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Tuesday, 30 November 2010

Where was the Anglian shoreline?

This is the conventional picture of the distribution of land, sea and glacier ice during the Anglian Glaciation, around 450,000 years ago.  This is in my view the best "candidate" for ice reaching Salisbury Plain.  Two things to bear in mind. 

First, the ice edge is shown following the Bristol Channel coasts of Somerset, Devon and Cornwall.  this cannot be right, because we know of glacial deposits well inland, around the Mendips and beneath the peaty sediments of the Somerset Levels.

Second, the assumption of dry land everywhere in the English Channel takes no account of isostatic loading.  If the ice limit is pushed maybe 30 - 40 km southwards (which would be more in tune with the geomorphological evidence) then we have a mechanism for considerable depression of the SW Peninsula and for sea water to flood into the English Channel to more or less the position of the present coastline.  Only in this way can we explain the giant erratics and other anomalous features of the English Channel coastlands.


Robert Langdon said...

Brian - You views seem to be quite 'main stream' (with a little tinkering) including accepting that Anglian Glaciation, around 450,000 years ago reaching no further down 'Hornchurch' in Essex.

The problem is if this is the greatest southerly effect of ice on Britain - how are the North and South Downs formed? I read that the 'traditional' theory was the water from Ice Caps carved out these 'Dry Valleys' not excluding Stonehenge Region - and you would need a long and sustained water flow over over 200' to damage the chalk downs to that extent.

The second 'problem' is that currently 'whole' of Britain seems to be suffering 'isostatic rebound' in some form or other - so it is not possible that what is attributed to the Anglian Glaciation, around 450,000 years ago was in fact the last Glaciation 12,000 years ago?

And therefore the Anglian Glaciation completely covered Britain - For I believe there are new findings that show the Anglian carved out the English Channel, which would explain the Downs in question.

Robert Langdon

BRIAN JOHN said...

Gosh -- I am flattered to be accused of being very conservative! Nice change from all the other stick I get, for rocking the boat too much.

I have never read anywhere that the Downs are supposedly affected by glacier ice. The dry valleys of coombes are generally attributed to rising and falling water tables, and maybe to enhanced runoff at times of permafrost, when there may have been a lot of snow banks around. Because we are talking about chalk, solution has been a major factor as well. Mind you, I wouldn't eliminate the idea of a semi-permanent thin cover of firn (and maybe some rather stagnant ice) at times when the ice sheet edge was not far away, on the north side of the Thames valley. By the same token, Dartmoor and Exmoor might have had thin ice caps.

The present isostatic uplift is of course attributed to the after-effects of the Devensian glaciation. But we can assume that there will have been an almost identical pattern of isostatic recovery (and eustatic rise) after each of the big glaciations -- dependent upon the pattern and rate of ice melting.

Did the Anglian ice cover the WHOLE of the UK? I'd like to see some evidence... and the evidence I have seen doesn't seem to point that way. Most of the UK, yes. All of the UK? Don't think so.

Robert Langdon said...

Interesting - water again!!

So the dry velleys in the south are from water levels - ok I can accept that!

When was this huge flood that was powerful enough to cut 200' valleys in chalk? Taking into account the soil layer - I think you geologists call 'head' - is less than 1 meter thick? A layman would suggest recently!


BRIAN JOHN said...

I said water tables -- not water levels. Different thins entirely.

Which huge flood? You don't need huge floods to create valleys on chalk, or on any other rock type, for that matter.

The soil layer is not the same as "head" -- the latter term is used for an accumulation of periglacial slope deposits. It has certain characteristics such as sharp clast shapes, upslope origin for everything, and pseudo-bedding.

Soil layers on chalk are often quite thin on interfluves etc -- but at the base of slopes and in valleys solifluction can lead to great thicknesses.

Anonymous said...

Head and angular clasts That is bad news
If head clays were used in pots would they llok like clay pots tempered with cruched (angular0 rock temper.
GCU In two minds.

BRIAN JOHN said...

Are you looking for clay on Salisbury Plain -- ie on the chalk? There is plenty of clay around the edges of the chalk scarp -- Kimmeridge Clay, Oxford Clay etc. I'm sure I have also seen some refs to "clay pits" on the chalk -- will try to chase this up.