The Local Geology

These are nice simple and yet informative short descriptions (from Natural England) of the local geology in Wilts, Somerset and the Bristol and Bath area:

Wiltshire: http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_ID39.aspx

Somerset: http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_ID30.aspx

N Somerset, Bristol and Bath: http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_ID31.aspx

Thoroughly recommended.

Stonehenge: Local Bluestones from the west

I am far from being an expert on the geology of Somerset and Wilts, but on looking at the geology maps I am encouraged to do a bit of speculation. Let's refer to everything at Stonehenge that is not sarsen stone as BLUESTONE, since that's what people have been doing for years. There are hundreds if not thousands of bluestones of all shapes and sizes at Stonehenge that could well have come from the west and north-west, within 80 km of Stonehenge, and which could well have been carried by ice, if it did indeed come in from the Bristol Channel in the direction(s) discussed in earlier posts. So which "local" rock types might we expect to be represented in the Stonehenge bluestone assemblage? Look at the map and key above.

For a start, we might expect erratics of Upper Greensand, Corallian, Portland beds (Upper Oolite), Middle and Lowe Oolite beds (including some limestones and some sandstones), Upper Triassic beds (marls, sandstones, conglomerates), Carboniferous Limestone (from the Mendips) and even Coal Measures (mostly sandstones and shales, with interspersed coal-bearing beds). There are also a few outcrops of Old Red Sandstone on the flanks of the Mendips, to the west of Bristol and along the North Devon Coast -- and a few small exposures of igneous rocks too. I have forgotten here about the clay beds, on the assumption that they would not have contributed large blocks and monoliths to an erratic load or a till cover (they would have been broken down for the most part into silt and clay fractions). In short, sandstones, gritstones, and limestones galore COULD have been entrained, transported and finally dumped by overriding ice from the west.

There are lots of all of these rocks at Stonehenge, but archaeologists have tended to assume (a) that they have been imported manually by the Stonehenge builders, and (b) that they have therefore come from quite close at hand -- ie from the Vale of Wardour or from the Eocene outcrops to the south of Salisbury Plain's chalk downlands.

Here is a question for the geologists: Have any of these "local bluestones" found at Stonehenge been definitely fixed to the rock outcrops to the south or south-west of Stonehenge, or could they equally well have come from the west or north-west? Chilmark, in the Vale of Wardour, is often mentioned as an Upper Greensand source for "Chilmark stone" -- but is that just wishful thinking on the part of geologists and archaeologists who had no great desire to look for sources further afield, or in another direction?

Where did the Packing Stones and Mauls come from?

On the left, typical packing stone debris and rubble -- this sort of material has been found, in vast quantities, during every excavation. Some of the stones are far too large to have been used as hammer-stones or mauls -- and this is a fair question: would this material have been carried from "quarries" many kilometres away, just to have been used as packing stones? Unlikely, in my view......

Following on from my last post, here is an extract from Chapter 6 of my book:

Almost every stone -- whether sarsen or bluestone -- that was pulled and pushed into a vertical position at Stonehenge was stabilised with packing stones and chalk rubble often containing flints. There is a phenomenal amount of this “clutter” at Stonehenge, and as far as we can see it has never been systematically analysed. An examination of Richard Atkinson’s photo archive from the 1958 excavations is very revealing; photo after photo shows rounded and sub-angular stones packed around the monoliths, sometimes with bits of antler thrown in for good measure. Some of the stones are as big as a human skull. The heaviest one found thus far weighed 30 kg, but Richard Atkinson thought that some mauls were as heavy as 60 kg when in use. (How would you actually use a maul which was that heavy?) Christopher Green refers to “chips and fragments of almost all the rock types that have been identified in the structure” -- including sarsen and bluestone. He says that in the course of 22 excavations at Stonehenge over 11,500 stone fragments have been recorded; some of this material is packed into stone holes as a sort of matrix between bigger stones, but many other fragments simply littered the ground surface in the past, prior to being incorporated into the soil horizon by earthworm activity and so forth.

In the excavations of Gowland and Hawley early in the twentieth century, they recorded 447 mauls and hammerstones, mostly in pit fillings, which had been used for the shaping of the sarsens. The tooling marks on the surface of the great monoliths are still visible. Some of the tools were made of a very hard type of sarsen which is densely cemented by microcrystalline silica, and which was therefore a tough and resistant tool. Other packing materials are more intriguing, since they are made (according to William Hawley) of Jurassic oolitic ragstone and glauconitic sandstone. Rock of the former type was supposed to have come from near Chilmark, around 20 km to the south-west of Stonehenge, in the Vale of Wardour. The latter was supposed to have come from the Upper Greensand beds in the same area. Dr Green thinks that these stones were collected there and carried to Stonehenge in a separate “stone collection” exercise. However, that is a matter of debate, and nobody has seriously questioned Hawley’s suggested provenances for the stones. These rock types also outcrop to the west, in the western part of Wiltshire and in Somerset, and Geoffrey Kellaway pointed out in 1971 that they could have reached the Stonehenge area as glacial erratics. The Upper Greensand outcrops around Warminster, about 22 km to the west -- not far, as it happens, from the site of Boles Barrow. Also, it is doubtful that these two stone types would have had any value as hammerstones, since they are generally too soft and only become hard when impregnated with calcium carbonate, for example by being placed in a chalk pit. Would piles of these soft sandstone and limestone rocks have been fetched all the way from the Vale of Wardour just to be used for stabilising standing stones in their sockets? That seems very doubtful to me.

The Great Erratic Hunt

Wiltshire geology map and cross-section. Courtesy Wilts Geology Group. With a bit of luck, if you click on either image you should get an enlargement on your screen. If there are Jurassic or Lower Cretaceous erratics in the vicinity of Stonehenge, my theory is that they should have come from the west or north-west.

If glacier ice approached Stonehenge from the west, how far eastwards did it extend? One theory that I have put forward with Lionel Jackson (in EARTH magazine) is that the ice actually came quite close to Stonehenge, with a line of "bluestone" boulders extending eastwards from the Chalk Escarpment across the gently undulating chalk downs -- marking the position of the junction between Welsh ice (on the north side of the junction) and Irish Sea ice (on the south side). We argued that this line or "train" of erratics, like the Foothills Erratic Train in North America, would have been quite easy to follow -- and would have been known to the Neolithic tribes inhabiting the chalk downs. So they collected them up for their building project, at first close to Stonehenge and then successively further and further away, until either the stone supply ran out or they ran out of energy. I still think that is a reasonable theory, in terms of glacial theory -- especially if we think in terms of the modelled ice extent described in earlier posts on this blog.

But how likely is it that the stone collectors will have collected ALL of these erratics? Would there not have been some, deeply embedded in the ground, or maybe hidden in thick copses of trees or undergrowth, that they will have missed? Quite so -- I freely admit that it would be good to find some more erratics still in position to the west of Stonehenge. Well, we have Boles Barrow bluestone boulder, for a start -- but that was a "collected boulder" too -- and not in its original position.

Are we too preoccupied with spotted dolerite? I wonder what other stones there might be on the Plain, or in buildings or stone walls, to the west of Stonehenge? And maybe we should think much more about erratics derived from the rocks which outcrop to the west -- for example the limestones and sandstones of west Wiltshire, Somerset and the Mendip area?

Stonehenge -- lessons from Avebury

Avebury and Stonehenge -- two of a kind? Both manufactured from a convenient litter?

Came across this from a chapter in a book on the Marlborough Downs:

"The origin of the stones in the Avebury stone circles is often considered to derive from one of the accumulations that still exist on Overton or Fyfield Downs to the east of the site, each stone being dragged a distance of several kilometres up and over Hackpen Hill. Such suggestions are merely a response to where boulders can be found in abundance today. However, as noted above stone was certainly available locally and may have been present on the site itself. There is no reason for the stones to have been hauled such a distance. Local accumulations of sarsen are or were certainly present here and it is likely that the valleys were once covered with boulders even though only isolated examples now survive. The farmer of land east of the West Kennet Avenue confirmed that a ridge c1.0m high and 30m wide `consists of a seam of sarsen that runs through the field' (Barker 1985, 21). Thus the land for this massive construction would need to be cleared first and any boulders within the immediate vicinity removed. Given this scenario the stones for the circle may well have come from the immediate vicinity of the henge, some potentially from the interior."

(Ch 8. Some observations on perception, consolidation and change in a land of stones.
David Field)

I think that's rather interesting -- confirming what many other archaeologists and geologists have said about Neolithic people being opportunists and scavengers, rather that highly organized long-distance stone hauliers. It makes a great deal of sense to assume that both at Avebury and Stonehenge the builders simply used what was on the spot, and cleared a litter of large stones from the landscape. As I have said elsewhere, after they had used up all the stones in the immediate vicinity, they cast about further and further afield, until they finally ran out of stones, ran out of energy, or got bored. Was either monument (Avebury and Stonehenge) actually FINISHED? I doubt it.

Silbury Hill -- one bluestone, or two?

Just a reminder -- this is Pete Glastonbury's photo of the Silbury Hill bluestone at the AK Museum in Avebury. it's very small -- just the size of a thumbnail -- so we should call it a flake or a fragment rather than a stone or a boulder!

Rumour has it that there is a bit of a spat blowing up over the lithology and provenance of the one (or two?) pieces of "bluestone" found on Silbury Hill by our old friend Richard Atkinson. Are they really fragments of spotted dolerite like those found at Stonehenge? Or are they hornblende schist? Watch this space......

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.