Icy breath in Death Valley

This is fun -- one of the strangest natural mysteries appears to have been solved.  We never cease to be amazed by natural processes the consequences of which which have been observed and puzzled over, but not previously understood.......

So here we are in Death Valley, one of the hottest places on Earth, having to come to terms with the work of ice.

These boulders, on surfaces with hardly any gradient, appear to move along for no apparent reason, leaving long trails behind them.  The work of aliens?  The trouble is that people have normally looked at them in the summer, when temperatures can be up over 40 degrees C.  The research team decided to look at them in the winter, when temperatures are often well below zero -- and lo and behold, they saw them move!  What happens is that very thin sheets (just a few mm) of ice form on the saturated playa floor, with temperatures sometimes not low enough for freezing down to the ground beneath.  So the thin ice sheets rest on a film of water, and when the wind blows, the ice sheets move gently, applying enough pressure on stones and boulders to overcome friction and move them too.  There isn't much friction because the playa floor sediments are saturated and slushy -- so the stones move along and leave "plough marks" behind them.  In the spring, the water all evaporates away, and desert conditions reappear, and lots of people come along and take photos like the one above!

Here are two other images from the original research paper, showing how changes of wind direction cause the thin ice sheets to move in different directions:

 

 One rock moved 224m during a single winter.  Notice that not all the stones and boulders were moved during the period of observations.  Individual stone movement depends on a variety of factors -- including the depth to which it is embedded in the sediments, the mobility of the slushy surface layer, the depth of the water film, the thickness and coherence of the ice sheet, and the strength and persistence of the wind. 

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This is a report from the Nature web site, with a link to the article itself in PlosOne journal:

http://www.nature.com/news/wandering-stones-of-death-valley-explained-1.15773

Nature | News

'Wandering stones' of Death Valley explained
Scientists spot ice shoving rocks on Racetrack Playa in California, resolving a longstanding geological enigma.

Alexandra Witze
27 August 2014

Richard Norris & Jim Norris

Ending a half-century of geological speculation, scientists have finally seen the process that causes rocks to move atop Racetrack Playa, a desert lake bed in the mountains above Death Valley, California. Researchers watched a pond freeze atop the playa, then break apart into sheets of ice that — blown by wind — shoved rocks across the lake bed.

Until now, no one has been able to explain why hundreds of rocks scoot unseen across the playa surface, creating trails behind them like children dragging sticks through the mud.

“It’s a delight to be involved in sorting out this kind of public mystery,” says Richard Norris, an oceanographer at the Scripps Institution of Oceanography in La Jolla, California, who led the research with his cousin James Norris, an engineer at Interwoof in Santa Barbara, California. The work was published on 27 August in PLoS ONE1.

Geologists previously speculated that some combination of wind, rain and ice would have a role. But few expected that the answer would involve ice as thin as windowpanes, pushed by light breezes rather than strong gales.

Visitors to Death Valley have to go out of their way to visit Racetrack Playa, which sits 1,130 metres above sea level and is a bumpy three-hour drive from the nearest town. The researchers began studying the region in 2011, setting up a weather station and time-lapse cameras and dropping off rocks loaded with Global Positioning System (GPS) trackers. The rocks were designed to start recording their position and speed as soon as something made them move.

What was not clear was how long the Norrises would have to wait. Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, has been studying the playa since 2007 as an analogy to lake beds on other planets. He had little faith that the GPS-equipped rocks would move in a time frame that anyone would capture. “I thought it was going to be the most boring experiment in the history of science,” he says.

But when the researchers travelled to the playa in December 2013 to check instruments and change batteries, they found a huge ice-encrusted pond covering about one-third of the 4.5-kilometre-long playa. After several days of camping, they decided to sit above the southern end of the playa on the morning of 20 December. “It was a beautiful sunny day, and there began to be rippled melt pools in front of us,” Richard Norris says. “At 11:37 a.m., very abruptly, there was a pop-pop-crackle all over the place in front of us — and I said to my cousin, ‘This is it.’ ”

They watched as the ice began moving past the rocks, mostly breaking apart but also shoving them gently. The rocks began to inch along, but their pace was too slow to spot by eye. “A baby can get going a lot faster than your average rock,” Richard Norris says.

But when the ice melted away that afternoon, they saw freshly formed trails left behind by more than 60 moving rocks. And on 9 January, James Norris returned to the playa with Lorenz and was able to record video of the roving rocks. “This is transformative,” says Lorenz. “It’s not just an anecdotal report, but we have before and after pictures, and meteorological information simultaneous with the event.” By the end of the winter, the farthest-moving rock had travelled 224 metres.

Racetrack Playa rocks move rarely — “maybe a few minutes out of a million,” Lorenz says. And the two events the scientists saw, with thin ice panes shoving the stones across a wet playa, do not necessarily explain every instance of rocks moving there. “But this breaks the back of the problem scientifically,” Lorenz says. “It is ice shove.”

Solving the Racetrack Playa mystery is not exactly a major scientific breakthrough, Lorenz says, but the work does show the rare combination of conditions that allow rocks to move seemingly on their own. And ice shove can have notable effects — in 1952, it uprooted enough telephone poles at a lake in Nevada to break a transcontinental telephone line.

One person who is happy to see the latest results is Dwight Carey. As a university student in the 1970s, he helped with an experiment in which two rocks were placed in a corral on the playa. Over the course of a winter, one stone moved out of the corral, unobserved, and the other did not 2.

The new explanation “makes sense to me”, says Carey, who is now an environmental regulatory consultant in Brea, California. “Eventually you’re going to get enough force on the pile of ice behind the rocks to be able to move them.”

References

Norris, R. D., Norris, J. M., Lorenz, R. D., Ray, J. & Jackson, B. PLoS ONE 9, e105948 (2014).
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0105948

Sharp, R. P. & Carey, D. L. Geol. Soc. Am. Bull. 87, 1704–1717 (1976).

Man dances at Stonehenge

A video is doing the rounds from those who are horrified that somebody did some dancing on top of the sarsens....  Shock!! Horror!!  Sacrilege!!

I wonder what happened to him?  Maybe he just fell off...... but it was pretty clever of him to get up there in the first place. (This was during the Summer Solstice celebrations this year, when almost anything goes.)

Rhosyfelin radiocarbon rumours

It's all perfectly simple.  If organic material from beneath the "proto-orthostat" has a radiocarbon age in excess of 5,000 yrs BP -- making the sediments Mesolithic or Palaeolithic in age -- that would support both those who believe in the "quarry hypothesis" and those who don't.  On the other hand, if the sediments on the flanks of the big stone, or on top of it, prove to be considerably older than 5,000 yrs BP, the quarrying hypothesis falls.

 In 2013 the diggers went down at least a metre beneath the base of the "proto-orthostat", exposing local till (probably Devensian in age) and hitting bedrock -- a large mass of while quartz.  They have clearly taken samples from these basal sediment layers -- and we would all dearly love to know what the radiocarbon ages of any organic materials might have been.....

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

Myris asks for more info on the radiocarbon dates.  It's interesting that he says he has heard nothing about them at all, except from this blog.  Even more interesting that a good friend of his is actually a partner in this project, and if he knew anything, he would certainly have passed it on......

From my report of the 2013 MPP lecture in Moylgrove:  Then Mike moved on to talk about the latest discoveries associated with C14 dating.  He gave no actual dates, did not tell us where they had come from, and gave us no idea how many dates there are from the past and present excavations. Secrecy prevails.  So we were forced to take everything on trust.  Anyway, he claimed to have a sequence of dates ranging from the Neolithic to the Iron Age, some of which were associated with a series of hearths near the position of the Iron Age hearth discovered last year.  I assume that they have found charcoal or other organic materials that have been dated.  It looked as if the hearths were in more or less the same place, in a nice sheltered position beneath the crag, just above the grassy floor of the valley.  Perfect camping places for hunting groups, or for quarrymen, or for jolly family barbeques on a summers evening, depending on your preferences........  That was all quite interesting.  There was no mention of the big pit or stone hole which got so much attention last year.  Wonder why?

As far as I can recall, this sort of statement has been repeated several times, in other contexts -- presumably referring to radiocarbon dates from the 2011 and 2012 digging seasons.  So it's on the record that there are Neolithic and Iron Age dates which presumably support the MPP hypothesis in some way.  So to the rumours, which I have picked up on from two different directions.  Where there is an obsession with secrecy, rumours proliferate......

One rumour is that certain samples collected last year, which were assumed to be of Neolithic date, have actually given radiocarbon dates which place them in the Mesolithic or early part of the Holocene.  The other rumour is that this has caused some internal difficulty regarding the "Neolithic Quarry" hypothesis, and for that reason the dates have been withheld pending further investigations.  I assume somebody has gone back to the lab and said "Surely these dates can't be right?  Might there be some mistake?" and that the lab has said "We stand by the results.  No contamination, as far as we can see."  (It happens all the time.  I have been involved in this process myself, when the radiocarbon date from a rhinoceros scapula from Caldey Island proved to be very "inconvenient" to certain people....... and when C14 shell dates I obtained from fluvio-glacial deposits in Pembrokeshire were similarly disbelieved by people who should have known better.)

As I have said many times before on this blog, if the sediments beneath the famous "proto-orthostat" at Rhosyfelin contain charcoal or other organic materials which can be dated to the Neolithic, c 5,000 BP, that would support the MPP thesis of a Neolithic "quarrying floor" or surface on which the heroic quarrymen went about their business.  As for the sediments in contact with the flanks of the big stone, or overlying it, these would of course have to be later Neolithic, Bronze Age or later for the quarrying hypothesis to stand up to scrutiny.  My own guess is that there may have been quite young deposits on top of the stone, because its upper flattish surface was not buried very deeply beneath the present ground surface.

If, on the other hand, the material on the flanks of the stone and on top of the stone proves to be Mesolithic or substantially older than 5,000 BP, the Rhosyfelin diggers have a problem, because the hypothesis collapses.  They might try and argue that the sediments are disturbed, and that older organic materials have been incorporated into younger sediments, but from what I have seen of the sedimentary layers, there doesn't seem to be any sign of disturbance.  To repeat my old mantra -- it all looks perfectly natural to me, and is therefore nothing to get excited about.

I may be totally up the creek on all of this, and dates may shortly be released which support everything the quarrymen have said in public -- in which case I will eat humble pie.  But until then, I will continue to ask for less speculation and more evidence.  And, of course, for the immediate publication of the dates.

Stonehenge Empire

I flagged up this multinational co-production about a year ago, and it looks as if it is now made, and is probably in the final stages of editing.  There will be two 60-minute parts -- probably on BBC2 this autumn.  Not yet in the schedules, as far as I can see.......

Interestingly, this doesn't seem to involve National Geographic -- but Smithsonian is in there, and I think I am right in saying that in the past the Smithsonian has generally sponsored the work by the Darvill / Wainwright tribe rather than the MPP tribe.  So it will be interesting to see which "international team of experts" has been involved in putting this latest extravaganza together.

I see no sign of a new Nat Geog Channel documentary on Stonehenge -- and the last one, a few years back, which based the whole programme on MPP's theories, was heavily panned on the basis that there was virtually no evidence in there, but a great deal of fantasy.  Maybe Nat Geog Channel has had its fingers burned, and is waiting for something more reliable before starting advance publicity for a new documentary?

If there is any gossip from the big wide world about all of this, let us know.....

How much do the Rhosyfelin partners know?

Here is a screenshot from the University College London web site, relating to the "Stones of Stonehenge" project. It is otherwise called the "Preseli Stones of Stonehenge" project, and currently has funding from: National Geographic Society, Royal Archaeological Institute, and Society of Antiquaries.  It has grown from the Stonehenge Riverside Project, which is now finished, having had a budget of c £500,000.  The budget of the present project is unknown, although somebody must know........ and we do not know how much public money is involved, apart from the fact that many of the "project partners" are paid for their time from the public purse and probably contribute equipment and laboratory / technical assistance via assorted university departments.  You and I, through our taxes, are the ones who pay for all that.  The three major cash funders are educational / charitable institutions which do not advertise the sizes of their grants disbursed to project leaders like Prof MPP.

In the light of the ongoing mystery regarding the research results from three seasons of digging at Craig Rhosyfelin, and the lack of any public statements relating to the radiocarbon dating results of many samples submitted, one wonders how much the project partners know.  In the old days, when I used to be involved in joint projects, we who were partners circulated our results between ourselves, and had an ongoing process of data reporting and information exchange.  Nowadays that process is easy, thanks to things like Dropbox and Cyberduck.

Mike prides himself on the manner in which he consults and bounces ideas around amongst his colleagues.  That's one of the things that comes out clearly from his latest book.  So I have to assume that all of those on the list above are in possession of the radiocarbon dating results from Rhosyfelin, or have at least had a resume or report from MPP............ so why has NOBODY said ANYTHING?

Regenerated glaciers

 

    

Regenerated glaciers are unusual in that they are able to survive in anomalously low altitude situations because they are physically separated from their ice and snow sources.  The glacier in the top photo -- an unnamed glacier in the Franz Josef Fjord complex of East Greenland -- exists right down at sea level.  The lower photo shows the Supphelle Glacier in Fjaerland, Sogn, Norway.  The lower part of this glacier is only 60m above sea-level, which makes it the "lowest glacier in southern Norway."  In both cases the glaciers are sustained by broken glacier ice which tumbles down a cliff face before becoming compressed and reconstituted down below.  These glaciers are difficult to research, given the constant torrent of ice fragments coming down from above.......  hard hats are not of much use in places such as these, since some of the ice blocks are enormous.....

In the Greenland glacier there is a huge mass of dead ice covered with moraine down at the base of the fjordside.  There is much rockfall debris as well, and it may be that this feature is partly a rock glacier and partly a normal glacier made of ice.

The ice in the lower part of Supphellebreen is much cleaner.  But it is on its way out -- it's melting very fast, both because of warmer conditions down on the valley floor and also because the supply from the icefall above is gradually being cut off.

Mike Pitts has got it wrong

Not for the first time, I'm currently a bit fed up with the constant barrage of unreliable information appearing in the media about Stonehenge and the bluestones.  People who should know better are just plain CARELESS -- not good enough.  So I am in letter writing mode.  Following the publication  of Mike Pitts's piece in the BBC Focus magazine in July, I wrote this to the Editor of the journal.  Sadly, they only publish letters up to a maximum of 120 words, which is not exactly sufficient for the development of an argument or the presentation of evidence.  So I have rejigged mine in a much shorter version, and I hope that will now appear.

This is what might have been published if the journal had had more space available!

=========================================

Letters for Publication,
Focus magazine ‘Reply’,
Immediate Media,
9th floor, Tower House,
Fairfax Street,
BRISTOL, BS1 3BN

17th August 2014

Dear Editor,

Bluestones:  the glacial transport theory is still alive and well

It was good to see an article on Stonehenge and the bluestones in the July 2014 issue of the magazine.  The author, Mike Pitts, has got some things right and some things wrong, and since this is a magazine which encourages respect for science and technology, I hope you will allow me to make certain corrections. 

Pitts says that the thesis of glacial transport of the bluestones is  " .......an old idea, dismissed by mainstream science but still championed by a few." Forgive me for saying so, but that is complacent, condescending nonsense.  The glacial transport hypothesis has NOT been dismissed by "mainstream science" -- whatever that is.  A few geologists and geomorphologists  (for example Scourse, Green and Bowen) have expressed their doubts about the glacial hypothesis in print, and others (for example Williams-Thorpe, Kellaway, Jackson, and yours truly) have written in support of it.  The debate goes on.

Pitts says that the idea of glacial transport is now discredited because the new geological work on the provenances of some of the bluestones shows that they have come from the northern slopes of the Preseli Hills in Pembrokeshire, and not from the southern slopes.  He suggests that if ice had entrained and transported the stones, they should have come from the south-facing or down-glacier slopes, given that the Irish Sea Glacier overrode Pembrokeshire from the NW and flowed onwards to the S and SE.  I am not sure where Pitts got this idea from, but it demonstrates an unfortunate misunderstanding of the complexities of glaciological theory. 

Let's try to explain.  When glacier ice flows across a landscape and inundates everything, there is indeed a tendency for plucking or "quarrying" to occur on the down-glacier flanks of obstacles, which explains the difference between the smoothed and striated up-glacier flanks of roches moutonnees (which are often gently-sloping) and the broken or fractured down-glacier flanks which are often transformed into steep cliffs because of the process of block removal.  This is explained in glaciology by reference to bed pressure and the flow law, with basal melting occurring up-glacier where rock surfaces are under compression, and freezing and thawing -- and the "dragging" away of blocks -- occurring down-glacier where rock surfaces are subject to pressure release and tension. It is probably that theory to which Pitts is referring when he speculates about the origins of bluestones and their mode of transport.

Then things get more complex.  In certain situations (for example where there is a major ridge or mountain range aligned transverse to the direction of ice flow) other processes come into play. In the lee of the obstacle there may be a vast expanse of "dead" or stagnant ice which may have very little effect upon the landscape and which might be overridden by more active glacier ice flowing up over the mountain ridge and then away in whichever direction the ice stream dictates.  So in the Preseli Hills case, the most active ice, capable of erosion and the entrainment of blocks of bedrock, may have had no contact with the tors and other landforms of the southern slopes.  On the northern slopes, in contrast, if the ice was cold and subject to large horizontal compression strain rates, shearing or thrusting (analogous to faulting in solid rock) could well have occurred, and there would be a mechanism also for the partial destruction of tors and the entrainment of large slabs of bedrock.  We see evidence for this type of thrusting -- and the incorporation of vast quantities of bedrock debris -- close to the snouts of certain present-day glaciers, especially in association with the formation of push moraines.  We also see signs of it well inland from the edges of the Greenland ice sheet.

Then things begin to get even more interesting.  Recent modelling work on the British and Irish Ice Sheet by glaciologist Alun Hubbard and various colleagues has shown that it exhibited a sort of "pulsing" behaviour, with alternating surges and ice surface collapses especially in the ice sheet's western sector.   James Scourse and colleagues have shown that one such surge carried glacier ice as far south as the Scilly Isles only about 20,000 years ago.  One of the features associated with surges is thrusting, as fast-moving ice encounters older and more sluggish ice that happens to block its path.  It remains to be seen what relevance this observation has for the entrainment of erratics on Preseli. 

Let's put this on the record:  the preferred locations for the deep glacial quarrying and entrainment of bedrock slabs, monoliths and other debris when the Preseli Hills were deeply inundated by ice would have been the NORTH SLOPES, and not the south-facing ones. 

Sincerely,

Dr Brian John

Methane vents in Siberia

This is a photo of one of the vents in Siberia -- approx 30m across, and probably 50 - 70m deep. It's rather spectacular -- and in my view not a pingo. I'll coin the term "methane vent" for it -- I don't think anybody has given these things a name before now.

This is an interesting article from Nature -- another consequence of global warming as permafrost melts in some areas where it has until now been rock solid, with annual melting of just a thin "active layer."  There is pretty catastrophiv lelting as well in parts of Arctic North America.

http://www.nature.com/news/mysterious-siberian-crater-attributed-to-methane-1.15649#/ref-link-1

Nature | News

Mysterious Siberian crater attributed to methane

Build-up and release of gas from thawing permafrost most probable explanation, says Russian team.
Katia Moskvitch
31 July 2014

A mystery crater spotted in the frozen Yamal peninsula in Siberia earlier this month was probably caused by methane released as permafrost thawed, researchers in Russia say.
Air near the bottom of the crater contained unusually high concentrations of methane — up to 9.6% — in tests conducted at the site on 16 July, says Andrei Plekhanov, an archaeologist at the Scientific Centre of Arctic Studies in Salekhard, Russia. Plekhanov, who led an expedition to the crater, says that air normally contains just 0.000179% methane.
Since the hole was spotted in mid-July by a helicopter pilot, conjecture has abounded about how the 30-metre-wide crater was formed — a gas or missile explosion, a meteorite impact and alien involvement have all been suggested.
But Plekhanov and his team believe that it is linked to the abnormally hot Yamal summers of 2012 and 2013, which were warmer than usual by an average of about 5°C. As temperatures rose, the researchers suggest, permafrost thawed and collapsed, releasing methane that had been trapped in the icy ground.
Other researchers argue that long-term global warming might be to blame — and that a slow and steady thaw in the region could have been enough to free a burst of methane and create such a big crater. Over the past 20 years, permafrost at a depth of 20 metres has warmed by about 2°C, driven by rising air temperatures1, notes Hans-Wolfgang Hubberten, a geochemist at the Alfred Wegener Institute in Potsdam, Germany.
Hubberten speculates that a thick layer of ice on top of the soil at the Yamal crater site trapped methane released by thawing permafrost. “Gas pressure increased until it was high enough to push away the overlying layers in a powerful injection, forming the crater,” he says. Hubberten says that he has never before seen a crater similar to the Yamal crater in the Arctic.
Larry Hinzman, a permafrost hydrologist at the University of Alaska in Fairbanks and director of the International Arctic Research Center, says that such craters could become more common in permafrost areas as the region heats up.
In Siberian permafrost, large deposits of methane gas are trapped in ice, forming what is called a gas hydrate. Methane remains stable and frozen at certain temperatures, but as the permafrost warms, and its internal strength decreases, it may be less able to withhold the build-up of sub-surface gases, he says, leading to a release.
But such gas hydrates normally occur at depths of at least 100 metres, says Carolyn Ruppel, a geophysicist in charge of the gas hydrates project at the US Geological Survey in Woods Hole, Massachusetts. The depth of the Siberian crater is not known. When Plekhanov and his team tried to measure its depth with a video camera tied to a 50-metre rope, the camera did not reach the bottom. But the video footage suggests that the depth to a pool of water at the bottom of the crater is around 70 metres, Plekhanov says. The water could add considerably to that dry depth, he adds.
To confirm what caused the crater, Plekhanov says that another visit is needed to check the methane concentration in air trapped in its walls. That will be difficult, however: “Its rims are slowly melting and falling into the crater,” Plekhanov says. “You can hear the ground falling, you can hear the water running, it’s rather spooky.”
Since the crater was reported, local reindeer herders have noted a similar but smaller hole nearby. Although the hole is yet to be confirmed, scientists worry that the release of trapped methane could threaten local industry and communities. “If [a release] happens at the Bovanenkovskoye gas field that is only 30 km away, it could lead to an accident, and the same if it happens in a village,” says Plekhanov.
To avoid such an event, the Russian team has now suggested drilling holes into the permafrost to release the pressure artificially. But Hinzman says that it would be extremely difficult to do so, if not impossible, as one would have to know exactly where the build-up was in the first place.

Naturedoi:10.1038/nature.2014.15649

References
Romanovsky, V. E., Smith, S. L. & Christiansen, H. H. Permafrost Periglac. 21, 106–116 (2010).

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 We keep on discovering new things on Planet Earth.  The methane vents also occur on a much smaller scale in Alaska, where they are seen in winter lake ice .   The bubbling up of methane from rotting vegetation on the lake floor -- or from melting permafrost -- is sufficient to keep the lake ice from melting.   Here the features are generally less than a metre across, and of course they are ephemeral -- when the lake ice melts, they disappear.  The dominant form is the circle, as you might expect.  Here are two pics found on the web:

Review of Mike Parker Pearson's 2012 book

 One of the pages from the book.  A little less certainty and a little more circumspection might have been appropriate......

This review, having been in the Cambridge system for the best part of a year, is now on the web site of The Antiquaries Journal:

Stonehenge. Exploring the Greatest Stone Age Mystery. By Mike Parker Pearson. pp 406, Simon & Schuster, London, 2012.
ISBN 9780857207302. £25 (hbk).

Brian John

The Antiquaries Journal, FirstView Articles
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=9243563&fulltextType=BR&fileId=S0003581514000043


This is a real curate's egg of a book. It isn't just about Stonehenge, but about the Stonehenge Riverside Project which has been investigating the landscape around the monument for almost a decade. So there is a great deal about Durrington Walls, the River Avon, the Cursus and “Bluestonehenge.” I'm not sure where the "target readership" of the book is -- it is too chatty, amiable and rambling in style to be an academic tome, and too detailed and specialised to be aimed at a general readership. The presentation is not very wonderful either -- I can't think of any other recent Stonehenge book that is less attractive to look at or use, printed as it is on a bulky cream paper which renders all illustrations flat and dull. There are abundant maps, photos and diagrams, but they are unnumbered and some are not even referred to in the text, causing one to wonder whether they were added as afterthoughts.

The text is at times clear and informative, at other times quite muddled, hopping about in time and space, and taking what another reader has referred to as “bizarre side roads” such as the Druids, earthworms, the politics of research funding, and the reburial of prehistoric human remains. The book could have done with much tighter editing.

As for the contents, I have to admit to major concerns. Parker Pearson appears to have little respect for academic rigour, and a much greater liking for the process of telling a good tale. The book’s narrative is full of first-name bonhomie, giving the impression that conclusions on important matters are simply arrived at via jolly chats between good mates over a pint or two in the nearest available hostelry. In chapter after chapter, I could just not work out where evidence ended and speculation began -- and over and again I had to conclude that hypotheses were being used as substitutes for facts, and that many matters simply assumed to be correct had never been through a proper peer-review process. When a senior academic publishes a book of this type, one has a right to expect accurate and impartial presentation of field evidence to be followed by the outlining of working theories, with hypothesis testing next and tentative but still testable conclusions to round things off. It appears that Parker Pearson does not do scientific method........

The “sinking stomach” feeling comes in this book even before the Introduction is over and done with, in definitive statements about the quarrying, shaping and transporting of Preseli bluestones. There is no reference to alternative explanations; speculations are stated as facts. So the book goes on, with many interesting pieces of evidence and insights spoiled -- for this reader at least -- by unsupported assumptions just as things are getting interesting. For example, the section on “Bluestonehenge” (Chapter 15) asserts that there were 25 pits that contained bluestones, which were later transferred to Stonehenge. But as I see it, and as other commentators have also pointed out, there is no evidence for 25 pits and no hard evidence that any of them actually held bluestones. In Chapter 16 the author asserts with great conviction that Stonehenge is where it is because of the discovery by its Neolithic builders that there were “periglacial stripes” which were aligned precisely with the midsummer solstice sunrise and the midwinter sunset. These features happen to have been revealed in a dig within the Avenue; but the author does not even try to convince us that they are unique, let alone significant. The need for a good story has simply trumped academic rigour. In Chapter 17 (on the origins of the bluestones) Parker Pearson’s enthusiasm for a good story once again runs away with him, and in attempting to discredit the “glacial transport theory” he misrepresents the arguments of proponents and even uses a map of the wrong glaciation to reinforce his scepticism.

Nowhere in the book does Parker Pearson question the thesis that there were about 82 bluestones imported from West Wales, or that the sarsens were transported from the Marlborough Downs area, or that the stone monument was actually completed. That’s a pity, since other authors are nowadays - quite rightly - testing these assumptions.

When he comes to his section on the “bluestone quarries” at Carn Goedog and Rhosyfelin the author really gets enthusiastic. At the latter site “.... we realized that we had not just a prehistoric quarry but a perfectly preserved one -- the Pompeii of prehistoric stone quarries.” Dramatic statements like that should be made sparingly by academics; and I have seen no evidence of any quarrying activity at Rhosyfelin, where the rockfall material examined thus far seems to me to be entirely natural. I am not aware of any peer review of Parker Pearson’s ideas about “the Rhosyfelin megalith quarry” prior to their publication in this book. And I am not aware that any geomorphologist has ever been involved in any discussions over site interpretation. That is regrettable, to say the least. Then he gets even more enthusiastic, fantasising about whole communities moving stone circles from Wales to Stonehenge as part of some grand political unification project. Again, who cares about evidence when you have such grand and all-encompassing ideas to play with?

There are too many other highly dubious statements to recount in the space of a brief review. Suffice to say that this book is a profound disappointment, since Parker Pearson is a good communicator who spoils everything with his tendency to make sweeping assertions about certainties that do not exist. To quote the author himself (p 308): “We cling onto what we think are certainties and it can be difficult to recognize when a mistake has been made earlier, back down the line, because it has taken on the status of incontrovertible fact.” Quite so.

Brian John

Past Lecturer in Geography, Durham University

Another evening of jolly tales from the Stone Age.....

Here we go again -- the latest blockbuster from MPP will be on 17th Sept 2014 (presumably at the end of the 2014 dig?) at Castell Henllys, which is the site of the National Park's Iron Age Village.  (Not far from Castell Mawr, as it happens.)   I think we know what he'll be saying....

See what I mean about the National Park now being completely absorbed into the process of myth propagation for commercial purposes?  Note that the entry fee this time is £3.50.....  and Rhosyfelin is, as we assumed, being promoted as a key archaeological site of local and even global significance.  Hmmm.....

I somehow doubt that I'll be getting a response from Phil Bennett to that letter I wrote!

Are the pink granite erratics from Lundy?

The Devil's Slide on Lundy Island -- pink granite galore........

So could the pink granite erratics at Flat Holm and Saunton have come from Lundy?  The answer is "yes" --  nothing is impossible unless the laws of physics (known and unknown) are broken.  For the ice of the Irish Sea Glacier to have picked up Lundy Granite and taken it 80 km or more to Flat Holm it must have flowed west towards east with a slightly northwards component as well.  For it to have reached Saunton, that would have been pretty straightforward -- the coast is only about 25 kms to the east of the island.  

But we must also beware of simplistic interpretations here -- the erratics might have followed zig-zag courses, and they might have been picked up and dumped several times by glaciers, sea ice or other processes before ending up where we see them today.

Shap granite erratic near Redcar

Glacial erratic, Shap Granite, found on beach between Marske & Redcar (at NZ 62912 23492) after rough seas (Jan. 2010) had removed sand. Characteristic rectangular feldspar phenocrysts are visible. GPS is 16cm long.

I found this photo on the web of a well-known erratic boulder -- purportedly from Shap -- on the beach near Redcar.  It looks very different from the pink erratics at Flat Holm and Saunton........  and this one has of course travelled across the Pennines from near the west coast to its resting place on the east coast.  That's no great surprise -- it fits with what we know about ice movements in the UK.

The conspiracy of Rhosyfelin fantasists

 Glacial erratics galore at Rhosyfelin -- thrown out from last year's dig onto a waste pile, presumably on the grounds that they were of no interest.......  a certain symbolism there, methinks.


Back in October of last year, I sent this letter to Phil Bennett of the National Park Authority here in Pembs.  In spite of a reminder from me, the letter has never even been acknowledged, let alone replied to.   It's a sad state of affairs when even a body like the NPA becomes so wedded to an unsupported fantasy that it appears to care nothing for scientific accuracy........  and just as EH has a powerful commercial reason for keeping the human transport / quarrying myth alive at the English (destination) end, it appears that here at the Welsh (source) end, commercial considerations also count for more than evidence and truth.

I have sent the letter to Phil one more time..........


Dear Phil

Thank you very much for your talk to PTA last night.  It was a pity it was curtailed by those technical glitsches -- I was similarly afflicted when I spoke to the PTA about Rhosyfelin etc in a quite different venue, over a year ago.  The Rhosyfelin goblins at work again........

I enjoyed most of what you said, and would like to have raised a number of issues with you if you had stayed a bit longer!  So I'll do it now.  My points relate to Rhosyfelin.

1.  You are really rather careless in referring to the site with 100% confidence as a Neolithic Quarry.  If I may say so, that's not good practice for an experienced archaeologist -- because no matter what MPP may say about his discoveries, what he seems to have discovered is a camp site used over a long period of time, with a pile of broken scree in the vicinity.  I have seen NO convincing evidence that this is a Neolithic quarry site, let alone one that has something to do with Stonehenge, and at the very least you should reflect this uncertainty and ongoing debate in what you say to uninformed (and often very gullible) audiences.......

2.  You said that two of the standing stones at Stonehenge had been provenanced back to Rhosyfelin by the geological detective work of Richard Bevins and Rob Ixer.  NOT TRUE.    None of the Stonehenge orthostats has been traced to Rhosyfelin.  What the two geologists have done is trace some of the rhyolite debris in the Stonehenge Layer to localities in and around Craig Rhosyfelin.

3.  You referred to the big "orthostat"  as being self-evidently quarried because of the position in which it is lying.  NOT TRUE.   It is not aligned parallel with the rock face, and it is simply a large stone that has fallen from the face just like all the others above it, beside it and below it.

4.  You mentioned the fact that the orthostat had been broken and had therefore been abandoned by the quarrymen. That's an unsupportable speculation -- ALL of the stones that have come from the rock face and from the crags above it are broken, to a greater or lesser degree!  You could probably put many of them back together again if you were determined enough...........  There's nothing unusual about the big one.

5.  You referred to the big stone as being supported or underpinned by other stones, deliberately placed there by the quarrymen. That again is an unsupported assertion.  As far as I can see, the stones beneath the big "orthostat" are lying in perfectly natural -- almost random -- positions, exactly where they fell.  They appear prominent today because the archaeologists have taken away all the debris surrounding them -- in other words, what we see now is an "archaeological artifice."

6.  You referred to the scratches or striations on that stone near the lower end of the "orthostat" -- and asserted that they were not natural, but were caused by big stones being dragged across them.  NOT TRUE.  Those apparent "striations" are in my view nothing more complicated or significant than outcropping foliations,  just like the ones we see on the surfaces of many other stones in the bank of scree and rock debris.  Those "striations" run in all sorts of different directions, right round the compass, as you would expect in a jumble of fallen rocks.

I could go on, but will resist.  I know you mean well, and that you have picked up on most of the things you have said directly from MPP and the other archaeologists involved in this dig, but it really does nobody any good when mythology is perpetrated in this way.  I appreciate that you are trying to encourage people to take an interest in archaeology and to value Pembrokeshire's rich heritage, and that's entirely laudable -- but when myth is turned into "fact" with the willing assistance of you and many other professionals, it does a profound disservice to archaeology -- which comes over as being unscientific and driven by fantasies and ruling hypotheses.    Archaeology should be accessible and popular, but it should also be truthful -- and it should accord due respect to the views of people from other disciplines including glacial geomorphology.  What is going on at Rhosyfelin is much too incestuous for anybody's comfort.......  I get the impression that everybody who turns up there has come to worship at the shrine, and not to ask hard questions.

http://forum.pembrokeshireu3a.org.uk/index.php?topic=1413.0

So please, when you have your sold-out big archaeology day in November, will you please encourage MPP to be rather more nuanced in his presentation than he has been in the past, and to allow for a degree of uncertainty in this business of the "Neolithic Quarry"?

All good wishes

Brian

That pink Saunton erratic

Luckily those Geocache people have been hunting for the Saunton pink erratic lately, and so there are a few nice photos on the web.  Here are three of them -- click to enlarge. 

According to Natural England, the erratic weighs about 12 tonnes and is deemed to have come from western Scotland.......

The geology of Stanton Drew

I have been looking at the big report from 2010 on the stones of Stanton Drew, in the hope that this will give us some guidance on the role of the Avon Ice Lobe on moving stones about in that general area.   Stanton Drew lies south of Bristol and west of Bath, to the north of the Mendips.  There are three circles, all somewhat ruinous.  This is the reference:

Stanton Drew 2010
Geophysical survey and other archaeological investigations
Report compiled by Jude Harris
Bath and North-East Somerset (BANES)
www.bathnes.gov.uk
Bath and Camerton Archaeological Society
www.bacas.org.uk
Contributors:  John Oswin and John Richards, Richard Sermon, Vince Simmonds

http://www.bathnes.gov.uk/sites/default/files/sd_2010_report_low_res.pdf

The main rock types represented in the stone circles are as follows:

Descriptions:

Oolitic Limestone Jurassic age
At the Stanton Drew site at least four principal rock types have been identified and are briefly described below as an Oolitic Limestone of Jurassic Age circa 205–142 Ma (figure 6.2). These rocks are a pale grey-yellow colour.

Silicified Dolomitic Conglomerate of Triassic Age, circa 248–205 Ma
These rocks have a wide range of colours from pale pink to orange-pink with some bright, sometimes ochreous orange, through to dark rust, and purple-red blotches, the red and orange colour is indicative of the mineral iron content of these Triassic rock types (figure 6.3). The rocks have a glassy, metallic appearance and feel and the surface has been described as pitted, pock-marked, frothy, knobbly and gnarly.

Dolomitic Conglomerate also of Triassic Age
This is a weathered pale grey-pink and has a lesser degree of silicification (figure 6.4). The varying clasts range from rounded to sub-angular fine to coarse gravel, pebbles and cobbles of limestone and sandstone. There are also some silicified fossil fragments from the remains of limestone clasts within the conglomerate and the stones again have a substantial cover of lichen.

Pennant Sandstone of Carboniferous Age  circa 354–290 Ma
These rocks are of a pink to fawn colour and distinct bedding layers are clearly visible, in particular cross-stratification which is typical of material that has been laid down in deltas (figure 6.5). There is a layer of sub-rounded to rounded fine to medium gravel of quartz.

Summary:

Within the Great Circle the vast majority of visible stones comprise a silicified Dolomitic Conglomerate, with the remaining other rock types comprising Oolitic Limestone, Pennant and a Dolomitic Conglomerate that has a lesser degree of silicification. The stones that form the Northeast Circle and the Avenues comprise mostly silicified Dolomitic Conglomerate and a small number of Oolitic Limestone. The orange to rust-red colours of the silicified Dolomitic Conglomerate do not look out of place at Stanton Drew, matching well with the local red sandy soils and the Triassic Mercia Mudstone that underlies the monument site. The majority of the stones in the SSW Circle comprise silicified Dolomitic Conglomerate, although at least one stone is of the local sandstone, possibly from the sandstone bands that are found within the Mercia Mudstones of this area and are visible in the local environment. The stones used in the construction of ‘The Cove’ comprise a Dolomitic Conglomerate that has been silicified but to a lesser degree. Within some of the limestone clasts are the silicified fossil remains of Siphonodendron, a rugose coral of Lower to Upper Carboniferous age (Black 1970). In the light some of the silicified clasts within the Dolomitic Conglomerate can be seen to sparkle due to the quartz crystallisation. The stones of the cove lack the vivid oranges and rusty-reds of the more silicified rocks found within the Circles and Avenues and they are greyer in colour.

So far so good.  But then note this:

"As the perception and cognition of landscape is altered by the construction of a monument, then the actual physical landscape is also altered. The monument becomes part of the landscape while the landscape then becomes materialized in the monument. The materials that are used to construct the monument might have been selected and gathered from specific sources within the landscape and are incorporated into a new form as part of the monument. The social and ritual performance of monument construction can alter entire landscapes (Goldhahn 2008: 59). When considering the monuments at Stanton Drew, their place within the landscape, of which they have become a part, is a significant factor. This also applies to the individual stones that remain a part of that landscape. When describing the geology and landscape of the stones and surrounding areas it is with these considerations in mind."

This is profoundly disappointing.  There is a lot of good work in this Report, and a huge amount of technical detail, but the authors seem to be so preoccupied with the archaeological fantasies of the moment that nobody -- even the geologist involved -- appears to have asked the fundamental questions:  Where did the Stanton Drew rocks come from?  Did they all come from the west?  Could they all be glacial erratics?  Were they in the area well before the stone circles were built, and could their presence here have been the key determinant in the siting decisions of the builders? 

Until those questions are answered, what on earth is the point of wild speculations concerning social priorities and rituals?

Those mysterious pink erratics

Where did these mysterious pink granite erratics come from?  The one in the top photo is on the island of Flat Holm in the Bristol Channel, and the one in the bottom pic is from Saunton in Devon.  Both are quite close to sea level, and both are resting on wave-cut platforms.  The mobile phone in the lower photo gives the scale.

I assume nobody has ever done any petrography on these.... but there are a number of possible sources of pink granite erratics in the UK -- some in Wales, some in Ireland.  But the main possible source is in northern England, where the Shap granite outcrops.  Could that really be the source?  Here is a close-up:

Over to the geologists.......

The Avon Ice Lobe

This useful map from the GCR volume on SW England shows most of the key Quaternary sites in this area.  It also shows the terrain -- and that's rather useful too. I am increasingly convinced that when the last glacier ice to affect this area came in from the west, it split into several lobes, as suggested a long time ago by Kellaway (1971).  This would be entirely natural, from what we know about the behaviour of ice sheet edges:

So although the hill masses like Exmoor, Mendip and Dartmoor might well have supported their own cold-based ice caps at the time of the Greatest British Glaciation (GBG) the hill masses would in ant case have acted in a diversionary role, forcing incoming ice streams along pre-existing lowland corridors.

We can speculate, therefore, that two main ice streams affected Somerset:  in the north the Avon Ice Stream and in the south the Parrett Ice Stream.  I have speculated before that the latter might well have reaches at least as far east as Glastonbury, and maybe all the way to Salisbury plain, while the northern branch reached at least as far east as Bathampton Down and Kinsdown, to the east of the city of Bath.

This is a map of ancient glacial (?) deposits to the east of the city, from a 1995 publication by Donovan.

Here are some notes:

Donovan, DT. 1995 High level drift deposits east of Bath.  Proc Univ Bristol Spelaeol. Soc. 1995, 20 (2), pp 109-126

Varney (1921) recorded on Bathamption Down deposits including angular and sub-angular flints, flint pebbles (often broken by frost), quartz pebbles (white, pink, brown), fine and coarse sandstone, in flat pebbles, occasional pieces of conglomerate containing quartz pebbles, fragments of Palaeozoic shales, as seen in Coal Measures, rare pieces of coal, rolled and subangular pieces of Oolite (local).

Other geologists have found ORS and Pennant material as found in S Wales.  Upper Palaeolithic sandstones  -- from S Wales?  Palaeozoic shales as in Coal Measures?

Donovan speculates that these deposits are from very ancient tills from which most limestone fragments have been weathered away.  Some sandstones may also have been destroyed, leaving only the most resistant (highly silicious) rocks in any quantity. Flint and chert pebbles and larger clasts have come from old Cretaceous deposits and residues in the Bristol-Bath area?  Coal fragments could survive in glacial deposits, possibly all the way from the South wales Coalfield.  The clay matrix is also reminiscent of till -- these are not river deposits.  The residual deposits are a mixture of Tertiary residual clays and glacial deposits?  If there was a glaciation here, it was probably Early or Middle Pleistocene.
http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=KNN
These deposits are classified to the Kenn Formation.  At Kenn, the glacial materials are overlain by Cromerian (MIS 15) deposits and by Holocene silts and clays.  Geographical limits:   Restricted to Avon and north Somerset; occurrences at Kenn, Court Hill, Nightingale Valley, Failland Ridge, Bathampton Down and Bleadon Hill.

Another "glacial" site is in the railway cutting beneath Hampton Rocks.

This starts to get interesting.  If, as appears to be the case, there are no known Pembrokeshire erratics in the area affected by the Avon Ice Lobe, this means that the ice might have come partly from the Welsh Ice cap, carrying Coalfield erratics with it.  If you look at the map at the top of this post, that would make sense.  There must have been some Irish Sea Ice involved, because there is a large erratic of "white limestone" from Northern Ireland at Kenn, which has to be accounted for.

But it looks increasingly apparent that Kellaway was right, and that the main thrust of the Irish Sea Glacier into Somerset was to the south of Mendip and across the deep embayment now occupied by the Somerset Levels.   Watch this space.....

Bluenose Lake thrust moraines

This is a very interesting paper which illustrates graphically the effects of thrusting in situations of compressive flow.  the authors describe an area where Wisconsin glacial ice is still present under a thick cap of till neat the Arctic coast of NWT.  They describe a situation in which the ice has butted up against a transverse hill mass, leading to compressive flow and shearing.  In the diagram above, the lower layers of ice are relatively inactive, and they are being overridden by more active ice layers moving along shear planes.  Large amounts of debris are transported along these planes by thrusting, and strips of moraine are visible at the ice surface where these planes emerge from the glacier.  When the ice has melted away, these ridges are up to 100m high -- very dramatic landscape features.

It follows from this work that the erosion of the bedrock is going on on the upslope section of the hill mass against which the ice is pressing.  If we transpose this situation to the Preseli Hills area, the erosion zone would be equivalent to the north of the uplands, not the south. 

Source:
Géographie physique et Quaternaire, 1999, vol. 53, n° 2, p. 287-295, 9 figs.
LA MORAINE DU LAC BLUENOSE (TERRITOIRES DU NORD-OUEST), UNE MORAINE À NOYAU DE GLACE DE GLACIER
by Denis A. ST-ONGE et Isabelle McMARTIN

Carn Meini from Carn Bica

A really fine photo from Google Images, showing the crags of Carn Meini (Carn Menyn) in the middle distance.  The photo is taken from Carn Bica, adjacent the to the ridge track called "The Golden Road".  The green path just to the right of the centre of the photo runs down to the oval stone setting of Bedd Arthur. 

More on thrusting, shearing and glacier tectonics

Thrust planes outcropping at glacier surfaces.  The top photo is from a glacier in Alaska, and the bottom photo is from the Swiss Alps.  In both cases the angle of the thrust is quite steep, and large amounts of debris have been carried to the glacier surface. In the top photo, right in the centre of the picture, we can see that a very large sub-rounded boulder has been brought to the surface. Click to enlarge.

In the "FOCUS" article from which I quoted the other day, Mike Pitts says that the idea of glacial transport is now discredited because the new geological work on the provenances of some of the bluestones shows that they have come from the northern slopes of the Preseli Hills in Pembrokeshire, and not from the southern slopes.  He suggests that if ice had entrained and transported the stones, they should have come from the south-facing or down-glacier slopes, given that the Irish Sea Glacier
overrode Pembrokeshire from the NW and flowed onwards to the S and SE.  I am not sure where Mike got this idea from, but it demonstrates an unfortunate misunderstanding of the complexities of glaciological theory.

Let's try to explain.  When glacier ice flows across a landscape and inundates everything, and where there is a basal-sliding regime with ice more or less at its pressure melting point, there is indeed a tendency for plucking or "quarrying" to occur on the down-glacier flanks of obstacles.  What we may well find, in areas of hilly terrain, are (a) smoothed and striated up-glacier flanks (which are often gently-sloping) on whaleback forms or roches moutonnees; and (b) broken or fractured down-glacier flanks which can be transformed into steep cliffs because of the process of block removal.  This is explained in glaciology by reference to bed pressure and the flow law, with basal melting occurring up-glacier where rock surfaces are under compression, and freezing and thawing -- and the "dragging" away of blocks -- occurring down-glacier where rock surfaces are subject to pressure release and tension.  It is probably that theory to which Mike is referring when he speculates about the origins of bluestones and their mode of transport.

A classic roche moutonnee -- Lambert Dome, Sierra Nevada, California.  The ice has flowed from right to left.  On the right, a gently sloping smoothed and striated face, and on the left the cliffed face where plucking or quarrying has occurred.

Then things get more complex.  In certain situations (for example where there is a major ridge or mountain range aligned transverse to the direction of ice flow) other processes come into play.  This is well covered in glacial geomorphology text books such as that by Benn and Evans (2010).  Thrusting can occur in relatively thin ice or in thick ice where there is a frozen-bed regime, with faults or shear planes rising from the glacier bed sometimes inclined at angles as steep as 45 degrees. Sometimes it seems that these thrusts occur at the junction between sliding ice and "static" ice which is frozen to its bed. (It's never actually static, because ice is always deforming, but it can certainly be very sluggish.)  A requirement for thrusting is that there should be large horizontal compressive strain rates in the ice (Moore, Iverson and Cohen, in Geophysical Research, 2010).  Thrust planes are of course capable of carrying eroded blocks and debris upwards towards the glacier surface -- and this process is seen in glaciers today in Svalbard, Antarctica, Sweden and Arctic Canada.  There is an abundant literature. To make things even more complicated, folding can also occur in the ice, through the process of internal deformation.  Benn and Evans show that some debris is carried in overthrust ice slabs, and other debris moves along on the thrust plane itself, sometimes assisted by meltwater lubrication.  In the former situation the blocks and other debris may be "encased" in ice and effectively protected from further abrasion, while in the latter case clasts and larger blocks may be broken, striated or abraded.  Neil Glasser, Mike Hambrey and others have shown this thrusting and debris transport going on in non-surging polythermal Svalbard glaciers.

 Triple thrust planes outcropping at the surface of the Chistochina Glacier in Alaska.  These features appeared after an earthquake in 2002, illustrating how brittle ice can behave in response to stress.

Thrusts on the surface of Variegated Glacier, Alaska.

Let's apply this to the Preseli situation.  In the lee of a substantial obstacle like a mountain range there may be a vast expanse of "dead" or stagnant ice which may have very little effect upon the landscape. It could well be frozen to its bed, because the ice overriding mountain crests tends to be thin.  Later on, as a glacier thickens during the waxing phase of a glacial episode, the stagnant ice "plug" can be overridden by more active glacier ice flowing up over the mountain ridge and then away in whichever direction the ice stream dictates.  So in the Preseli Hills case, the most active ice, capable of erosion and the entrainment of blocks of bedrock, may have had no contact with the tors and other
landforms of the southern slopes.  On the northern slopes, in contrast, if some of the ice was cold and subject to large horizontal compression strain rates, shearing or thrusting (analogous to faulting in solid rock) could well have occurred, as described above.  There would also be a mechanism also for the partial destruction of tors and the entrainment of large slabs of bedrock.  We see evidence for this type of thrusting -- and the incorporation of vast quantities of bedrock debris -- close to the snouts of certain present-day glaciers, especially in association with the formation of push moraines.  On this blog we have already looked at the spectacular landscape at the snout of the Thompson Glacier on Axel Heiberg Island.  We also see signs of thrusting and debris incorporation well inland from the edges of the Greenland ice sheet.

 This extraordinary photo shows part of the Thompson Glacier push moraine on Axel Heiberg Island.  The gently-sloping "shelves" are capping patches of detached dead ice, and they are remnants of the thrust planes within the glacier snout along which debris has been transported.

A "blocking ridge" near the edge of the Greenland ice sheet, SW Greenland, inland of Nuuk.  The ice from the ice sheet is flowing broadly from NE to SW here, and the ridge is more or less transverse to this direction of flow -- and is thus acting as a barrier.  The pattern of crevasses is highly complex, but it is almost certain that the moraine on the northern side of the ridge is supplied by thrusting within the ice and the upward carriage of debris.  If you look at the western end of the ridge, and then at the ice surface to the north, you can see a series of long linear crevasses transverse to the direction of ice flow -- almost inevitably these will be the surface expression of thrust planes within the ice.

In some cases, for example at the North Norfolk coast, it can be shown that overriding ice frozen to its bed is capable of dragging away and incorporating into the basal ice layers huge slabs of bedrock which may be more than a kilometre in length.  Such features are very spectacular when exposed in present-day cliffs; they are referred to as "chalk rafts."  The incorporation of large slabs of bedrock could have happened in North Pembrokeshire as well, so long as the "freezing bond" between glacier bed and bedrock surface was very strong, and so long as there were pre-existing weaknesses (rock fractures or maybe a contact between permafrost and unfrozen ground) in the ground being over-ridden by ice.  This is something worthy of greater investigation in Pembrokeshire.  For example the geologists might address this question: was the dense fracture pattern at Craig Rhosyfelin adequate for the bulk extraction of large masses of bedrock by this mechanism?  The idea is an attractive one.......

A famous "chalk raft" incorporated into glacial sediments at Overstrand, Norfolk.

Then things begin to get even more interesting.  Recent modelling work on the British and Irish Ice Sheet by glaciologist Alun Hubbard and various colleagues has shown that it exhibited a sort of "pulsing" behaviour, with alternating surges and ice surface collapses especially in the ice sheet's western sector.   James Scourse and colleagues have shown that one such surge carried glacier ice as far south as the Scilly Isles only about 20,000 years ago.  One of the features associated with surges is
thrusting, as fast-moving ice encounters older and more sluggish ice that happens to block its path.  It remains to be seen what relevance this observation has for the entrainment of erratics on Preseli and in the area between Preseli and the north coast.

So let's put this on the record:  the preferred locations for the deep glacial quarrying and entrainment of bedrock slabs, monoliths and other debris when the Preseli Hills were deeply inundated by ice would have been the NORTHERN SLOPES, and not the south-facing ones.