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Thursday, 16 June 2011

Devensian erratic flushing from South Wales



This map, which I have used before on this blog, shows the maximum extent of glacier ice in South Wales during the Devensian or last glaciation, around 20,000 years ago.  If -- as seems possible -- a scatter of Pembrokeshire erratics was left across the South Wales coastlands at the end of the Anglian Glaciation, or later glaciations, these erratics might well have survived in central and South Pembrokeshire and on the western tip of Gower, because these areas were ice-free.  However, a series of Welsh Outlet Glaciers flowed down the main valleys from the uplands of mid-Wales during the Devensian, in the valleys of the Towy, Taf, Tawe, Neath, Rhondda, and xxx, to name but a few.  These glaciers expanded out to the present coastline and indeed beyond it, taking on the character in some cases of piedmont lobes.  Within the limits of these lobes, the glacial deposits that remain are of Welsh origin, with till that is recognizably different from the Irish Sea till, and with a suite of erratics transported from the north.  In such circumstances, the chances of any Irish Sea till surviving were very small -- and most if not all of the erratics left during the previous Irish Sea Glaciation must have been "flushed" or moved southwards, into the area now submerged in the Bristol Channel.  If there are bluestones that were laid down in a trail from Preseli to Somerset, they will be found, if anywhere, on the sea bed.

4 comments:

Constantinos Ragazas said...

Brian,

This map continues to mystify me, especially the 'ice free' area depicted in it. It is inconceivable to me that such an area would be 'free of ice' so close to so much ice! But I will accept it for now as an indisputable fact.

Do I understand then correctly your theory is the bluestones of Stonehenge were carried to Salisbury Plain by earlier glaciations (say 400,000 years ago) and the expected trail of bluestones from Preseli to Somerset were 'flushed in the sea' along the coastline much latter by the Devensian glaciers 20,000 years ago? And that, according to you, explains the absence of such glacier 'trail of erratics' as would be expected?

What happened to the rest of the 'trail of erratics' from Somerset to Salisbury Plain? How were these erratics flushed in the sea?

For the record, I have a different explanation that can explain the unexplainable!

Kostas

BRIAN JOHN said...

Kostas -- this "ice-free" area was not free of ice -- just free of moving glacier ice. Within it, there would have been extensive snowfields and seasonal ice on rivers and lakes -- and a lot of periglacial activity.

I'm not claiming there WAS a trail of erratics. All I'm saying is that if there was one, it would have been flushed out to the area now submerged. I still think the best analogy is that of the Darwin Boulders mentioned in some of my posts.

Constantinos Ragazas said...

Brian,

Thanks for clarifying your position. But if you are not claiming there was a trail of erratics, why give an explanation of a 'flushing of erratics' that can only explain what might have happened to part of such trail but cannot explain what would have happened to the rest of the trail of erratics. It only opens up your argument to criticism by those that oppose your glacier theory and wish to exploit logical flaws in your arguments. In my humble opinion!

You write, “ this "ice-free" area was not free of ice”

Looking at the map in your post again, the area designated as 'ice free' covers part of Bristol Channel. Was the Channel frozen at times according to your description of this periglacial area? Or was this body of water not yet existing at the time 20,000years ago?

Kostas

BRIAN JOHN said...

Kostas -- you are making a mountain out of a molehill here. I referred to a "scatter of erratics" -- they could have been in till deposits, or left at the surface -- from earlier glaciations. All I'm saying is that there MIGHT have been a trail of erratics -- and if there was, these later pulses from the Welsh glaciers would have flowed across these areas of older sediments and moved them out or redistributed them.

As I have explained many times before, at the peak of the Devensian Glaciation there would have been no sea water in the Bristol Channel. All the sea-level curves show this. There would have been some isostatic depression, even beyond the ice edge, but probably not enough to cause sea water to flood in.