Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click

Saturday 31 August 2013

Catastrophe Country

This is landslide territory -- the western fjords (Vestfirdir) of Iceland -- the north-western peninsula of the country, where I spent a good deal of time in the 1970s.  The map shows the locations of more than a hundred major landslides.

A cluster of landslides beneath the plateau edge near Sudureyri.  M = moraines.  L = landslides.

Landform map of a larger area including that covered in the satellite image above. 

Why are there so many landslides beneath the steep cliffs of this area?  Well, the topography is very suitable, with very steep basalt cliffs around the edges of the plateau country that makes up the interior of the peninsula.  Steep basalt cliffs seem to be very amenable to collapse.  This area is geologically much older than that part of Iceland where most of the volcanic activity now occurs, but it is quite possible that earthquakes have triggered many of the landslides since the end of the last glacial episode -- so most of them have occurred since about 12,000 years ago.

Another mechanism for slope collapse and landsliding on a large scale is pressure release following deglaciation.  When a glacial trough has been filled with ice, and when that ice melts away, the ice which has been pressing against the sides and base of the trough is of course no longer there to "support" rock which may be fractured or unstable -- and collapses become more or less inevitable, especially if there are minor tremors associated with isostatic adjustment following the disappearance of the ice load.

Now for some spotted rhyolite......

Up on Carningli today I found several boulders of the rock which I call rhyolite -- it is typically very fine-grained and even glassy in appearance, and has this typical grey-blue colour.  The weathered surface of rhyolite boulders is often cream or white.  But this one is distinctly spotty, rather like the famous spotted dolerites.

There are some spotted dolerites on Carningli too.  There are also quite wide expanses where rhyolites outcrop at the surface-- but I'm note sure where this one has come from.  The whole mountain is littered with erratics, many of which have come from within a mile or so of the place where they are now found.

I'm not sure if there are any spotted rhyolites at Stonehenge -- maybe Myris knows.....

Thursday 29 August 2013

A mega-canyon beneath the Ice Sheet in North Greenland

This is really interesting!  high-definition ice--penetrating radar has revealed this splendid canyon beneath the ice sheet in North Greenland.  With the ice sheet stripped away, the top image shows the canyon winding its way northwards towards the coast.  According to Prof Bamber, it is up to 800m deep and about 10 km wide. (Actually, that is less impressive than you might think -- the trough of Nordvestfjord is over 3,000m deep in places, from plateau surface to fjord bottom.)  But the impression is that this is a very old fluvial system.

The lower image shows the exit of the canyon, which is now currently occupied by the Petermann Glacier.  There is now much speculation that the canyon is very old indeed, and that it acts as a conduit for meltwater beneath the ice sheet, in turn affecting the behaviour of the ice.

In case you wonder what this has to do with Stonehenge, let me enlighten you.  If Salisbury Plain ever was overridden by glacier ice, it would have been by a very large ice mass (part of the British-Irish Ice Sheet) moving cross country from the west.  We are still learning about what happens beneath these large glaciers.  Sometimes they slide and scrape things on their beds, amd sometimes they don't.  Slippage and freezing-on are the two scenarios normally talked about, associated with ice at its pressure melting point (sometimes called "temperate" ice) and ice which id below the PMP (referred to as "polar" ice).  But in recent years it is becoming clear that the situation is much more complex than that, and that there can be sub-glacial lakes and also conduits or canyons which can affect ice behaviour at great depth.    This is where it becomes important for the transport and deposition of the bluestones -- every bit of research like this one tells us a bit more, and gives us more to think about......

Vol. 341 no. 6149 pp. 997-999
DOI: 10.1126/science.1239794

Paleofluvial Mega-Canyon Beneath the Central Greenland Ice Sheet

Subglacial topography plays an important role in modulating the distribution and flow of basal water. Where topography predates ice sheet inception, it can also reveal insights into former tectonic and geomorphological processes. Although such associations are known in Antarctica, little consideration has been given to them in Greenland, partly because much of the ice sheet bed is thought to be relatively flat and smooth. Here, we present evidence from ice-penetrating radar data for a 750-km-long subglacial canyon in northern Greenland that is likely to have influenced basal water flow from the ice sheet interior to the margin. We suggest that the mega-canyon predates ice sheet inception and will have influenced basal hydrology in Greenland over past glacial cycles.

Those were the days.....

Thanks to Mike Pitts for flagging this up -- and to Julian Richards and EH, from whose collection this comes.  Great pic!  Midsummer in 1976.   Now that the road and the old Visitor Centre are going, will we see the like of this again?  I doubt it.......

Monday 26 August 2013

"The scientific consensus" -- the last resort of the charlatan?

 HH Thomas -- the man who single-handedly invented the scientific consensus about the Stonehenge glacial erratic theory being "impossible"

In the context of an interesting exchange of messages with a good friend, I was struck by one of his comments about the glacial erratic theory and Stonehenge.  I'll quote his words:  "........ to be honest, I thought that this had been knocked on the head decades ago."

When I had calmed down, I got to thinking about where these myths come from -- and of course I recalled that the scientific establishment has been using this sort of argument for years, as a means of eliminating dissent and silencing mavericks.  What it does is to simply say "Well, it is scientifically accepted that ....."  or "........ the scientific consensus shows that......." without ever giving us chapter and verse.  And having said it once, you repeat it over and again until it becomes a sort of mantra.  Others pick it up and do the same.  If you repeat a falsehood often enough it becomes accepted as the truth.  The Royal Society does it.  They all do it -- including the archaeology establishment.  So my friend clearly genuinely believes, on the basis of chats with his colleagues, that the glacial erratic theory has been "disproved" years ago.  By whom?  Nobody quite seems to know........ so it is assumed that it has been disproved by "the scientific consensus"  -- a faceless and anonymous phantom.

When you look at it in detail, what is this "scientific consensus" on the glacial erratic theory being knocked on the head?  Well, it is not a consensus at all.  Who are the key players?  They are HH Thomas, Atkinson, Scourse and Green.  One geologist, one archaeologist, and two geomorphologists who are often mistakenly referred to as glaciologists.  I have repeatedly shown that their evidence is as flawed as their arguments.  On the other hand, those who have argued for the glacial theory include Jehu, Kellaway, Williams-Thorpe, Burl, and yours truly.  And if you look at the manner in which the evidence on the ground is interpreted, there is even less of a consensus.......

So, if anybody out there is listening, and if you hear somebody trotting out the nonsense that there is a "scientific consensus that the glacial erratic theory is dead",  just challenge him or her by asking what the basis of that so-called consensus might be, and by asking for the evidence upon which it is based.

More on the Glaciation of Dartmoor

 A reconstruction of the Dartmoor Ice Cap after 1400 years of buildup -- around the time of the Devensian maximum ice coverage in the UK -- around 20,000 years ago.  From the paper by Evans et al, 2012.

Earlier in the year I posted in item relating to the glaciation of Dartmoor -- commenting on a direct assault by a senior geomorphologist called Allan Straw on the paper by Evans et al (2012).  This was the paper which I also covered in 2012,  and which provided what was in my view very convincing evidence of a whole suite of landforms on the moor which could only be interpreted as the results of glacial ice on the landscape. (Do a search on the blog if you like -- put "Dartmoor Glaciation" into the search box.

Well, Evans et al have now hit back hard in a paper in Quaternary Newsletter 130 (June 2013), arguing out the issue point by point and effectively reinforcing everything in the original paper.  Allan Straw has also been given the right of reply, but he does not make a very good fist of it, and as far as I am concerned, this is game, set and match to Evans, Harrison, Vieli and Anderson.  The evidence and the theory are just too strong to wither away in the face of an attacker who provides no new evidence and who simply says "well, this and that MIGHT possibly be explained by other mechanisms......"

Last time round, somebody asked in a comment what this all as to do with Stonehenge?  I repeat the simple point that if glacial processes were possible in the Devensian on quite a large scale on Dartmoor and probably Exmoor as well, as I have often suggested, then it is well within the bounds of PROBABILITY that Stonehenge was glaciated as well at some stage during the Pleistocene. 

Anyway, it's good to see that there is robust debate in geomorphology, out in the open, where it should be............ 

Thursday 22 August 2013

Where did the Stonehenge Sarsens come from?

 Reconstruction of the site of Stonehenge in ‘Phase 1′, before Stonehenge, showing a rather too liberal scattering of sarsens in the area (Picture courtesy of David Field at English Heritage).

Thanks to Tony for drawing this to my attention -- it's an interesting 2012 conversation between Edward Pegler and David Field, published on Ed's blog site here:
Reproduced below again, for anybody who might not be familiar with the site -- full acknowledgement as to the source.

Edward and I had an interesting debate on this as well, back in 2010 -- in the context of a chat about bell curves.

Over the last year or two there seems to have been a strong drift towards the idea that the Stonehenge sarsens were collected up in the vicinity of Stonehenge, and didn't need to be transported very far.  We have touched on this before, many times, in this blog.  I also support the idea strongly in my book, since I have seen no convincing evidence that the big sarsens were collected up and transported all the way from the Marlborough Downs, or from anywhere else more than a mile or two away.  Geomorphologists like Mike Summerfield and Andrew Goudie have in general concurred with this.


The sarsens of Salisbury Plain – A conversation with David Field

by Edward Pegler on 22 May, 2012

Apart from the obvious ones at Stonehenge, are there many sarsens on Salisbury Plain? David Field thinks there are.

I went to see a talk before Christmas at the Devizes Museum, Wiltshire, England. It was given by David Field of English Heritage, and was all about the development of the landscape around Stonehenge through the millennia. After the talk I got into a brief e-mail conversation with Dr Field about the Sarsens of Salisbury Plain. This started really due to a question about just how many sarsens there were at Stonehenge before Stonehenge was built. For anyone who’s interested, here’s the full transcript.

David: I’m pleased that you enjoyed the talk. Yes, coupled with the recent work by Mike Parker Pearson and the Stonehenge Riverside Project there’s enough material accumulating to provide a new platform for Stonehenge studies for a while to come. I’ve attached the sketches of Stonehenge. There’s a lot wrong with them as you’ll no doubt soon notice, not least the missing bluestones, but we’ll work on this in the coming months and come up with something more precise.

Me: Just having a look at the sketches, they are lovely. Ok so there are a few details that are odd. But what strikes me as most odd is all the broken up sarsen lying around the area in Phase 1. I know it’s an idea that appeals to some but this amount of sarsen is very unlikely for Holocene Salisbury Plain. The evidence for it would be present as smaller sarsens not just in the Stonehenge area but widely distirbuted in the surrounding valleys due to solifluction. But perhaps more important to me seems to be the absence of large sarsens encorporated into the long barrows of Salisbury Plain. Comparison with the Marlborough Downs and the Cotswolds would suggest that such burial mounds would have used them.

David: Yes, the artist put too many in. I just wanted a thin scatter with a cluster by the mound. The next version will look a little different. There are, however, quite a few smaller sarsens around on Salisbury Plain and some were indeed incorporated into long barrows. Knook Barrow had a cairn of sarsen, Arn Hill long barrow had a standing stone, Corton long barrow had a ‘massive boulder’. Cunnington said that sarsens can be found all over the downs beneath the turf and that farmers plough them up in the area north of Stonehenge (Larkhill west of barracks) from time to time. There is a long barrow there (Figheledean 31-see attached) with three in the ditch and another six in a line where they were disturbed when the military built a rifle range. Quite a few around Bulford, aside from the Cuckoo stone (attached), Togstone and the one in the river, there is one from a round barrow that had a burial beneath an ‘immense sarsen’ and a number of others noted on early maps. One of the King Barrows formerly had a sarsen circle or kerb around it. Today the Imber to Chittern valley has many small boulders and cobbles on the slopes and in the stream and presumably many more were once visible when the area was cultivated.
As you rightly say, none of these are large in trilithon terms, but then neither are any of those on the Marlborough Downs where they rarely exceed a couple of metres – three at the most. The big ones there seem to have been reserved for the Cove and blocking stones at West Kennet. The survival of many on the Marlborough Downs can be put down to lack of agriculture (it’s a degree colder there than Salisbury Plain) for they get in the way of ploughs and soon get cleared and broken up or buried. You can trace the clearance process at Overton/Fyfield from undisturbed sarsens on the summits, to the clearance to field edges to create ‘Celtic’ Fields in the Bronze Age on the upper slopes, to the development of lyncheted fields that cover the sarsens around the edge in the Roman and medieval periods on the lower slopes. If the same processes took place on Salisbury Plain where there was widespread agriculture in Roman, medieval and post-medieval times there will be many other sarsens buried beneath the field lynchets.
So where did the big ones come from?”

Me: Not expecting a reply to this, but I guess the big sarsens must have been more common in the past in certain areas. This must largely depend on ?Miocene distribution of groundwater flow during southern England’s sub-tropical flat phase before the Plio-Pleistocene uplift. Extensive silcrete development, I guess, would be along natural fluid-flow pathways in the ground, so it would probably occur in lines.
Once uplift had taken place that variable thickness silcrete layer would have been broken up by flexure of the landscape, creating joints along directions related to the stress field. If the silcrete were thicker in certain places then the jointing would be spaced out further. Hence thicker stones would have larger sizes.
All this means is that you’d get small, possibly linear, zones where big chunks of silcrete, appropriate either for Avebury or Stonehenge, would end up at or near the surface. On highs I should imagine that they’d be exposed. In the valleys I should imagine they could end up under alluvium, although the larger ones might tend to move toward the surface (as happens in debris flows). Overall, many, through natural processes, would move down slope.
From my own observations of the Avebury area the concentration of larger remaining sarsens seems to be in the Lockeridge, West Overton area now, although obviously there are quite a few on Fyfield and in Clatford Bottom. Trying to work out their former distribution here seems quite difficult as so many have been broken up, but there must have been a fair few large ones here as many of the walls are made out of squared blocks, presumably from larger blocks broken up in the eighteenth century. I should imagine that the alluvium of the upper Kennet valley formerly contained possibly the most spectacular ones in this area and perhaps quite a lot of them. Perhaps some are still there at the valley edges.
As for the Stonehenge area what interests me is the statistical pattern of distribution. I think it would be interesting to see a cumulative frequency plot of sarsen size ranges from the two areas (not including the stones of each monument).

David: Interesting. I think that regarding statistical pattern of distribution based on size frequency the Marlborough Downs would have it as there are so few around Stonehenge. But the difference between a three metre and five metre sarsen is quite dramatic and I imagine that just one in either place could shift the balance. Its perhaps worth noting, though, that the heelstone is not exactly small and because of its shape few people will argue that it was brought from far.
Heavy mineral and grain size analysis carried out in the 80s on some of the Stonehenge sarsen indicated that it did not match samples from Clatford Bottom and Piggledene on the Marlborough Downs. Not that this really demonstrates much as sandstones can differ in composition quite dramatically over relatively short distances, except to perhaps note that, for the moment, it provides no support for the idea of a Marlborough Downs origin.
Have you seen the valley in West Woods? It was the location of a sarsen industry. As you walk up the valley you can see the hollows from where the sarsens were dug along with causeways where the trucks could load and, higher up the valley, sarsen boulders still in situ.

Me: Interesting too. I didn’t know about the heavy mineral analysis. Funnily enough me and Steph were walking Fyfield yesterday and I tried to apply the concepts I had made up to the blocks I saw. I didn’t get much joy. It’s exceptionally difficult in many places due to the clearing of blocks from fields but it’s intriguing how adjacent blocks seem often to have very little obvious relation with each other. I think they must have been washing around near the surface for millions of years and who knows what their history was during this time, let alone how much they were moved or broken up in the last few hundred years.
I remember seeing a concentration of sarsens near the Wansdyke to the west of West Wood but I didn’t know that there were some in the wood themselves. Whereabouts is that?

David: Try Hursley Bottom cSU153666

Me: As for Stonehenge, I think it would be good to compare the heavy mineral contents with those of the Avebury sarsens themselves. But to be honest I would no longer be surprised it they turned out to be quite different. I did, I admit, have a long discussion over the glacial or non-glacial origin of the ‘bluestones’ with someone called Brian Johns a while back. Again, the statistical evidence of size distribution was what swayed me toward a source at least in the Bristol channel area rather than as a moraine deposit.

(Conversation held between 20th Dec 2011 and 3rd January 2012)

Additional References

Nash, D.J. et al., 1998 Drainage-line silcretes of the Middle Kalahari: ananalogue for Cenozoic sarsen trains? Proc. Geol. Assoc. 109, 241-254.
Discusses the origins of sarsens as non-pedogenic (not formed in soil profiles) silcretes, suggesting that they may have been formed as linear bodies in gentle river valleys millions of years ago, not as a continuous sheet over the entire landscape, as has often been suggested. The evidence for this is based on petrology, not on their distribution (however, the sarsens arrangement in the landscape near Avebury, running NNW-SSE, may indicate the same thing).
Arthur, P. 1961 Sarsen stones of Stonehenge: How and by what route were the stones transported? What is the significance of their markings? Science 133, p1216-1222.
Argues that the sarsens were transported from the Avebury area via Lockeridge and down to the head of the Avon. I haven’t seen the paper BUT, nailing my colours to a flag, I think that this is the best story I’ve heard (maybe I wouldn’t if I’d read it). I’d probably add that I think the Kennett valley near Lockeridge may well be where the stones for Stonehenge came from. My route would take them between Adams Grave and Knapp Hill, down to the Avon at Marden and then down the river. The obvious problem with this is that you’d have to use the river only when it was high, e.g. wet winters or exceptionally wet summers.

Next Rhosyfelin dig coming up?

Interesting that my "Long History of Rhosyfelin" has now had 328 reads on Scribd -- so somebody out there obviously finds it interesting enough to look at.  But I have had no formal responses from any of the leaders of the dig to any of the points I have raised, or indeed relating to the accuracy (or otherwise) of what I say about the stratigraphy.  Thunderous silence.

I think the reluctance to debate this is really rather sad...........  so I wonder what is going on?

The only people honest enough to discuss a few points of fact and interpretation are a couple of the volunteer diggers, off the record.

From what I can gather, the next dig starts on 31 August.  I have no idea whether the team is going back to Rhosyfelin, or whether the key place this year will be Castell Mawr, where work was started last year.  Has anybody got anything to share?

Wednesday 21 August 2013

Acts of God

 Draft cover image -- in the end, it probably won't look anything like this.......

Sorry I haven't been paying much attention to this blog for the last 3 weeks -- been writing my latest novel.   It's called "Acts of God" and I finished the first draft last night after a mammoth writing session.  Four chapters and 16,000 words in one day -- so today I have been chilling out, if you'll pardon the expression.  It's a chiller thriller set in East Greenland -- as you might have guessed.  Nothing whatsoever to do with Stonehenge, although I was tempted to put in an episode where the heroes find a complete Stonehenge embedded in the ice, at the bottom of the Greenland Ice Sheet.  But I thought better of it.  Enough fantasies about Stonehenge already.........

If anybody is interested, I have a Facebook album with the same title as the story, with some fabulous images from the locations in which the story is set.

You can find it here:


And now for a galloping glacier......

One photo and two maps of a very strange glacier in East Greenland, on the northern flank of Nordvestfjord.   The satellite image (top) shows the Løberen Valley as it is today, with no glacier other than a pathetic remnant up towards the top of the photo.  The map in the middle shows Løberen Glacier with the snout a little way in from the coast.  And the map below shows the same glacier as a tidewater glacier, reaching all the way to the coast and calving icebergs directly into the sea. (It's the glacier above the first letter "d" in "Nordvestfjord".) 

You might be forgiven for getting confused here, or for thinking that the Danish mapmakers are rather slapdash.  But the truth of the matter is that Løberen is one of the elite surging glaciers of the world, rocketing forward (well, in glacial terms anyway) and then retreating catastrophically every now and then.  In fact, the Danes who made the maps and looked at all the aerial surveys for this area were so impressed that they called it "the sprinter" or "the runner" partly as a joke -- and that has now been accepted as the official name.

The middle map shows Oxford Gletscher -- that's the one we worked on in 1962, and we gave it its official name.  We did ice temperature measurements on it, and were mystified because there were some very strange anomalies.  What we did not realise at the time was that Oxford Gletscher also surges occasionally, and the traces of that surging behaviour were still there in the ice temperature profile.  If we had gone up the fjord a bit further, to Løberen, we would have had a much more exciting time, because in 1962 it was more or less at its surging peak, reaching the fjord.

As far as we know, the glacier was in advancing mode in 1950 - 1967, moving forward erratically through that period -- and since when it has retreated by at least 7.5 km.  We don't know how fast the glacier was moving in the main surge phase, but some glaciers can increase their velocity 100-fold during a surge, moving at more than a metre per day.  In one surge in Iceland, the glacier ice was observed to be moving at 5 m per hour.  Here it is quite possible that Løberen was advancing at a rate of c 1 km per year at the peak of the last surge.  That would give a peak advance rate of c 3m per day.

Of course, since the glacier was terminating in the sea for the period 1960 - 67, we have no means of knowing how much further it might have advanced, if it had had a nice gentle glacial trough to flow along.

How's this for a giddy glacier?

Hold onto your horses, folks -- this switchback of a glacier is really rather amazing.  Makes one almost feel giddy just looking at it.  But of course it's a bit of an optical illusion, looking up the glacier from quite low down, with a foreshortening effect......

It's Korridoren Glacier in East Greenland.  The belts of medial moraine are quite something, telling us that the ice has come from six or seven different tributary glaciers.

Flower power from outer space

Thanks to Rob for sending this little gem.......

Metro Paper 20th August.  
Space balls.
An ancient Egyptian necklace on show at London’s Petrie Museum originates from outer space. Beads set in the piece were made from iron meteorites that were hammered into thin sheets. The astral elements include cobalt and geranium according to researchers at University College London.

Sea Ice is rather interesting.....

Came across these two rather nice aerial photos of sea ice off the coast of East Greenland -- just to give us some idea of the almost infinite variety found in nature.

The top photo shows very thin plates of ice -- maybe just a few inches thick -- with rafting occurring under the influence of winds and currents.  The slabs are literally sliding over one another.

The lower pic is of old shorefast ice which is breaking up into discrete floes -- this can be a foot or more thick.  Note that in some areas there have been so many collisions between the floes that there is a lot of brash ice.  But there is no glacier material here.  All the ice in both photos has come from the direct freezing of sea water.

Saturday 10 August 2013

Stonehenge: complete or incomplete?

Thanks to Phil Morgan for drawing this to my attention.  It's an article by Mike Pitts in the latest edition of "British Archaeology" (Sept-Oct 2013, p 6) in which he draws attention to the find by Tim Daw that during the UK summer drought there were dried-out patches showing up in the Stonehenge turf.  There were lots of them, of all shapes and sizes -- some revealing the locations of old excavation pits, trenches, and paths.  But some of them seemed to coincide with the places where sarsens 17-20 might -- or might not -- have been located.  The illustrations below show the locations of these 'burnt opatches' on the Stonehenge map, and on the ground during July 2013.

Photos:  acknowledgement to British Archaeology.  Please go to the magazine for the full article. In the photo, the patch in the foreground coincides with the supposed location of sarsen 17.

This of course brings up the great debate about the completion (or otherwise) of the stone monument -- a matter argued about since Inigo Jones in 1655 assumed it WAS complete, and Wood in 1747 argued that it wasn't.  I have personally argued that the monument was never completed, since I could not see any sound evidence that it was -- and many others have agreed with me on that, including a number of current archaeologists.  Well, in the light of this latest evidence I don't think I would go so far as Mike Pitts does, with the headline "STONEHENGE DISPUTE SOLVED AFTER 260 YEARS."    But the evidence IS quite persuasive.   So one must keep an open mind.

However........  There are still a number of issues to be sorted before this matter really is resolved.  The "burnt patches" are quite large -- much larger than the "footprints" of the assumed sarsens.  They are also rather irregular, not being in PRECISELY the right places.  Rather large pits might have been dug by the Stonehenge builders in preparation for some big sarsens -- but that does not mean that the sarsens ever were found and installed.  Also, we have the widely-cited evidence from geophysical surveys that shows no pits or sockets here -- how does that evidence square with what has shown up during the 2013 summer drought? (See the chapter by David and Payne in "Science and Stonehenge.")  Again, the marks shown on the photo may simply show where past excavations have taken place, conducted by people who were looking for fallen and buried stones, or simply for the sockets -- and naturally they would have dug and searched in the places where the sockets were assumed to have been located. 

Interesting debate coming up, I suspect....

Thursday 8 August 2013

Not a bad little cliff......

Acknowledgements to Joshua Lavigne and Grant Dixon for these two photos -- go to their web sites for more images.  These show Mount Asgard on Baffin Island -- famous in the climbing fraternity but also in glacial geomorphology circles -- showing how vertical downcutting can occur in certain circumstances by a combination of frost-shattering and glacial trough deepening (and transport away of lots of debris) by glaciers in close proximity.

Monday 5 August 2013

Another nice erratic

I came across this rather nice photo of a large erratic on a glaciated surface near Syd Kap, in Scoresby Sund, East Greenland.  It's good to have the human being for scale.......  this will have been carried in the Devensian Glaciation by the huge glacier coming out of Nordvest Fjord.  The human being would not have been there at the time......

Saturday 3 August 2013

Glaciers, protection and erosion in the uplands

 It's not very often that you get a straight piece of glacial geomorphology making it onto the prime news pages on the BBC web site -- but here is something from today.  The report is quite interesting, and the paper presents some neat observations which back up glaciological theory quite nicely -- showing that cold ice protects the landscape, that temperate or warm ice erodes, and that even more erosion and downcutting occurs on the mountain slopes above and at the edges of the ice -- with the glacier acting as a conveyor belt for transporting away shattered rock debris.  We see this very often in ice fronts, where ridges of terminal and lateral moraine are shown to be very thin indeed -- we might even call them superficial, with dark rock material extending to a depth of just a few metres, and with clean ice lower down.

Not sure I would agree with the suggestion that cold ice in Alpine snowfields protects the landscape so much that this helps to explain the height of the Alps (which are still being lifted tectonically)...... but this is nonetheless a very welcome contribution to the literature.

Alpine glaciers 'protect mountain peaks from erosion' 

By Simon Redfern Reporter, BBC News

Instead of wearing mountains down, evidence from Europe's high Alps shows that glaciers shield summits from erosion, acting as a protective lid. French scientists studied erosion on Mont Blanc, western Europe's highest peak, below and around its glaciers. Cold ice at the highest points froze to the mountain rock and played little part in erosion, the team said.  In contrast, water and rain eroded glacier-free areas 10 times faster than areas protected by the glacier.

"Mountains don't grow to infinity, so there must be another mechanism which has lowered the summit of Europe” --  Prof Fritz Schlunegger Bern University

The research was part of Cécile Godon's doctoral research at Université de Savoie, located on the edge of the French Alps, and appeared in the journal Earth and Planetary Sciences Letters.  The study focused on the Bossons glacier, which flows down the northern face of Mont Blanc towards the French town of Chamonix.  Rock debris, carved from the mountain at the toe of the Bossons glacier and sediments washed out in high mountain streams were compared with erosion in nearby glacier-free areas.  The researchers found that the cold glacial ice protected the mountain from erosion, rather than promoting it.

Rising ice

These results may explain the high altitude of the Alps. Driven by the tectonic collision of Europe with Africa, the high alpine bedrock is rising about one millimetre each year.  Glacier-free areas of the Alps erode at a similar rate but where the mountains are protected by ice, the peaks wear away at one tenth that rate.

Fritz Schlunegger, from Bern University, Switzerland, was not involved in the work and commented: "This group has used sediments at the end of Bossons glacier to determine where erosion is happening beneath the ice.  "Most material has been derived from the non-frozen part the glacier, while higher up towards (the summit of) Mont Blanc - where the glacier is frozen to the ground - erosion is much less," he told BBC News.  "This is really the first time, according to my knowledge, where this has been convincingly shown in a quantitative way and using a natural example.

"However, mountains don't grow to infinity, so there must be another mechanism which has lowered the summit of Europe. According to (Dr) Godon's findings, this erosion is not related to glaciers, so we still have to think about other possibilities."

Around the globe, mountain glaciers - especially those at low latitudes - are retreating in response to climate change, scientists say.  Reports earlier this year indicated that glaciers around Mount Everest had lost more than one eighth of their area in the past 50 years, and the snowline had retreated 180 metres up the mountain sides.    Dr Godon's results suggest that changes like these could change the shapes of the world's highest mountains, and that climate and mountain landscape are intimately linked.
The Bossons glacier protects Europe's summit from erosion

by C. Godon, J.L. Mugnier, R. Fallourd, J.L. Paquette, A. Pohl, J.F. Buoncristiani
Available online 28 June 2013


The contrasting efficiency of erosion beneath cold glacier ice, beneath temperate glacier ice, and on ice-free mountain slopes is one of the key parameters in the development of relief during glacial periods. Detrital geochronology has been applied to the subglacial streams of the north face of the Mont-Blanc massif in order to estimate the efficiency of erosional processes there. Lithologically this area is composed of granite intruded at ~303 Ma within an older polymetamorphic complex. We use macroscopic features (on ~10,000 clasts) and U–Pb dating of zircon (~500 grains) to establish the provenance of the sediment transported by the glacier and its subglacial streams. The lithology of sediment collected from the surface and the base of the glacier is compared with the distribution of bedrock sources. The analysis of this distribution takes into account the glacier's surface flow lines, the surface areas beneath temperate and cold ice above and below the Equilibrium Line Altitude (ELA), and the extent of the watersheds of the three subglacial meltwater stream outlets located at altitudes of 2300 m, 1760 m and 1450 m.
Comparison of the proportions of granite and metamorphics in these samples indicates that (1) glacial transport does not mix the clasts derived from subglacial erosion with the clasts derived from supraglacial deposition, except in the lower part of the ice tongue where supraglacial streams and moulins transfer the supraglacial load to the base of the glacier; (2) the glacial erosion rate beneath the tongue is lower than the erosion rate in adjacent non-glaciated areas; and (3) glacial erosion beneath cold ice is at least 16 times less efficient than erosion beneath temperate ice. The low rates of subglacial erosion on the north face of the Mont-Blanc massif mean that its glaciers are protecting “the roof of Europe” from erosion. A long-term effect of this might be a rise in the maximum altitude of the Alps.

Thursday 1 August 2013

Ploughing in West Kennet Avenue?

More thanks to Pete G for yet another pic -- this time a painting from 1895, showing plouging furrows running along the line of the Avenue.  According to Pete, the alignments exposed in the current dig run across the Avenue at an angle -- so this might rule out the ploughing hypothesis, unless these deep marks date from medieval or even earlier ploughing......  after all, many parts of rural Britain were transformed by ridge and furrow farming practices in the Middle Ages.

A veritable cornucopia of spotted dolerites

I drove past Bethel Chapel in Mynachlogddu today, and was forcefully reminded of the way in which spotted dolerites from the nearby outcrops of Carn Meini etc have been used in local architecture.  The whole of the chapel facade (finished in 1875) is built of shaped blocks of spotted dolerite -- many different types.  Big spots, little spots, scattered spots, dense spots, white ones and cream ones, and so on and so on.

This stone must have been incredibly difficult to work, but somehow or other the faithful builders of the chapel have managed to make most of the blocks rectangular.  The only stones that are NOT spotted dolerite are the window cills and lintels, and those curved window surrounds -- those are all, I think, made from soft grey local slate which is very easy to work and shape.

I dare say that Rob or Richard could give a comprehensive geology lesson on spotted dolerites without having to move away from the churchyard.  I don't know enough about these different types to know which outcrops they were taken from -- but there are historical records of decisions being made by the chapel deacons to go up onto Preseli to collect stone, and of horses and carts coming down with loads specifically for the chapel restoration.

Here are some close-ups.  In some cases the weathered surface has been left, giving a nice rusty or buff colouring, and in other cases the blue-grey of the fresh worked surface is quite striking:

You can click to enlarge any of these photos.  What interests me here is the question of WHY, in or around 1870 -75, the congregation here chose local spotted dolerite for their chapel facade.  Did it have any particular religious or spiritual significance for them?  I doubt that very much -- chapels were generally built with stone that was cheap and durable, and attractive -- they obviously wanted their chapel to be striking, as an offering to the glory of God.  But if sandstone or limestone had been the local rock, they would have used that instead.  And it WAS cheap -- they needed to pay nothing for it, for there it was, within a mile or two, up on the common, ready to be carted away.

Interestingly enough, in the years 1946-48, during the famous local episode called "The Battle of Preseli", local ministers and political leaders created a great campaign to resist the efforts by the MOD to turn the whole of Preseli into a military training range, by building on HH Thomas's thesis of "the sacred stones."  It suited their campaign strategy very well to claim that the spotted dolerites were sacred, and always had been, and that it would be an outrage and an insult to the sensitivities of the Welsh if the MOD had gone ahead with its plans.  Shock!  Horror!  Military firing range planned for sacred Welsh mountains......!!

Anyway, it worked.  Churchill and the rest of the Government of the day were swayed by this emotional and pseudo-spiritual argument, and chose Castlemartin and the Brecon Beacons instead.  That doesn't alter the fact that the campaign to save Preseli was based on spiritual mumbo-jumbo, conjured out of thin air by the bards and religious leaders of the day.......  all credit to them, for in reality there never was any great feeling in history that either the spotted dolerite or Preseli had anything sacred about them.  They had MYTHOLOGICAL connections, of course, since the uplands figure prominently in the Mabinogion -- but so do many other places in Pembrokeshire, and you could argue that Cwm Cych, Narberth and the Pembrokeshire Islands were much more "special" than Preseli in the minds of the medieval storytellers.

Periglacial -- or something else?

Many thanks to Pete G for a new photo of the site at West Kennet.  As he suggests, this shows the "stripes" far better.  On the basis of this visual evidence, I'm prepared to accept that they MIGHT be periglacial in origin.  The high concentrations of flints are striking -- something that doesn't occur in the ridges in the Stonehenge Avenue?  One question -- do they run straight downslope?  If not, there may be a problem with this interpretation.

Are they ploughing marks, as one contributor has suggested?

As I said in reply to Pete's comment, they may also be hollows and ridges attributable to solutional activity on the bedrock surface beneath the regolith -- or they could be the outcropping strike planes of flint-rich layers in the chalk -- and therefore structurtally controlled bedrock features rather than down to any unique cold-climate environment.

Hopefully more evidence will be forthcoming.