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Wednesday, 27 February 2013

New OSL dates for North Somerset sediments



Six new dates have been obtained from sediments in the Gordano Valley, North Somerset, as part of a research project by Anne Bridle from West of England University in Bristol.

This area is well known for the presence of glacial and related deposits, proving that the ice of the Irish Sea Glacier extended into North Somerset on at least one occasion and possibly more.

In a brief note ("Optical Ages of a Pleistocene sequence in the Gordano Valley, North Somerset, UK"  Quaternary Newsletter Vol 129, Feb 2013, p 72) the researcher says that samples were taken from a sequence of sands, gravels and "clay drift" beneath the recent peat layer.  The "clay drift" may be till, but the author doesn't comment on this.  The dated samples all come from the sands and gravels, and they range somewhat erratically between 91,000 and 62,000 BP -- which places all of them into the Early Devensian and possibly the preceding interglacial.  It's a pity that the OSL technique is the only one used here, since all of the new methods of dating non-organic materials really need checks from other techniques so that systematic errors can be eliminated.  So we don't know as yet whether these dates can be taken at face value.

But if, for the moment, we accept that they are reliable, this means that the till (?) at the base of the sequence cannot be Devensian, and must therefore be Anglian -- as we have suggested on this blog many times before.

So although there may well have been small ice caps on Dartmoor, Exmoor, Mendip and other upland areas of the South-West, it does not look, according to current evidence, as if the Devensian Irish Sea Ice Sheet transgressed across the Somerset coast........

2 comments:

Constantinos Ragazas said...

Brian,

A 'thought experiment'. Suppose for a moment glaciers came rolling down and across the Bristol Channel carrying Deventian-age sediments with them (clay, sand, pebbles, etc.) to North Somerset. And retreated leaving behind this till. Would we can the till Deventian?

Kostas

BRIAN JOHN said...

Yes, of course we would. The Devensian covered a long time -- over 70,000 years. In the Devensian tills I examined as a research student, there were bits of Tertiary lignite and shells dated at c 38,000 years BP -- but the till was laid down around 20,000 years ago.