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.