Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Monday, 31 January 2011

Glama -- nearly an ice cap

Further to my earlier post about the Glama and Dranga plateaux in Iceland, I found this satellite image which shows a terrain (Glama) just beneath the glaciation limit.  Probably the picture was taken in spring or early summer -- I would guess it was May.  Or maybe we are seeing the first winter snows, in October?  See the earlier posts, dated  9th Jan 2011 and 11th Jan 2011.  Anyway, this is a nice illustration of what a great deal of Southern England might have looked like through many of the cold phases of the Pleistocene -- most recently during the Older Dryas and Younger Dryas, when a lot of snow -- in depressions and valleys and against escarpments -- might have survived from one winter to the next, with parts of the "snow blanket" melting out each summer when average July temperatures will have been around +10 deg C.

Saturday, 29 January 2011

Prudden on Holwell

I have had a look at Hugh Prudden's "Somerset Geology - a Good Rock Guide" (downloadable pdf) and found this:

23.  HOLWELL  ST 7245   5 km S W of Frome
Fissures with Triassic and Jurassic infills-tectonics

There are old quarry workings down a track to the right of the pub (ST 729 452).  A large multi-stage fissure runs the length of an old quarry wall at the eastern end of quarry used as a car park.  Part of the fill has collapsed enabling Triassic, Rhaetian and Jurassic sediments plus the mineralisation to be examined at close quarters.  The sediments appear to have entered the fissures in both a lithified and partly lithified state.  Deformation of the sediments seems to have been associated with the opening of the fissure and the intrusion of the sediments.  There are footpaths to the nearby village of Nunney which has a nice pub, castle ruins (Doulting and Bath Stone) and sarsen stones (Tertiary silcretes) on a wall and near castle at the bottom of the hill leading from Nunney Catch.

Park in lay-by to the east of the Bear Inn.  Please first contact Mr R Bullus of Valley Sawmills, Holwell, Nunney, Frome for permission to visit the site.

A viewing platform has been built overlooking Colemans large quarry on the north side of the workings. Park in lay-by at sharp corner on the road the Whatley at ST 7231 4529 and walk some 200 m NNW along footpath.  There is a fascinating view both of the quarry activity and the undulations of the same unconformity as seen at Tedbury Camp quarry.  In addition the view includes the overlying Jurassic and Cretaceous terrains to the east.  There are few better places where one can appreciate geological time and space.

I would imagine therefore that Hugh would interpret the Holwell deposits as being intruded sediments from the Jurassic, Triassic and Rhaetian which have entered fissures resulting from faulting.  Even more intriguing -- that explanation raises as many questions as it answers -- must definitely take a look.

Something strange at Nunney

Over the years I have seen a number of references to a strange deposit at Nunney, a small village in Somerset which lies due west of Stonehenge.  The above map shows Geoff Kellaway's reconstructed glacial route, showing the Irish Sea Glacier passing over Nunney on its way to Stonehenge.  leaviung aside for the moment the queestion of whether the ice actually did reach Stonehenge, what is the deposit at Nunney?  Rodney Castleden describes it as a "drift deposit" -- a reddish-brown sandy clay containing both chalk and flint fragments, which means it is not directly related to the underlying bedrock.  Castleden suggests that the deposit has come from the EAST -- from the chalk escarpment -- and that the material was transported in mudflows during the cold phases of the Ice Age.  Hmm -- that sounds a bit strange to me.  On the other hand, Kellaway's explanation is equally difficult, since he thinks that the chalk and flint blocks have come from some chalk outcrop at an unknown location to the north or west.

In his famous (infamous?) Nature article in 1971, Kellaway described the Nunney deposit in some detail.  Actually, it is at Holwell, just outside the village at grid ref ST72554523, where there are large limestone quarries.  The deposit was observed when the land was being cleared for one of the quarries.  The reddish clay was seen to be up to 1m thick, resting on Inferior Oolite (Jurassic) limestone.  It was thin and patchy, and was partly decalcified, but the deeper pockets contained rounded and subangular fragments of white chalk (thought to be from the Middle Chalk), flint, chert, quartzite, Carboniferous and Jurassic limestone.  Kellaway thought that this deposit was related to other glacial deposits on the limestone uplands on the eastern margins of the Mendips, at an altitude of c 152m.  He also thought that at one time the ice extended at least as far east as the "western approaches of the vale of Pewsey."

What are we to make of the Nunney (Holwell) deposit?  Well, it so happens that my son and his family live in Nunney -- next time I go to see them, I am minded to pop over to those quarries and to see if any of that strange reddish deposit is still to be seen, forty years after Kellaway.......

It might well be a till, incorporating fragments from relict weathering deposits or residuals which accumulated to the west during a long process of denudation.  Another possibility is that the deposit incorporates gash breccia material which has come from the Nunney Fault.  Watch this space....

Wow!! The Stonehenge Legacy

As part of my service to mankind, herewith an extract from a review / PR puff for the latest blockbuster, published by Sphere at £6.99.  Sounds as if there are some very nasty druids on the prowl.  Worth reading?  Sounds much too violent for a sensitive soul like me.......... I'll pass on this one.

The last big Stonehenge blockbuster was Bernard Cornwell's "Stonehenge" which was panned by the critics and by readers.  It'll be interesting to see if this one does any better.

"Ancient codes, ritual sacrifice, serial killings, terrifying suspense and the eternal, brooding mystery of Stonehenge.
Is this the trailer for a new blockbuster film? No, it’s a dazzling hybrid thriller from debut British author Sam Christer which has all the exciting semiotics of a Dan Brown adventure, the high-octane action of Indiana Jones and the old-fashioned detective work of Patricia Cornwell.
But what really marks out Christer’s rollercoaster tale of deadly modern-day druids is his lively and well-drawn cast of characters and a plot that maintains a relentless pace and a simmering menace.
There’s a bunch of fanatical stone worshippers, a feisty woman police detective, a young archaeologist seeking the truth about his father’s death and the innocent victims marked out for sacrificial death.
Add a few twists and turns, an unfolding mystery, a smattering of supernatural and a race against time and all the ingredients are there for a classic page turner........."

Thursday, 27 January 2011

The strata do not lie

The two diagrams above come from a paper of mine which was published in 1973.  Click to enlarge.

The top diagram is a model showing how the "sediment record" varies at various points along the long profile of a big glacier such as the Irish Sea Glacier. Close to the ice shed area, ice will be present for a very long time -- almost from the outset of a glacial episode through to the final catastrophic wastage.  This means that there will be thick glacial deposits and not much in the way of periglacial deposits or head. About 250 km downstream, there will have been a long period of permafrost / periglacial conditions before the ice arrives, and so there may be thick periglacial head accumulations, overlain by an almost equal thickness of till and other glacial materials.  Closer to the eventual ice edge (at the outer limit of glaciation) there may be even thicker head and just a thin layer of till -- since ice is present for no more than a few centuries -- or even less.  Beyond the ice limit there will be no till -- just a thick series of head deposits of different sorts, representing fluctuations of periglacial climate -- sometimes wetter, sometimes drier, sometimes colder etc etc.

This is of course a model -- and it takes no account of bed conditions or glacier thermal regimes -- which can lead to accelerated erosion in some places at some times, and freezing-on and effective protection of the bed (and underlying sediments) at other times.  Also, fluvioglacial and other sediments can accumulate well beyond a glacier edge -- for example in valleys occupied by meltwater streams, and in the sea or in lakes.  So variable erosion and deposition rates and types have to be built into the model, which becomes more and more complex as we go on........

But if we look at the real sediment sequences we see that they do match the model reasonably well.  The two sequences -- from Abermawr in North Pembrokeshire and Poppit on the Teifi Estuary -- show a thick head sequence beneath the Irish Sea Till which marks the last glaciation of the area.  This till must have accumulated over a very long period of time while the glacier was "waxing" -- maybe 15,000 years or more.  The 4 facies of head at Abermawr show that there were climatic oscillations during this time -- and probably during the Early Devensian as well.

How long was the glacier ice present at these two sites?  Maybe a few centuries?  The ice was active enough to erode away some of the head layers -- in places we can see how there is an unconformity, with the junction between till and head clearly truncating or cutting across the "pseudo-bedding" of the head layers.  So the ice was flowing and eroding -- which means that the glacier was warm-based and sliding.  We must be talking about the period around 20,000 yrs BP.

After this glaciation, what do we see?  Well, first a wasting ice phase, as the glacier melted away -- with flow tills and abundant fluvio-glacial deposits here and elsewhere in West Wales.  Then, after a weathering episode, another cold spell, which must represent the Older Dryas - Younger Dryas period, around 12,000 - 10,000 years ago.  This was a short phase, and a thin layer of head accumulated, and incorporated much glacial material -- so other authors and I have referred to it as "rubble drift" to differentiate it from the clean "main head" below.  Then we have sandy loam, shown by analysis of sand grains to have been wind-blown at a time of bare ground surface conditions, strong dry winds, and abundant exposed glacial and fluvio-glacial materials on the land surface.  The modern soil is developed on and in this sandy loam deposit which caps the stratigraphic sequence.

The stratigraphy does not lie -- and it cannot be explained in any other way than that outlined above.

Sunday, 23 January 2011

On frozen gravels

Superficial deposits around Mendip

In trying to find out what the glacial and non-glacial deposits are around the Mendip Hills - Salisbury Plain area, I came across this old map drawn by Alan Fells of Bristol University about 50 years ago!  It's actually quite useful.  Click to enlarge. 

There are strange gravels all over the place, many of them not yet properly analysed and classified.  These gravels occur both within and beyond the assumed limit of glaciation in Southern Britain.  The term "Taele Gravels" used to be used for gravels of uncertain origin that were at one time frozen.  It's a bit misleading to refer to them as "periglacial gravels" -- because sorted gravels are not formed by freeze-thaw processes as such, but laid down by by flowing water and then frozen.  (That having been said, I have seen some head deposits in West Wales that are remarkably fine-grained, and which appear to have been laid down by trickling water, bit by bit, on a frozen surface........) 

In some periglacial river systems with perennial flow, gravels can be laid down each summer and frozen each winter, building up gradually over time and maybe incorporating involutions, ice wedges or other diagnostic features.  Erratics may be found in these gravels, if glacial or fluvioglacial deposits are to be found in the source areas or catchments of the rivers.  Landslide, slump and solifluction deposits (often called "head" or "rubble drift" can also be incorporated into the taele gravels, providing layers or lenses of coarser material.  Similar conditions must have occurred right across Southern England.

From a survey of the geology of Surrey:
"Whilst no true glacial (moraine) deposits such as chalky boulder clay are to be found on the North Downs or further south, the furthest extent of the earlier ice sheets over southern Britain during the period of glaciation (intermittent between 500,000 and 15,000 years ago) remains somewhat ambiguous. There is however much better evidence for glacial outwash gravels (Plateau Gravels such as cover Epsom Common, Great Bookham Common, Chobham, and at Newdigate) and permafrost chalk soils and rubble (Coombe Rock) and frozen gravels (Taele Gravels as found at Great Bookham, Ashtead, Epsom, Brockham and Betchworth).
River gravels such as occur in the Thames River Terraces (Flood Plain, Taplow, and Boyn Hill Terraces) can occupy large areas. The earlier terraces which may have been formed more than 250,000 years ago were deposited at a higher level before the downcutting of the bed of the river. Most of the surface geology within the Bos. of Runnymede, Spelthorne, and parts of Elmbridge relate to these terrace gravels of the Thames and this has also become an important economic resource in terms of sand and ballast gravel for the construction industry. Gravel extraction, both old and modern, has resulted in the discovery of Pleistocene mammal remains including many examples of cold-living fauna such as woolly mammoth, woolly rhinoceras, red deer, reindeer, wild horse, aurochs etc. Most of these finds have been of disarticulated remains since these would often (though not neccesarily) have been washed downstream by the river. Although modern methods of gravel extraction through dredging does not favour the preservation and discovery of such fossils, remains of mammoth teeth and tusks, bones etc. are still deposited with museums and form a useful if partial record of animal distribution. River Terraces and gravel deposits, though much reduced in comparison, occur on most of the large Surrey rivers - in particular the Mole, the Wey, the Blackwater and others. Excavations for gravel or construction have produced similar finds of Pleistocene mammals, mammoth teeth being amongst the commonest finds deposited in nearby museums. Not all such finds have been from river terrace deposits and locality details in these cases can prove extremely important."

Saturday, 22 January 2011

Snowblitz theory revisited

My thoughts about (a) the extent of snowcover over Southern England during glacial phases, and (b) the existence of small thin ice caps over the uplands of Cornwall, Devon and Somerset reminded me that back in the 1970's there was a lot in the literature about the "snowblitz" theory.  The leading proponent of this was Nigel Calder, who made a popular BBC TV programme in 1974 and wrote an associated book with the title "The Weather Machine."

To put it in context, at that time a number of climatologists were pointing out that there had been three decades of global COOLING (look at the above graph) -- triggering speculation that the current interglacial was coming to an end, having lasted for about 10,000 years (That would be par for the course.)  At the time, I subscribed to that idea myself, because sea ice extent was increasing, and many glaciers worldwide had started to advance after decades of retreat.  So, we thought, we can now expect a gradual slide towards the next glacial episode.  Indeed, some politicians were warning that governments in the most vulnerable parts of the world should immediately start laying up food stocks in anticipation of coming food shortages!!  Others were arguing that we would have to spread soot onto the polar ice sheets and sea ice in order to encourage melting and counteract the effect of global temperature decline.

Another part of the debate revolved around the idea that global temperature could "flip" with great speed from a warm phase to a cold phase or vice versa.  Data from many sources were showing a number of episodes in the past when temperatures had rocketed up or down in the space of just a few centuries.  How on earth could this happen, given the massive inertia of the global climate?  Were global catastrophes (like volcanic eruptions or meteorite strikes)  to blame?  There was a lot of talk about catastrophism at the time -- and people like Hubert Lamb and George Kukla became involved in intense debates about what the MECHANISMS for these rapid changes might be.  And then Nigel Calder came along with his nightmare scenario involving the snowblitz theory.  It wasn't taken all that seriously by the experts, but at the time it was a seductive idea because we were all into systems analysis, and we all talked about positive feedback mechanisms etc.  In essence, Calder argued that we could be plunged into a new Ice Age (by which he meant a new glacial episode) within a couple of hundred years by an inexorable process which started with a dusty or cloudy episode shutting off solar radiation from extensive polar and mid-latitude areas during a sequence of winters and summers.  Heavy snowfalls would thus continue to lie on the ground from one winter to the next, with the high albedo of the ground surface radiating back solar radiation and thus cutting off summer warming of the ground surface.  Heat loss would be exacerbated by the establishment of a high-pressure block accompanied by clear skies for many months at a time.  According to Calder, winter snowfalls would then simply accumulate, with inadequate summer heat to melt them off -- and the ice age would be triggered off by positive feedback.  Hubert Lamb contributed to the debate by calculating that the thickness of the snowcover over the land surface might accumulate at the rate of 50 cm per year -- enough to give us 100m of accumulated snow over 200 years.  This in turn would be sufficient to create firn and then glacier ice at depth.  So hey presto!  You have an ice-covered landscape, about which mere mortals would be able to do nothing at all........  remember that disaster movie from 2004 called "The Day After Tomorrow"?  You get the general idea.....

 The idea of the imminent new ice age has gone -- in fact it did not survive beyond about 1978, when clear signs began to emerge of the "cool decades" coming to an end -- leading, as we all know, to an inexorable and accelerating period of climate warming, linked to man-made CO2 increases in the atmosphere.  Look at the above graph, which shows the cool phase in a longer context.

Since the 1970's there has been a huge amount of research on the topic of rapid climate change, much of it centred on the Younger Dryas cooling episode.  The main question has been "Can GLOBAL climate (as distinct from regional climate or weather patterns) change as rapidly as 6 or 7 deg C in the course of a single millennium, or is there a theoretical limit to the rate of change at about one deg C per millennium?"  Evidence from ice sheet cores, sea-floor sediments and elsewhere has flooded into the public domain, some of it contradictory but generally leading to this conclusion: cold episodes (glacial phases) can end extremely rapidly, as a result of catastrophic ice sheet and glacier melting, with positive feedback mechanisms coming into play.  So a global warming of more than 5 deg C in a millennium does seem to be possible -- and is supported by field evidence.   On the other hand global cooling is a much slower process, and an equivalent cooling of 5 deg C is now thought to take at least 5,000 years.

Maybe there are more surprises out there, waiting to be uncovered, but for the moment the snowblitz theory is very much out of fashion.  There is something to be said for a high-latitude snow accumulation scenario during the growth of the Laurentide Ice Sheet in Northern Canada (proposed by Jack Ives) and during the growth of the Greenland Ice Sheet (Fristrup and others) -- but the survival of an extensive lowland cover in high latitudes is favoured by the rhythm of the seasons, with very short summers and very long winters.  In mid-latitudes, in contrast, it would be difficult to make the heat flux equations stand up to scrutiny -- and the feeling is that even during a glacial episode, except maybe with an ice sheet edge quite close, winter precipitation would almost always be approximately balanced by summer melting.

Tuesday, 18 January 2011

Uniformitarianism and Occam's Razor

 Sir James Hutton

Uniformitarianism is the principle (expounded by James Hutton, Charles Lyell and others) that present-day rocks and landscapes can be interpreted by reference to presently observable processes, and that these processes have always been in operation through space and time.  In its earliest form, the Principle of Uniformitarianism denied a role for catastrophic (high intensity and intermittent, unpredictable or erratic) events, as a reaction against the Biblical mythology of Noah's Flood, but earth scientists have long since recognized that catastrophic events (even extraterrestrial ones) DO happen, and that they are observable either on the surface of today's planet Earth or in the geological record -- so catastrophism does nothing to destroy the validity of the Principle.
    As far as the science of landscape is concerned, our principle is this:  If a past phenomenon can be understood as the result of a process now acting in time and space, do not invent or invoke an extinct or unknown or supernatural  cause as its explanation.  By the same token, if a landscape can be understood by reference to known physical processes, even if they have varied through space and time because of climate change or crustal movements, do not bring in "invented" processes which are unverifiable through observation.

Stephen Gould (1987):  “We should try to explain the past by causes now in operation without inventing extra, fancy, or unknown causes, however plausible in logic, if available processes suffice.”

This is known as the scientific principle of parsimony or Occam's Razor.  This is a good definition: "Of two equivalent theories or explanations, all other things being equal, the simpler one is to be preferred".  

"Strict uniformitarianism may often be a guarantee against pseudo-scientific phantasies and loose conjectures, but it makes one easily forget that the principle of uniformity is not a law, not a rule established after comparison of facts, but a methodological principle, preceding the observation of facts . . . It is the logical principle of parsimony of causes and of economy of scientific notions. By explaining past changes by analogy with present phenomena, a limit is set to conjecture, for there is only one way in which two things are equal, but there are an infinity of ways in which they could be supposed different." (Hooykaas 1963)

One doesn't want to get too involved in the philosophy of all this, apart from saying that in my past life as a scientist I sought always to look initially for the simplest possible natural explanations for phenomena, with working hypotheses formulated and later often falsified and modified.  (I was greatly influenced too by Popper's ideas on the role of falsification in science.)

It's because of all this that I am steadfastly unimpressed by the theories (aired by Kostas and Robert on this blog) which seek to "explain" Stonehenge by reference to sloping Neolithic sheets of ice or Mesolithic landscape inundations by water.  Not only are these theories unsupported by observations on the ground and by reference to equivalent processes operating today, but they are also entirely unnecessary and overly complicated.  According to Occam's Razor, the theories are not actually needed -- and for that reason they are best disregarded.  Of course, one understands why the theories have been put forward -- there is an insatiable desire out there in the big wide world for NEW THINGS to be written about the "mystery of Stonehenge".  I suppose I am guilty of that crime myself!!

On the principle that the simplest possible explanations of Stonehenge are the ones to go for, until they have to be replaced, I also feel strongly that Herbert Thomas and Richard Atkinson behaved in a fashion that was scientifically indefensible, and even reprehensible, by failing to give due consideration to the simplest and most logical explanation of how the bluestones got to Stonehenge -- as glacial erratics, as suggested by Judd, Jehu and others in the late 1800's and early 1900's.  They wanted fame and notoriety, so they both dumped or disregarded the Principle of Uniformitarianism and went for a wacky and unsupportable theory of human transport that had no empirical evidence to back it up it a century ago, and still has none today.

We do not NEED floods of Biblical proportions, or conveniently sloping Neolithic sheets of ice, or gangs of Neolithic hauliers, to get bluestones moved from the west of Wales to the chalk downs of Wiltshire -- the downland landscape, and the presence of bluestones on or near Salisbury Plain, can be explained quite simply and logically by reference to natural processes which are well observed and well documented.

Monday, 17 January 2011

Preseli -- bleak and barren in the Neolithic?

 Carn Goedog -- "woodland carn"

One of the assumptions that underpins the "bluestone transport" theory is that the eastern Preseli Hills in the Neolithic period looked much the same as they do today.  Indeed, a bleak and barren (treeless) landscape is almost a prerequisite for the easy collecting and downslope  transport of bluestones from the imaginary bluestone quarry at Carn Meini down into the valley of the eastern Cleddau river.  Herbert Thomas, Richard Atkinson and others were also greatly taken by the idea that the spectacular -- and even awesome -- skyline profile of the Carn Meini tors was in part responsible for the reverence accorded to these rock outcrops by Neolithic traders and engineers.  And this, in turn, may have driven the desire to collect the bluestones and cart them off to Stonehenge.

But what if Preseli was partly wooded at the time?  What if the tors, currently so clearly defined on the skyline,  were partly obscured by tall trees?

There is not a lot of pollen or other evidence to tell us what the Neolithic and Bronze Age landscape was really like -- but the educated guess among landscape historians is that 5,000 years ago there had not been much clearance of the wildwood forest, and that the slopes and summits of Preseli were quite thickly wooded.  The woodland was certainly more scrubby and patchy than that of the lowlands,  where we can reasonably describe it as jungle.  But there had not been much clearance by burning or tree felling -- there was just not sufficient population pressure in this area, unlike Salisbury Plain.

One interesting point is that "Carn Goedog" (now thought to be the main spotted dolerite source area for the Stonehenge bluestones) means, when translated, "Woodland Carn" -- and indeed there are bluebells around it -- another sign of a woodland habitat into relatively recent times.

The wide open spaces that we enjoy on Preseli today are man made -- or made by grazing animals.  The grassy moorland with acid bogs and flushes is not a "climax vegetation" pattern at all, but one which we can put down to many generations of grazing by sheep, cattle, goats and horses on the common land.

I have always said that the lowlands of Pembrokeshire (with deep valleys, impenetrable woodlands, cataracts, boggy areas and rocky river beds with many shoals) would have been virtually impossible to have negotiated with a single stone transport exercise, let alone 82 of them........ and I am increasingly convinced that the Preseli uplands would also have been hostile, rather than favourable, environments for the collection and moving of very large stones.

More on Bedd Arthur -- was it a Bronze Age barrow?

Bedd Arthur, located on the hillside not far from Carn Meini, is often cited as a sacred site which supports the proposition that there was something "special" about the Carn Meini area, thereby providing justification for the fetching and carrying of bluestones over a great distance.  As I have mentioned before, the monument at Bedd Arthur (Arthur's Grave) is an ellipse rather than a circle, and made of small stones (mostly ashes and rhyolites) found in the locality.  I have always thought of it not as a ceremonial stone setting but as the last remains of a burial mound or barrow, with one "chopped off" end that makes one think of a portal dolmen.  The stones lean inwards, suggesting that they were laid onto the flanks of a mound -- in the middle of which, presumably, there was once (if this is a Neolithic site) a small burial chamber.  If there was once a cromlech there, it may now be collapsed, and embedded beneath the turf -- or alternatively it may have been taken away by local farmers during their "quarrying" operations in the area, as they hunted for gateposts, lintels, slabs and sills.  Alternatively, if this is a Bronze Age site, there may have been a small cist grave in the centre of a low mound.

In looking at other Pembrokeshire sites with similarities, I rediscovered this one -- the Dyffryn Syfynwy stone setting not far from Henry's Moat.  This is another elliptical setting of 12 small stones, some of them fallen -- but here the stones are clearly arranged around what is left of a large barrow, which must at one time have held a burial site.  Here the consensus is that the stones were erected vertically around the edges of the barrow, and were not resting on the sloping barrow surface -- but the similarities between these two sites are in my view quite persuasive.

One of the most interesting things about the Bedd Arthur site is that the builders of the monument were not in the slightest bit interested in the preferential use of spotted dolerite -- although it was quite abundant in the immediate neighbourhood......

Sunday, 16 January 2011

Carn Meini summit stone

Noticed something in NP Figgis's book "Prehistoric Preseli" -- he says that "a recent study of human environments" suggests that the highest stone on the Carn Meini outcrop was deliberately placed there to enhance the significance of the site.  He says this proves that Neolithic people moved stones like this for a non-functional (ie ceremonial, ritual or ornamental) purpose. Hmmmm....

He also says that there is a corrugated earthfast stone nearby which looks as if it was used as a polishing stone -- for polishing tools.  He says that in 1986 another possible polishing stone was recorded at Carn Meini.

Not sure who is responsible for these ideas -- maybe the Darvill / Wainwright SPACES project?  They all sound very dodgy to me.  For a start, there are six or seven outcrops in the Carn Meini area, all with rocky summits with a jumble of stones on top of them  I know these summits well -- and don't know of any stones that look "unnatural" in their placing.  As for corrugated stones, corrugated surfaces are everywhere -- this is a natural consequence of the weathering of the dolerites, as internal "banding" or foliations are picked out by weathering processes.  Polished surfaces?  Haven't seen anything up there which looks remotely like a man-made polished surface. 

Sorry chaps -- I put all of these "phenomena" down to wishful thinking -- all part of the process of seeking to demonstrate that Carn Meini was special in some way, and therefore worthy of reverence as the source of those famous magical bluestones.

Friday, 14 January 2011

Inside the Neolithic mind

I was looking recently at Rodney Castleden's very readable and entertaining -- and wonderfully fanciful -- tome called "The Making of Stonehenge" -- and marvelling at his attempts to see into the minds of the builders of Stonehenge.  In one place in the book he suggests that the bluestones were stolen from West Wales as a deliberate act of aggression, designed by the powerful tribes of Wessex as a snub (or a demonstration of political power and technical expertise) to the lesser tribes of West Wales and Ireland.  Elsewhere he suggests that the stones might have been taken as a diplomatic gift from these poorer tribes to the ruling elite on Salisbury Plain.  Then again, he thinks that the stones might have been part of a process (over quite a long period of time) of diplomatic exchanges of "prestige" gifts.  Herbert Thomas and Richard Atkinson speculated on similar line many years ago -- indeed they both found quite attractive the idea that there was once a bluestone circle on the slopes of Preseli, made of locally derived stone monoliths,  which was for some reason dismantled carefully and shipped off across the Bristol Channel regardless of the physical challenges that there might have been.  More recently, MPP, MP, TD and GW have all speculated on these lines, referring occasionally to "tribute stones" or totems brought eastwards from West Wales as part of some ceremony involving ancestor worship.  I have blogged about this before.  I almost feel a novel coming on..........

To come down to earth with a bump, one needs to take a look at Steve Burrow's book called "The Tomb Builders" which concentrates on Neolithic Wales between 6000BP and 5,000BP.  This is exactly the period during which the bluestones at Stonehenge must have been shipped, if you are into that style of thinking.  In Chapter 4, on the availability of materials for the erection of megaliths and the building of tombs, he says: "There are no instances in Wales where it can be demonstrated that megaliths were carried great distances to build a tomb at a specific location."  Again:  "..... construction involving megaliths was usually carried out near the source of raw materials."  Again: "... tombs are located within easy reach of a supply of boulders or quarry stone that could be used to form their large covered cairns." Then finally, having examined the evidence from all of the megalithic monuments in wales, he concludes: "..... the evidence suggests that they (the tomb builders) were keen to minimise the amount of effort their enterprises involved without compromising the ambition of tomb building.  Fortunately, in a land as rich in stone as Wales, this still allowed them many opportunities to site tombs at locations that would add both drama and significance to their enterprises."

So our Neolithic ancestors were pragmatic, opportunistic, focussed on economy of effort, and adaptable as far as building materials were involved.  They were also scavengers in the sense that they collected and used stones wherever it was most convenient to do so.  Stone availability was a prime consideration in determining where tombs were built and where they were not.  So much for ley lines, astronomical observations, earth energies and sacred geometry.....

The strange thing is that Steve, having talked so much good sense about Neolithic megaliths, cannot find the courage to part company with his peers in the archaeology establishment.  He talks of the supposed transportation of the Preseli bluestones from Wales to Wiltshire for the building of Stonehenge as "the remarkable exception."  Ah, the conservatism of the academic mind, and the importance of peer group pressure!  I wonder what the archaeology establishment would have done to Steve if he had had the courage of his convictions and concluded:

"All of the evidence from Neolithic Wales suggests that the tribal groups of the time had neither the technical resources nor the desire to carry large numbers of stones from Pembrokeshire to Wiltshire.  If there are lessons to be learned from Wales, we have to conclude that the builders of Stonehenge used whatever large stones they could find in the immediate vicinity of the monument."

Wednesday, 12 January 2011

Hauling bluestones through the jungle

This map, from Bell and Walker 1992, is a useful reminder of the physical conditions that prevailed around the time that the bluestone settings were being worked on at Stonehenge.  It shows the approximate dates at which the dense woodland was cleared by Neolithic and later tribal groups.  According to the conventional wisdom (which I have always thought very unwise) the bluestones "arrived at Stonehenge" around 4,600 yrs BP.  There is good evidence that around that time there was a wide area of open grazed grassland on the downs of Salisbury Plain -- and that although many of the river valleys were still thickly wooded, there were also areas of quite well cleared land to the west (in Somerset) as well.

However, Wales was a different matter.  The landscape was covered with thick deciduous woodland in which oak and hazel predominated.  Clearance was very patchy indeed, and many observers have noted that Neolithic settlement sites were concentrated on hills and ridges where land clearance was easier and where grazing of animals presented fewer difficulties.

The "bluestone expeditions" so beloved of the archaeologists would have been exceedingly difficult in the circumstances, with huge obstacles presented by dense trackless woodlands, boggy areas, steep slopes and rushing rivers in deep valleys, with many cataracts and areas of exposed rock outcrops and boulder fields.  Not to mention muddy tidal estuaries and storm-lashed cliffed coastlines!

Common sense tells us that these stone collecting expeditions are nothing but fanciful modern creations.  If our Neolithic ancestors had any sense at all, they would have collected up all the stones they needed from as close to Salisbury Plain as possible.  Which of course is precisely what they did..........  and when they ran out of stones, the building project was abandoned.

Tuesday, 11 January 2011

More on Devensian climate oscillations

 Click to enlarge the graphs.
In trying to work out what might have happened in SW England during the Devensian glaciation, here is a reminder of some of the data in the influential paper by Dr Alun Hubbard and colleagues published in 2009.  The graphs above might look complicated, but they show how the climate oscillated between about 40,000 years ago and 10,000 years ago.  In the middle part of the Devensian it is now thought that the climate cooled in Great Britain sufficiently for ice to form in the uplands, and for several pulses of glacier expansion to occur prior to 27,000 BP.  But in this period there were also 6 interstadials or warmer episodes with greatly increased melting followed by glacier retreat.  The British - Irish ice sheet almost melted away around 27,000 BP, and then (because there was a relatively prolonged period of increased accumulation without any major interruptions) the ice sheet grew and grew until it reached its maximum volume and extent around 20,000 BP.  There was a phenomenal rate of growth over 7,000 years, followed by a catastrophic collapse.  The authors refer, over the whole lifetime of the ice sheet, to "binge-purge cycles" of gradual accumulations followed by rapid ice evacuations.
Beyond the ice edge, for example in periglacial SW Wales or SW England, what we see in the period 40,000 BP to 27,000 BP is a series of relatively short-lived oscillations between hard permafrost and boreal conditions, with some of these episodes lasting no more than 500 years. Then, from 27,000 to 20,000 BP the climate was continuously cold -- and I surmise that conditions must have been just right for small ice cap formation over Exmoor, Dartmoor and other uplands, and for periglacial processes to continue to operate unabated over the lower-lying areas including Salisbury Plain.
The challenge is to see whether this model actually fits with the sediment accumulations and characteristics of coastal and inland sites in Cornwall, Devon, Somerset and Wiltshire.
Quaternary Science Reviews
Volume 28, Issues 7-8, April 2009, Pages 758-776
Quaternary Glaciodynamics
Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet

Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper and Martyn Stoker

The Sensitivity of Little Ice Caps

In trying to learn some lessons about how sensitive small ice caps may be on undulating plateau surfaces, we are lucky to have such good data from NW Iceland.  Historical records and maps go back a long way, and on the above map I have marked four different maps of ice edges, the earliest of which is from Knopf in 1734.  That was between two of the coldest phases of the Little Ice Age.  Note that between 1734 and 1844 the ice caps of Drangajokull and Glamajokull both expanded.  They probably reached their greatest Little Ice Age extent around 1850 -- at a time of great hardship (and glacier advances) across Europe.  But 50 years later there had been a catastrophic glacier retreat, which continued through to 1913-14.  By that time Glamajokull had all but disappeared.  A century later, there is still nothing there but some permanent snow-patches.

Drangajokull has survived.  Why?  That's an interesting question, since the two plateaux of Glama and Dranga are at more or less the same altitude -- with extensive areas over 700m and summits at around c 920m.  But Icelandic glaciologists have pointed out that the "glaciation limit" -- which roughly equates to the firn line or ELA (equilibrium line altitude) -- is at around 1000m on Glama but at around 800m on Dranga.  So on Glama, with a slightly warmer and more oceanic climate, ablation exceeds accumulation, and glaciers can no longer survive.  On Dranga, in contrast, there is (or was until recently) a rough state of balance.

There are some interesting lessons here if we want to understand what happened on the hill masses of SW England around the peak of the Devensian and other glaciations.

Monday, 10 January 2011

The Devensian climate in the South-West

About 50 years ago (OK -- that really dates me!) I concluded that the stratigraphy of the deposits around the coasts of SW Wales and SW England could best be interpreted as showing a big early glaciation -- with the Irish Sea Glacier flowing across Pembs and up the Bristol Channel and onto the coasts of Somerset, Devon and Cornwall; an interglacial period in which sea-level rose a little above that of today; and finally a rather complex period of cold climate culminating in a short-lived glaciation.

Since 1965 (when I finished my doctorate) nothing has appeared in the literature to make me change my mind.  In the stratigraphic sections above, look at the items numbered (4) -- these are quite complex and thick layers of solifluxion deposits, indicative of prolonged cold-climate conditions.  There are at least 4 different facies, and the beds contain at least one weathering horizon which seems to suggest a warmer interval.  We find similar deposits on the coasts of SW England too.

Click to enlarge the diagrams.

If one tries to fit these deposits (4) into a known framework of climate change during the Weichselian / Devensian, we can refer to this period as Early and Middle Devensian.  It lasted for about 45,000 years, and there were some periods of boreal climate and others of tundra / permafrost climate, when the ground would have been deeply frozen and when solifluxion processes would have been very important during the summer thaw season.  Since solifluxion layers do not simply accumulate layer on layer ad infinitum, but sometimes "flush out" older deposits and rework material already present on a solifluxion slope, matching up these thick pseudo-bedded layers to specific climate phases is difficult......

But at last, in the Late Devensian, the ice of the Irish Sea Glacier arrived, dumping glacial deposits (made from recycled sea floor deposits, for the most part) on top of the accumulated solifluxion layers -- but generally not flowing with such strength that these old layers were everywhere removed.  In many embayments (such as Abermawr, West Dale, Druidston, Poppit and many others) what seems to be almost a full sequence is preserved -- and we can also see what happened when the glacier ice melted out, leaving a cap of flowtills and thick deposits of sands and gravels behind.

Then there was another short-lived and very cold periglacial phase which led to the creation of frost-heave features, fossil ice wedges etc.  It's difficult to decide whether this was the Older Dryas phase, or the Younger Dryas, or a combination of both.

So that's the story -- and I think that the stratigraphy of coastal and inland deposits backs it up.

Did Devensian ice reach Lundy?

This map, by Prof Frank Mitchell, published in 1968, shows pretty conclusively that the ice of the Irish Sea Glacier came in from the west and overrode the lowest part of the island, up to a limit of about 100m above sea level.  The dotted area shows "glacial gravels" with abundant erratics, and the black blobs show tors that Mitchell considered to have been overridden and moulded by ice as it crossed the island.  He thought that areas above c 100m remained ice-free at this time, since there are no glacial deposits on the southern part of the island, and the tors have not been destroyed.  So he thought that the southern part stood up above the ice as a nunatak, as ice flow streamed round the upstanding upland (remember that the sea was not there at the time) and continued eastwards.

Interesting info.  There are two great questions:  Was this glaciation Devensian, or earlier?  And if the ice surface altitude was c 100 m in the vicinity of Lundy,  did it have the energy to reach the adjacent coast, on the other side of Barnstaple Bay, about 20 km away?

Sunday, 9 January 2011

Those Little Ice Caps

What did the little ice caps of Dartmoor, Exmoor and the other uplands of SW England look like?  Well, when they were reasonably healthy, they probably looked like Drangajokull, a small ice cap in Vestrirdir, NW Iceland -- shown in the lower of the two photos above.  The current ice cap is about 20 km long and 6-7 km wide , and it was at one time much larger.  Look at the lower right quadrant of the photo -- the barren area with lots of lakes has clearly been deglaciated during the last 100 years or so.

And how long did they survive for? Little ice caps like this can be created, and can then disappear, very quickly indeed.  It would be quite normal for ice caps like this to develop over a period of 500 years or so, and then to disappear again in maybe 200 years.  Because they sit on upland plateaux, a cooling of climate, accompanied by a fall in the firn line or equilibrium line to a level below the edge of the plateau can trigger rapid accumulation and growth, with very little loss of mass through melting.  On the other hand, if the firn line rises maybe 50m because of climate warming, suddenly the whole of the ice cap becomes vulnerable, and there can be catastrohic melting.

The Glama Plateau (top photo) is a case in point.  It's not very far from Drangajokull, but it is very vulnerable.  In the Middle Ages there was no ice cap here.  Then came the Little Ice Age, and a plateau ice cap started to develop in the 1500s.  By the time of the coldest part of the Little Ice Age (c 1750 - 1850) it may well have looked like Drangajokull.  Then the little Ice Age came to an end, and catastrophic melting occurred.  By 1900 it was mostly gone, and now there is just an assortment of snow-patches, with a rocky barren terrain and lots of lakes.

So what about the Exmoor and Dartmoor ice caps?  Because they were effectively beyond the southern limit of ice sheet glaciation, they will have experienced glacial conditions only for a very short period of time. In western Scotland, at the centre of the British - Irish ice sheet, it's reasonable to assume that ice covered the landscape in the Devensian for more than 20,000 years; but the ice that reached the coasts of Devon and Cornwall may only have been there for around 1,000 years or less.  I would guess that the little ice caps of the South-West were in existence for 2,000 years at the most, around 21,000 - 19,000 BP.  They just sat there, as Glamajokull did in the Little Ice Age, and then melted away.  The effects on the landscape were minimal and even protective, since periglacial processes under the ice would have been temporarily stopped.  Maybe, beyond the ice edges, there was a phase of very intense cryoturbation and ice wedge formation.  And maybe, when the ice melted, there would have been another phase of  periglacial slope development, with a good deal of summer melt and even resumed water flow in the river valleys.

Saturday, 8 January 2011

Anglian Glaciation Map

Here is my attempt to portray the Anglian Glaciation ice limit in Southern Britain, with the local ice caps incorporated.  If you had looked down from a satellite at the time, you would probably not have been able to pick out the edge of the Irish Sea ice on or near the Bristol Channel coasts of Devon, Cornwall and Somerset.  A snow-covered landscape would have extended almost as far south as the South Coast.  The dotted areas on the map represent thin cold-based ice on the uplands of the Cotswolds (C), Mendips (M), Exmoor (E), Blackdown Hills (B), Dartmoor (D) and Bodmin Moor (B).  Between these ice-covered areas there may have been some areas free of ice and firn -- but these areas, like the area beyond the ice edge, will have been affected by long seasonal snow-cover and intense periglacial activity.

This is a modification of a map which I posted some months back.

A Glacial Map of Southern England

Here's my latest attempt to summarise all the ground data (stratigraphy, geomorphology, geology, sediment studies, pollen analyses, radiocarbon dates etc) and modelling which has appeared in the literature in recent years.  There will inevitably be tweaks to the ice edge positions shown.  Click on the map to enlarge it.

I have taken the Anglian Glaciation (450,000 yrs BP?) to be the Greatest British Glaciation (GBG) because that is where most of the dating information seems to point.  At that time there was a powerful component of ice coming down from the North Sea and also a very powerful Irish Sea Glacier which pushed up the Bristol Channel and flowed around the edges of the Mendips as far east as Bath, Glastonbury, Street and maybe Nunney -- so the Somerset Levels and much of the lowland of Somerset was inundated.  That would have been the ice stream that carried the Stonehenge bluestones.  Whether the erratics were carried all the way to Stonehenge, or were left some distance to the west, is still an open question......  Beyond the ice edge -- and blending into it -- there would have been several small, thin local ice caps (cold based or frozen to their beds) that played a more or less protective role.  On the map I have just shown thin ice on the western uplands -- maybe the North and South Downs and other areas further east were also affected.)

The isostatic loading of Southern England at this time would have been sufficient to depress the land surface -- and the Channel coasts -- to the level at which ice rafting of giant erratics onto a shoreline at more or less its present position would have been possible.  (This is an issue that has been largely avoided by most geomorphologists -- but I have devoted a few posts to it.)  All of Southern England beyond the ice edge will have been affected at this time by intense permafrost conditions -- and in many areas there will have been a more or less permanent snow-cover, melting out for maybe just a month or so each summer.  River systems will have operated intermittently and seasonally, but fluvial erosion might have been considerable during warmer episodes, with much river terrace formation.

During the Devensian, the British Ice Sheet was more lop-sided, with much less ice flowing in from the Scandinavian Ice Sheet and the North Sea.  The Irish Sea Glacier affected the northern and western coasts of Pembrokeshire and pushed some way up the Bristol Channel, and it may just have reached the Bristol Channel coasts of Devon and Cornwall.  My guess is that the Somerset coast was NOT affected by the Irish Sea Glacier at this time.  The glacier reached the Scilly Isles and also extended further out into the south-western approaches -- maybe forced by a very powerful ice stream from Southern Ireland.  Again there would have been small, thin cold-based ice caps over the uplands of  Somerset, Devon and Cornwall, and extensive permafrost across the whole of Southern England.

My instinct is that during the Devensian there was not sufficient isostatic loading in Southern England for the land surface to be depressed to -135m; the result was that any ice rafting would have dumped Devensian giant erratics at a level well below present RSL -- so they are offshore and out of sight.  (We know they are there on sandbanks and shoals, since there are records of them in the literature.....)

So that's my latest hypothesis -- all comments gratefully received.

PS.  Re the erratic boulders in the Channel, see my post of 8 June 2010.  It included this extract:

Boulders, Salcombe Fishing Grounds, English Channel
Hunt (1880, 1881, 1883, 1885) found a considerable number of foreign blocks in the Salcombe fishing grounds, some 30 to 50 km south of the Devon coast. Of 40 blocks described, there is granite, microgranulite, serpentine, syenite, gabbro, diorite, basalt, "diabase" (dolerite), trachyte, gneiss, quartz grit, conglomerate, sandstone and chalk flints and other rock types. They are discussed further by Prestwich (1892). The serpentine is precisely like the Cornish varieties. Surprisingly the other igneous rocks could not with certainty be ascribed to the English or French coasts. The gneiss resembled Hebridean gneiss from Scotland.

Friday, 7 January 2011

The Glaciation of Devon and Cornwall

There has been an interesting debate on this topic over the years, with some support in the 1940's and 1950's for glacial activity on the higher parts of the SW Peninsula, then a period in which glaciation went out of fashion, with most Pleistocene landscape change put down to periglacial and fluvial processes, and then within the last few years a growing recognition of the role played in many parts of the world by thin, cold-based glaciers in protecting the landscape at times when there may have been intense glacial streaming and erosion in the lowlands and in glaciated troughs.  Interestingly enough, the authors of the Geological Conservation Review for SW England (1998) dismissed the idea of glaciation in the SW out of hand, while getting quite tangled up in the debates about the giant erratics on the coast, and the glacial deposits found in pockets along the Bristol Channel coasts of Devon, Somerset and Cornwall.  Not entirely logical, you might say -- and I might agree.

Stephan Harrison, in the book called "The Glaciations of Wales and Adjacent Areas" (2005) takes a much more nuanced approach, and argues for intermittent small, thin ice caps over Dartmoor, Exmoor and other uplands -- and he cites a good deal of evidence which is difficult to interpret except by reference to glacier ice.  More recently the modelling exercises undertaken as part of the BRITICE project has also led the research team to the view that there WERE thin ice caps over the uplands of Dartmoor, Exmoor, Preseli and the Yorkshire Moors at the peak of the Devensian Glaciation, around 20,000 years ago.

If you look at the maps above you will see that there is --  and was -- considerable high ground across the SW Peninsula, capable of supporting extensive caps of snow and firn for long periods, and even true glacier ice during the coldest and snowiest parts of the Devensian.  For the most part, the ice will have been largely immobile and frozen to its bed, but there may have been some movement (and the creation of till and even morainic features) in the deeper valleys on E- and NE-facing slopes.

I postulate that c 20,000 years there were thin ice caps on all of the "high plateax" and "high hills" areas shown on the top map.  Lower plateax and hills might well have had a more or less continuous cover of snow and firn patches, which occasionally melted out during warmer episodes.  In the lowlands and valleys there would have been less extensive snow cover, but still permafrost -- so periglacial processes would have been active.

Wherever the edge of the Devensian Irish Sea Glacier might have been (somewhere in the Bristol Channel,  maybe quite close to the coastline) the intensity of glaciation in the SW Peninsula might not have been great enough to depress the land surface sufficiently for iceberg transport of erratics in the manner seen during earlier glacial episodes.

Wednesday, 5 January 2011

Last Glaciation Protective Snow or Ice Cover in Southern England

There is a lot in the literature about the manner in which certain areas beyond the edges of active glaciers or ice sheets can support thin, cold-based and largely static ice which does very little to the landscape apart from providing a "protective blanket" over a landscape previously affected by permafrost.

I came across this example from Scoresby Land - Jameson land in East Greenland, where there are a number of "landform" and depositional puzzles which have caused confusion among geomorphologists.  Lena Hakansson has tried to date erratic boulders and rock surfaces (using cosmogenic dating methods) and found that the distribution of ages could only be adequately explained by postulating intermittent exposure to radiation, with snow or ice cover shutting off this exposure at certain key times -- in particular coinciding with the peak of the last glacial episode.

So she has postulated the above scenario, with active glacier ice streaming along the broad fjord of Scoresbysund (right hand edge of the diagram) and with a more or less stagnant cover of snow and ice blanketing the landscape to the east.  This blanket, with ice up to 200m thick in places, is shown by the light blue area in the cross-profile, with frozen ground beneath (dark blue).  The interesting thing is that this blanket has capped or sealed older glacial and periglacial deposits, as well as scattered erratics, effectively preserving them from further erosion or disturbance for the duration of the snow / ice blanket.

This sort of scenario seems to fit southern England pretty well, for a period of 5,000 years or more near the peak of the Devensian glaciation.  I still think it's quite possible that the uplands of Dartmoor, Bodmin Moor and Exmoor might have supported thin covers of firn or glacier ice which coalesced with the Irish Sea Ice coming onto the coasts of Somerset, Devon and Cornwall from the north and west -- but further south and east what did the landscape actually look like?  At times, parts of the ground surface must have been snow free, allowing the formation of periglacial features and permitting rapid solifluxion on steep slopes, but  there must also have been episodes when the WHOLE landscape must have been snow-covered even during the summer months -- but with inadequate precipitation for the development of glaciers.  Snow and firn maybe -- but probably no glacier ice.

When I was in Antarctica I worked on a snow-covered (and ice free) peninsula called Byers Peninsula, on Livingston Island.  Not far away was the edge of the ice cap.  Only at the height of the summer season (for maybe a week or two) does the snow cover melt back sufficiently to see what's underneath it.  When we were there, early in the summer season, we plodded about in the snow and missed all sorts of details (and quite a complex landscape) which were somewhere down there, beneath our feet.......