed journal.  I get a distinct impression that the article will be so obsessed with the so-called quarry and the pseudo-proto-orthostat that it will not even mention Devensian till and fluvio-glacial sediments, or Holocene slope deposits and rockfalls.   But the interpretation of the deposits at Rhosyfelin is so absurdly simple that it could have been done in the course of a 30-minute visit by an A level student or first-year geography undergraduate, by reference to any one of a multitude of text-books and on the basis of just a little field experience.  By the same token, a description of the site written by me or anybody else would also be so absurdly simple that no self-respecting geomorphology research journal would waste space by publishing it.  I have, as readers of this blog will know, already put a brief description of Rhosyfelin onto the web site called SCRIBD.  So when people say "put up or shut up" or "publish and be damned" my answer is that it is already done.  I have had enough peer review already from people whose opinions I respect.  We do not need a battery of whizzo techniques to show that a till is a till, and that a rockfall is a rockfall.  What we do have, if you will allow me to blow a trumpet here, is a considered view from somebody who was joint author of the UK's best-selling glacial geomorphology textbook of all time.  One might have thought that would be worth something.......

So yes, a number of us will produce a joint note on what is to be seen at Rhosyfelin.   It remains to be seen whether anybody will want to publish it.   Before long there will be a RIGS designation too.  I'll keep you posted on that.

But I have a really bad feeling here -- on the basis that a big article on the Rhosyfelin dig will be published, with an enormous media fanfare, with a number of geologists involved in it, but without any input at all from anybody familiar with the glacial and periglacial deposits of West Wales.  The authors will take joint responsibility for the contents of the article -- they will all have seen it in draft form, and they will all (I sincerely hope!) have been given opportunities for changing phraseology or addressing defects or omissions.  The referees -- whoever they were -- will also be culpable if they have failed to give the article proper scrutiny, and if they have failed to ask hard and obvious questions about the glacial history of the area.  So they sink or swim together.

Lifejackets, anybody?




Saturday, 4 April 2015

New paper on Bluestone 38

 
 
Altar Stone (top), Stone 38 (middle) and Stone 48 (bottom) -  thanks to the Stones of Stonehenge site for the photos.  These stones are neither rhyolitic tuffs nor dolerites, spotted or otherwise.  Look at the extraordinary markings on the surface of the Altar Stone -- more on this later.....
The new paper refers to Stone 38 and related debitage.

More images:

Stone 80 (Altar Stone)
http://www.stonesofstonehenge.org.uk/search/label/Altar%20Stone

Stone 38
http://www.stonesofstonehenge.org.uk/search/label/Stone%20038

Stone 48
http://www.stonesofstonehenge.org.uk/search/label/Stone%20048

There's a new paper from Ixer, Bevins and Gize on the subject of Bluestone 38 and the related debitage at Stonehenge -- which might, or might not, have come from the stone itself. Once again, an interesting piece of work which increases our understanding of the widsespread provenances of the weird bluestone assemblage.

Hard ‘Volcanics with sub-planar texture’ in the Stonehenge Landscape by Rob A. Ixer, Richard E. Bevins and Andy P. Giże
Wilts Arch & Nat Hist Mag 108 (2015), pp 1-14

Volcanics with sub-planar texture -- twelve fragments have been examined. They are "small, often sub-rounded, rather than angular flaked, fragments throughout Stonehenge and its environs ....."  And there is quite a wide distribution of fragments across the landscape.  Stone 38 itself is a strange little one, quite unsuitable to be counted as a fallen orthostat -- it's more like a mis-shapen boulder, and like  Stone 48 it's best described as a glacial erratic, pretty worthless for incorporation into a megalithic monument, but simply used because it was there........

Quote: These volcanics comprise two groups namely Volcanic Group A, friable rocks with abundant white mica and a strong metamorphic fabric and Volcanic Group B, hard rocks that are partially characterised by an unusual mineralogy including two forms of graphitising carbon. Only twelve Volcanic Group B samples have been recognised from the Stonehenge debitage but they share the same petrography as Orthostat SH38. Spatially, as with the debitage from the Altar Stone and Orthostat SH48, they are widely, if thinly and randomly, distributed throughout the Stonehenge Landscape. Temporally, almost none of the debitage, from all three bluestone orthostats, has a secure Neolithic context. 

Quote:
 
-->
Conclusions 

Orthostat SH38 and twelve pieces of debitage that constitute the new Volcanic Group B class of debitage are sufficiently uniform in terms of their mineralogy, grain size and textures that it seems probable that they are all from the same rock rather than just from the same outcrop.

Although this debitage is numerically rare it has a wide spatial distribution in the Stonehenge Landscape notably within the Darvill and Wainwright April 2008 excavation and Heelstone Ditch but also including within Trench 45 in The Avenue and Aubrey Hole 7 in Stonehenge. Although a lithic with graphitising carbon was found from close to the Stonehenge Greater Cursus no SH38 debitage has been recognised from there with any certainty. The SH38 debitage distribution is similar to that found for orthostat SH48 but is more extensive than that for the Altar Stone.

The temporal distribution of the SH38 debitage is very similar to that for SH48 in that most pieces are found from post Neolithic contexts but are less ‘bunched’ than that from the Altar Stone.

The newly reported SH38 debitage has extended the range of petrographical features beyond those seen in orthostat SH38, notably to include the presence of large zircons, rare earth-bearing minerals, tube pumice and a significant fine-grained siliceous component. This in turn suggests that were the single geochemical analysis for SH38 (Thorpe et al. 1991) and taken from a very small sample , to be augmented by new analyses from the present samples, a geochemistry that was closer to the bulk geochemistry for SH38 could be achieved. An enhanced petrography plus a more representative geochemistry would help to narrow the possible geographical sources for the orthostat. On present knowledge this is still expected to be found within the Ordovician Volcanic sequences, in the north Pembrokeshire area but the net is tightening.






Wednesday, 1 April 2015

Rhosyfelin Quarry -- going, going.........gone?


Had a pleasant visit to Craig Rhosyfelin this morning in the company of geomorphologist Dr Dyfed Elis-Gruffydd and geologist John Downes. We examined many of the features of the site and hunted hard for a quarry.......

Quite a few earth scientists have now visited the site privately or in university field study groups, leading U3A field trips etc -- and I think I am right in saying that not one of them has seen anything suggesting human quarrying activity.  Two groups have visited the site in recent weeks.

The "features" cited by Prof MPP as indicative of quarrying, such as rock props, rails, wedges, hammer stones, scratch marks etc are examined with incredulity, and are universally interpreted as entirely natural.

The consensus is that what we have here is an assortment of Holocene deposits dating back to the Devensian glaciation -- rockfall debris, till, fluvio-glacial and fluvial sediments, some stillwater sediments, slope deposits and modern soil.  What we are also agreed upon is that there is a lot of organic material incorporated into the rockfall and slope deposits, because trees and shrubs on the crag and in the little meltwater channel have played a key role in the degradation of the crag -- thus many rockfalls will have dragged down roots, trunks and branches over a very long period of time, to be incorporated in the organic-rich sediments that "enclose" or incorporate the rock debris including the famous pseudo-proto-orthostat..

Rhosyfelin -- place of beauty, or corporation rubbish dump?



Far be it from me to cause any mischief, but I was down at the site this morning, and wondered what the official view might be of the appearance of the dig.  When you look skywards, you see something of great beauty (top pic), but when you lower your gaze, you are confronted by something like a corporation rubbish dump (bottom pic).  The plastic is shredding every time there is a high wind, and a large piece of it was discovered the other day in the garden of the cottage next to the site. 

Apparently somebody came from the National Park Dept not long ago, and it seems that he was responsible for lovingly wrapping the pseudo-proto-orthostat in new tougher plastic sheeting.  Why?  Is it likely to be harmed by the rain? Answers on a postcard to the National Park office please.......