The North Pembrokeshire Devensian Trimline

I've been pondering on this matter lately, after another walk on Carningli yesterday.  I'm still investigating where the trimline might be along the north Pembrokeshire coast which shows us the maximum elevation of Devensian glacier ice coming in from Cardigan Bay and St George's Channel.

 The zone referred to as being occupied by the "The South Wales End Moraine" -- characterized by extensive glacial deposits, fluvio-glacial landforms and deposits, and pro-glacial lakes. Click to enlarge.

Many years ago Charlesworth identified the "South Wales End Moraine" as marking the maximum extent of glacier ice during the last glacial episode.  It consists of a scattered assemblage of mounds and terraces of fluvio-glacial material, located for the most part between Fishguard and Cardigan.  I spent years studying this "moraine" and it's not really a line at all, since the sands and gravels (including some patches of ice-contact materials) are in a band of country some 15 km long and 8 km wide.  I have always thought that the ice progressed well beyond this "zone of deposits" and that it should therefore not be referred to as an end moraine.  But how much further?  Danny McCarroll and others think that the north coast was glaciated because the tors and craggy hills on Pencaer and along the coast between Fishguard and Dinas are rounded or ice moulded, whereas  those further inland, on Dinas Mountain and on the Carningli upland, are frequently delicate, with steep crags and balanced boulders etc.  They appear to have been affected by frost action but not by overriding ice.

So somewhere there is a trimline...........  as I have suggested, the ice appears to have affected Carningli up to an altitude of c 300m, possibly leaving behind the Pont Ceunant moraine after a retreat downslope on the north-facing mountainside.  I have found more evidence of overriding ice almost as high as the summit crags.

Evidence of ice action on Carningli.  Top and bottom photos:  ice smoothed undulating rock slabs near the summit.  Middle photo: smoothed rock slabs with crescentic gouges, on the south side of the mountain near the western summit.  If these really are Devensian features, there must have been ice at least 30 metres thick.....

Other evidence comes in the form of perched blocks.  There are two spectacular ones in the area -- one on Carn Edward (alt 280m), to the west of Carningli, and the other on one of the four tors which are called Carnedd Meibion Owen (alt 240m).  These blocks are extremely unlikely to have been emplaced by periglacial action or any other mode of tor formation;  they must have been stranded or let down by melting glacier ice.


Yet more evidence coming into the frame is that relating to the pro-glacial lakes referred to as Llyn Teifi and Llyn Nevern -- occupying the lower parts of those two valley systems.  Charlesworth (see above maps) was the first one to refer to these lakes, in 1929 -- but he thought that they formed during ice retreat or deglaciation, which is when we would expect vast quantities of meltwater to be pouring off the wasting ice.  New work, however, suggests that the lakes were impounded by ADVANCING glacier ice, since lake deposits including fine silts and clays with varved sequences are overlain by coarser stream sediments and even by till.  So the glacier coming in from the coast rode over these lake sediments and passed further inland.  Etienne and others (2006) think that the ice eventually flowed right over the Carningli upland and blocked off the whole of Cwm Gwaun, with lakes flowing over a succession of cols to cut overflow channels in the vicinity of the ice edge.  They think that the overflowing water utilized the older and larger channels of the Gwaun-Jordanston channel system -- which were inherited from earlier glacial episodes.  I think I would agree with much of that, although I won't be entirely convinced about all those lakes until we have varved sediments to look at.

So if the ice margin pressed as far inland as Eglwyswrw and Brynberian, with meltwater impounded up against the north face of Mynydd Presely, we can set the altitude of the ice edge at somewhere around 200m.  Now that is very important indeed, as we shall see.......

http://brian-mountainman.blogspot.com/2010/06/new-dates-shed-light-on-old-ice-limits.html

New technique confirms Pont Saeson source for some bluestones

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	I have been reminded that a paper due out in March has now been placed on the web.  It confirms, through the use of zircon chemistry, that some of the rhyolites in the Stonehenge area have come from the Pont Saeson area on the northern flank of Mynydd Preseli.  This had earlier been suggested by Rob Ixer and Richard Bevins on the basis of other methods of analysis.

Stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics

Richard E. Bevins,  Nick J.G. Pearce and Rob A. Ixer

Journal of Archaeological Science
Volume 38, Issue 3, March 2011, Pages 605-622

References and further reading may be available for this article. To view references and further reading you must purchase this article.

Abstract

The source of the bluestones at Stonehenge has long been debated, and while there is general consensus that the so-called spotted dolerites are derived from a relatively small number of outcrops exposed in the highest parts of Mynydd Preseli, in southwest Wales the source of the rhyolitic component has attracted relatively little detailed attention. This is largely because unlike the uniqueness of the spots in the coarser grained doleritic rocks, the rhyolites are fine-grained in character and lack any obvious distinctiveness, especially in hand specimen. This makes their provenancing difficult. A recent study, however, suggested that there was a close lithological similarity between the informally-termed ‘rhyolite with fabric’ bluestone component and rhyolitic rocks from the Ordovician Fishguard Volcanic Group exposed in the Pont Saeson area of north Pembrokeshire. This study aims to see if the chemistry of zircons, which are present in both sets of samples, could be used to support the petrographical association. Analyses for certain high field strength elements (including the rare earth elements) obtained by LA-ICP-MS showed that indeed the analyses were nearly identical when compared using a range of statistical approaches, including similarity coefficients, statistical distance, and principal component analysis, while showing clear differences to sample sets which had no reason to be correlated with the Pont Saeson samples. There are two important conclusions arising from this study. Firstly, the identification of the Pont Saeson source of the ‘rhyolite with fabric’ bluestone from outcrops in low ground to the north of the Mynydd Preseli will without doubt lead to fresh debates about the mechanisms of transport of this component of the bluestones to the Stonehenge site. Secondly, the chemistry of zircons may well prove to have a wider application in the provenancing of fine-grained rhyolitic rocks which have an archaeological context.

Druids in hard hats

I'm still not sure of the details on this, but rumours are spreading westwards from Salisbury Plain that English Heritage is bringing in new rules (in response to the Health and Safety regulations) which will require Druids (and others?) who get special permission to stand among the stones to wear hard hats.  The Druids are not best pleased -- to be seen wearing full ceremonial gear, and yellow hard hats at the same time, would not be good for their image -- and might indeed take something away from the ceremonials themselves.  They have my full support on this.

It's interesting that EH seems to think that if you stand under a sarsen lintel, in the midst of the great trilithons, and it then falls on you, you will be MIRACULOUSLY protected by your yellow hard hat!  Ah, the wonders of bureaucracy....

Garry Lavin's rolling wickerwork basket

There has been more publicity (on the BBC web site) for Garry Lavin's theory of how the big monoliths were transported from A to B.  Typical headline:
"Rewriting the ancient history of Stonehenge"
http://brian-mountainman.blogspot.com/2010/12/wickerwork-cradles-used-for-bluestone.html
http://www.bbc.co.uk/news/world-europe-isle-of-man-12386804
http://www.safari.org.uk/

Apparently he has tried out his theory, and found that a 40 kg stone (ie weighing less than 2 bags of cement) can be rolled along in its wicker cradle, and also floated.  On the Safari web site -- and on the video clip -- there is mention of a "three-quarters of a tonne" stone being used in the experiments.  Garry is having lots of fun on this one.....

There is also a video clip from the Discovery Channel:
http://watch.discoverychannel.ca/clip415360#clip415360

The Latest BBC Prehistoric spectacular

Been watching the latest Neil Oliver spectacular on BBC2 -- Part 1 of the new History of Ancient Britain.  Programme 1 of Series 1 -- looks as if we are in for a long journey, folks.  Interesting and well produced, but I wish we could have had 90% history and 10% Neil Oliver, instead of the other way round.......

Some of the commentary was far adrift -- our friend totally screwed up on the late-glacial, and some of the simplifications re the Loch Lomond readvance were positively appalling.  Does nobody check this stuff before it goes out on air...?

Next week we will be at Carn Meini and Stonehenge, among other Neolithic locations.  I already see a tendency towards fanciful interpretations and interpolations (cf the Red Lady of Paviland) -- I dread to think what we will get about our wacky ancestors in the next programme about the bluestones!

The Carningli Nunatak

Most of us who are thinking about where the edges of the Devensian Irish Sea Glacier might have been are now convinced that on the north Pembrokeshire coast the ice pressed inland as far as the upland ridge of Carningli - Dinas mountain.  Danny McCarroll (see previous post) thinks that the ice did not override the summit of Carningli, and has the cosmogenic dates to prove it.  But I still have not managed to find out PRECISELY where the relevant rock samples were collected, and I am still convinced that there are fresh glaciated slabs very near the summit.  The question is "how fresh?"  Are they fresher than the glaciated slabs at Carn Meini and on other tors on Mynydd Preseli?  More work needs to be done to sort this out.

Because there are hummocky areas of sands and gravels on the south side of Carningli, near the entrance to Cwm Gwaun, I think that the ice overrode at least a part of Carningli, and that it might have looked like the above photos around 20,000 years ago at the height of the Devensian glaciation.  I also think that the south flank of the mountain was the site of a large windscoop, within which a substantial scree bank was able to accumulate.  How long it took for the whole scree bank to form is another question -- it's possible that it was forming throughout the cold parts of the Devensian when the "main head" was accumulating on the coast and on hillsides everywhere.  But why is there no thick scree bank on the NORTH side of Carningli?  I suspect that the ice might have "cleaned out" whatever scree or rockfall debris there was.

Not far away, at Pont Ceunant, we have the "Pont Ceunant moraine" which may represent the maximum position of the Devensian ice on the north flank of the mountain, or it may simply represent a "stadial" or stillstand position where the ice edge remained in the same place for decades or maybe even centuries.  See the post for 3 May 2010.

The Irish Sea Ice Stream

 Ice directions around 23,000 years ago -- note the Irish Sea Ice Stream or Glacier pushing as far south as -- and even beyond -- the Scilly Islands.

I'm grateful to Prof Chris Clark for copies of two recent papers which deal comprehensively with the growth and wastage of the British-Irish Ice Sheet during the Devensian glaciation.  The illustrations above are acknowledged as from these papers.

The relevance of these papers for our understanding of glacial effects in SW and S England is obvious -- although we may not be talking of the same glacial episode, there is no reason to think that earlier glaciations will have been hugely different from the Devensian with respect to the overall history of the glacial event, the positioning of major glaciers, ice domes and ice streams, or glacier dynamics.

So what are we to make of the Irish Sea ice stream as portrayed?  The first thing to notice is that it is very similar in its dimensions to that suggested by Prof James Scourse with reference to the glacial deposits and radiocarbon dates obtained from the Scilly Islands.  The latest thinking is that the ice stream pushed southwards in a massive surge around 23,000 BP,  pushing southwards from its earlier position by around 300 km, before then retreating at a rate of over 140m per year towards St George's Channel, leaving a temporary lake behind, which was still there around 18,000 BP.  The latest generation of glacial geomorphologists will no doubt discuss this at some length -- but my reaction is exactly the same as when I saw James Scourse's reconstructed ice lobe pushing far out into the Celtic Sea and the SW Approaches.  That is, I don't like the look of it.  I don't believe it is glaciologically feasible to have an ice stream (even a surging one) pushing this far out from the main body of the ice sheet and flanked only to the west by ice-covered or glaciated terrain.  Ice streams, as I understand them, move through the outer parts of an ice sheet, constrained by either terrain or by pressure of flanking ice, and more or less perpendicular to the position of the ice edge or the transition into shelf ice.  There is no deep trough to channel or direct the glacier southwards and to prevent its lateral spread. 

The Bristol Channel cannot have remained ice-free with such a glacier pushing southward through St George's Channel, and I think it quite possible that the ice reached Lundy Island and the coasts of Devon and Cornwall at the same time as it reached the Scilly Islands.  This makes the interpretation of the glacial deposits on the Bristol Channel coasts even more crucial to our understanding of what went on in this part of England.


Sources:
Clark, C.D., et al., Pattern and timing of retreat of the last British-Irish Ice Sheet, Quaternary Science Reviews (2010), doi:10.1016/j.quascirev.2010.07.019

Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model
of ice sheet growth, retreat and dynamics
Sarah L. Greenwood, Chris D. Clark
Quaternary Science Reviews 28 (2009) 3101–3123