Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Thursday, 15 April 2010

Letter to Ned -- the glaciers are still winning

Dear Ned,

OK -- let's look at your "Neolithic statistics". Yes -- glaciers can and do pick up material of all shapes and sizes. But your first graph is faulty -- there is not a smooth linear relationship between stone size and frequency. Your Fig 1 is sheer fantasy. In some areas of glacier deposition there may be large quantities of very large boulders and virtually no till or finer matrix. Even in till sheets there may be bimodal peaks on the graphs of particle size. Then you say: "But when the ice sheet finally melts it dumps everything, from grain to boulder, in the same place." That is sheer fantasy too -- glaciers do not do that. They entrain material in ways that are highly variable, depending on basal ice conditions, ice velocity, and the nature of the underlying terrain and lithology. Sometimes the basal ice will hardly move at all, and may be frozen onto the bedrock -- and ice above it will move along shear planes or through internal deformation, carrying entrained material with it.

Next point: " this jumble of rubble will tend fit tidily in one thing though – the statistical bell curve of size distribution by weight." Sorry -- but that is sheer nonsense.

Then: "So the ice sheet will be picking up thousands of tiny pebbles at the same time as it picks up one large boulder." Sorry -- wrong again.

Then: "On the other hand those millions of tiny pebbles, when you weigh them, don’t weigh that much." Oh really? Ned, have you tried to weigh a sheet of lodgement till maybe 10m thick and extending over maybe 100 km2? Come off it! But Fig 2 shows that big stones tend to be heavier than small stones -- statement of the obvious -- are we supposed to be surprised by that?

So to your bell curve (Fig 3) which is supposed to hold the key to this mystery. Is it supposed to show us that the total weight of small stones in a glacial deposit is relatively small, whereas the total weight of medium-sized stones is rather large, and whereas the total weight of very large boulders or monoliths (like the bluestones at Stonehenge) is much smaller again. You seem to think this is an ideal "ice sheet dump distribution". Sorry again, Ned -- but that is fantasy.

Then to the title "Ice sheets lose" -- I would agree that what we have on Salisbury Plain is a graph that would show a series of spikes, for example associated with the bluestone monoliths at Stonehenge, or the massive numbers of bluestone fragments in the "Stonehenge layer". But to suggest that there "should be absolutely thousands of smaller bluestones lying around" is absurd -- as I have explained both in my book and on this blog.

Quote: "Alternatively, if the bluestones were the “average” stone size (B on the bell curve), then there should be much larger stones lying around. There aren’t. Perhaps the Stonehenge builders chopped these larger stones into bits. Maybe. Whatever, even in this case you’d still find a large number of pebbles lying around. You don’t." This all falls down on the faulty assumption which underpins this whole speculative piece. I doubt that you will find a particle size distribution for an area of glacial deposits anywhere in the world that would remotely fit your hypothetical bell curve.

Quote: "Notably, the Neolithic long barrows of Salisbury Plain, which are older than Stonehenge, do not include one single one of these theoretical large bluestone boulders in their construction." Wrong -- Boles Barrow did contain a bluestone boulder, and for all we know there may be other bluestone boulders in other long barrows as yet unexcavated.

Your heading: "People win… at least partly." I beg to differ -- the glaciers are still winning.

Contrary to what you seem to think, Ned, you have not demolished the idea of glacial transport and glacial deposition, since your hypothesis is based on faulty mathematical modelling. Please go off and read some of the abundant text books on glacial deposition and the nature of glacial sediments, and then maybe have a fresh look at the problem.


Edward Pegler said...

Dear Brian

Thanks for the comments. I've briefly replied on my blog and I've also ammended one line to clarify what I said, which was a bit unclear at one point. I'll get back to you about the rest.

By the way, I'd love to meet up sometime to talk about this. Sadly, I was in Newport recently but I didn't know where to find you (though I did buy your book)


Brian said...

Thanks Ned -- by all means let's discuss this. It is a very interesting way of looking at the "problem" of the bluestones. I'll have a look at your blog again -- let's hope we can move along to a greater understanding of what REALLY happened....