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Sunday, 5 December 2010

Devensian Isostatic depression in the South-West

 Little Milford, on the shores of Milford Haven.  Sea-level studies in areas such as this show remarkably similar results to studies in Cornwall and on the English Channel coasts, suggesting that the whole of the SW part of the British Isles has a similar eustatic / isostatic history -- and therefore behaves as a block. 

So where does all this stuff about eustasy and isostasy get us?  It's actually quite important.  To summarise.

 This is the overall pattern of eustatic sea-level rise at the end of the Devensian Glaciation, as agreed by most experts from a wide range of disciplines:

Around 14,500  years ago, sea-level stood around -100 m, but a rapid rise of 40 m occurred up to 13,000 BP at a rate of 3.7 m per century.  A second major melting phase at 11,000 BP raised eustatic sea-level to around – 40 m by the beginning of the Holocene (10,000 BP) at a rate of 2.5 m per century, by which time global ice volumes had been reduced by over 50%.  

Holocene sea-level then rose in the Bristol Channel area from -35 m OD at 9,500 BP to 2-5 m OD at 5,000 BP.  The rate of sea incursion gradually declined.  Around 7,000 years BP sea-level was around 8 - 10 m lower than it is today, depending on whose curve you are using (there is generally assumed to be a margin of error of + or - 1m)

After around 6,000 BP the marine incursion into coastal areas of northwest Europe took place more slowly.  In Neolithic times, the sea was at c -6m OD, and in Bronze Age times about -4m OD.  About 2,000 years ago (the time of the Romans in the UK) sea-level was probably about 2m below that of today -- but it may have been higher.  

The configuration of the British coastline around the time of Christ was similar to that of the present day, except that it was more indented due to the drowning of wetlands and estuaries which have subsequently silted up.  As mentioned in earlier posts, the evidence is difficult to interpret in some estuarine and fenland environments because of the effects of storm surges, changes of coastal configuration resulting from the breaching of dune barriers or pebble beaches, and from the compaction of sediments.

In all parts of the UK where isostatic equilibrium has prevailed, or where there has been an overall depression of the crust during the Holocene, it should be possible to "read" this record -- or parts of it -- in the coastal sediments.  In the northern parts of the UK, as indicated in the last post, isostatic effects have caused chaos -- and where the ice load was heaviest isostatic depression was massive, and isostatic recovery has proceeded at a rate that has outstripped the eustatic rise.  But what about the areas south of Cardigan Bay?  This is where, in the models, the hinge line is supposed to have occurred, with virtually no glacio-isostatic depression and therefore no rebound.  It is assumed that this is where the ice margin was located at the time of maximum Devensian ice extent in the domain of the Irish Sea Glacier.  Further to the south, we are into the "forebulge" area, where land is expected to have been HIGHER than it is today around 20,000 years ago and then sinking gradually in compensation for the recovery of the landmass further north.  This is what the maps of current crustal downwarping show (see the last post)..........

This means that the further towards the SW we move, the lower (with respect to a fixed datum) should be the sediments dated to particular phases of the Holocene.  If the Scilly Isles and SW Cornwall really are sinking at a rate of c 2m per millennium, and if the rate of sinking was even faster in the past (in compensation for the faster isostatic recovery further north) then this should show up clearly in coastal sediments.  In other words, there should be pretty large ANOMALIES!

What we find, when we look at the evidence from a vast range of studies from West Wales to the South Wales coast to the Severn Estuary to the Somerset Levels to the south coasts of Cornwall, Devon and Dorset, is that there are virtually no anomalies.  The sea-level curves come out conforming to the normal eustatic curve over and again, with variations of maybe a metre or so on either side of the mean curve -- within the margin of error.  There is an intriguing thought -- namely, that the assumed rate of coastal sinking on the South Coast of around 1 mm per year or 1 m per millennium is more apparent than real, since most of the observations on which this thesis is based have come from areas with compacting sediments!

Two key references:  
Mid- to late-Holocene relative sea-level change in southwest Britain and the influence of sediment compaction, Robin J. Edwards, The Holocene, 16,4 (2006) pp. 575  587
Relative sea-level change and postglacial isostatic adjustment along the coast of south Devon, United Kingdom, Massey, AC et al, J. Quaternary Sci., (2008) (ISSN 0267-8179)

At the moment, the sheer conformity of the observations from across this region suggest to me that this was not a hinge line and forebulge area at all, but an area that acted during and after the Devensian Glaciation as a more or less stable block.  This is what you would expect if Devensian ice really did reach the Scilly Isles, as suggested by Prof James Scourse and others.  The strange tongue of ice that he suggests as flowing down the centre of the Celtic Sea is glaciologically most unlikely, and in the Aberystwyth Devensian modelling work the eastern edge of the Irish Sea Glacier is shown pressing onto the Cornwall and Devon coasts.  If the ice edge had anything like an equilibrium profile, it must have been several hundreds of metres thick in the Bristol Channel -- whether or not it encroached inland.  It all fits..........

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