Mike Pitts has got it wrong

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

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

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

17th August 2014

Dear Editor,

Bluestones:  the glacial transport theory is still alive and well

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

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

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

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

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

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

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

Sincerely,

Dr Brian John