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

Tuesday, 26 June 2012

More on the Dartmoor Ice Cap

I missed this earlier, but there is a nice video on the BBC web site here:

I assume the recording comes from the BBC South-West regional news programme.  The piece features Stephan Harrison (above) and contains some interesting background material -- simply explained, for those who find the learned article heavy going!

The model reproduced above is part of a computer simulation -- the image shows what the ice cap may have looked like after 1210 years of development.

Monday, 25 June 2012

A possible source for the Altar Stone?

Note from Phil Morgan, for which many thanks:

I've attached a photo of of Hay Bluff, the Senni Beds of which could, arguably, be a candidate for the source of the Altar Stone. The base of the Senni Beds is indicated by the thin yellow line, and the  bed thickness in this area is about 160m.
As you probably know, the boundary between England and Wales runs along the crest of the Bluff, so if this striking feature turns out to be the source of the AS, then I wonder which nation will claim possession rights?
As usual, you are welcome to use the photo if you wish.

Thanks to Phil for this.  We have had a number of posts on this blog about the Altar Stone, referring frequently to the rejection of the idea that it came from the Cosheston Sandstones of Milford Haven.  Rob Ixer is sure it has come from the Senni Beds, which are extensive in South Wales.  The preference at the moment, among the geologists, is to provenance the Altar Stone to the Towy Valley or somewhere else around the western fringes of the Brecon Beacons and the South Wales Coalfield.  That makes sense to me, since valley glaciers flowing broadly southwards could well have introduced this stone -- and maybe others -- into the path of the Irish Sea Glacier which later on flowed broadly eastwards towards the coasts of SW England.

Could the stone have come from the Senni Beds of the Welsh Borders?  Personally I doubt it, but it's quite possible that erratics from the Hay area could have been carried to the Stanton Drew area and even onto the flanks of the Mendips.  What I don't know is how much variation there may be within the Senni Beds, and how accurate the provenancing of the Altar Stone can be.

Perhaps Rob or another geologist can enlighten us?

Chris's Review of the new MPP book

As requested by Tom, here is Chris's full review of the new MPP book.

Chris Johnson's review of the new MPP book

The author gives selected results from the extensive explorations of recent years that he has been leading. These titbits are interesting to the specialist but newcomers to the subject will find themselves lost and confused. Specialists find themselves wondering what else has been discovered that Mike Parker Pearson chooses not to mention.

The relentless pursuit of Mike's theory that Stonehenge is a graveyard becomes wearisome. Human remains are indeed present but relatively few considering the number of people involved over a thousand year period. Mike explains this by asserting that the scarcity of remains is evidence for a dynasty, a ruling elite. He fails to explain why a ruling dynasty could have exercised such control over its workers at a time when Britain was thinly populated and there were abundant natural resources such as deer to sustain any clan that did not fancy the hard work of constructing monuments.

On the positive side Mike advances good arguments for reconsidering chronology, including the placement of the Bluestones from Wales in the first phase. He makes a good case for a link with the recent excavations at Durrington Walls, but fails to address the distinct contrasts with the Avebury monument which is close by. This superficial treatment is frustrating for the serious student. Alternative theories for, say, the transport of the Welsh stones by glaciers are considered and dismissed, in this case failing to take account of the latest evidence. Likewise the theory of a healing connection with Prescelli is quickly dismissed on the basis of Mike's opinion. The book is heavy on opinion and this is not what one expects from a leading academic.

All in all a disappointing book - it could and should have been a lot better, especially considering the amount of public money that has been invested in the projects he has been leading. Still, stonehenge obsessives will find it required reading if only for the dribble of facts emerging from recent scientific work.   

Friday, 22 June 2012

On Stone Age politics

This item on the BBC web site is no doubt based on a Sheffield University press release, timed to coincide with the publication of the latest MPP book. These ideas about political unification and so forth seem to me to be fantastical, but quite in tune with the modern style of archaeology, in which fact and fantasy are blended together in such a way that it becomes difficult for those of independent mind to sort out what sort of factual basis there is for all the speculation.

I'm not surprised by the MPP /SRP "conclusions" (which should really be called "suggestions") since they were aired in last year's controversial lecture in Newport which I criticised heavily in this blog.

Anyway, here is the report, for what it is worth......

Stonehenge was built to unify Britain, researchers conclude

22 June 2012

Building Stonehenge was a way to unify the people of Stone Age Britain, researchers have concluded.

Teams working on the Stonehenge Riverside Project believe the circle was built after a long period of conflict between east and west Britain.

Researchers also believe the stones, from southern England and west Wales, symbolize different communities.

Prof Mike Parker Pearson said building Stonehenge required everyone "to pull together" in "an act of unification".

The Stonehenge Riverside Project (SRP) has been investigating the archaeology of Stonehenge and its landscape for the past 10 years.

In 2008, SRP researchers found that Stonehenge had been erected almost 500 years earlier than had originally been thought.

Now teams from the universities of Sheffield, Manchester, Southampton, Bournemouth and University College London, have concluded that when the stone circle was built "there was a growing island-wide culture".

"The same styles of houses, pottery and other material forms were used from Orkney to the south coast - this was very different to the regionalism of previous centuries," said Prof Parker Pearson, from University of Sheffield.
"Stonehenge itself was a massive undertaking, requiring the labour of thousands to move stones from as far away as west Wales, shaping them and erecting them.

"Just the work itself, requiring everyone literally to pull together, would have been an act of unification."

Stonehenge may also have been built in a place that already had special significance for prehistoric Britons.

'Centre of the world'

The SRP team found that its solstice-aligned avenue sits upon a series of natural landforms that, by chance, form an axis between the directions of midsummer sunrise and midwinter sunset.

"When we stumbled across this extraordinary natural arrangement of the sun's path being marked in the land, we realised that prehistoric people selected this place to build Stonehenge because of its pre-ordained significance," said Mr Parker Pearson.

"This might explain why there are eight monuments in the Stonehenge area with solstitial alignments, a number unmatched anywhere else.

"Perhaps they saw this place as the centre of the world".

Previous theories suggesting the great stone circle was inspired by ancient Egyptians or extra-terrestrials have been firmly rejected by researchers.

"All the architectural influences for Stonehenge can be found in previous monuments and buildings within Britain, with origins in Wales and Scotland," said Mr Parker Pearson.

"In fact, Britain's Neolithic people were isolated from the rest of Europe for centuries.

"Britain may have become unified but there was no interest in interacting with people across the Channel.

"Stonehenge appears to have been the last gasp of this Stone Age culture, which was isolated from Europe and from the new technologies of metal tools and the wheel."

Wednesday, 20 June 2012

The myth of "periglacial Dartmoor"

I have been having another look at the Evans et al paper on Dartmoor and pondering on the manner in which an "established wisdom" actually becomes established........

For many years I have been intrigued by the fact that professional geomorphologists, over several generations, have simply accepted the idea that the uplands of southern Britain (we'd better not call them mountains) have been affected by periglacial conditions but not by glacier ice.  Hardly anybody has been prepared to stand out from the crowd apart from Stephan Harrison, arising from his work on Exmoor -- maybe because the "periglacial paradigm" was argued so forcefully by famous establishment figures such as Prof David Linton.  But over and again we have seen "definitive" statements in the specialist literature to the effect that SW England has never been affected by glacier ice, and that everything in the landscape can be explained by reference to oscillating periglacial and more temperate conditions during the course of the Pleistocene.  Since geomorphologists have been saying this sort of thing, it is perhaps not surprising that archaeologists have seized upon their statements and have assumed that everything is sorted, and that nobody talks nowadays about glaciers affecting the counties of Somerset, Devon and Cornwall.

A number of things have contributed to the questioning and eventual destruction of the myth.  After Stephan's work on Exmoor suggested very strongly that glacial conditions were possible -- and had indeed existed on the uplands of the South-West, a number of researchers (including me, on this blog) pointed out that the Exmoor findings were perfectly in line with the evidence of glacial action in Somerset, the evidence of till at Fremington, the giant erratics on the coasts of Devon and Cornwall, and the occurrence of till on Lundy Island. (Use the search facility on this blog if you want more detail.)  Then along came glacial modelling, which allowed the reconstruction of ancient ice sheets and ice caps through the use of increasingly sophisticated data sets and calculation procedures.  This work, based at Aberystwyth University and elsewhere, showed that it was possible -- and indeed probable -- that the Irish Sea Ice Sheet had pushed across the coasts of SW England on more than one occasion, with the more extreme models showing glacier ice covering most of Wiltshire.  When James Scourse and others showed that Irish Sea ice had reached the Scilly Isles in the Devensian or Last Glacial Maximum,  only about 20,000 years ago, the computer models were shown to be essentially quite reliable. 

One would have thought that on the basis of all this new work, geomorphologists would have been happy to go on the record to state unequivocally that the Irish Sea Glacier HAD affected the counties of Somerset, Devon and Cornwall and that local ice caps must also have been present during the Pleistocene.  But when I asked senior geomorphologists to sign up to a simple letter to British Archaeology laying out the facts, only a couple of them agreed;  some of them refused, and most never bothered to respond.  I was disappointed and surprised by that, since it seemed to suggest that geomorphologists were reluctant to become involved in "the Stonehenge problem" even in a peripheral way, and that they did not want to be involved in anything that might upset their colleagues in Archaeology departments!  Whether these senior academics were afflicted by apathy or diplomacy, their silence simply served to encourage senior professors like MPP, GM and TD to believe that there was a consensus among earth scientists that no glacial processes had ever affected South-West England.  So, somewhat disgusted by the wimpish tendencies of some of my senior ex-colleagues, I decided to submit the letter under my own name -- and to his eternal credit, Mike Pitts published it in British Archaeology:

So there we are then.  Has the myth of "periglacial Dartmoor" and the "ice-free South-West" finally been laid to rest? I doubt it -- too much academic capital has been invested in it for any instant change of attitude.  And don't let's forget that many archaeologists still accept that whatever Chris Clark, James Scourse and Chris Green may have written in rather influential articles in the past MUST be true.  They may not have noticed it, but the world has moved on -- and thanks to David Evans, Stephan Harrison and their colleagues we now have an influential article in the peer-reviewed literature that sits easily with all of the other field evidence which we have examined on this blog over the last couple of years.

Listen carefully, archaeologists.  The South-West of England has been affected by the ice of the Irish Sea Glacier, and by local ice caps, on several occasions during the Pleistocene.  This means that it is perfectly feasible for glacial erratics to have been carried from West Wales, Ireland and even Scotland on more than one occasion and dumped well inland of the Bristol Channel coast.   Got that?

Below I reproduce the Introduction to the recent article by Evans et al -- with key phrases highlighted in bold type.  It's a very interesting comment on how academic thinking (even in mainstream scientific disciplines) can be affected by fashions, conservatism and senior academics who have reputations to protect, to the point where perfectly sound observations presented by people who are not a part of the academic establishment can be ignored and -- dare I say it? -- even sneered at.


The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap

in the British Isles

David J.A. Evans, Stephan Harrison, Andreas Vieli, Ed Anderson


Although the granite uplands of Dartmoor (Fig. 1) have long
been considered to be relict permafrost and periglacial landscapes
that lay beyond the limits of Quaternary glaciations (Linton, 1949;
te Punga, 1956; Waters, 1964, 1965; Gerrard, 1988), the notion that
glaciers had developed in these areas was entertained by some
early researchers
(Ormerod, 1869; Pillar, 1917). The evidence pre-
sented at that time was largely circumstantial even anecdotal. For
example, Ormerod (1869) reported that “he had not seen any
glacial markings on the Dartmoor granite, but that Professor Otto
Torrell, when visiting the Moor with him last autumn, gave an
unqualified opinion that many of the gravels were the remains of
moraines” (p. 99). In contrast, Somervail (1897) suggested that the
absence of small lakes on Dartmoor was incompatible with former
glaciations and wrote: “It is true that various attempts have from
time to time been made by various observers to refer certain
phenomena occurring on Dartmoor to local glaciation. None of
these, however, are, I think, the result of true glacial action, but
must be referred to the more common operations of running water.
During the cold of the Pleistocene period, and at its close, the floods
from melting snows would perfectly accomplish all the distribution
and arrangement of that deposit of angular rocky debris
surrounding Dartmoor so frequently referred to ice. The same cause
would also equally well explain these accumulations of scree
matter filling some of the valleys, which some have regarded as the
remains of ancient glacial moraines” (p. 388). Pillar (1917) later
argued that Dartmoor should have been glaciated given its prox-
imity to the Pleistocene ice sheets: “As the meteorological condi-
tions must have been the same in these contiguous areas, it seems
somewhat strange that land in such close proximity should be
considered as outside the range of Ice influence” (p. 179).

A more systematic and empirical approach was taken by Pickard
(1943), who argued for the former existence of extensive glaciers
and small ice caps on Dartmoor during the Quaternary based on
a large collection of varied features, some more convincing than
others. In particular he presents evidence of amphitheatre-like
valley heads or incipient cirques, glacially “worn boulders” and
grooved rocks, “moutonnée rocks”, perched boulders, moraines
comprising ridges of blocky debris, and potential glacifluvial
gravels. Although this evidence has never been directly refuted, the
predominant view since the 1950s has been that the Dartmoor
landscape of summit and valley-side tors is the product of peri-
glacial mechanical weathering and slope processes
that have
exploited zones of rotten granite and exposed large coherent
bedrock residuals, which because of their dilatation joints or
pseudo-bedding resemble corestone stacks. The efficacy of these
cold climate processes, which must have operated over a large
proportion of the Quaternary, was enhanced by pre-existing granite
breakdown through deep weathering in the tropical climate of the
Tertiary (Linton, 1955) and/or pneumatolysis during much earlier
periods of deep thermal activity (Palmer and Neilson, 1962; Eden
and Green, 1971). Despite the greater substance of the large
volume of work undertaken since Pickard’s paper, it is not clear
how the present consensus regarding the absence of glacial ice on
Dartmoor came about.
There are several possibilities, the first of
which is the assumption largely championed by Linton (1955) that
the development of tors required a lengthy period of ice free
conditions. Palmer and Neilson’s (1962) view that the Dartmoor
tors reflected prolonged periglacial action, rather than the opera-
tion of tropical deep chemical weathering processes as Linton had
argued, may also have served to further alienate notions of Dart-
moor glaciation and consolidate the periglacial paradigm. The
second possibility is that Pickard’s views may not have been taken
seriously. His 1943 paper was his Presidential address for the
Devonshire Association and was, presumably, not refereed. More-
over, although it appears he had a strong interest in natural history
and geology, he was an opthalmist by training and was likely
regarded as an enthusiastic amateur by later geomorphologists.

However, recent research on Exmoor (Fig. 1) by Harrison et al.
(1998, 2001) has demonstrated the existence of tills and associ-
ated glacial landforms in the vicinity of The Punchbowl, a north-
facing valley near the village of Winsford. The tills have been
deposited at altitudes down to 255 m asl probably by a glacier snout
that flowed into The Punchbowl from a small ice cap located on the
summit plateau of Winsford Hill (426 m asl). The presence of glacial
ice at relatively low altitudes on Exmoor enabled Harrison (2001) to
speculate on the likelihood that Dartmoor had been similarly
glaciated at times during the Pleistocene.
The Exmoor glaciation
evidence is entirely predictable considering that areas of high
terrain, such as those located in SW England, are likely to have been
cold enough to host small ice caps and glacierets during full glacial
periods when British-Irish Ice Sheet limits extended as far south as
the Isles of Scilly (Scourse, 1991; Scourse et al., 1991; Scourse and
Furze, 2001; Hiemstra et al., 2006, Fig. 1). Indeed, numerical
modelling exercises invariably create ice masses over the SW
English uplands during the Last Glacial Maximum
(LGM) (Hubbard
et al., 2009) merely because the environmental boundary condi-
tions for the model will develop glacier ice where the local equi-
librium line intersects the topography. Moreover, the plateau-style
of topography on Exmoor and Dartmoor is conducive to the accu-
mulation of snow on the broad summits in addition to its buildup in
the deeper valley heads due to snowblow
(cf. Manley, 1959; Sissons
and Sutherland, 1976; Sissons, 1979; Sutherland, 1984; Mitchell,
1996; Rea and Evans, 2003, 2007; Coleman et al., 2009). There-
fore the style of glaciation will be similar to the plateau icefield
glacial landsystem, wherein predominantly thin, largely cold based
and protective ice on upland surfaces would drain into valley heads
radiating from the plateaux to form locally thick, warm-based
snouts capable of eroding the substrate
(Rea et al., 1998; Rea and
Evans, 2003, 2007). Such erosion would have been responsible for
the production of The Punchbowl on Exmoor and potentially the
overdeepened valley segments around the high summits of
northern Dartmoor, identified by Pickard (1943) as evidence for
glacial modification.

The evidence for former glacier ice in marginally glacierized
terrains, such as those represented by Exmoor and Dartmoor, is
likely to be subtle for a number of reasons. First, the predominantly
thin ice located on low-angled slopes would only generate low
shear stresses and low flow rates, so the creation of well developed
bedrock erosional forms is unlikely. Additionally, the coarse crys-
talline nature of the Dartmoor granite is not suitable for the pres-
ervation of striae and other small scale glacial erosional features.
Second, the absence of high bedrock cliffs above the accumulation
zones of plateau icefields precludes the provision of extraglacial
rock debris, which together with the resistant nature of the granite
substrate would have resulted in small glacial debris loads and
hence weakly developed moraines and tills. Glaciers would have
instead only been able to incorporate periglacial slope deposits,
which after at least several hundred thousands of years of weath-
ering and gelifluction will have reached significant thicknesses in
valley bottoms (Waters, 1964). Moreover, such deposits would have
developed into large rock-fronted lobes and possibly rock glaciers
in some valley heads in the absence of, or between, glaciations,
similar to the stone runs of the Falkland Islands (Joyce, 1950;
Clapperton, 1975; Hansom et al., 2008; Wilson et al., 2008).
Harrison et al. (1996) have suggested such an origin for some
boulder lobes on Dartmoor. These rock-fronted, lobate forms, once
their ice content was removed, would resemble thin till sheets and
moraines in some settings. Finally, glacier ice that is frozen to its
bed effectively protects even delicate periglacial landforms and
sediments which therefore can survive one or more glaciations (cf.
Clapperton, 1970; Whalley et al., 1981; Dyke, 1993; Kleman, 1994;
Kleman and Borgström, 1994; Rea et al., 1996a, b). On Dartmoor, the
tors and clitter fields were initially regarded by Linton (1949, 1955)
as features that could not survive glaciation but he later modified
this view by arguing that they could be protected by thin and slow
moving ice. This principle of limited glacial erosion has since been
used to explain the preservation of tors in the Cairngorms (Sugden,
1968; Hall and Phillips, 2006; Phillips et al., 2006) and the presence
of preglacially weathered in situ bedrock and weathering pits in
glaciated east and northeast Scotland (Hall and Sugden, 1987; Hall
and Mellor, 1988; Hall and Glasser, 2003).

Although Dartmoor has been long established as an exemplar of
a mature periglacial landscape, no systematic assessment of
potential glacial landform evidence has ever been undertaken
likely due to the overwhelmingly strong periglacial sediment and
landform signature. Given the recent advances in our under-
standing of plateau icefield landsystems and the preservation of
tors and associated deposits beneath cold based ice, the systematic
survey and mapping of the Dartmoor landscape for potential glacial
evidence is warranted. We now show evidence that northern
Dartmoor (Fig. 1) was glaciated and discuss the evidence for
glaciations in the context of alternative views of landscape inheri-
As the resolution of these issues is critical to the recon-
struction of regional glaciation levels and palaeo-equilibrium line
altitudes in marginal glacierized terrains and hence the refinement
of boundary conditions for numerical ice sheet models, the
central aim of this paper is to assess the nature of the Dartmoor
landscape in the light of current understandings of plateau ice

The Bluestone Collection of Arthur Dennis Passmore

Thanks to Pete Glastonbury for drawing attention to this interesting fellow:

"I spent some time in Wiltshire Heritage Museum today and heard some things about the antiques dealer and amateur archaeologist AD Passmore (1877-1958).
Seems he was the plague of the Ministry of Works in his time. He found pieces of bluestone all over the place and collected them up, including a large sized piece near where the car park is now.
His collection is scatted over several museums but I have been told that if someone were to looking into his works they would find a lot of references to the bluestones he claims to have found.
There are some links to his collections in the Bodleian but his paperwork is scattered in various museums.
Devizes has a good amount.
His note's on Silbury mention finding the large bluestone chunk.
Archaeologists of his time hardly credited him at all".

Can anybody else provide information about the man and his rocks?

Friday, 15 June 2012

MPP's New Stonehenge Book


 Here is the Amazon info for the new book by Mike Parker Pearson.  It's only just out, but already it has attracted one very hostile review.  Being of a mischievous disposition, I can't resist adding it onto the end of this post........
We all have to live with hostile reviews.  If you venture into print, you can expect all sorts of people with all sorts of agendas to come after you.


Stonehenge: Exploring the Greatest Stone Age Mystery

Mike Parker Pearson

• Hardcover: 416 pages.  Price £25.00
• Publisher: Simon & Schuster Ltd (7 Jun 2012)
• ISBN-10: 085720730X
• ISBN-13: 978-0857207302
Kindle edition also available
(NB  Only published on 7th June -- but there are 5 used copies already available from Amazon.........)

Book Description

Publication Date: 7 Jun 2012

Our knowledge about Stonehenge has changed dramatically as a result of the Stonehenge Riverside Project (2003-2009), led by Mike Parker Pearson, and included not only Stonehenge itself but also the nearby great henge enclosure of Durrington Walls. This book is about the people who built Stonehenge and its relationship to the surrounding landscape. The book explores the theory that the people of Durrington Walls built both Stonehenge and Durrington Walls, and that the choice of stone for constructing Stonehenge has a significance so far undiscovered, namely, that stone was used for monuments to the dead. Through years of thorough and extensive work at the site, Parker Pearson and his team unearthed evidence of the Neolithic inhabitants and builders which connected the settlement at Durrington Walls with the henge, and contextualised Stonehenge within the larger site complex, linked by the River Avon, as well as in terms of its relationship with the rest of the British Isles. Parker Pearson's book changes the way that we think about Stonehenge; correcting previously erroneous chronology and dating; filling in gaps in our knowledge about its people and how they lived; identifying a previously unknown type of Neolithic building; discovering Bluestonehenge, a circle of 25 blue stones from western Wales; and confirming what started as a hypothesis - that Stonehenge was a place of the dead - through more than 64 cremation burials unearthed there, which span the monument's use during the third millennium BC. In lively and engaging prose, Parker Pearson brings to life the imposing ancient monument that continues to hold a fascination for everyone.

Review by TW Flowers  (who decided to put the knife in pretty quickly, by the look of it.....)

It seems that every day brings a new hypothesis, and every day some other hypothesis gets proved wrong. That is why early archaeologists had the professionalism not to speculate. Sadly those professionals are long gone.
Not so long ago, Professor Wainwright, head of the British Antiquarian Society of London used our televisions in an attempt to brainwash us into believing that Stonehenge was a place of healing similar to Lourdes of France. Wainwrights idea died a death in a matter of a few short months. This latest speculation, trumped up by Professor Pearson is that Stonehenge was a place for the dead.

Wherever did Professor Pearson get this idea from? Did he get it from another professor, one of the members of the Time Team perhaps, or did it come from a learned member of the Open University? Or did he take the kind of vote that archaeologists call `A consensus of opinion'? NO; it seems that the brains of our most educated aren't good enough, because this latest offering comes from as far away as Madagascar and from a modern-day megalith builder of that island called Ramilsonina. Pearson might just as well have gone to the moon.

It wouldn't be so bad were it not for the fact that Pearson and Ramilsonina's hypothesis was outdated a long time ago. And Pearson knows it.
As head of the `Stonehenge Riverside Project' Pearson has every right to produce a book that tells us everything he and his team have discovered, a team which included archaeologists and students from Manchester, Bournemouth, Sheffield, Bristol, Preston, Birmingham and many more.
The result is that this book is so heavily loaded with Pearson's pet life-to-death theory that no newcomer with a passing interest in Stonehenge - and therefore unable to sift fact from fiction - should read it.
The fact is that archaeologists are so embarrassed by a multitude of past mistakes that several of them have taken to lying, and are therefore well past being worthy of trust. Full of distorted facts, this book should never have been written.
As for corruption - don't just take my word for it; take the words of Mick Aston:   "Archaeology in Britain is a shambles from top to bottom... I'm not proud of the Time Team, it hasn't worked. And I'm totally dissatisfied with my time at Bristol University."   (Professor Mick Aston on why I had to leave the Time Team. British Archaeology Magazine, March/April 2012.)

By the way, the 7.5 megalithic yard (measured internally) Seahenge, did have 56 above ground posts to represent the moon, but archaeologists saw to it that by counting the posts from ten, they succeeded in making a miscount of one short at 55.

All in all, Pearson's book is a deliberate attempt to scotch a better, more universal theory of Stonehenge - and there is a better one out there - Pearson attempting to close the door on all fresh thought - and he knows it.

Thursday, 14 June 2012

The Dartmoor Ice Cap

One of the photos from the new paper.  This is one of the most spectacular glacial landforms on Dartmoor -- a terminal moraine ridge (right centre of the photo)

Grateful thanks to Dr Stephan Harrison for giving me sight of the PDF of this important article.  This is the full reference:

The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap in the British Isles
David J.A. Evans, Stephan Harrison, Andreas Vieli, Ed Anderson
Quaternary Science Reviews 45 (2012) 31-53

This is a long and detailed paper which provides very convincing evidence for glaciation on Dartmoor during the Devensian or Last Glacial Maximum.  The four authors are also convinced that there was some ice remaining in the Younger Dryas, and that this ice was quite active.

In fieldwork and from photographic evidence, the authors identify till and fluvio-glacial deposits, lateral and terminal moraines, hummocky moraine features, meltwater channels, ice-smoothed bedrock slabs, roches moutonnees, eroded tors, streamlined drift forms and other features -- adding up to a very convincing picture of a small and relatively thin ice cap which was at one time a coherent single feature and at other times broken into several segments.  At its greatest Devensian extent, it was about 10 km in diamater, and almost circular.  Here are some of the illustrations from the paper:

 A substantial lateral morainic ridge (arrowed)

 Ice-smoothed granite slabs

In addition to the presentation and analysis of the field and mapping evidence, the authors seek to understand the dynamics and dimensions of the Dartmoor Ice Cap that was responsible for most of the landforms described.  They create a number of models which match very closely the evidence on the ground.  Here are two of them:

Suggested ice thicknesses for the Dartmoor Ice Cap after 1400 years of development.  Note that in places the ice is more than 140m thick, with a maximum thickness of c 180m where ice was flowing rather than stagnant.
Suggested flow rates for the ice of the Dartmoor Ice Cap.  Much of the ice was almost stagnant, but in places, in the main outlet valleys where glaciers have been located, ice movement was up to 15m per year.  That is still very slow compared with many outlet glaciers from ice caps in the polar regions today.

The authors attribute most of the described features to the Devensian glacial episode, but they do admit that some of them might be composite, having been modified during a number of phases of glacial action.  I would have liked a little more consideration of the extent of "fossil" or "inherited" glacial features from earlier glacial phases such as the Anglian --  but perhaps that work will come later.  For the moment, this is a very significant paper which deserves attention.  I'll return with further comments on the implicatioins for the glacial history of Southern England.

In the meantime, here are some key extracts:


The granite uplands of Dartmoor have traditionally been considered to be relict permafrost and peri-
glacial landscapes that lay beyond the limits of Quaternary glaciations but a variety of landform evidence indicates that a plateau icefield existed on the northern part of the moor, constituting the southernmost independent ice cap in the British Isles. Overdeepened or weakly U-shaped valley segments fringing north Dartmoor document an early, extensive phase of glaciation but the most convincing landform evidence relates to more recent, valley-based glacier occupancy. A moraine ridge on the Slipper Stones represents the most unequivocal palaeo-glacier on north Dartmoor with a palaeo-ELA of c.460 m above sea level (asl), although this relates to the youngest and most restricted phase of glaciation. A longer term ELA is likely to be represented by the Corn Ridge proto-cirque at 370-410 m asl. More extensive valley glaciers are recorded in each of the major drainage basins of north Dartmoor by arcuate and linear bouldery ridges and hummocky valley floor drift, which are interpreted as latero-frontal moraines deposited by outlet lobes of a plateau icefield. Recession of these lobes is marked by inset sequences of such ridges and occasional meltwater channels. Plateau ice was predominantly thin and protective, and snowblow and preferential accumulation in valley heads facilitated the modest glacial erosion and debris transport recorded in the landforms and sediments. It is proposed that the highest plateaux have been occupied by ice for the longest cumulative period of time throughout the Quaternary (“average glacial conditions”), explaining the distribution of different tor types on northern Dartmoor. This also explains the lack of tors on the most expansive of the highest plateau terrain (ice dispersal centres) as the product of: a) average glacial conditions preferentially removing tors or dampening their production rates; b) the
survival of high relief (Type 1) tors during glaciation if they occupy summits too narrow to develop
significant plateau icefields and/or ridges that are bypassed by faster moving ice in adjacent deep valleys; and c) the survival of subdued (Type 2) tors in areas glaciated less regularly during the Quaternary.  Simple ice flow modelling indicates that a plateau icefield type glaciation is required for significant ice flow to occur and confirms thin ice cover, in particular on narrow summits, thereby supporting the explanation of tor class distribution. The modelling allows us to spatially correlate the geomorphological evidence of margin positions into two major stages and further indicates a strong altitude-mass balance feedback leading to an ice cap that is not in balance with its climate and with an extent that is limited by the length of the cold phases rather than their severity.


7. Age of glaciation

While at present there are no dates on the glaciations described
here, on morphostratigraphic grounds we can speculate on the
likely ages of the glacial phases. We hypothesise that the devel-
opment of the plateau ice cap and associated valley glaciers
occurred during the LGM at a time when the Late Devensian ice
sheet reached the northern Scilly Isles to the west and south of
Dartmoor. The modelling suggests that such more extensive ice-
fields with significant ice flow require several hundreds to thou-
sand of years to build up. We further suggest that the restricted
glacial advance producing small niche and cirque glaciers in the
West Okement Valley occurred during the Younger Dryas. In this
scheme the long term ELA at Corn Ridge of 370-410 is slightly
higher than that reconstructed by Harrison et al. (1998) for the
glaciation of the Punchbowl on Exmoor which had an ELA of 334 m
asl but remains similarly undated. Similar low values are recorded
in parts of South Wales such as the Black Mountains with Younger
Dryas ELAs around 290-350 m asl (Barclay, 1989). Recent unpub-
lished work (Hägg, 2009) reports the use of cosmogenic nuclides to
assess the age of tors and long term erosion rates on northern
Dartmoor. He used the dates to derive higher long term catchment
denudation rates than present values. Although he recognized the
possibility that Dartmoor may have been glaciated especially in the
central plateau region where our modelling predicts the existence
of plateau icefields, most of his sample sites were on the fringes of
this region and therefore do not help test this assertion. However,
high rates of catchment denudation may result from glaciation
given that burial of surfaces under ice would provide shielding from
cosmic radiation and thus lead to underestimation of surface age or
overestimation of erosion rates (Hägg, 2009, p. 233). Clearly,
cosmogenic nuclide dating on selected surfaces will be crucial to
provide future information on the age of the Dartmoor glaciations.


8. Conclusions
The extensive glaciation of Dartmoor is proposed in this paper
based upon the interpretations of several geomorphic criteria and
simple numerical flow modelling which strongly indicate the
development of a plateau icefield landsystem. The glacial evidence
is subtle; glaciers flowing at low gradients produce low driving
stresses. Nevertheless, we argue here that the glacial evidence
comprising moraines, drift limits meltwater channels, ice-scoured
bedrock and glacial sediment is compelling. The ice cap repre-
sents the southernmost independent Pleistocene ice mass in the
British Isles and is hypothesized to have last developed during the
LGM and Younger Dryas. The palaeo-glacier reconstruction with
which we have the greatest confidence, the Slipper Stones niche
glacier, represents a palaeo-ELA of around 460 m OD based upon
the MELM method. When combined with the altitudes and
breadths of the main plateau surfaces of north Dartmoor on
a “Manley curve”, this palaeo-ELA indicates that upland ice would
have been thin and protective and outlet glaciers would have
received significant nourishment from snowblow into valley heads,
although caution must be exercised when using only one glacieret
to determine a regional ELA. If the weakly developed bedrock
amphitheatre on Corn Ridge is accepted as a cirque, its floor altitude
of 370-410 m OD could be regarded as a long term ELA, thereby
making plateau icefield development more viable on north Dart-
moor. The flow modelling allows a 3-dimensional reconstruction of
a potential ice cap which spatially verifies the mapped marginal
moraine features. Due to the low surface gradients of the Dartmoor
plateau, the numerical modelling produces a plateau-style rather
than a valley glacier style of glaciation in order to initiate significant
ice flow. As a further consequence of the low surface gradient, such
a potential ice cap is exposed to a strong feedback between surface
mass balance and altitude which suggests that the reconstructed
glaciation extents do not relate to steady state conditions and that
these extents are limited by the length of cold phases. Furthermore,
the modelled ice extent, thickness and flow speed are in general
consistent with the development and preservation of the observed
tor style distribution. Our findings further support the earlier work
on glacial deposits and landforms on Exmoor and suggest that
other high moorland in the region (such as Bodmin Moor to the
west) may also have hosted ice masses during the Quaternary in
response to the sub-Milankovitch or millennia-scale excursions to
cold climate which seem typical of the last glacial stage and which
could have driven the repeated buildup of ice caps that were
limited by the length rather than severity of cold phases.