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Sea Level around the coast of Britain in the Holocene

23 August 2010

Ian Shennan of the Sea Level Research Unit at Durham University has written and co-authored a number of books and articles on the subject but as with all modern science, computers have predictably been seconded to provide a knowledge base, and modelling, as always, has been developed as an aid to see how small changes over many years may have panned out at different points in time. There is of course nothing wrong with that – it is an extremely useful tool, supposing all the information is fed into the models. Hence, any geological possibility not recognised in the consensus theory is omitted – such as axial changes. That is not to say there have been major axial change episodes in the Holocene but some people do think so and therefore may regard the models as irrelevant. In spite of that the model has sprung some very interesting information particularly in respect of isostatic changes as a result of glacial rebound. This includes glacier isostatic local contributions with a redistribution of ocean mass as an observable effect – sea level change to the uninitiated (see http://gsa.confex.com/gsa/2009AM/finalprogram/abstract_160868.html ).

In addition, the Geological Society of London issues Special Publications and at their website (google Geological Society) there is an extensive article by Shennan and Andrews which can be downloaded free, An Introduction to Holocene land-ocean interaction and environmental change around the western North Sea.

At www.geosociety.org/gsatoday/archive/19/9/article/i1052-5173-19-9-52.htm there is an article by Shennan, Milne and Bradley on the same subject. Basically, Scotland was depressed by ice during the last glacial episode but since then has gradually rebounded upwards, causing southern and eastern Britain to sink. Hence, Mesolithic sites in the south have been drowned but in Scotland shell middens are still a common landscape feature in the west (they were wiped out by a tsunami in the east). The article basically discusses the models in the context of continuing isostatic effects and the threat it poses to coastal communities. Exactly the same idea appears at http://planetearth.nerc.ac.uk/news/story.aspx?id=555 and defines those regions affected.

Steve Mitchell, member and contributor of various articles to SIS journals, has written about sea level change around Britain and has used such models extensively (but see back issues) but has up till now mainly used the work of Dawson et al. The Shennan sea level curve model has a 40m difference between Somerset and the Firth of Tay some 8000 years ago – around 6000BC. He used a combination of Shennan’s modelling in a graph to show that as a result of exponential sea level rise it would mean Neolithic people would have been able to walk into Britain with their cattle, pigs and sheep, carrying seed of wheat and barley, and therefore the idea they arrived by boat is unnecessary. In orthodox archaeology and geology Britain became an island at around 6000BC (date varies due to model in use) so is Shennan showing that exponential rise leads to his slower rise in sea level? – and is the older orthodoxy due to geological field work rather than modelling? Shennan is saying that as Scotland was depressed in the early Holocene, southern Britain was somewhat upwards of where it is now. This opens a raft of possibilities – such as the Scilly Islands and the Channel Islands being much larger units of land for a longer period of time and both may have been attached to the mainland for a longer period of time than currently envisaged. However, in contrast Scotland would have been a much wetter place in the early Holocene – but is this true? The Orkney Islands for example could have been connected to the mainland when Scotland was depressed and in rising up the mainland became separated from what are now the islands – which is what archaeology has discovered to some degree. But why didn’t Orkney rise up with the mainland if it had been one single land mass. What caused the break? This suggests not all of the variables have been inserted into the model but in many ways it brings the actual drowning of a large land mass where the Scillies are now just a remnant to a fairly late period – the post Roman era (as suggested by Basil Cracknell in 2006). It may also explain why the Goodwin Sands was an island between Kent and the French coast equally as late.

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