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Thursday, 1 April 2010

Gravels, pebbles and Bluestones


Near the confluence of the Nadder and Avon -- the river terraces in this area were examined by Chris Green

One of the main reasons for scepticism about glacier ice reaching as far east as Salisbury Plain is the scepticism of geomorphologists Jim Scourse and Chris Green. Both of them are utterly convinced that the ice of the Irish Sea Glacier never got this far -- and they expound their reasons in the big "Science and Stonehenge" book published in 1997. Jim uses a glaciological argument (which I will examine on another occasion) and Chris uses the evidence of pebbles in terrace gravels to demonstrate that there is no "foreign" material in the river catchments of the chalk downland.

Now I have great respect for Chris (he and I were contemporaries in Oxford) but I do wonder how sound his evidence actually is. I have looked again at the "Nature" article which he published in 1973 which purported to show a "complete lack of glacially derived material in the Pleistocene river gravels of the Wylye, Nadder and Avon" -- and I wonder how he can come to that conclusion. He concludes that there is less than 1% of far-travelled material in the area around the confluences of the three rivers, and downstream from there in the Avon valley. He claims to have analysed 50,000 pebbles, but leaving aside the difficulty of actually spotting pebbles of far-travelled sandstone, dolerite or rhyolite in around 30 washed samples of terrace gravels, he seems to have used vein quartz as the key indicator of "foreign rock presence." Now that in my estimation is a rather dubious thing to do -- and I assume that vein quartz was used because it is white, and is easy to spot quickly. He seems to assume that if there are no vein quartz pebbles present, there are no other foreign rocks either. He also refers to small numbers of pebbles of quartzites, grits and cherts, and says that these could have all come from "nearby Tertiary formations." That is true, but they COULD also have come from much further away, to the west! Those rocks occur in very low percentages (generally far less than 1%) and from many samples they are entirely absent.

Chris's work leaves us with many questions, for example relating to his ability to spot far-travelled pebbles, to the use of vein quartz as a key indicator, and to the clustering of the sampled terrace sites around Salisbury and downstream from there in the Avon Valley. We also need to highlight the fact that far-travelled materials are now popping up all over the place on Salisbury Plain -- as indicated in Chapter 6 of my book. The number of sources for these pebbles and flakes must now be approaching 30 -- and while some may be fragments of broken or shaped bluestone monoliths, we should remember that Atkinson and others have referred to the fact that most of those at Stonehenge are "undressed" or in their original states. What percentage of the total stone population on the interfluves is "foreign" and from the west? Is it higher, or lower, than the percentage of foreign stones in the river terraces? We won't know the answer to those questions until a lot more work is done.

Chris does NOT address the problem of the foreign stones which have been found, ever since excavations began, in the neolithic and Bronze Age sites dotted about on Salisbury Plain. Nor does he examine the pebbles present in the river banks of the middle and upper reaches of the three rivers in question.

Finally, I would dispute Chris's contention that the apparent (more apparent than real?) lack of far-travelled pebbles in the terrace gravels disposes, once and for all, of the idea that glacier ice once reached Salisbury Plain. It does nothing of the sort. Most of the material in glacial deposits is rather local, and in many of the tills and outwash deposits I have seen, really far-travelled materials are quite rare. As I have mentioned in other posts, if an ice margin is cold-based, there may be virtually no meltwater production and virtually no deposits to show which areas were ice covered and which were not.

We know that there are quite high percentages of foreign stones in the Thames terraces around Reading (up to 30% in some cases), and as Chris indicates, these are good indicators of the presence of glacier ice from the north during the "Anglian" and "Wolstonian" glaciations. Many of the foreign pebbles in the Thames terraces appear to have come from a deposit called "the Northern Drift" -- assumed by many to be an ancient till deposit. Can we -- and should we -- draw close parallels between the Thames Basin and Salisbury Plain? How similar were the glacial and non-glacial conditions around Reading and around Amesbury? How did the Downland rivers behave during the colder episodes of the Ice Age? Since they are flowing -- at least in part -- on chalk, was their flow continuous or intermittent? If it was intermittent, what effect did this have on pebble transport and terrace construction?

This is an interesting dilemma, worthy of more discussion and more field research..... and in the meantime I'll stick with my belief that the ice DID reach Salisbury Plain on at least one occasion. That belief is supported by the abundant erratics that we all know about on the chalklands, and by the presence of affirmed glacial deposits to the west and north-west of Salisbury Plain.

19 comments:

Anonymous said...

Hi Brian, interesting post, can you elaborate a bit more on "the abundant erratics on the chalklands (?), and the presence of affirmed glacial deposits to the west and north-west of Salisbury Plain".
Do these deposits include material of a South Wales geological provenance?
Cheers Ed

Brian said...

Thanks Ed -- I hesitate to keep on saying "read the book!" but I'll say it anyway! Around page 107. Using the term "erratic" for any stone or fragment that is in the "wrong" place (ie well away from its source) they do seem to be very abundant on Salisbury Plain. From the recent geological work by Rob Ixer and Richard Bevins, who seem to have been looking at material from all the recent digs, there must be stones from over 30 different locations now found in archaeological contexts. Most have a South Wales provenance, but some may be from sites currently unknown. It has been known for many years that the clay-with-flints contains many stones of "unknown provenance." On pages 118-122 of the book I list some of the known "glacial" sites in the South-West. One thing that intrigues me is that all the stones, fragments and monoliths that have been found inside and outside of archaeological contexts on Salisbury Plain appear to have come from the west. Nothing from the north, east or south.......

Anonymous said...

Hi Brian,
"nothing from the North" - are you not including the Sarsens as a monolith which came from Marlborough Downs - 25 miles north of Stonehenge?
And I think someone (?) identified a small number of limestone packing stones as coming from a quarry 3 miles southeast at Hurdcott.
Ed

Brian said...

I have seen nothing to convince me that the sarsens were transported by Neolithic tribesmen all the way from the Marlborough Downs -- that's another Atkinson myth. Sarsens are notoriously difficult to locate to a specific outcrop. I prefer to believe the geologists who have argued that the sarsens at Stonehenge were collected up from a sarsen litter, maybe within a radius of a few miles of Stonehenge. Sure, some impressive effort must have been involved in gathering them to one place and putting them into the monument. But I think the monument was never finished -- they ran out of sarsens, just as they ran out of bluestones. Don't I recollect that the packing stones are also difficult to provenance -- and that they could have come from the west too? (Actually I'm not very worried about them -- easy to carry, and if they wanted them enough, they would have gone to fetch them.) My main point is that the builders were scavengers and opportunists, who would instinctively have used whatever raw materials or tools they could find, as close to their building site as possible.

Anonymous said...

Brian,
I didn't say the sarsens were transported by neolithic tribesmen, I happen to agree with you that they were probably 'litter' close at hand on Salisbury Plain.
The point being they came from the north by whatever means - so we cannot say that all the stones on Salisbury Plain came from the West.
Ed

Brian said...

OK -- I stand corrected! Since we are being just a little pedantic here, one might say that the stones in the "sarsen litter" might also have come from N, E, W and S of the site of Stonehenge. We have no idea where exactly they came from -- maybe within a radius of a few miles? Some of them might be true glacial erratics, in that they might have been caught up in the ice advancing from the west, and nudged a little closer to Stonehenge than they were to start with.

Anonymous said...

Sorry Brian, I didn't mean to be pedantic, but the crux of your debate is were them stones came from, so it is impotant we get it right.
Ed

Anonymous said...

Brian,
In your earlier comment (02/04) you say that 'the stones in the "sarsen litter" might also have come from N, E, W and S of the site of Stonehenge'.
But by what mechanism, as in your book, The Bluestone Enigma, p.129, you state:
'There is no solid evidence of glacier ice ever having travelled northwards or westwards'.
Cheers, Ed.

Brian said...

I stick to that statement, unless somebody shows me some convincing evidence to cause me to change my view. The point about the sarsen "litter" is that it has nothing to do with glaciation -- the point I was making was that (as Chris Green pointed out) Salisbury Plain around Stonehenge is relatively "clean" as far as big sarsens are concerned. He says that shows that the sarsens came from Marlborough Downs. I say that is circular reasoning, and that the fact that the Plain is "clean" shows that the builders of the monument scouted about to W, N, E and S for the stones that they needed, gathered up as many as they could, and then abandoned the project for some reason or other, before it was finished.

Anonymous said...

I apologise Brian, I thought you were referring to "litter" as in left overs from glacial deposits.
However, there is evidence of glaciers moving North and West - in Canada for example the orientation of drumlins, (elongated, tapered hills, which indicate the direction of ice movement from the the blunt end towards the tapered end), found north of Lake Ontario and north of Lake Erie, in the Tavistock-Woodstock area, indicate a north-westerly glacial flow direction.
Kind regards, Ed

Brian said...

I should have been more specific! I should have said "no evidence that glaciers have moved towards the north and west IN SOMERSET AND WILTS!!" Of course there is nothing to stop glaciers moving in any compass direction -- it depends what part of a glacier mass we are talking about. Welsh ice coming off the uplands of Wales has of course moved westwards towards Cardigan Bay, and northwards towards the North wales coast from the uplands of Snowdonia, in various phases of the glaciations.

Kostas said...

Brian, the gravel and pebbles that pack the sarsens are evenly all around each of them? Are they also found, perhaps in smaller quantities, at other places inside the monument?

Brian said...

Others will know the answer to this better than I, but my impression is that there is a jumble of packing stones, hammer stones, mauls and even debris including antler picks etc just chucked into the holes to stabilise the standing stones. As I understand it, this includes sarsens and bluestones. Look at some of the photos from the Atkinson archive etc -- the rubble seems to occur everywhere. Literally thousands of stones.

Kostas said...

Thanks Brian, I value your comments. Are these Atckinson archives available in the internet? If so, do you have the URL handy to post this on your blog? It may be of interests to others as well.

Brian said...

As I recall, you go to the English heritage site and search for "Atkinson + Stonehenge in the collection of photos. There are some quite intriguing ones!

Kostas said...

Brian, after some considerable effort I was finally able to view the Atkinson photos of his 1953/1964 restoration/excavation of Stonehenge. These did not sufficiently answer my questions, but they did illuminate Prof. Atkinsons' love for photography and for photos of himself. I was specifically looking for gravel/pebble packing around the base of the huge sarsens. What I saw instead is that the entire ground (after a thin top soil) is uniformly filled by such soil mix. In some photos there do appear to be packing of larger stones around the base, but what impressed me about these is how little they aged over their 5000 years existence. They still maintained very sharp and irregular edges and seem to have survived intact under considerable pressure. One other interesting mention in one of the photo captions. I quote, “Cutting C48 along the Avenue showing periglacial stripes.” What in your view could this mean, Brian!

Brian said...

Not sure that you would expect pebbles to "age" just because they have been in the ground for a long time. The only thongs that will "age" pebbles (or round off their edges)are attrition during transport (in a stream, or in waves at the edge of the sea, or in a glacier) or chemical rotting in an aggressive environment. If these pebbles and flakes and fragments are broken up prior to use, they will remain virtually unchanged for 5,000 years or much longer than that.

The periglacial stripes? There has been much discussion of these. I've not looked at them personally, but there certainly are soil stripes and even ridges and furrows in present-day periglacial contexts -- permafrost may be a prerequisite.

Kostas said...

I understand your point Brian about buried pebbles not aging, but these were sizable stones (perhaps 6 to 10 inches big) in positions where they would get considerable pressure if they were to support sarsens upright. Their edges were so impressively sharp that drew my attention and amazement. What evidence can prove that these were originally placed by Neolithic builders and not by restoration and preservation efforts done much later? Say by the Romans? And could such heavy upright stones survive for so long being supported by such packing? Just the natural seasonal shifts in the soil it seems to me would have neutralized their effectiveness. What portion of the sarsens was 'foundational'? From the pictures it doesn't seem that these foundations were very deep.

Brian said...

Ending this one now, Kostas. we are getting nowhere! Not sure why you are som preoccupied with the sharpness of the edges of packing stones etc. Pressure doesn't round off stone edges -- only abrasion of chemical rotting does.

The sarsen sockets varied in depth, depending on the size of the stone and the requirement to keep the tops level, so that lintels could be fitted on.