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Wednesday, 28 September 2016

Hell Bay Moraine, Bryher, Isles of Scilly


There is a bit of discussion about the nature of a ridge of large boulders that runs part of the way across a narrow peninsula of land leading to Shipman Head on the Isle of Bryher.   I need to revise my opinion!

This is what I said in my working paper:

The Hell Bay Moraine on the west side of the headland runs straight downslope, and is made of a jumble of erratic boulders and slabs mixed with angular blocks of local origin.  This feature is shown on topographic maps as "castle ramparts" -- or as an Iron Age or Bronze Age defensive feature.  However, initial examination suggests that it is natural.  The boulders and slabs are much larger than those normally used in defensive ramparts, and many of them are faceted and abraded. 

I have been scrutinizing the satellite imagery, and I have enlarged a section of it above.  What we can see in the centre of the photo is a ridge made up of a litter of very large blocks.  Some of these may have moved downslope from the granite outcrops at the top of the photo.  The cliffs are just off the edge of the image. But I am still convinced that most of this material is contained within a very large moraine.  But look carefully to the right of it and you can see two roughly parallel lines of smaller boulders, with a continuation southwards in the form of an accumulation of even smaller blocks -- just the sort of material that Iron Age people all over Britain used for building their fortified defensive banks.  This was completely invisible to me on the ground when I walked across it -- maybe the vegetation was more extensive and higher than when this image was taken.

So now I think that I'm correct, and so are the archaeologists.  Moraine AND a man-made defensive feature.

So there we are then.  Nice compromise.

Stonehenge as it might have been -- or maybe not....



Stonehenge 2,800 BC, copyright Peter Dunn.  Gouache on Art paper.  Original for sale.

On this blog we are always happy to help the poor and needy, and having had my attention drawn to Peter Dunn's very skilled artwork I'm happy to help him to sell some of it.  Info here:

http://www.sarsen.org/2016/07/peter-dunn-exhibition-and-sale.html

and here:

http://www.mcneillgallery.com/index.php?s=Peter%20Dunn

The illustration above presumably shows Peter's interpretation of what Stonehenge looked like in its pristine or "perfect" form.  There are a number of notable features.  The Altar Stone is shown as a big upright slab or pillar.  Then we have a small bluestone circle contained within the setting of sarsen trilithons.  We have two mini-trilithons made of bluestones -- that's a fair assumption, since some of the working on dolerite bluestones suggests this sort of usage at some stage.  Then we see a sort of mini-avenue of bluestones flanked by a double bluestone setting -- and I wonder where the evidence for this has come from. 


My main problem with the portrayal is the assumption that all of the bluestones in the bluestone circle and around the mini-avenue were pillars.  We know that they were not.  Most of them were boulders and stumpy slabs.  And as we have said before, there is nothing to show that Stonehenge was ever completed, with this stone setting or any other.

The tip of the Rhosyfelin spur


Somewhere out on the tip of the Rhosyfelin spur is the location of sampling point number 8 -- where the characteristics of the foliated rhyolites were deemed by Ixer and Bevins to be identical (sort of) with the characteristics of some of the foliated rhyolite debitage at Stonehenge.  None of the slides made from the samples are identical, but that did not stop the geologists claiming that they had provenanced some of the fragments at Stonehenge "to within a few square metres."  The rest, as they say, is history.

Of far greater importance is the  character of the bedrock surface here.  Quite spectacular.  Click to enlarge the photo.  Many of the rock surfaces are heavily weathered and abraded, suggestive of either ice action or (more likely) an episode of intensive abrasion by heavily-charged glacial meltwater in a dead-ice environment.  There is similar abrasion on the rock surface at the base of the spur, which is what leads me and various colleagues who have visited the site (I hesitate to call them senior academics since Myris will get upset) to think that meltwater -- possibly flowing subglacially -- has at some stage in the Devensian flowed over the spur and down the small channel on the right under hydrostatic pressure.  What are the dates for this episode?  Maybe the samples collected for cosmogenic dating will help us to tell the story.

Cosmogenic dating at Rhosyfelin

The "pseudo-proto-orthostat" at Rhosyfelin.  It is a rock-solid certainty that the two weathered surfaces shown here will have greater exposure ages than the surface on which the bucket is sitting.


Last month I reported on the planned sampling programme at Rhosyfelin -- and it is pretty certain that the sampling has now been done by Derek Fabel, with MPP and Richard Bevins in attendance.  Interestingly, a few weeks ago I received a message from a quite senior academic unconnected with the dig that my involvement (or even suggestions for sampling locations) would not be welcomed since those involved "know what they are doing."   Let's hope so.  All will be revealed in due course, but I really do hope that Derek has taken samples designed to test the minimum ages of some of the weathered surfaces and some of the surfaces affected by rockfalls, as well as the famous "recess" from which MPP thinks a bluestone pillar was extracted somewhere between 6,000 yrs BP and 5,000 yrs BP.

I have already done many posts on this blog about the potential -- and the pitfalls -- of cosmogenic dating in general, and the 36Cl method in particular.
One previous post:
http://brian-mountainman.blogspot.co.uk/2016/08/rhosyfelin-more-good-news-from-rock-face.html

Of course, all dates from this site will be MINIMUM DATES, since whatever is measured by way of cosmic bombardment on specific rock surfaces will simply represent the period of time that has elapsed during which those surfaces have been open to the heavens.  We cannot know the extent to which said surfaces have been affected or covered by seasonal snowbanks, standing water, a temporary regolith, vegetation (bracken, gorse, brambles, shrubs and trees) or even rockfall debris;  there may have been many episodes, quite unconnected to human interference, when the "cosmic clock" has been stopped.  So we will always get minimum ages for the exposure of surfaces.

In those circumstances, it will be very difficult indeed to accurately date block removal and headwall exposure in a place like Rhosyfelin, in a thickly vegetated environment.  The top of Carningli would be, in theory, a much better place in which to get sensible age determinations!  A date of 5,000 yrs BP on a Rhosyfelin rock surface, for example, might falsely suggest that there was an "event" at that time such as the physical removal of a block.  On the other hand the block might have been eroded away or taken away much earlier that that, followed by centuries or millennia of overshading by gorse bushes or shrubs or even burial beneath rubble or soil.  Then a rockfall or landslide, or even a wildfire,  might expose the surface to cosmogenic bombardment and weathering for the first time.......  To make matters even more complex, evergreen overshading plants such as gorse or conifers will cut off most radiation whereas plants that die back or lose their leaves in winter will allow winter light to penetrate to the ground beneath them.

In this context it is not surprising that 36Cl and 10Be dates are often very erratic, as discovered by Danny McCarroll and his colleagues when they undertook a dating programme on rock surfaces in west Wales in 2010:

http://brian-mountainman.blogspot.co.uk/2011/03/glaciation-of-carningli-dating-problem.html

Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream.
Danny McCarroll, John O. Stone, Colin K. Ballantyne, James D. Scourse, L. Keith Fifield, David J.A. Evans, John F. Hiemstra, Quaternary Science Reviews (2010) 1-9 

The Carningli dates were all over 100,000 yrs BP, and the dates from rock exposures on the north Pembrokeshire coast came out at over 35,000 yrs BP.  Danny and his colleagues were greatly exercised, in their discussions of the dates, by the problem of "inherited ages" on rock surfaces that had been intermittently or partly eroded by overriding ice.

 So cosmogenic dating of rock surfaces is far from perfect, but better than nothing.  One date, from MPP's famous recess, may give us some guidance on the likelihood of quarrying and the reliability of the block removal hypothesis, but a spread of dates from maybe 20 sites would give us much more data to work with.



Just for fun, here is a list of the possible MINIMUM dates that might be obtained from various parts of the Rhosyfelin spur.  For the heavily weathered and abraded surfaces (for example, at A) we might well get exposure ages of over 100,000 years; for areas at the base of the rockface (areas marked D) that have been eroded by meltwater or ice and then covered by accumulated rockfall debris and tree growth, we might see exposure ages of less than 10,000 years; for areas high on the crag in the "gorse and bracken zone (marked C) we might get ages less than 18,000; and for areas on the rock-face subject to post-Devensian rockfalls (marked B) we might see a wide variety of exposure ages, ranging from 18,000 years to the present day.  The "freshness" of the rock surface should give a guide.

The dates are eagerly awaited.........



Tuesday, 27 September 2016

Back to normal service



Thanks for the patience during the breakdown of the mail service!  It was a password authentication problem -- a typical Apple Mac issue.......

If I have missed any submitted comments, apologies.  Some messages might have come in and been accidentally dumped while I was sorting things out.  Anonymous messages will ALWAYS go straight into the bin -- I don't even get to see them.

By the way, since some were asking for reports of the MPP talks last week, I got this today from a very senior (retired) academic: 

"We very much enjoyed your talk at Castell Henllys last week. We also attended the talk the following day by Mike Parker Pearson which was interesting but had a far more uncritical, one might almost say sycophantic, audience. The debate at the end of your talk was enjoyably extensive and pointed by comparison. It was especially disappointing that despite the juxtaposition of the two talks and theories MPP did not even acknowledge the existence of an alternative to his view."


So there we are then.

Monday, 26 September 2016

Mail problems

Apologies if there have been any comments submitted in the last 24 hours -- my mail programme is giving problems -- nothing in and nothing out.  Please bear with me -- working on it!

Brian

Saturday, 24 September 2016

Meini Gwyr



 Illustration from Pam Figgis's book on Prehistoric Preseli

We have been thinking about what the archaeologists were hoping to find at Pensarn when they started their dig a few weeks ago.  Tony has mentioned possible matches in Anglesey.  Maybe they were hoping to find something like this?  This is Meini Gwyr, close to the Glandy Cross petrol station, at SN14172658.  At present all you can see is two small standing stones and a faint raised embankment or circular cairn with a lower area in the centre.  But the excavations revealed an interesting structure with at least 17 standing stones, a passage down the middle, and kerbs or revetments.  There do seem to be segments and kerbs like this at Pensarn as well.......

This site is of course interesting because it is a part of the Glandy Cross complex, flagged up as one of the most important Early Bronze Age ritual complexes in West Wales.  Herbert Thomas knew about it, and of course this is the same area as his "Cilymaenllwyd" which he speculated as being the possible location for a proto-Stonehenge.  So MPP and his colleagues are by no means the first people to think about a large stone monument being erected in Pembrokeshire and then shipped off later to Stonehenge for some mysterious reason or other...........

The trouble is that in spite of much searching, no circle of sockets of other evidence has ever been found for a big stone circle in this area -- and neither has anybody found a stone working area or "Preselite" tool-making factory, although both have been mooted many times over the years.

From the Coflein web catalogue of sites in Wales:

Site Description
This is an interesting example of an embanked stone circle, a monument type not common to south Wales. Its occurence here shows that the Glandy Cross area was of exceptional importance in the Neolithic and Bronze Ages. The circle consists of a broad, low, roughly circular bank 36.6m in diameter with a narrow entrance on the west. There is no ditch, and excavations by Grimes in 1938 confirmed there had never been one. Two stones of an original 17 still survive on the west side, 1m and 1.7m high respectively, standing 6.5m apart.

Information from Rees, S. 1992, A Guide to Ancient and Historic Wales - Dyfed, Cadw/HMSO, page 38.
For fuller discussion see: T. Kirk and G. Williams, ‘Glandy Cross: A Later Prehistoric Monumental Complex in Carmarthenshire, Wales’, Proceedings of the Prehistoric Society, 66 (2000)

T. Driver, RCAHMW, 16th April 2010.

--------------------------



More info (Dyfed Archaeological Trust):


Meini Gwyr, also known as Buarth Arthur, is an embanked stone circle probably dating to the transition between the late Neolithic and early Bronze Age periods. The site is likely to have been used for religious rituals.
According to a late 17thC account by Edward Lhuyd, there were then fifteen stones in the circle ranging in height from three to six feet, but a further seven or eight were thought to have been 'carried off'. Apparently, there was also an entrance lined by smaller slabs.

The site was partially excavated in 1938 by Professor W.F. Grimes. Unfortunately most of the records were destroyed in a bombing raid on Southampton in 1940. The plan is based partly on ground and air photographs of the excavation. Grimes established that the circle, some 60 feet in diameter, originally consisted of 17 stones which, like the two surviving ones, were set at an angle into the inner slope of the bank about 3 feet height and 120 feet in the external diameter, with no trace of a ditch. The excavations confirmed that the entrance through the earthwork was formerly flanked by upright stones, set in a trench. The bank was set with stone curb extending for some 30 feet on either side of the entrance, in front of which was a clay-filled pit containing a large quantity of charcoal. There were no features or finds recorded from the interior, though this was only partly examined. Some fragments of early Bronze Age pottery came from a hearth set in a deep depression on the southeast bank.

Meini Gwyr stands at the centre of 'West Wales' most important complex of Neolithic and Bronze Age ritual and funerary monuments, lying on a ridge-way linking the wester end of the Preselis to the eastern Cleddau river and Milford Haven. This was a route by which the bluestones for Stonehenge may have been transported. Included in the complex are several Bronze Age burial mounds and cairns or various forms, and a 'henge' monument (akin to early elements at Stonehenge). Also, there is the site of 'Yr Allor' ('The Altar') comprising two, formerly three standing stones some 200 yards west of Meini Gwyr and apparently known by the 17thC. These stones may be the remains of a chambered tomb.

Carn Meini, a source of the bluestones lies only 3 miles to the north. The site's name - 'Meini' ('large stone') and 'Gwyr' ('crooked') may refer to the varying size, shape or angle of the stones set in the circle. These were not 'bluestones' but another form of volcanic rock. Many such boulders are found locally and were originally deposited by glacial action. The alternative name 'Buarth Arthur' ('Arthur's Yard') is an example of a common legendary association of this figure with prehistoric stone monuments and is not regarded as significant.