Lecture on the Stonehenge Bluestones

Happy to give a puff for this one -- a lecture by Richard Bevins at the Wilts Museum in Devizes.  Thanks to Tony for the alert.....

LECTURE: Old Stones, New ideas: Sourcing the Stonehenge Bluestones

2:30 pm, Saturday, 21 November, 2015
Wiltshire Museum, Devizes

A Saturday afternoon lecture by Richard Bevins.

Stonehenge is arguably one of the most famous prehistoric monuments in the World. It is renowned for the enormous size of the sarsen monoliths used in its construction which comprise the Outer Circle and Outer Horseshoe. It is generally agreed that these stones were sourced from the Marlborough Downs area, some 30 km to the north of Stonehenge. However, a set of smaller stones, comprising the Inner Circle, the Inner Horseshoe and the Altar Stone, are exotic to the Salisbury Plain area; these are the so-called bluestones, and have been the subject of investigations since the latter part of the 19th Century. Early petrographical studies recognised that the bluestones largely comprise a range of altered volcanic, intrusive and tuffaceous rocks with rarer sandstones but could not provide a definitive source.

However, it was the seminal paper by H.H. Thomas in 1923 that persuasively demonstrated that the spotted dolerite component of the bluestones could be sourced to outcrops exposed towards the eastern margin of Mynydd Preseli in southwest Wales, citing the tors Carn Meini and Cerrigmarchogion as the most likely sources. Thomas also argued that other lithologies in the bluestone assemblage, notably the rhyolites and the ‘calcareous ash’, could be sourced in the same locale, in particuar from Carn Alw and the northern slopes of Foel Drygarn respectively.

The first major investigation of the geochemistry of bluestone assemblage was by Richard Thorpe and team who compared whole rock wavelength-dispersive X-ray fluorescence spectrometry analyses from both orthostats and debitage at Stonehenge with whole rock analyses from Mynydd Preseli
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Using petrography, mineral chemistry and whole rock geochemistry Richard Bevins and Rob Ixer have re-examined the proposed source of the bluestone rhyolites and determined that Carn Alw, as proposed by Thomas, is not the source of bluestone rhyolite; instead they argued that the majority of the rhyolite debitage from the Stonehenge Landscape (but not the four rhyolitic/dacitic standing or recumbent orthostats) comes instead from a prominent outcrop called Craig Rhos y felin, located on low ground to the north of the Mynydd Preseli range in the vicinity of Brynberian. More recently they have re-examined the spotted and non-spotted dolerites and concluded that a large % of the dolerite fragments and cored samples from Stonehenge come from Carn Goedog rather than Carn Meini.


Biography
Dr Richard Bevins as Keeper and Head of the Department of Natural Sciences at the National Museum Wales in Cardiff is responsible for Strategic leadership for collections and research related activities within the Department.

Qualifications, memberships and relevant positions:
BSc (Hons) Geology (Aberystwyth University), PhD (Keele), Fellow (Geological Society of London), Chartered Geologist (CGeol), Fellow (Society of Antiquaries of London), Honorary Lecturer (School of Earth & Ocean Sciences, Cardiff University), Chair, Geological Society of London’s Geoconservation Committee, Member of the Geological Society of London’s External Relations Committee, Chair of the British Geological Survey’s National Geological Repository Advisory Committee.

References
Primary research area is centred on the the Caledonian igneous history of Wales and related areas, as well as on their low-grade metamorphism. More recent work has focussed on extending the petrology and geochemistry of altered igneous rocks from Pembrokeshire into a re-examination of the source of the Stonehenge bluestones.

Selected Publications:
Bevins, R.E., Ixer, R. A. & Pearce, N.J.G. 2014. Carn Goedog is the likely major source of Stonehenge doleritic bluestones:evidence based on compatible element geochemistry and Principal Component Analysis. Journal of Archaeological Science, 42, 179-193.

Ixer, R.A. & Bevins, R.E. 2014. Chips off the old block: the Stonehenge debitage dilemma. Archaeology in Wales, 52, 11-22.

Bevins, R.E., & Ixer, R. A. 2013. Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical re-appraisal of the petrographical account of H.H. Thomas. Journal of Archaeological Science, 40, 3293-3301.

Bevins, R.E., Ixer, R. A., Webb, P.C. & Watson, J.S. 2012. Provenancing the rhyolitic and dacitic components of the Stonehenge landscape bluestone lithology: new petrographical and geochemical evidence. Journal of Archaeological Science, 39, 1005-1019.

Parker Pearson, M., Pollard, J., Richards, C., Thomas, J., Welham, K., Bevins, R.E., Ixer, R., Marshall, P. & Chamberlain, A. 2011. Stonehenge: controversies of the bluestones. In L. García Sanjuán, C. Scarre and D.W. Wheatley (eds) Exploring Time and Matter in Prehistoric Monuments: absolute chronology and rare rocks in European megaliths. Proceedings of the 2nd European Megalithic Studies Group Meeting (Seville, Spain, November 2008). Menga: Journal of Andalusian Prehistory, Monograph no. 1. Seville: Junta de Andalucía. 219-50.

Bevins, R.E., Pearce, N.J.G. & Ixer, R. A. 2011. Stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics. Journal of Archaeological Science, 38, 605-622.

Links:
www.researchgate.net/profile/Richard_Bevins2

Saturday afternoon lectures start at 2.30pm and last approx. one hour.

Our Lecture Hall is accessible via a lift if required, has a hearing loop and air conditioning.

Booking:

Essential.

Booking Options
Book online using Paypal
Telephone – 01380 727369
Email – hello@wiltshiremuseum.org.uk
Visit – Wiltshire Museum, 41 Long Street, Devizes

Ceibwr connected pot-holes and cemented sediments

The subsidiary Channel (with linked pot-holes) -- view towards the sea.  Note the foxy-red cemented sediments on both flanks.

One of the larger meltwater channels at Ceibwr.  The channel is on the right.  To left of centre is the ridge which separates the channel from the larger Ceibwr Bay channel.  Note the foxy red cemented sediments on the tip of the spur.  The inner end of the subsidiary channel is seen at the right edge of the photo.  In the foreground we see the bedrock floor of the channel.  Photo: Ben Ashton.

Yesterday I was over at Ceibwr, near Moylgrove on the N Pembs coast, during a pleasant walk along the clifftop.  I was struck once again by the rather interesting landforms and sediments there.  There are two big meltwater channels separated by an elongated ridge.  One of the channels is now occupied by an arm of the sea (Ceibwr Bay) and the other is dry and occupied by the roadway and car parking area.  This dry channel is fascinating, because it contains a smaller channel which is cut into its floor at the seawards end -- as seen above.  Really this small channel is a series of connected pot-holes, excavated out at a time of very turbulent meltwater flow, with large erratic boulders rolling about in enclosed hollows and excavating out very soft Ordovician shales.  Similar boulders can be seen in the foreground in the photo above.

It's tempting to think that the main channels here are of Anglian age and that they have been modified during the Devensian at a time of more limited meltwater flow.  But that would be too simple.  There are solidly cemented tills, frost-shattered periglacial accumulations and fluvioglacial deposits plastered on the inside of the "newer" small channel, and then fresh till sitting on top of these red-stained cemented deposits.  The fresh till has to be of Devensian age.  So how old are the cemented deposits?  Might they date from the Anglian glaciation?  If they do, that means the small channel might also date from the Anglian, and it might be even older.

Solidly-cemented deposits are common in the Moylgrove-Ceibwr area, maybe because groundwater here is very rich in iron and manganese.  So the reddish and black cement is made essentially of iron oxide and manganese oxides.  How long does it take for a deposit to be turned into concrete?

I'm increasingly convinced that these cemented, stained and rotten deposits (for example at Llangolman and Ceibwr) are very old, but they do need to be dated through the use of one of the modern techniques.  Must work on that........

The sediments at the head of the subsidiary channel.  Foxy-red stained and cemented till and other materials below, and grey uncemented fresh Devensian till above.  The boulders at bottom left are residuals left behind when finer sediments are washed away during extreme storm wave conditions and by runoff during periods of heavy rain.

Stained and cemented sediments at the roadside, on the hillside above the subsidiary channel.  Broken Ordovician shales below.

The crust of cemented till resting on bedrock at the Witches Cauldron, near Ceibwr.  Here the cement is so hard that it has resisted wave attack while much of the softer black shale bedrock beneath it has been eroded away. There's a lot of manganese oxides here, so in places the sediments are stained black rather than foxy red.

There is a lot of discussion of the channels and the cemented sediments in this paper by Glasser et al (2004):

Glasser, N. F., Etienne, J. L., Hambrey, M. J., Davies, J. R., Waters, R. A. & Wilby, P. R. 2004 (August): Gla- cial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol. 33, pp. 224–237. Oslo. ISSN 0300-9483.

I don't agree with all that the authors say -- they don't sufficiently differentiate the uncemented and cemented deposits, and they refer simply to a "pre-late Devensian" episode of sedimentation and cementation.   If I understand them correctly, they seem to think that the cemented materials are mostly fluvial in origin, and relate to a period of very ancient fluvial valley erosion and sedimentation.  I think the cemented deposits are much more varied than that, with ancient till included.

(Note:  thanks to Myris for correcting me!  I have changed the text to refer to manganese oxides....)

Sarsen locomotion

One of my pics from the other day -- showing the glass-fibre sarsen stone on its cradle and rollers, just outside the new Visitor Centre.  No doubt these very large stones were moved from assorted locations (either near or far away) to their present positions, and we have reported often on this blog the latest experimental archaeology work in this field.  What interests me more than the rollers, cradles and sledges is the nature of the rope.  The ropes used to hold the stone in position here are very magnificent -- and very modern and shiny.  But what would ropes have been made of in the Neolithic?  Nettles?  Brambles?  And how strong would they have been?  And how long would they have taken to make?  And how long might they have been in order for very large groups of men to haul extremely large stones across country?

The burning of the books?

After the frustration of watching Wales trying to score a try against the Aussies at Twickenham, we travelled yesterday from Sevenoaks to Nunney, and the nice lady who lives inside our Tom Tom decided that we should travel past Stonehenge.  We didn't have much time, but popped into the new Visitor Centre and took a look at the gift shop and some of the new exhibition items.  We didn't bump into Chris, but maybe that's not surprising since there were thousands of others there at the time......

Anyway, we checked on the availabilty of "The Bluestone Enigma" and were not really surprised to find that it was nowhere to be seen.  We asked the staff about it, and were told that the decision had been made some time ago by EH to discontinue stocking it, in spite of the fact that it had been selling well.  They hadn't informed me of that fact.

I'm not sure whether this signals a burning of the books, or a disinclination to stock anything perceived to be uncomfortable or subversive, or even a decision to cut down on titles more than 5 years old, or even an attempt to take out lots of books so as to make more room for "popular" merchandise.  I'll be generous and assume that the latter is the most likely scenario.  The book section seems to me to have been dramatically reduced in size.  There are lots of other titles I expected to see there -- but could not see a trace of.

Anyway, I'll try and find out what the politics of this might be, and will report back!

Rhosyfelin Mill?

"Rhosyfelin" means "the moor by the mill" or some such thing, and I have always wondered where the mill was or is.  I think we found it a couple of weeks ago, a few hundred yards up the valley, lost in the trees and somewhat ruinous.  To get there, you just follow the public footpath from the archaeological dig site and the ford.

This is far more worthy of archaeological investigation than the "Neolithic Quarry" since it has some quite interesting features which appear to be man-made.  In the winter it will be far easier to explore, when the vegetation has died down a bit.  But there is a sort of walled depression in front of it, which brings to mind the old fulling mill at Pandy, in the Gwaun valley.  ("Pandy" means fulling mill.)

So my working hypothesis is that this is a fulling mill, and not a woollen mill or a corn mill.   I wonder if the National Geographic will sponsor a full-scale research project?  Hmm -- I suspect not.  That worthy journal is probably not that interested in old mills which had stinking ponds full of urine in front of them........

Red sarsens

On the matter of red sarsens, Pete Glastonbury has kindly given permission for me to use this interesting pic -- of red sarsens in a cottage wall. What strikes me is the distinctly foxy red colour -- typical of the iron oxide staining which can occur on rock surfaces when they are underground.

What intrigues me rather more is the pinky colour that sometimes appears on rocks -- presumably that must also be due to oxidation processes, involving different mineral combinations?

Reddish erratic in Savernake Forest?

Out of the blue, I have received a letter from a gentlemen who once did some work in Savernake Forest, which lies SE of Marlborough.  He was asked by a well-known geomorphologist to investigate and take photos of an apparent erratic boulder with dimensions c 18" x 18' x 12" which lay in the depths of the wood.  He recalls that it was sub-rounded in shape, and it had a reddish colour -- in other words, it was very distinct from the chalk bedrock in that area.    A reddish bluestone?

The grid ref is approx 423166.  The location is more than 25 km to the NE of Stonehenge.  The gentleman concerned is trying to find his photos taken at the time.  I will also try to discover more details......

Has anybody else got any knowledge of possible glacial erratics in that area?  Or has anybody else come across any records that might be of interest?