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, 30 November 2016

Oxford Gletscher, East Greenland

Something from the distant past! I was going through the official Greenland place-name gazetteer, looking for some photos, when I came across this: "Oxford Gletscher 71Ø-369 (71°32.8 ́N 25°16.7 ́W; Map 5). Glacier in the south Stauning Alper, draining south into the east end of Nordvestfjord. Named by the 1962 Oxford University expedition, which undertook survey work on the glacier. Oxford University is one of the world’s oldest and most prestigious universities, whose origins go back to the early 12th century. Uranus Glacier has also been used." 

 Dave Sugden and I named the glacier (and it was accepted by the authorities) after we did some rather difficult surveying and ice drilling work there in 1962. We were quite convinced that our ice temperature readings were rubbish, but actually we had discovered that the glacier was subject to surging behaviour. We were young and inexperienced, and did not know how to interpret the temperature anomalies.........  Hardly anything was known about surging glaciers at the time.

Talfynydd, Preseli -- unrecorded hut circle?

This apparent hut circle is located on the spur of Talfynydd, on the south side of the main Mynydd Presely upland ridge.  The grid reference is SN 127318.  Not far away to the NE is the tor called Carn Sian.  There is a gentle west-facing slope here, and the site looks out over Cwm Cerwyn.  This feature is 7-8m across, and as we can see there is a slight embankment littered with rather large boulders.  The feature is not perfectly circular, but slightly elongated downslope.  I have searches through all the Coflein and Archwilio records, and can find no trace of it.

Is it a Bronze Age hut circle?  There are certainly many others in the area -- but most of the interesting prehistoric remains on Talfynydd are on the eastern side of the ridge, whereas this one is on the west-facing (weather!) side.  There are of course similar features on Carningli as well.  Given its exposed location, I wonder whether this might have been an animal enclosure? But against that idea, there are no guiding walls anywhere near.........

Does anybody else know the site?  It's almost exactly 50m north of the site of the tragic Liberator crash of 19 September 1944, in which five airmen were killed.  The patch of bare ground (where the plane was burnt up deliberately after the crash, since it would have been very difficult to remove the debris from the mountain) can be seen on the image below, towards the bottom.  The "hut circle can also be seen quite clearly.

Tuesday, 29 November 2016

Cwm Cerwyn

One doesn't often see Cwm Cerwyn in the right light conditions to appreciate its form and location.  It's the deep cwm or hollow on the lee (eastern) flank of Foelcwmcerwyn or Preseli Top.  In this photo the winter sun is very low, and the greater part of the cwm is in shadow.

There are slight morainic landforms on its floor, and till is exposed in some of the stream cuttings -- so I think that this is where the last trace of glacier ice in Pembrokeshire was located.  I reckon there might have been a Younger Dryas (Zone III) glacier here, at the same time as other small glaciers were being regenerated in other parts of the British uplands.  That's only about 10,500 years ago.........

The location is perfect for a small niche or corrie glacier.  East facing, plenty of shade in the winter part of the year, and on the downwind side of an upland plateau -- excellent for the accumulation of windblown snow and for its conversion into ice.

I also suspect that the whole of the area shown in this photo was covered by a Preseli Ice Cap during the Late Devensian glaciation about 22,000 years ago, when the ice of the Irish Sea Glacier was pressing against the northern flank of Preseli, just over the horizon to the right......

Understanding of Stonehenge transformed for the first time this week

Oh dear-- yet another transformation in our understanding of Stonehenge. I wish these people would back off on the purple prose -- but I suppose it is in the DNA of all archaeologists........  But this does look quite interesting.  It will be intriguing to see what emerges about this early episode, which seems to pre-date the "stone using phase" by a few centuries..........


New discoveries rewrite Stonehenge landscape

28th November 2016

Archaeologists have found new evidence that rewrites the history of the Stonehenge landscape.  One of the newly-discovered sites even predates the construction of the world famous monument itself.

At Larkhill, the discovery of a Neolithic causewayed enclosure - a major ceremonial gathering place some 200 meters in diameter - dating from around 3650 BC radically changes our view of the Stonehenge landscape. About 70 enclosures of this type are known across the UK, although this is only the second discovery in the Stonehenge landscape, with the other further to the northwest at Robin Hood's Ball on the Salisbury Plain Training Area. In the Wessex region they occur on hilltops and, along with long barrows, are some of the earliest built structures in the British landscape.

FASCINATING FINDS - 700 yrs older than Stonehenge:

The Larkhill enclosure has produced pottery, worked flint, a saddle quern, animal bone and human skull fragments, all placed in the ditches which define the enclosure. Sites of this type were used for temporary settlement, to exchange animals and other goods, for feasting and other ritual activity, including the disposal of the dead. The objects found in the ditches reflect these ceremonial practices. The Larkhill causewayed enclosure is around 700 years older than Stonehenge and is part of a landscape that included other large earth and timber structures such as long barrows and cursus monuments. Its builders shaped the landscape into which the stone circle at Stonehenge was placed, which was already special long before Stonehenge was constructed. The causewayed enclosure at Larkhill shows that they had the social organisation necessary to come together to create significant earthworks, and the resources to support the work, as well as the people to carry it out.

Dr Matt Leivers of Wessex Archaeology told Spire FM:  "This is an exciting new find and one that transforms our understanding of this important monumental landscape."

While part of the site has been investigated, the majority of it lies within the Larkhill Garrison, where it remains unaffected by the current works.


At nearby Bulford, archaeologists have found a unique double henge, the only example known in Britain. The earliest phases were created around 2900 BC with circular enclosures formed by ditches dug in segments with openings to the north. In the Early Bronze Age (around 2000 BC) both henges were enclosed within continuous ditches, and perhaps buried beneath barrow mounds. From one of the Bulford henges a skull from a large dog or wolf, perhaps a working companion, a trophy from the hunt, or even a totemic symbol, was recovered.

Martin Brown, Principal Archaeologist for WYG told Spire FM:  "These discoveries are changing the way we think about prehistoric Wiltshire and about the Stonehenge landscape in particular. The Neolithic people whose monuments we are exploring shaped the world we inhabit: They were the first farmers and the first people who settled down in this landscape, setting us on the path to the modern world. It is an enormous privilege to hold their tools and investigate their lives."


Archaeological work on both sites is being managed and directed by WYG on behalf of Defence Infrastructure Organisation (DIO), with fieldwork undertaken by Wessex Archaeology.

The sites' development is part of wider plans to accommodate the 4000 additional Service personnel plus their families who will be based on and around Salisbury Plain by 2019 under the Army Basing Programme. In total, the MOD is planning to invest more than £1 billion in the area which will provide more than 900 new homes  for Service families, over 2,600 new bed spaces for single soldiers and the construction, conversion or refurbishment of 250 other buildings within bases, such as offices, garages, workshops and Mess facilities.

Carnedd Meibion Owen perched block

This is one of my favourite perched blocks, sitting on an ice-smoothed surface on one of the four tors of Carnedd Meibion Owen, above Ty Canol Wood.  Some perched blocks have nothing to do with glaciation, but in this context glaciation is the only realistic explanation, since the large boulder cannot have dropped or slid from anywhere else. 

It's dolerite, and has not moved far.  But I think it was moved into position by glacier ice during the Devensian glaciation.  This theory accords with the evidence of glaciation at Rhosyfelin, down in the valley below, and at Tafarn y Bwlch and many other localities up to the 330m contour.  It's all piecing together into a coherent picture.

Climate change in Norway

Here are some newly published photos from western Norway, showing the changes in the snouts of two of the outlet glaciers of the Jostedalsbre ice cap.  This is the largest ice cap in Europe, and the melting rate is getting scary.  The images have been published by the Norwegian weather service and the Norwegian Glacier Museum at Fjaerland, for whom I have done a lot of work over the years.  I observed both glaciers very closely over many visits.

Some years ago there was a temporary advance of the glaciers, caused by a short-lived rise in precipitation on the ice cap surface (that is one of the consequences of global warming).  Now, however, precipitation of snow is dropping off sharply, and metling rates are increasing at a frightening rate.  This is the result -- images from 1997 and 2016.

Monday, 28 November 2016

The matter of the "single Rhosyfelin monolith"

Now that Prof MPP seems to be backing away from the thesis that there was a Neolithic monolith quarry at Rhosyfelin, what do we have in its place?   Apparently (and we have to have this confirmed) the latest thought is that just one stone was taken from Rhosyfelin -- from the so-called "extraction point" near the tip of the spur and close to the sampling point 8 referred to by Ixer and Bevins.  We have already devoted a lot of attention to this issue on this blog.  Just type "extraction point" into the search box........

It's worth pointing out that that thesis is already well and truly falsified by the geological work.  Ixer and Bevins have shown that there are various types of rhyolite represented in the debitage and in the bluestone assemblage at Stonehenge -- and they have also shown various types of foliated rhyolite from the Pont Saeson - Rhosyfelin area.  In fact the famous "Jovian fabric" is pretty variable too, and I have disputed the contention that some of the fragments found at Stonehenge can be provenanced to "within a few square metres" since nothing at Stonehenge matches exactly the geochemistry and the thin section information given to us for Point 8.

Take a look at this post:

The more I look at the published thin sections and associated geochemical information, the more convinced I become that the closest match of all is between sample OU18, taken from the Stonehenge Avenue, and sample PS10 taken from an outcrop on the valley side a few hundred metres south of Rhosyfelin, where there is a sharp bend in the river gorge.  These are the two samples:

 Sample OU18, from the Stonehenge Avenue
Sample PS10, from the valley side south of Rhosyfelin

Note that the fabric alignment is not the same on the two slides, and that the interconnecting lattice structures are on a larger scale in sample PS10 -- but otherwise the fabric is very similar indeed. 

How much longer will it be before the quarrying fraternity comes to terms with the fact that we have at Stonehenge a scatter of fragments (and maybe one or two monolith stumps) that appear to have come from the Pont Saeson - Rhosyfelin area, having been entrained and transported by overriding ice?

Sunday, 27 November 2016

Those bluestone quarries -- on the way out?

Here is a report on the NPA Archaeology Day by our roving reporter Chris Johnson, to whom we offer thanks and felicitations.

I'm particularly interested in the signs that the archaeologists are beginning to withdraw from the proposition that Rhosyfelin is a Neolithic quarry.  It appears that they too have come to recognize that there is not actually any evidence in favour of it...... and that maybe it's best to quietly dump the idea.

There are other things in this report too, which are of interest.  I have inserted some numbered references and added my notes at the end.


Chris's report on the NPA Archaeology Day:

Crossing the Cleddau and approaching Haverfordwest from the south-west, the Preseli ridge garlands the horizon with its four gentle peaks, looking down on the farmlands and the town in a firm but friendly way. On a clear day like Saturday the hills look closer than the 30 kilometer distance and I am struck, not for the first time, how significantly the hills must have figured in the lives of the farmers in the Neolithic who were working these lands and the Mesolithic hunter gatherers who preceded them.

The Preseli mysteries loomed large too in the Annual Archeology Day organized by the National Park in which Volume 1 of the Pembrokeshire County History was launched with major contributions from Tim Darvill, Heather James, Kenneth Murphy, Geoffrey Wainwright, and Elizabeth Walker.

Among the several interesting presentations was a glimpse into recent work done upstream on the Cleddau where considerable finds of stone tools have been made dating back to the Mesolithic. Previous concentrations of Mesolithic activity have been identified on the coast, but now it looks like early inhabitants were using the river valleys too. I was pleased to hear that investigations are in progress to establish the presence of rhyolite as a material for the Mesolithic tools, flint of quality being rare in this area (1).

As we know, the Eastern and Western Cleddau have their origins in the spring waters and rainfall from the Preseli. Tim Darvill presented results from his excavations on Carn Menyn, one of the four peaks, in which he has convinced himself that early man was engaged in "quarrying" for thousands of years beginning in 7000 BC. First metamudstone was sourced, then spotted dolerite, and finally, in the run up to the Bronze Age, metamudstone again (2). There is convincing evidence for Mesolithic people building lives and traditions around the rivers flowing from Preseli and the mountains themselves (3).

Prof. Darvill comes in for some criticism on this blog, and did indeed rise to the bait regarding the healing properties of Preseli stones with an audience member who has convinced himself that his heart attacks have resulted from NOT having stood underneath the capstone of Pentre Ifan in the years of his attacks. Everybody listened politely. Tim's thesis for healing seems to reside in the observed general proposition that beliefs can cure when held sincerely, the stones themselves having no intrinsic properties apparently. He keeps talking about folk memory and I must say as someone with folk who have memories in the locality I wish he would be more specific because we have no idea what he means.  (4)

Still, Darvill managed to remain on the scientific side of the fence while giving a thoroughly informative and entertaining talk, venturing into "Spaces" and quoting Waldo Williams "It is the pearl pledged by time to eternity". There are few academics who can get away with saying "We should not shy away from magic in archaeology" but he managed. He also gave a first class classification of the "tomb" monuments in Pembrokeshire which I will save for another day. (5) Suffice it to say that Carn Alw is worth a visit. Apparently a massive boulder there has been raised 60 centimetres in dolmen times (5th millenium)  (6) and there are field systems where crops were grown on the heights in iron age and perhaps before.

Prof. Parker-Pearson (MPP) closed the show with a virtuoso performance. He started with a reconsideration of previous generations of experts directed primarily as a hefty swipe at the glaciers as an agent. As Chris Clarke apparently told him, "Glaciers do not move uphill!" (7)  H.H. Thomas apparently called it a "theory contrary to all sound geological reasoning". (8)  There is no room for doubt or compromise or even acceptance of a minority view. (9) The audience seems prepared to suspend disbelief too; why spoil a lovely story that is good for tourism and makes us all feel a bit special to be from Pembrokeshire. (10)

The experts cited by MPP also assume that the bluestones were used undressed in the Aubrey holes around 3000 BC and they were later dressed in the same fashion as the sarsens between 2620 and 2480 BC. (11) He also dropped into the dissertation that the Boles Barrow bluestone has been examined closely and found to have been worked by metal tools (12) which would disqualify it from being an original feature of the long barrow and furthermore undermine the belief that bluestones had been on Salisbury plain earlier that 3000 BC. There is a slight inconsistency in this narrative because of course MPP believes the stones were transported from Preseli for the monument so how could there be a theory that they arrived earlier than needed?

MPP is elegantly withdrawing from the proposition that Rhosyfelin is a quarry. (13)  "Maybe only one lift of a single block". The charcoal under the picnic table - the stone which caused all the excitement - is dated to 2000 BC and so the "quarry" operated for longer, quite clearly according to the evidence :)). Of course it could not possibly have dropped onto an old fireplace. When challenged from the chair Mike admitted that "quarry" is perhaps the wrong word, perhaps extraction would be better or basically the stones were picked up, which explains the lack of tools. (14)  Moving on quickly, the real quarry is at Goedog. And additionally, Ixer/Bevins have identified Cerrig Marchogion as the source of three Stonehenge dolerite pillars - lots more to do! (15) Some more dating will be done at Rhosyfelin but the roadshow has moved on. (16)

MPP spent time explaining how he knows Goedog is a quarry with use of photos. Apparently the pillars could be prised simply from the rock face - hence a minimum of tools. They were then lowered by ropes ( twisted honeysuckle maybe, as at Seahenge) onto wooden sledges parked on prepared terraces. (17)  Apparently the path off the mountain is a two metre wide roadway. (18) I confess I could not see any of this on the slides shown, but then I have never seen a Neolithic quarry, but then again nor has anybody else. Mike did say that Rhosyfelin was the first in Europe and now that is no more. Trust me, I am a professor from London and a jolly nice chap.  (19)

Next year activities are directed to find the proto-Stonehenge circle. Maybe Bedd-yr-Afanc is going to prove to be a Bryn Celli Ddu transformation, and then there is the mysterious circular enclosure north of Goedog which, I believe, Brian and I walked over last year and which Hugh pointed us towards. MPP is keeping it secret. Personally I look forward to the investigations as these are both fascinating sites. (20)

On transportation Mike is super positive. "Only 10 students can move a 1 ton block". "Don't need rollers". (21)  His computer crashed before he could show the video (sic).

This year's adventure at Pensarn was skipped over - perhaps it might be published in my lifetime? To cut a long story short, they thought it might be a passage tomb but it turns out to be a 27m diameter Cairn. There has been some development and reuse of the monument, standing stones, etc but Mike did not dwell on this as it was off-topic - although on another day and in another place it might be considered a spectacular find. MPP also slipped in that the various excavations north of Rhosyfelin have been unproductive too, in the sense of failing to supply the evidence he was hoping for. (22)

Enough for now. I enjoyed the day and the Pembrokeshire County History is a worthy project and worth buying if you like this stuff. Very likely substantial revisions will be made in the next ten years.

Chris Johnson, 27.11.2016


(1)  Interesting point.  This seems perfectly logical too me, and I look forward to reading the evidence when published.  We have said since we started work at Rhosyfelin that the foliated rhyolite is very suitable for the manufacture of disposable blades and cutting tools -- and that this maybe explains why people camped herein the Mesolithic and later.

(2)  We have already examined the evidence for this claim, in earlier posts on the blog.  It is in my view not supportable by the evidence presented.

(3)  I completely disagree with that contention.  It is pure fantasy.

(4)  Ah, who needs evidence when you have faith?

(5)  There are already classifications in print.  It will be interesting to see whether this new one is any different.

(6)  I wonder if there is any evidence for this?

(7)  Chris Clark is a good glacial geomorphologist, and he would never have been so stupid as to make such a statement.  Of course glaciers flow uphill if circumstances are right.  In every area affected by big glaciers erratics are found which have been moved uphill from their source areas.  I do wish MPP would stop trotting out this sort of nonsense, and attributing it to others -- who would be apoplectic if they were to hear him!

(8) HHT did his fieldwork about a century ago.  He knew virtually nothing about glaciology and his ideas on glacial geomorphology were seriously out of tune with those of his own peers.  I wish people would stop citing him as some sort of expert on the glaciation of  the British Isles.

(9)  I would be interested to know whether MPP actually acknowledged the existence of the peer-reviewed articles by my colleagues and myself..........

(10)  Sadly, deference instead of scrutiny, yet again.

(11)  Sounds reasonable.

(12) I would like to see the published evidence in support of this contention.

(13)  Is common sense breaking out, I wonder?  Now our friend Myris is going to have to revise all his comments on this blog and refer to "possible extraction sites" instead!

(14)  Quite soon we will see Rhosyfelin referred to as "a source area for some of the foliated rhyolite fragments found at Stonehenge."  When that happens, we will have 100% agreement on the form of words that should be used.

(15)  No they haven't.  Ixer and Bevins have suggested Cerrig Marchogion as a source area, but they cannot be certain about it.  There are other possibilities too.

(16)  Probably this refers to the samples taken for 36Cl dating -- we await news of what those dates are.

(17) Don't believe a word of it.  All fantasy.

(18)  Ditto.

(19)  Same old tactic.  Encourage deference and evade scrutiny at all costs......

(20)  Forgive me for sounding weary, but for how much longer do we have to put up with this flogging of a dead horse?

(21)  Quite incorrigible, isn't he?  See a multitude of previous posts.....

(22)  If he is now admitting this in public, we have some progress.

Wednesday, 23 November 2016

Rhosyfelin -- National Park still peddling myths

 I'm rather intrigued that the Pembs Coast National Park seems to be incapable of doing a simple piece of editing. In spite of abundant correspondence between me and Phil Bennett (the NPA archaeologist) the description of the PDF walk leaflet still has the following text:

"This craggy outcrop of stone lies within a deep secluded valley. It is here that at least one of the famous bluestones were quarried and later taken to Stonehenge – most probably by land on sleds hauled by oxen. The rock outcrop can be viewed from the road or nearby public footpath. Please keep to the public footpath, access to the rock outcop is by permission of the landowner."

I am not amused.  You can find some of my correspondence with Phil by putting his name into the search box.  However, I gain some solace from the fact that Phil is apparently no longer employed by the National Park, but has become an "independent archaeologist".  We wish him well in his new career, and hope that whoever takes over from him in the NPA office shows rather more respect for scientific accuracy than he ever did.

Rhosyfelin Archwilio Record amended

The Archwilio Record  (Dyfed Archaeological Trust) record for Rhosyfelin has been amended.  It's better than it was, but is still not acceptable.  The site description is missing.  It is still flagged up (preferred type) as a Neolithic Quarry -- or is it?  The heading is really rather garbled.

In the text it should say:  ".....the site is interpreted by some as the source for some of the Stonehenge rhyolite."

And the reference to our paper is wrong.  It should be as follows:

Brian John, Dyfed Elis-Gruffydd and John Downes.  2015. OBSERVATIONS ON THE SUPPOSED "NEOLITHIC BLUESTONE QUARRY" AT CRAIG RHOSYFELIN, PEMBROKESHIRE".  Archaeology in Wales 54, pp 139-148.  

And regarding the dating, it should say:
Charcoal fragments are the primary dating evidence for the Mesolithic - Medieval use of Craig Rhosyfelin.

It's a slow job, getting these archaeologists to do their job properly........

Primary Reference Number (PRN) : 106534
Trust : Dyfed
Community : Nevern
NGR : SN11663615
Site Type (preferred type first) : Neolithic Quarry / NOT APPLICABLE Natural feature
Legal Protection :

Summary :
Natural rock outcrop that may have been quarried during the Neolithic. Some of the rhyolite debitage at Stonehenge closely matches that found here; thus the site is interpreted by some as the source for the Stonehenge rhyolite. Charcoal fragments are the primary dating evidence for the Neolithic use of Craig Rhosyfelin. There is ongoing scholarly debate regarding the use of the site as a Neolithic quarry. DAT 2016

Description :

Sources :
Many , 2014 , Correspondence between geologists and K. Murphy
Ixer, R & Bevins, R , 2011 , Craig Rhos-Y-Felin, Pont Saeson is the Dominat Source of the Stonehenge Rhyolitic 'Debitage' , Archaeology in Wales : 50 : 21-32
John, B et. al. , 2014 , Observations on the supposed 'Neolithic Bluestone Quarry' at Craig Rhosyfelin, Pembrokeshire
Coleman-Philips, C , 2015 , New theory casts doubt on Stonehenge bluestone links
Coleman-Philips, C , 2015 , Original Stonehenge 'was built in Preselis'
John, B et. al. , 2015 , Quaternary Events at Craig Rhosyfelin, Pembrokeshire , Quaternary Newsletter : 137 : 16-32
Parker-Pearson, M. et. al , 2015 , Craig Rhos-y-felin: a Welsh bluestone megalith quarry for Stonehenge , Antiquity : 89 : 1331-1352

Events :

Related PRNs :

Saturday, 19 November 2016

Brynberian Gorge -- a paradise for Neolithic hunters and gatherers?

Four sites within the Brynberian Gorge, downstream of Rhosyfelin, which would have been perfect camping locations for Neolithic hunting and gathering parties.

In previous posts on this blog I have argued that the evidence of human occupation at Rhosyfelin and Carn Goedog (including hearths, burnt hazel nut shells, charcoal and other organic remains, some of which have been radiocarbon dated) is entirely consistent with the use of these sites over a long period of time by people who depended to some degree on hunting and gathering.

I have made the same point in public talks and in these articles:  

The essential point is that not one of the radiocarbon dates obtained thus far from these two sites has a secure archaeological context that might tie it to quarrying activity.  The dates are more or less randomly scattered across a long time scale, and if there are one or two "clusters" of dates they are at quite the wrong times to provide any support for the quarrying hypothesis.  One might even argue that far from supporting the quarrying thesis, the dates actually falsify it.

Let's just look at the likelihood that the Rhosyfelin occupation traces are unique.  No chance.  I went for a walk along the Brynberian Gorge yesterday, and was struck yet again by the fact that it must have been a paradise for the Neolithic tribes who inhabited this area.  In the Neolithic people were moving towards a more sedentary way of life, with animal husbandry and the growing of crops supporting larger and more or less static communities.

The evidence shows that the elm decline was continuing -- probably related to land clearance activities -- but that the ecology of the Brynberian area was not that different from today. Mixed deciduous woodland must have predominated, with settlement clearances appearing on the uplands and on land below the 300m contour that was well drained. Hunting and gathering were of vital importance for the maintenance of food supplies, and while fishing activities might have declined as compared with the Mesolithic, hunters must have spent much time in hunting for the larger animals such as elk, red deer, roe deer, wild boar and aurochs. All of these animals must have been common in the forests and deep wooded valleys of North Pembrokeshire: but small animals would also have been hunted -- foxes, squirrels, badgers, hares, geese, swans, and even small birds. There must have been fishing for trout, sewin and salmon in the Nevern and Brynberian rivers, and shellfish would have been gathered from the estuary. Eggs were collected too.

Not all of the hunting was for food supplies -- some animals were hunted for their skins, horns, bones and sinews and anything else that could be utilised.  Berries and other fruits and nuts were essential parts of the diet, and withies were collected for basket-making while birch bark was collected for the weatherproofing of buildings.  Brambles and nettles were collected for rope-making.  It is quite possible that rhyolite rock outcrops were used for obtaining sharp-edged cutting tools and that dolerite outcrops were used for axe manufacture.  And so the list goes on.

In the Neolithic summers were warmer and drier than they are now, and winters were about 2 deg C colder.  Hunting and gathering would have had strong seasonal rhythms, and a lot of the hunting would have been concentrated in the winter months when farming activities were reduced.  The valleys were the natural places in which these activities would have been concentrated, since they provided shelter from winter storms and relatively easy hunting conditions (far easier than on the open uplands).  Some authorities think that these valleys were also used for the overwintering of herds of sheep, goats and cattle, and the frequent traces of ancient stone walls in unexpected places might support this idea.

There must have been hundreds of temporary camp sites strung through valleys such as the Brynberian Gorge, used at intervals throughout the year but maybe with more intensive winter use.  Sites beneath sheltering rocky crags may well have been preferred.  From this point of view Rhosyfelin is perfect -- but there are scores of other perfect sites in the Brynberian Gorge alone.  At the top of this post I have pictured some of the suitable crags with dry and safe camping sites beneath them.  In the past I have mentioned the crags at Felin y Gigfran (on the Afon Nevern, below the confluence with Afon Brynberian) and also other crags upstream from Rhosyfelin.  A veritable hunting and gathering paradise.........

The archaeologists who have been so obsessed with the Rhosyfelin quarrying hypothesis appear not to have considered any of the points raised here -- and have assumed that the occupation traces discovered are unique and uniquely related to quarrying activity.  If they want to support their very dodgy assumptions they need to demonstrate that Rhosyfelin really is unique, and that all the other "favourable camping sites" in the gorge were NOT used in similar fashion.

I am 100% convinced that if they were to dig in the other places for which I can give them grid references, they will find hearths, charcoal, hazel nuts and maybe even traces of tool-making which can be dated to the Neolithic.

One of the classic Stonehenge papers......

I have discovered that this classic paper on Stonehenge is now available on the web:

On reading it again I am impressed by its clarity and its scholarship, and am saddened by the thought that Richard Thorpe died before it was published.  We are talking about an article published in 1992, so the info in it is 25 years old.  Therefore a lot of the detailed geology has been shown to be inadequate, and has been replaced by the more modern work by Rob Ixer, Richard Bevins and others.  Nowadays the "rock terminology" has changed too.  The science of bluestone provenancing has moved on.

But this remains one of the clearest and most concise discussions of the evidence for and against the bluestone "human transport" hypothesis.  It is also very careful and even meticulous in its presentation of evidence and in the citation of sources.   Bravo!

Friday, 18 November 2016

New modelling and the Devensian limit

The modelled extent of the Celtic Ice Sheet,with basal thermal regime assumptions.  The red line shows the commonly assumed Devensian limit, which has now been shown to be incorrect in many areas.  The orange areas show where enhanced erosive capacity probably occurred.  Note that there was no great potential for erosion in St George's Channel and the Celtic Sea approaches.

This new paper (Patton et al, 2016) has just been published online.  It's very influential, and will be much cited!  It's a big and complex paper, and I have yet to fully digest its contents, but there are a few things that spring out on an initial reading.    First, the British and Irish Ice Sheet (BIIS) is referred to here as the Celtic Ice Sheet (CIS).  Second, the maximum Devensian ice extent is deemed to be about 22,700 years ago -- that's rather earlier than many other papers have suggested.  Third, the authors support the conclusions of other authors that the behaviour of the Devensian Irish Sea Glacier was highly erratic and dynamic.

This is what they say:

The sensitivity of this region to temperature and precipitation fluctuations (Fig. 6B–C) supports the contention that this ice sheet was highly dynamic, and led to a complex geomorphological palimpsest with multiple advance and retreat cycles (Greenwood and Clark, 2009, Clark et al., 2012 and Hughes et al., 2014). In this context, model output supports and resonates with recent evidence of expansive marine-based limits found on Porcupine Bank, west of Ireland (Peters et al., 2015), and at the Celtic Sea shelf break south of Ireland (Praeg et al., 2015). However, our model results presented here indicate that these far-field limits likely relate to short-lived surge-phases of ice stream activity, associated with enhanced orographic precipitation input to the Irish ice sheet, rather than long-term stable ice-sheet limits.

 Given that the modelling work here covers a vast area and is portrayed on small scale map illustrations, we cannot read too much precision into the ice limits for Southern Great Britain, but what interests me is that when the new evidence from Dartmoor, the Scilly Isles and the South-west is brought into the frame, the Devensian Irish Sea Glacier is shown on nearly all of the models as filling the Severn Estuary, the Bristol Channel and the Celtic Sea approaches. The ice is even shown pushing well into Somerset -- and in some scenarios as far east as Salisbury Plain.  Bear in mind that this modelling is for the Devensian -- I am quite convinced that when similar modelling is done for the Anglian Glaciation, the ice will be shown as being even more extensive.......

Glacial transport of bluestones, anyone?

Henry Patton, Alun Hubbard, Karin Andreassen,  Monica Winsborrow,  Arjen P. Stroeven. 2016.
The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing.  Quaternary Science Reviews, Volume 153, 1 December 2016, Pages 97–121

Note:   Late Weichselian = Late Devensian



The Eurasian ice-sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum (LGM), after the Antarctic and North American ice sheets. Despite its global significance, a comprehensive account of its evolution from independent nucleation centres to its maximum extent is conspicuously lacking. Here, a first-order, thermomechanical model, robustly constrained by empirical evidence, is used to investigate the dynamics of the EISC throughout its build-up to its maximum configuration. The ice flow model is coupled to a reference climate and applied at 10 km spatial resolution across a domain that includes the three main spreading centres of the Celtic, Fennoscandian and Barents Sea ice sheets. The model is forced with the NGRIP palaeo-isotope curve from 37 ka BP onwards and model skill is assessed against collated flowsets, marginal moraines, exposure ages and relative sea-level history. The evolution of the EISC to its LGM configuration was complex and asynchronous; the western, maritime margins of the Fennoscandian and Celtic ice sheets responded rapidly and advanced across their continental shelves by 29 ka BP, yet the maximum aerial extent (5.48 × 106 km2) and volume (7.18 × 106 km3) of the ice complex was attained some 6 ka later at c. 22.7 ka BP. This maximum stand was short-lived as the North Sea and Atlantic margins were already in retreat whilst eastern margins were still advancing up until c. 20 ka BP. High rates of basal erosion are modelled beneath ice streams and outlet glaciers draining the Celtic and Fennoscandian ice sheets with extensive preservation elsewhere due to frozen subglacial conditions, including much of the Barents and Kara seas. Here, and elsewhere across the Norwegian shelf and North Sea, high pressure subglacial conditions would have promoted localised gas hydrate formation.



Maximum ice sheet coverage was 5.5 × 106 km2 ∼22.7 ka BP.

The EISC grew to 7.2 × 106 km3, equivalent to c. 17 m of global sea level lowering.

Maximum ice extension was asynchronous - 2–5 ka later east of the main ice divide.

Subglacial erosion patterns reveal potential for widespread landscape preservation.

The optimal reconstruction reveals a moderately thick ice complex with nunataks.



In this study we use a higher-order, thermomechanical ice sheet model to reconstruct the build-up of the Last Glacial Maximum Eurasian ice-sheet complex (37–19 ka BP), as well as its sensitivity to a variety of key glaciological and climatological parameter configurations. Boundary condition data for each semi-independent ice centre are forced separately across three contrasting zones of the ice-sheet complex, reflecting the disparate oceanographic and climatological regimes of the northern Eurasian and High Arctic domains.

The optimal experiment presented indicates rapid growth of the ice-sheet complex, with margins first present along much of the western Eurasian shelf break by 29 ka BP. The Celtic, Fennoscandian and Barents Sea ice sheets continue to expand south and east under further temperature cooling, reaching a peak extent and volume c. 22.7 ka BP (5.48 × 106 km2 and 7.18 × 106 km3 respectively).

Required climate and oceanographic forcings differ significantly across the three ice sheet sub-domains, ranging from maritime conditions across Britain and Ireland to a polar desert regime in Siberia. Minimum temperature suppressions required to drive extensive glaciation decrease northwards, indicating an apparent insensitivity to climate cooling across Arctic regions. Considerable precipitation gradients that simulate rain-shadow effects are also required to keep ice within known empirical margins in eastern sectors. A heterogeneous sensitivity to calving losses is applied across the domain to simulate differences in sub-surface ocean temperature and the buttressing effects of perennial sea ice/ice shelves.

Maximum LGM margins are not contemporaneous, with major ice-divide migrations forcing a relatively late incursion into eastern sectors c. 20–21 ka BP compared to c. 23–25 ka BP along western margins. Although rain-shadow effects amplify this asynchrony, the most compelling driver for this behaviour is a pronounced difference in topography either side of the major nucleation centres.

Flowsets previously reported in the literature and attributed with an LGM age closely align with general predictions of ice flow direction across Fennoscandia and the southern Barents Sea. However, the relative chronologies of some flow packages appear to be closely associated with the timing of large magnitude ice-divide migrations as the ice complex migrated eastwards. Ice flow from central sectors of the BSIS and northern Fennoscandia therefore dominated only during the latter stages of the LGM.

A relatively high enhancement factor of deformation ice flow is needed to force low-aspect growth of the ice complex, particularly for the Barents Sea ice sheet. Although the resulting ice cover is generally thinner than under conventional parameter values, comparison of ice profiles with cosmogenic-exposure age transects along the margins of Svalbard and Norway, as well as central Fennoscandia, reveal the optimum experiment to be a good fit, and an improvement on previous modelled reconstructions.

Crustal deformation imposed by the EISC at 21 ka BP matches closely to the broad patterns of observed uplift across the northern Barents Sea and Fennoscandia. During the LGM, maximum depression by the ice sheet was c. 290 m east of Svalbard and in northern Sweden. More limited ice cover based over the Scottish Highlands led to a maximum depression of c. 125 m.

Subglacial properties of the model reconstructions reveal that basal erosion by the EISC was widespread at the LGM, particularly beneath the major ice streams that drained the Fennoscandian and Celtic ice sheets. Conversely, cold-based conditions dominated across the Barents Sea and upland regions of Fennoscandia. Such conditions across the hydrocarbon-rich continental shelf of the Eurasian Arctic would have created a stable environment conducive for the widespread growth of sub-marine gas hydrate as well as paraglacial permafrost.

Thursday, 17 November 2016

Glaciers online

Here is a reminder, for those who might have forgotten, that there is an amazing resource on snow and ice, glaciers and all things glacial on the following site:

The Glaciers Online photographic record is frequently updated and improved, and the collection of classic photos is second to none.  Many thanks to the Swiss Educational site, and to Mike Hambrey from Aberystwyth, who provides much of the material from the UK end of the operation.

Monday, 14 November 2016

The joy of being falsified........

I thought I should share this.  Good for Danny!

Trimline Trauma: The Wider Implications of a Paradigm Shift in Recognising and Interpreting Glacial Limits. Danny McCarroll, Scottish Geographical Journal, 2016
Published 27 Feb 2016 

Danny's Confession

I must confess that being proven wrong is not a new experience for me. My interpretation of the very thick glacial sediments of western Llyn, in North Wales, as meltout till, for example, fitted a paradigm that was popular at the time but has since been falsified (McCarroll & Harris 1992). I also produced several papers arguing that carbon isotopes in Arctic tree rings could be used to reconstruct summer temperature (McCarroll et al. 2010), which my younger colleagues demonstrated was not true (Young et al. 2010; Gagen et al. 2011). However, as a scientist there is no shame in being wrong because falsification of hypotheses is how we make progress (McCarroll 2015). If you are afraid of being wrong you will never have the confidence to publish anything of interest. We should, however, try to learn from our mistakes.

The South Pembrokeshire problem

There's a problem in South Pembrokeshire.  It's probably not going to make the headlines, but it's quite an interesting problem if you happen to be a geomorphologist or an archaeologist, because it relates to the availability of land for settlement back in the Palaeolithic, around the time of the Devensian maximum ice advance. 

Over the years, I have published scores of maps on this blog, and most of them show an Irish Sea Glacier ice limit running along somewhere to the north of Mynydd Preseli and then looping down towards the mouth of Milford Haven (there are fresh glacial deposits at Druidston, West Dale, Mullock Bridge and West Angle) and then looping off again into Carmarthen Bay.  Here are a few examples:

You'll notice that in all of these maps, central and south Pembrokeshire are shown as being ice-free at the Devensian maximum, around 20,000 - 18,000 years ago.  The modelling of the waxing and waning of the Welsh Ice Cap, done by researchers in Aberystwyth University, has reinforced this assumption, showing Welsh ice pressing westwards in to eastern Pembrokeshire and with an intermittent Preseli Ice cap to the west interacting / coalescing with the ice of the Irish Sea Glacier when it was at its most advanced position.

But there are big problems with this scenario, which have been keeping me awake at night. (Well, not really, but it's a nice expression......) 

First, in the majority of published maps the ice edge is shown in a highly idealised form, sometimes as a virtually straight line.  Ice edges are not like this -- glaciers always butt up against obstacles and flow into lowlands, creating a fingered ice margin except in the rarest of circumstances.  So if the ice flowed in from the north-west and had a margin at an altitude of c 250m on the northern flank of Preseli, why would it have left the lowlands of the St David's Peninsula and central and southern Pembrokeshire ice free?  Clearly the ice would have flowed across these areas unless there had been an exceptionally steep ice edge -- and Celtic Sea researchers are now coming to the view that the ice surface gradient was remarkably low rather than remarkably steep.   Just look at the topography of Pembrokeshire and you will see that there is effectively no obstacle to Irish Sea Ice sweeping right across the county from the NW in the Devensian:

Second, the recording of unconsolidated till containing striated Old Red Sandstone cobbles and resting on carboniferous Limestone near the eastern tip of Caldey island means that Devensian ice must have crossed the island, travelling from west to east.  That means that the ice must also have crossed the Castlemartin Peninsula, even if actual traces of ice action on the limestone plateau are extremely rare.

Third, we now know that there are glacial deposits at around 340m on the northern flank of Preseli, not far from Tafarn y Bwlch.  It is reasonable to assume that the ice edge was at a similar altitude all the way eastwards towards Foel Drigarn and Frenni Fawr.  With ice movement from the north-west -- as demonstrated by the passage of the Broad Haven "super-erratics from Ramsey Island -- the ice must also have inundated the land surface of the western Cleddua drainage basin at least as far inland as Rosebush and Haverfordwest.  As it happens, fluvioglacial deposits are recorded from the Rosebush area, although I have not seen them myself.  When I put up a previous post on this, in 2014, I suggested that the deposits in the Rosebush sand pit had come from the Preseli Ice Cap -- now I am not so sure:

I have also interpreted the gravels at Llangolman as "probably Anglian in age" -- and I'm no longer so sure about that either! 

So should we now radically revise our opinions on the extent of Devensian ice in Pembrokeshire?   Well, maybe -- but we still have anomalies to cope with, including the remarkably fresh and fragile appearance of the tors at Maiden Castle, at the edge of the Trefgarn Gorge.  All of the fieldworkers in this area have assumed that they are so fresh that they cannot possibly have been overridden by Devensian ice:

 In contrast to Maiden Castle, the other tors and monadnocks of North and West  Pembrokeshire are heavily denuded and ice smoothed..........

Now another thing comes into the frame.  In a previpus post ion the Isles of Scilly, I referrd to an interesting article by Prof Danny McCarroll:

Trimline Trauma: The Wider Implications of a Paradigm Shift in Recognising and Interpreting Glacial Limits.  Danny McCarroll, Scottish Geographical Journal, 2016
Published 27 Feb 2016 

Trimlines mark the boundary between glacially eroded landscapes on low ground and landscapes dominated by evidence of periglacial weathering on higher summits. For many years the trimlines of Scandinavia, Britain and Ireland have been interpreted as marking the surface of the ice sheets at the maximum of the last glaciation, but recent cosmogenic exposure dating of erratics far above the trimlines in NW Scotland shows this to be false. The trimlines in that area must represent an englacial thermal boundary between warm (eroding) ice and cold (protecting) ice. It is now clear that even very experienced geomorphologists cannot necessarily tell the difference between terrain that has been recently glaciated and terrain that has not, because cold-based ice can leave virtually no trace. This calls into question not only the interpretation of high-level trimlines elsewhere, but also the mapping of the lateral limits of past glaciations, which are often based on similar or even weaker geomorphological and sedimentological evidence.

Danny refers to several of the areas which he knows well, and he says this about Pembrokeshire:

The ice limits that I know best are those in South Wales, and I now have very serious doubts about their veracity. In north Pembrokeshire, for example the proposed ice limit runs along the northern flanks of the Preseli Hills, leaving southern Pembrokeshire ice free. The evidence used to define that limit is exactly the same as the evidence we used to define the ice surface at trimlines. It is marked by a transition from bedrock that has been glacially scoured, and where glacigenic sediments are widespread, to a landscape dominated by blockfields and tors where glacigenic sediments are absent (Walker & McCarroll 2001). However, there are plenty of erratic boulders well to the south of the proposed ice limit, just as there are erratic boulders above the trimlines in Scotland. My best guess at the moment is that the north Pembrokeshire ice limit probably is the southern limit of the last ice sheet in that area and that the headwaters of the Cleddau remained ice free. That explains why that is the only river system in Wales that remains graded to well below the present sea level, producing the deep water port of Milford Haven. However, on a recent visit to the Castlemartin Peninsula, the limestone area south of Milford Haven, I noticed that the Carboniferous limestone is littered with erratic pebbles. There are also old records of large erratic boulders perched on the limestone, though most (perhaps all) have since been moved. Of course the erratics may have been deposited during an earlier glaciation, but I am not aware of any clear evidence to that effect. The concept of the ‘ice-free enclave’ of South Pembrokeshire should really be critically tested.

I am rather happy to go along with the sentiments expressed, although Danny is wrong to suggest that glacigenic sediments are absent outside the conventional Devensian limit.  I don't agree that the headwaters of the western Cleddau were ice-free in the Devensian -- I demonstrated relatively recent glaciation in the catchment in my doctorate thesis way back in 1965.......  And I am not sure what the depth of Milford Haven has to do with the Devensian -- it is clearly an immensely old inherited feature. 

But yes, let's really test the idea of this "ice-free enclave" -- and the evidence is stacking up that ALL of Pembrokeshire was probably affected by Devensian ice belonging to the Irish Sea Glacier, the Preseli Ice Cap, or the Welsh Ice Cap.  Watch this space.  In the meantime, here is a map that represents my current thinking on the position of the Preseli ice margin around 20,000 years ago:

 The thing that has really pushed me to this interptetation is the discovery of till not far from Tafarn y Bwlch when I was up there, wandering about with Chris a few months ago:

This till has come from the north, not from the south -- so it has nothing to do with a Preseli Ice Cap.  

Notice that I have postulated two ice lobes pushing through the mountain ridge via cols in the upland ridge.  The first of these, near Crymych, has already been suggested by the field surveyors of the BGS, and by other researchers.  The other one, in the Mynydd Bach col, is just to the west of Carn Goedog.  If there was an ice surface at around 340m, a lobe might well have pushed through.  But there is a rather interesting ridge-like feature right in the gap.  I've never noticed it before.   Is it a moraine?  As soon as I get a chance, I shall be up there to check it out..........

Sunday, 13 November 2016

Mynydd Preseli ice margin

I have been looking through this excellent report on the Teifi catchment, published in 1997.  I'll come back to it with more of the useful material culled from the literature by the authors.  But for the moment, this map caught my eye, since it is right in line with my current thinking on where the ice edge might have been in North Pembrokeshire at the time of the Devensian glacial maximum, around 20,000 - 18,000 years ago.

In line with much of the recent work done by BGS surveyors, they show Welsh ice affecting the upper catchment of the Teifi, as far downstream as Llandysul.  On its western edge, between Llandysul and the Ceredigion coast (which was not there at the time) there was a contact zone with Welsh ice to the east and Irish Sea ice to the west.  Carningli and Dinas Mountain are shown as completely inundated by the Irish Sea ice stream -- and more and more of the field evidence published on this blog now appears to support the idea that the ice edge lay along the northern flank of the Mynydd Preseli ridge, with the Brynberian Valley (including the Rhosyfelin area), Carnedd Meibion Owen and even Cwm Gwaun overridden.  This means that the morainic accumulations at Pont Ceunant, Cilgwyn and Tafarn y Bwlch may owe their origins to temporary stillstands or slight readvances during  a prolonged deglaciation phase.  The thick accumulations of sands and gravels in the Monington - Moylgrove area must also date from this episode of ice melting.

More fieldwork is required.  Watch this space........

R A Waters, J R Davies, D Wilson and J K Prigmore.  1997. A geological background for planning and development in the Afon Teifi catchment.    British Geological Survey Technical Report, WA197I35.102pp.

Saturday, 12 November 2016

Deference or scrutiny?

I forgot to report on the talk which I gave at our local micro-brewery on October 28th.  It was indoors and supported by a good audience -- and after a few glitsches we managed to get my laptop computer to communicate with a friend's digital projector -- so I was able to show my Powerpoint presentation.  My talk was very similar to that given to the Welsh archaeologists in Machynlleth a few months ago.

I enjoyed it -- and of course did my best to stir up a jolly discussion.  It got quite heated at times, with most of the ire directed at me by people who have heard too many of MPP's talks over the years.  So afterwards several people said how entertaining it had all been!

I spent part of the talk referring to the work done at Rhosyfelin and Carn Goedog by the geologists and the archaeologists, and part of it talking about the work done by Dyfed Elis-Gruffydd, John Downes and myself on the Quaternary sediment sequence and landforms.  Of course I was critical of those who are promoting the "Neolithic bluestone quarrying" hypothesis.  And as ever, nobody questioned any of our findings as published in our two peer-reviewed papers.

It was all very good-humoured, and what struck me was the ongoing determination of some people to follow the line that "there may be no evidence for long-distance monolith transport by human beings, but they were certainly very clever (think pyramids, Easter Island heads etc) and if they had wanted to do it they would have done it.  Therefore they probably did....."    I have no time for that sort of argument, and said so.

The other thing that was quite entertaining (for me as the speaker) was that one or two members of the audience were quite outraged that I had the temerity to question the evidence presented by those terribly expert geologists and archaeologists, who were deemed to be honest and highly-skilled academics simply doing their best to collect evidence in the field.  I should presumably have accorded them more respect and even deference......... 

I could have reminded these good people that I am rather expert too, and know what I am talking about, at least for most of the time.  But time was too tight for all that sort of stuff, so I contented myself with reminding the audience that when you do field research you are supposed to go through a process of data collection, data analysis, and interpretation before telling the world what your conclusions are.  In contrast, the archaeologists who have been digging in our area for six seasons have demonstrated a cavalier disregard for the norms of scientific field research, and they deserve to be slated for it.  (In fairness, the geologists have done things properly, so no problems on that score, apart with some quibbles on their interpretations.....)

I also reminded my enthusiastic critics that I have at least done MPP and his team the honour of referring to their work and examining it carefully.  On the other side of the argument, from all the reports I have had of MPP's talks in the last year or two, the glacial transport thesis is simply ignored or dismissed out of hand as being "discredited."   By whom, and on what grounds, is apparently never explained.

So a sad conclusion that I came to, following our convivial evening in the brewery, is that some members of the public believe that senior academics should be deferred to as "experienced experts who should be believed" -- rather than as field researchers whose work should be subjected to close scrutiny by their peer group and by workers from other disciplines who see things rather clearly because they are not wearing rose-tinted spectacles.  It obviously helps if you have the title "Professor" in front of your name! (Don't get me going on that one.......)

I always have, and always will, work on the basis that anybody who goes onto the record with his/her field results should welcome close scrutiny, and should expect his/her hypotheses to be tested to destruction, to be modified and rejected -- and to be replaced by better hypotheses.  My two colleagues and I have put our Rhosyfelin work on the record, and we welcome and await the scrutiny of others. If falsification follows, so be it.

That, fundamentally, is the difference between those who come from a scientific background and those who enjoy a quiet life lit up every now and then by the magic of fairy tales.


Friday, 11 November 2016

All aboard the magic raft......

Thanks to Dave and Tony for drawing to my attention an extremely silly letter just published in British Archaeology.  It's written by one Tony Lucas.  He correctly points out that the "bluestone quarrying" thesis is now effectively falsified as a result of those extremely inconvenient radiocarbon dates from Rhosyfelin that didn't fit with anything at all.......

However, the writer seems to believe the MPP's assertion that there is a "smoking gun" somewhere or other in the form of a mysterious proto-Stonehenge.  A little more scepticism might have come in handy.  How many more digging seasons do we all have to endure while the smoking gun search goes on?

Then we have the wondrous idea that the timber posts used to make the rafts for floating the bluestones were somehow invested with magic as a consequence of being used for such a noble purpose.  Having become magical and precious, these waterlogged massive logs were then dragged all the way to Durrington Walls and used for "a brief celebratory monument".   What this has to do with the "missing 500 years" I am not sure.....

Nice to see that fairy tales are still coming off the production line.

Wednesday, 9 November 2016

Erratic clusters and super-erratics

I have been revisiting this old article by Joakim Donner, which I have read before,  and have found within it something really rather interesting.  In describing the downstream transport of Jotnian sandstone boulders and cobbles during the Devensian glaciation of SW Finland, he shows us this map:

 The map shows that downstream of the outcrop, there is quite a wide spread in the erratic trail or apron, attributable to frequent swings in ice movement directions.  As you would expect, the greatest concentrations of erratics from the sandstone outcrops lie downglacier to the SE, with some concentrations of erratics above 40% in a zone about 25 km wide. Some of the erratics are more than 200 km away from the source, well off the area covered by the map.

But Donner's attention is drawn to two anomalies or sudden increases in erratic concentrations on the Salpausselka moraines II and III.  In one place there is a peak of over 30% and in another the peak is over 40%.  He explains in the text that he first considered these "clusters" or "anomalous concentrations" to be down to entrainment on, and transport away from, unknown outcrops of the parent rock in the vicinity.  But that line of reasoning was difficult to sustain, and then he noticed that the erratics in these concentrated zones were more angular and less weathered than those in adjacent areas where less than 5% of erratics were made of Jotnian sandstone.

His explanation?  The concentrations have come from the breaking up of super-erratics, maybe hundreds or thousands of tonnes in weight, which have initially been transported intact by the overriding ice and have then been subjected to sufficient stresses adjacent to these big morainic ridges to cause them to be broken up.

Sounds perfectly reasonable to me.  If in Finland, why not elsewhere too?


DONNER, JOAKIM, 1996. On the origin and glacial transport of erratics of Jotnian sandstone in southwestern Finland. Bull. Geol. Soc. Finland 68, Part 2, 72-83.


Late Proterozoic Jotnian sandstone erratics were transported during the last Quaternary glaciation from the source area in Satakunta at the coast of south- western Finland and the bottom of the Bothnian Sea to the southeast as far as Estonia, Latvia and Russia. The frequencies of the sandstone erratics show that they were transported greater distances than indicators of other rocks in the southern parts of Finland. In addition, high frequencies in small areas, south of Salo and in Bromarv, indicate that there are or were small separate source areas of Jotnian sandstone outside the main area. This is supported by the distribution of erratics of Cambrian sandstone and Ordovician sedimentary rocks in the same area. The tracing of possible small occurrences of Jotnian sandstone or Palaeozoic rocks is, however, difficult in an area with numerous faults and fracture zones in the Precambrian bedrock, where the depressions are covered by thick Quaternary drift.

Tuesday, 8 November 2016

Darwin's Boulders, Tierra Del Fuego

I found these excellent photos of Darwin's Boulders showing their size, and how they are scattered. Some of them come from a Durham University blog site.  It;s thought that the main concentrations or clusters belong to quite different glaciation episodes.

Entrainment events

This is rather an important topic -- and interestingly enough, it is not a topic covered properly even in the glacial geomorphology literature.

Let's see if we can encourage a debate on this.

Beneath the ice

It stands to reason that when a glacier is moving across a landscape, eroding material and entraining it into the body of the ice,  the process of entrainment is not continuous but intermittent.  I have talked a lot in previous posts about the processes that operate on the glacier bed, on its flanks and above its flanks on mountainsides and rocky peaks.  When abrasion us under way under a fairly steady basal thermal regime, small fragments of gravel, sand and even cobbles might be entrained on a small scale every day or even every hour.  But where larger chunks of rock are involved, fractures and entrainment events may only occur two or three times a year, when the relationships between ice and underlying bedrock (we are talking glaciology and rock mechanics) are just right.  The process referred to as "quarrying" or "plucking" kick in, and on the downstream side of rock fractures massive slabs of rock can be dragged away from their places of origin.  There may be a debate about how rapidly this "extraction" process takes place; we can probably assume that rocks on the glacier bed will not be moved away from their source positions at anything like the ice velocity as measured at the surface.  So entrained erratic blocks or slabs may only be moved at a rate of 50 - 100m per year, even if the ice is surging or flowing at a rate of 1km / yr.

Here's that quote again, from Prof Dave Evans of Durham University:

Quarrying (Prof David Evans)
Progress in Phys Geog 2004

Quarrying involves two separate processes: (1) the fracturing or crushing of bedrock
beneath the glacier;  and (2) the entrainment of this fractured or crushed rock.  Fracturing
of bedrock may take place where a glacier flowing over bedrock creates pressure
differences in the underlying rock, causing stress fields that may be sufficient to induce
rock fracture (Morland and Boulton, 1975; Morland and Morris, 1977). Fluctuations in
basal water pressure may also help to propagate bedrock fractures beneath a glacier
(Röthlisberger and Iken, 1981; Walder and Hallet, 1985; Iverson,1991a). Brepson(1979)
has successfully simulated the sliding of temperate ice over an obstacle in the
laboratory, and noted that large cavities form in the lee of obstacles, aiding quarrying.
Evacuation of rock fragments along joints in the bed is possible where localized basal
freezing occurs, for example as the result of the heat-pump effect proposed by Robin
(1976). Although Holmes (1944) originally argued that quarrying could occur beneath
both thick and thin ice, and outlined a theory based on pressure-controlled freezing of
meltwater in joints in bedrock, there is now general agreement that quarrying is
favoured beneath thin, fast-flowing ice (Hallet, 1996). Modelling studies indicate that
low effective basal pressures (0.1–1MPa) and high sliding velocities are the dominant
glaciological conditions required for quarrying because these conditions favour
extensive ice/bed separation (subglacial cavity formation)and also concentrate stresses
at points, such as the corners of bedrock ledges, where ice is in contact with the bed
(Iverson, 1991a; Hallet, 1996).

The Sleek Stone super-erratics, for example, appear to have been entrained by ice flowing over Ramsey Island and intermittently incorporating large blocks of bedrock initially made available by fracturing on the glacier bed, possibly in association with shearing within the ice mass.

Part of the biggest Ramsey Island super-erratic at Sleek Stone near Broad Haven -- estimated to have weighed over 100 tonnes before it broke into two.

In one of the classic examples in the literature, the roche moutonnee on the island of Rodloga Storskar in the Stockholm Archipelago,  we can actually see how a series of massive slabs have been sheared off the down-glacier side of a whaleback feature and dragged away by the overriding ice.  First, there comes the  fracturing, and then the extraction -- and these events are clearly intermittent.

See also this post about a huge fracture observed on the same island:

In this case, the glaciation came to an end before the down-glacier mass of rock could be broken up or dragged away.

On the ice surface

The intermittency of input or entrainment events is much easier to understand here, because the processes operating on glacier surfaces are much easier to observe.  In the case of the Foothills Erratic Train in North America, the material dumped onto the glacier surface has come from intermittent landslides or rockfalls high in the mountains, in one rather small area with a recogniseable rock type.  Once dumped onto the glacier surface, the glacier has simply carried the debris away, acting more or less like a conveyor belt: 

We are talking here not about continuous small rockfalls every spring (which might well have been happening too, of course) but about intermittent and widely-spaced catastrophic events, each one of which dumped thousands of tonnes of broken rock onto the glacier surface below.  The landslide that dumped the vast group of erratics later emplaced at Big Rocks was not the first to occur, but it must have been one of the biggest.

These rockwall collapses might not have been as spectacular as the earthquake-induced landslides that have affected the Lamplugh Glacier and the Sherman Glacier, but they must have been pretty impressive nonetheless.

 The effect of the Lamplugh Glacier landslide, 2016

The effect of the Sherman Glacier landslide of 1964

Something similar has happened in Tierra del Fuego, in the case of Darwin's Boulders.  They appear to have been dumped onto a glacier surface during a series of "pulses" or events which have given rise not to a long and continuous erratic train but to elongated clusters of erratics in various locations which cannot be traced all the way back to their places of origin.  So each cluster is discrete, and each one owes its origin to either a single landslide event of a series of events within a limited time frame.

Two of the discrete clusters of Darwin's Boulders

When we refer to mountain earthquakes and major landslides in the mountains, we must bear in mind that these might be directly glacier-induced.  The vast ice load of a large glacier, ice cap or ice sheet on a small part of the earth's crust can in itself induce earth tremors; and rockwalls adjacent to glaciers are especially vulnerable when the ice begins to melt at the end of a glacial episode.  A process called "pressure release" occurs, as the crust adjusts to a reduction in its surface load -- and catastrophic rockfalls or landslides can occur as a consequence.   This process of isostatic readjustment and unloading can continue for thousands of years after complete deglaciation.    But that's another story.........

The lessons to be derived from all of this?  Glaciers do not continuously pick up erratics from the bed during a glacial episode.  Sometimes they entrain bog blocks, and sometimes they do not.  You should never assume that there "should be" a continuous erratic train leading from A to B.  And you should not necessarily be surprised if there was and is a cluster of old and very weathered erratics from Pembrokeshire in the vicinity of Stonehenge, with not very much to be found between the source area and the final destination. 

I keep on trying to explain all of this to the archaeologists, but they are not very good at listening.