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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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Monday, 20 February 2017

Llansteffan and the Altar Stone



Thanks to Geraint Owen for further information on the Senni Beds.  He has studied the Craig Ddu section in detail, and from his descriptions, and the work of  Ixer, Bevins and Turner, I am now more than ever convinced that the Altar Stone might well have come from Craig Ddu near Llansteffan. Below I reproduce some of the key info on the Senni Beds from the big report by Barclay et al 2015.
 It's clear from the published work that the Altar Stone has significant differences from the Cosheston Sandstones around Mill Bay (Milford Haven) and significant similarities with the sandstones described from Craig Ddu.  The samples are not identical, but that is not surprising;  we do not know the precise positions from which all the samples have been taken, along a rock exposure c 400m long and 20m high.

For the moment, until further work is done, I think we can assume that the Altar Stone has probably come from the Llansteffan Peninsula and maybe even from the Craig Ddu exposures. That makes perfect sense, since the eastern Preseli hills and the Llansteffan Peninsula are on pretty well the same Irish Sea Glacier flowline.  All of the speculation about the Altar Stone having come from somewhere along the A40 road between Llandeilo and Sennybridge becomes redundant.

 ==========================

Big BGS report on ORS stratigraphy.  It provides a more formal correlation of Cosheston Group and Senni Formation:

BARCLAY, W J, DAVIES, J R, HILLIER, R D, AND WATERS, R A. 2015. Lithostratigraphy of the Old Red Sandstone successions of the Anglo-Welsh Basin. British Geological Survey Research Report, RR/14/02. 96pp.
It's a free download from the BGS website.

Extract:

4.2.5     Senni Formation

Name
Derived from the Senni Valley (Glyn Senni), Powys, south-central Wales. The name Senni Formation supersedes the traditional Geological Survey name Senni Beds (Cantrill in Strahan et al., 1904). In the Clee Hills, the formation was previously named the Clee Sandstone Formation.

Type area
Senni Valley [SN 930 209]

Partial type section
Waterfall exposures [SN 930 209] in a tributary of the Afon Senni (Nant Ystwyth) above Tyleglas.

Reference sections

1.    Heol Senni Quarry [SN 9154 2210], Powys provides a well documented reference section. It exposes about 40 m of grey-green sandstones, with minor siltstones, mudstones and intraformational conglomerates at the top of the formation (e.g. Edwards et al., 1978; Loeffler and Thomas, 1980; Dineley, 1999b; Barclay, 2005e).

2.    Cliffs at Craig Ddu on the western side of the Llansteffan peninsula, Carmarthenshire [SN 32441015](Owen, 1995). These reach a height of 20 m and have continuous exposure for 400 m of beds near the top of the formation.

3.    The stream draining north-westwards across Clee Liberty [SO 583 848–587 844] in the Clee Hills exposes about 140m of the formation (Allen, 1961) and provides a reference section for this area.

Lithology
Mainly of green and green-grey (locally red-brown and purplish green), very fine to medium- grained, micaceous sandstones, mainly channelised, cross-bedded and parallel-laminated, with green and red-brown siltstone and mudstone interbeds, some calcretes and intraformational conglomerates; the formation is characterised by the presence of vascular fossil plant remains and some soft sediment deformation is also present. In the Clee Hills, pale green sandstones are mainly arranged in fining-upwards conglomerate–sandstone–siltstone cycles, with subordinate red or green, sporadically calcretised mudstone/siltstone interbeds (Ball and Dineley, 1952, 1961; Allen, 1961; Greig et al., 1968).

Lower and upper boundaries
The lower boundary is placed at the base of green sandstones, which overlie red-brown sandstones and mudstones of the underlying Freshwater West (formerly St Maughans) Formation. Where mature calcretes at the top of the Freshwater West Formation are present (the Ffynnon/Abdon Limestones), the base of the formation is placed at the top of the uppermost calcrete. The upper boundary is placed where red-brown sandstones of the Brownstones Formation overlie the green sandstones of the Senni Formation, the junction being gradational. In the Clee Hills, Here, the lower boundary of the formation is placed at an erosion surface cut in the uppermost calcrete of the Upper Abdon Limestone, where it is sharply overlain by the basal green sandstones. The upper boundary is a gradational passage into generally coarser-grained
strata lacking in argillaceous beds and in which the sandstones are more variably coloured, these being assigned to the Brownstones (previously Monkeys Fold) Formation.

Thickness
300 to 450 m in the Brecon Beacons and Black Mountain, 150 to 200 m in the Black Mountains and 152 to 167 m in the Clee Hills.

Distribution
From Carmarthen Bay eastwards to the Black Mountain, Brecon Beacons and Black Mountains, and from there southwards to Abergavenny, wedging out north of Pontypool.

Depositional environment
High-discharge, mixed-bedload, sand-dominated, braided stream systems with relatively high sedimentation rates and water-table levels; although the formation is dominated by in-channel deposits, overbank floodplain silt and mud deposition also occurred.

Age
Latest Lochkovian to latest Pragian or Emsian in age.

REFERENCES

OWEN, G. 1995. Senni Beds of the Devonian Old Red Sandstone, Dyfed, Wales: anatomy of a semi-arid floodplain. Sedimentary Geology, Vol. 95, 221–235.

see also:

Also, RG Thomas's opus magnus on the Cosheston Group, which includes petrographic data:
THOMAS, R G, BARCLAY, W J, MORRISSEY, L, WILLIAMS, B P J, and ALLEN, K C. 2006. Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south-central Pembrokeshire, Wales. Geological Journal, Vol. 41, 481-536.


Rhosyfelin -- Coflein record is corrected




After lots of messages and a good deal of confusion, this is probably as right as it ever will be   -- it should now be almost acceptable to both sides of the argument!! 

I suggested further corrections, but these have been ignored.......


http://www.coflein.gov.uk/en/site/416247/details/craig-rhosyfelin-pont-saeson-craig-rhos-y-felin-rhyolite-bluestone-outcrop

Site Description

Fieldwork in 2011-12 led to the proposal that most of the foliated Stonehenge rhyolite debitage originated from a specific 70m long area called Craig Rhos-y-felin near Pont Saeson. Petrographical sampling by Dr Rob Ixer and Dr Richard Bevins found that 99% of these foliated rhyolites could be matched to rocks found in this particular set of outcrops. Rhyolitic rocks at Rhos-y-felin are distinctly different from all others in South Wales, which suggests that almost all of Stonehenge foliated rhyolites have a provenance of just hundreds of square metres. The results were of considerable significance, and were published in 2011 in the Journal of Archaeological Science.

Excavations between 2011 and 2015 identified Neolithic activity that may have been associated with megalith quarrying, but this is disputed by geologists, who believe that erratic materials were entrained at Rhosyfelin by over-riding glacier ice and transported south-eastwards towards Stonehenge.

Sources:

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

Ixer, R.A., Bevins, R.E., 2011, Chips off the Old Block: The Stonehenge Debitage Dilemma. Archaeology in Wales, 52, 2011, 11-22.

Brian John, Dyfed Elis-Gruffydd, and John Downes, 2015, Quaternary Events at Craig Rhosfelin, Pembrokeshire. Quaternary Newsletter 137, October 2015, p16-32.

Mike Parker Pearson, Richard Bevins, Rob Ixer, Joshua Pollard, Colin Richards, Kate Welham, Ben Chan, Kevan Edinborough, Derek Hamilton, Richard Macphail, Duncan Schlee, Jean-Luc Schwenninger, Ellen Simmons and Martin Smith (2015). Craig Rhos-y-felin: a Welsh bluestone megalith quarry for Stonehenge. Antiquity, 89 (348) (Dec 2015), pp 1331-1352.

L. Osborne & T. Driver, RCAHMW

Friday, 17 February 2017

Here comes Super-Stonehenge........



Talking of hokum, as we do quite often on this blog, wait for it....... here comes the latest blockbuster for Sky TV.  It's called "Britannia".  It will be broadcast this year, and featuring large will be a sort of Super-Stonehenge around which the drama involving Druids, Celts and Romans will swirl, no doubt with plenty of magic, monsters, and blood and gore.  Historical accuracy cannot be guaranteed.......

For those who like this sort of thing, here is the key info, put out by the publicity people:

Full credits for Britannia -- 10 episodes coming up in 2017

http://www.imdb.com/title/tt5932548/fullcredits?ref_=tt_ov_st_sm

Britannia is an epic drama set in 43AD as the Roman Imperial Army - determined and terrified in equal measure - returns to crush the Celtic heart of Britannia - a mysterious land ruled by warrior women and powerful druids who can channel the powerful forces of the underworld. Or so they say.

http://www.radiotimes.com/news/2016-12-07/first-look-at-david-morrissey-and-kelly-reilly-in-epic-new-historical-drama-britannia

Sky and Amazon partnership  -- filming in Czech Republic and Wales 2016.

The international distribution rights (excluding the US) for Britannia will be handled by Sky Vision, Sky’s international production and distribution arm. WME and Sky Vision negotiated the deal with Amazon.

Commenting on the show, Jez Butterworth said: “Besides being hard, hard warriors, the Celts have a belief system which makes them almost invincible. It’s a deep, heavy magic. Last time the Romans tried to invade, the mighty Julius Caesar took one look, turned round and went straight home. Now, almost a century later, the Romans are back. I’m fascinated in what happens when gods die. When an entire, ancient faith stalls, topples, collapses - and a whole new one grows in its place. New names, new faces, to suit the new times. Here we have a  war between two pantheons - the Roman gods v the Celtic gods. It’s the Heavyweight Clash of All Time, the one which most shapes who we are today. And we see it all from a human perspective, of individual survival, ambition, courage, lust, loss, revenge. All the stuff the gods have always loved us humans for the most!”

Kelly Reilly said: "I am thrilled to be joining the production of Britannia, a story rooted in the heart of our ancient history. Collaborating with great creative minds is what I love most about my job, and having one of Britain’s leading playwrights, Jez Butterworth, come on board to bring Celtic Britain alive has made this a fiercely exciting project. The vision is big and bold and beautiful, and I can’t wait to get my hands dirty.”

David Morrissey added: "I have been a fan of Jez's writing for a long, long time, so to get the chance to work with him on this epic tale is very exciting. His dialogue and storytelling are second to none. I am also really looking forward to working with the brilliant cast that has been assembled for Britannia - I just hope I get a nice horse!"

Anne Mensah, Sky’s Head of Drama said: “This is one of the most ambitious dramas we’ve ever made and one of the most exciting. Jez Butterworth has written an incredible drama that is entertaining and emotional but also asks important questions about what drives men and women to stand and fight. This is a battle for the heart of Britain like you’ve never seen before.”

Roy Price, Vice President, Digital Video and Amazon Studios, said: “We’re thrilled to be collaborating with Sky on our first series entirely produced overseas. Our customers will experience a cinematic world with bold characters anchored in a ruthless period of history.”

April 1st comes early this year....

Our friend Paul Sanday is doing another talk in Newport about his claim that he has found "The Welsh Stonehenge" -- full coverage in the local press.  Ah well, it's a free world, and I suppose his ideas are no wackier than many others featured in the Stonehenge debate over the years......

http://www.countyecho.co.uk/article.cfm?id=109765&headline=Geologist%20stumbles%20across%20%E2%80%98Welsh%20Stonehenge&sectionIs=news&searchyear=2017

I think Paul is having a jolly time here, just seeing how much outrageous nonsense he can get a gullible public to accept as "scientifically" based.  Not sure what this all does for the reputation of geology....... maybe that's a problem for geologists to address!

Wednesday, 15 February 2017

Wolstonian glaciation of west Wales: an hypothesis



In a previous post, I suggested that there is now so much evidence of strange goings-on in the glacial record of Pembrokeshire that it may well be time to reinstate the Wolstonian as a serious and extensive glacial event.  Here is my earlier post from a month ago:

https://brian-mountainman.blogspot.co.uk/2017/01/time-to-bring-wolstonian-in-from-cold.html

As I have suggested on many occasions, the old Wolstonian limit, as proposed by assorted researchers including Gibbard and Clark, does not make any sense, so anything has to be an improvement. To repeat -- my resoning is as follows:

1.  The orientation and scale of the Gwaun-Jordanston meltwater channel system cannot be explained as a product of Irish Sea ice coming in from the N and NW, either during the Anglian or Devensian glaciations.   The channels must have been eroded at a time when there was a strong hydrostatic gradient from E towards W --that is the undoubted direction of meltwater flow.  So there must have been Welsh ice flowing from east to west.

2.  There are anomalous striae on the north Pembrokeshire coast, which have always puzzled me since they run approx E-W.  All other striae match the expected directions of Irish Sea ice across the north Pembrokeshire coast.  I have not been able to work out the relative ages of the assorted striae, but some of the anomalous ones might well be of Wolstonian age.

3.  The glacial and fluvioglacial deposits that scatter the Devensian "ice free area" south of Mynydd Preseli have to be explained somehow.........  some are very rotten, as at Llangolman, suggesting a pre-Devensian age.

4.  The ancient till deposits at West Angle, Lydstep and maybe New Quay may well be of Wolstonian age.

5.  The frequency of Ceredigion grits and coarse sandstones in the glacial deposits of the Nevern Estuary in Newport suggests to me that they were deposited far out into Cardigan Bay during a Wolstonian glaciation, and then picked up and redeposited by Irish Sea ice during the Late Devensian.

6.  The deep downcutting of Milford Haven may also be partly a result of large quantities of glacial meltwater flowing westwards at the end of a Wolstonian glacial episode on the lines shown on the above map.

What do others think?  Comments very welcome.


Suggested Wolstonian limits of the Welsh ice cap.  It must have been similar to the modelled Devensian ice cap, but it was probably thicker, with more active ice streams in all of the major radiating valleys.  It was also substantially more extensive. It is assumed that the Irish Sea Glacier was less powerful than during the Anglian and Devensian glaciations -- but there must have been contacts between these two ice masses in the NW, N and NE sectors. 

Tuesday, 14 February 2017

Hardangerjokul -- on the way out......


This is a fantastic oblique image of Hardangerjokul in Norway.  As we can see, it is a perfect plateau ice cap, with just one outlet glacier that has shrunk do much that it no longer drops over the trough head near the left edge of the photo.  The ice in the centre of the ice dome is still about 300m thick, but the highest point on the ice cap has dropped by 15m since 1925, and the edge is retreating at a dramatic rate every year.  The Norwegian weather service is now predicting that the ice cap will melt away completely within 50 years, if the current rate of climate warming continues.......

Monday, 13 February 2017

Llandre Gravel Quarry -- where is the Penfro Till?


 The flooded Llandre Gravel Quarry, not far from Clynderwen, well inside the "Devensian ice-free area".  Gravels (heavily iron-stained) can be seen in the exposures on two sides of the quarry.  There us no current exposure of till.

Not long ago I  put up a post about the Penfro Till Formation, and wondered where its type locality is really located and just how old it is.  Is it, indeed, just one formation,or are assorted tills of various ages all lumped together rather unreliably?  I am beginning to suspect that the latter is the case.

https://brian-mountainman.blogspot.co.uk/2017/02/penfro-till-formation.html

I called in to have a look at the quarry today  -- it's very close to the road, near a big animal shed belonging to Llandre Egremont Farm and a few hundred metres north of the point where the road passes under the Gelli railway bridge. Grid reference:  SN 092202.  I wasn't able to achieve much, and I couldn't dig away at the gravels without risking life and limb........ but the gravels are at least 3m thick and are heavily iron-stained. But they do not appear to be anything like as rotten as the gravels exposed at Llangolman.  I'm not even sure that they are fluvioglacial -- they look like river gravels to me -- with most pebbles under 5 cm in diameter and with just a few larger clasts.  I think the gravel pit is cut into a remnant of an old river terrace with its top no more than 2m above the present valley floor.  I would hesitate, therefore, to give this site any great geomorphological significance, or assume that it will help us in unravelling the Ice Age history of this area.  I'll take a look at it again in the summer, when hopefully it will be dry enough for me to do a bit more work on the gravel faces.

Higher up the slope, adjacent to the animal sheds, there is an exposure of rotted bedrock and slope deposits or head -- but no gravels or till.

This is certainly not a good enough site to be a type locality for anything.  I might revise that judgment in due course, once I have hoofed about a bit more.

Postscript

I have discovered another gravel pit, at SN093203, about 300m from the one described above.  It's at an altitude of c 44m, on a broad spur above the river valley.  Let's call it Llandre No 2 quarry -- I shall go and check it out.......