How much do we know about Stonehenge? Less than we think. And what has Stonehenge got to do with the Ice Age? More than we might think. This blog is mostly devoted to the problems of where the Stonehenge bluestones came from, and how they got from their source areas to the monument. Now and then I will muse on related Stonehenge topics which have an Ice Age dimension...
THE BOOK Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it.... To order, click HERE
Tuesday, 11 January 2011
More on Devensian climate oscillations
Click to enlarge the graphs.
In trying to work out what might have happened in SW England during the Devensian glaciation, here is a reminder of some of the data in the influential paper by Dr Alun Hubbard and colleagues published in 2009. The graphs above might look complicated, but they show how the climate oscillated between about 40,000 years ago and 10,000 years ago. In the middle part of the Devensian it is now thought that the climate cooled in Great Britain sufficiently for ice to form in the uplands, and for several pulses of glacier expansion to occur prior to 27,000 BP. But in this period there were also 6 interstadials or warmer episodes with greatly increased melting followed by glacier retreat. The British - Irish ice sheet almost melted away around 27,000 BP, and then (because there was a relatively prolonged period of increased accumulation without any major interruptions) the ice sheet grew and grew until it reached its maximum volume and extent around 20,000 BP. There was a phenomenal rate of growth over 7,000 years, followed by a catastrophic collapse. The authors refer, over the whole lifetime of the ice sheet, to "binge-purge cycles" of gradual accumulations followed by rapid ice evacuations.
Beyond the ice edge, for example in periglacial SW Wales or SW England, what we see in the period 40,000 BP to 27,000 BP is a series of relatively short-lived oscillations between hard permafrost and boreal conditions, with some of these episodes lasting no more than 500 years. Then, from 27,000 to 20,000 BP the climate was continuously cold -- and I surmise that conditions must have been just right for small ice cap formation over Exmoor, Dartmoor and other uplands, and for periglacial processes to continue to operate unabated over the lower-lying areas including Salisbury Plain.
The challenge is to see whether this model actually fits with the sediment accumulations and characteristics of coastal and inland sites in Cornwall, Devon, Somerset and Wiltshire.