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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Friday, 30 September 2011

A few centuries of glacial action....

This image, from one of the modelling experiments reported, shows the percentage of time that the ice was present at any particular point. Note that the ice was present for a very short time indeed (centuries, not millennia) when it affected SW England.

Another look at the Hubbard et al paper from 2009. The authors of that article were reporting on the computer modelling experiments which gave a good indication of the behaviour of the British and Irish Ice Sheet during the Devensian (most recent) glacial episode.They argue that according to the best models available, feeding all all relevant climate and glaciological data, and incorporating ground evidence too, they see the ice sheet extending across parts of SW England as recently as 20,000 years ago -- maybe for no longer than a few centuries. That is a very short-lived incursion. Not many people accept that, arguing that the evidence on the ground just does not support such an incursion as far east as Salisbury Plain. I have my doubts too -- but I prefer to think of this as a model that might also have applied in the Anglian Glaciation, with a wet-based ice sheet advancing very rapidly eastwards, incorporating the small detached icefields over the uplands of Dartmoor, Exmoor etc before reaching its position of maximum extend -- and before melting away very rapidly.

Interestingly, they argue that it was a slight WARMING of the climate that caused this apparent surge behaviour, since increased water at the glacier bed would have led to accelerated forward movement.

"Another phase of warming at 20.8ka prompts a widespread mobilisation of wet-bedded ice across extensive lowland and shallow marine sectors yielding maximal ice cover across the domain at 20.1–19.9ka. Large, mobile, surge lobes dominate the southern sectors of the Irish Sea and North Sea, and the Atlantic coast and extend to the north coast of SW England, Norfolk and to the continental shelf edge, respectively." (p 767)

"The experiments presented also indicate significant excursions of wet-based ice into areas of southern England, where little evidence of recent glaciation has been found, This may not present such a major problem given that our model indicates ice was at this extended limit for less than 1 ka. The experiments also provide support for a possible glacial mechanism for the movement of Preseli erratics as a transport trajectory which overrides parts of northern Pembrokeshire and was subsequently deflected south-eastwards across the Bristol Channel into SW England, cannot be completely discounted."

'Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet.'
Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper, Martyn Stoker
Quaternary Science Reviews 28 (2009) 759–777

Look at the last sentence in that quotation.  That is interesting, coming from a group of glacial geomorphologists and glaciologists.  You will recall that  James Scourse and Christopher Green HAVE completely discounted the idea of glacial transport of the bluestones, and have even (ill-advisedly, in my view) used the word "impossible." 

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