Thursday, May 10, 2012

Why is Salisbury Plain so steppe-like? A prelude to another look at the Stonehenge mystery

Hello. Sciencebod is back again, after a break of some 4 months (I've been researching the Turin Shroud - see previous posting and link to my specialist site).

My new interest is Stonehenge - like... what was it for, and how did the bluestones get there, all the way from the Preseli hills in west Wales - reckoned to be 135 miles or so as the crow flies?

UPDATE: please try to read my latest thoughts on Stonehenge and Silbury Hill but you will need a strong stomach...

Non-colour coded

 Spot those famous  'bluestones' (colour-coded)

I've been getting acquainted with some of the facts and rival theories on Dr. Brian John's splendid and friendly site (and a paperback copy of his book is due to arrive in a day or two).

Stonehenge, just west of Amesbury, is situated a short way south of the approximate centre of Salisbury Plain, the latter being described as one of the largest tracts of chalk grassland in western Europe.

Straightaway that seems an odd description to this science bod. I thought the climax vegetation in Britain was supposed to be forest. What's holding Salisbury Plain back, so to speak? Or did it used to be forest, and if so, been unable to regenerate?  If the latter, then why not, apart from all those MOD tank tracks?  OK, so this may seem to be peripheral to the enduring mystery of Stonehenge, where I am still developing ideas, some of which agree with Brian John's, some of which do not (at least for now, but we may converge once I have read his book).

For starters, let's take a look at satellite pictures of Salisbury Plain, courtesy of Google Maps. I have started with a picture that includes most if not all the Plain, and then homed in progressively on the bare region in the centre. The fifth and last picture in the series looks for all the world like a green oasis-like area, but I shall resist the temptation to call Salisbury Plain a desert - not most people's idea of SW England. But I do believe the description 'steppe-like' is fitting and will be exploring this curious geology and/or history of previous land use/misuse in future posts. (Relax, MOD, it is not you I have in my sights, but yours and my ancestors, going back thousands of years).

Picture 1 (click to enlarge) : Note the bare butterfly-shaped area, most of which is left of centre - Salisbury Plain

Now zooming in (labels  and more pictures later)...

Picture 2 (above):

Picture 3 (above):

Picture 4 (above):

Picture 5 (above):

Even without Stonehenge, Salisbury Plain would be weird. Is its un-English- looking steppe-like weirdness (not always apparent if passing by on the A303) part of the reason why Stonehenge came to be where its is?  More to come...

Late addition: 11 May 2012. It has been suggested on Brian John's site, indeed by the blogger no less, that it is the military activity on the Plain that prevents regeneration.  Here's a photograph from Google maps that suggests otherwise.

 Notice the way that the tracks avoid the trees, but that the trees themselves look stunted. Those trees are not happy trees. Now why would that be I wonder. Prepare for another "out-of-the-box" hypothesis soon, one that adds a third possibility over and above bluestones being transported by human or glacial means.

Here's a clue: note Stonehenge. Note nearby grove of trees, happily growing in an otherwise unpromising chalky field with no real soil to speak of...

More to come...

And finally, missus and I visited Stonehenge and a bit of the Plain today. I'll write a short post later. In the meantime, it was good to see Brian John's book on display in the visitor centre.

My own copy was waiting for me on the doormat when we got back. I now have some serious reading to do (and may have to retract some or all of my  "out-of-the-box" ideas" re the local geology  - we shall see...)

Update: Sunday May 13 : here's the way my thinking ("hypothesizing") is going.

Look at the labelling in red that I have added to a standard textbook diagram. (Ignore the rest of the diagram re laccoliths etc at least for now).  I'm not suggesting, of course, that any of the Stonehenge monoliths are dikes that sprouted at that precise spot - but there may have been dikes within, say, a 10 mile radius that provided some or all of those 'bluestones' - about as heterogeneous a bunch of dolerites, rhyolites etc as one can imagine, that just happen to match some that are found  130 or so miles away in the Preseli hills. As the diagram makes clear - dikes/dykes (for once I prefer the US spelling) can in principle come up close to the surface (and become exposed with erosion) pretty well anywhere in principle, depending on the size of the underlying pluton and/or batholith.

Why do I think there is a sill under Salisbury Plain? That was not an ab initio hypothesis - quite the opposite in fact. It is/was an attempt to understand the unusual characteristics of the Plain where English landscape is concerned. (The Plain is described in wiki as one of the largest areas of 'chalk grassland' in Europe, aka calcareous grassland). My starting point was that there is something unusual about the drainage characteristics that make the Plain a difficult habitat in which deep-rooted trees can get established. An impermeant sill, maybe at a slant that allows quick run-off of percolated rain water, might provide an answer. More later.

Further afterthought: there's a well-known phenomenon in probability theory - namely that if you have 20 or so people in the same room, the chances of at least two people sharing the same birthday is surprisingly high  (said to be 50% (p =0.5) for 23 people in the same room ). I cannot help but wonder if the same effect is not operating with respect to the so-called cause-and-effect correspondence between the dolerites of Stonehenge and the Preseli Hills of west Wales .If there's a sizeable heterogeneity ("range of types")  within the bluestones at Stonehenge, and a sizeable heterogeneity of the bluestones at Preseli, then there is an enhanced probability of finding at least one close match. But that should not be an occasion for premature celebration unless one has looked at the heterogeneity of bluestones everywhere within a 200 mile radius of Stonehenge...

Update: Monday 14 May:

Here's a picture of Preseli rocks in situ erupting from the ground as tors (showing their origin as intrusive igneous rock that has become exposed by erosion):

Preseli  - Carn Menyn - presumed source of the Stonehenge bluestones

See the interesting objection made to the supposed Preseli/Stonehenge connection made by a commentator on the 'FreeRepublic' site from which I obtained the above picture (merely searching Google image files for ...  preseli bluestone... And it's geological, to do with quartz intrusions and fracture planes....

To save you, dear reader, the trouble of locating the particular comment, here's the key section from a kindred spirit:

..I also observed that where columns or pillars ARE present in the bedrock outcrops, when they are released and fall onto the scree they almost inevitably break across, because of transverse fractures or other weaknesses related to quartz veins etc.
So away with that particular piece of nonsense. I am quite convinced that there would have been NO reason for Neolithic tribesmen to "target" Carn Meini as an ideal quarry site. There was nothing special about the spotted dolerites (they outcrop over a very wide area), or about the stone shapes, or about the ease of access or stone extraction (other sites would have been easier).
For the best part of a century archaeologists have been indulging in special pleading with respect to Carn Meini. Once HH Thomas announced that that was where the spotted dolerites came from, one generation after another has sought to find justifications or reasons for the "choice" of the site. It's all there, in the literature......
To which yours truly would add - those pillar-like blocks from the same or related pluton or batholith could have erupted elsewhere, and have had a similar chemical composition petrology if the conditions at the other place of intrusion and final exposure had been similar. The crucial factor with an intrusive rock that forms when magma crystallised before reaching the surface is the rate of cooling. The slower the cooling, the bigger the crystals of feldspar etc. Who's to say that  dike A from Pluton1 did not crystallize at approximately the same rate as dike B from the same pluton, with 130 or so miles separating the two dikes?

Here's another picture I came across (the High Force waterfall, River Tees) which refers to dolerite sitting on top of limestone (the latter a close relative of chalk needless to say, both being calcium carbonate, CaCO3, albeit of differing chemical and biological source material):

The caption in the source refers to the dolerite being visible as darker rock (though it all looks much the same to me except for that single rectangle immediately above the top of the fall). Irrespective,  there ARE precedents for English dolerite making it through beds of calcium carbonate.... Maybe there are some closer to Wiltshire than NE England...

And finally, here's a geological map of England and Wales:

The red star shows the approximate position of Stonehenge, situated on Cretaceous chalk. But what lies beneath that chalk, shown in green (apart from magma if you go deep enough?)

1 comment:

sciencebod said...

Have just sent this - the latest in a long series of comments - posted to Brian John's site.

"Have just added a diagram to the end of my Salisbury Plain posting, Brian (standard geology textbook stuff ;-), showing where I think
there may be a sill and even an overlooked dyke or two
This is a tentative toe in the water, needless to say, but is an attempt to link the location of those bluestones to the local landscape (Salisbury Plain) that seems to have exceptionally sharp drainage.