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Saturday, 27 August 2011

The Celtic Sea Piedmont Glacier

Following my recent posts, I can see no great reason to maintain the pretence that the southern part of Pembrokeshire was totally ice-free during the Devensian.  The evidence from Caldey and elsewhere indicates that the same stratigraphic sequence of Late Pleistocene deposits occurs in South Pembrokeshire as in the north of the county -- ie with glacial and fluvio-glacial deposits overlying head and overlain by sandy brickearth or loess material which forms the basis of the modern soil.  Beneath this whole sequence is the raised beach, cemented solidly where it lies on Carboniferous limestone.

So I'm modifying the map which I pasted on this blog a few months back as below.  My old map is above, and the modified one is below.
If thin ice covered South Pembrokeshire and Caldey Island, it quite possibly coalesced with the ice coming down to Carmarthen Bay from the Towy and Taf valleys.  The signs are that it had a very flat gradient, and did very little, in this area at least, in the way of landscape modification.  I think the ice edge may well have reached the coasts of Devon and Cornwall in places -- maybe not everywhere.

It follows that in eastern Pembrokeshire and western Carmarthenshire there would have been a complex zone of coalescence between Irish ice, Irish Sea ice, and Welsh ice.  Where, when and how?  That is something to be worked on.....

As Henry Patton has shown with his Devensian ice cap modelling, the outermost ice limits in different zones may not have been reached contemporaneously -- the line (if we need one at all)  may be a composite feature, with pulses of forward movement here and there, and out of phase.

And I still think it quite possible that just as a small Preseli Ice Cap existed intermittently in the Devensian,  there may have been thin short-lived ice caps on the uplands of SW England, on Dartmoor, Exmoor, Bodmin Moor and the Mendips, for example.

So the ice edge may have been very fuzzy indeed, with no clear demarcation between forward-moving and far-travelled glacier ice from the west, locally-generated ice on high ground, and areas perennially and intermittently covered by extensive snowfields during the coldest phases.

5 comments:

Constantinos Ragazas said...

Brian,

The redrawing of your map is a silent admission that my skepticism of the “ice free” area was well founded!

I go further and claim the entire Bristol Channel was frozen during the Devensian.

You argue:

1)When the sea level was low (some 240ft low), Bristol Channel was dry. When the sea level was high, Bristol Channel was filled with water some 240ft deep.

2)The Irish Piedmont Glacier moved in from the west over dry land up to Caldey Island leaving behind the glacier till deposits you found on the easternmost tip of the Island.

An 'inconvenient' question:

If the lowering of sea levels was due to the formation of vast glacier ice (sucking up sea water) what happened to the relatively shallow coastal inlets of water, like Bristol Channel? Did they just drain of water? Under such prolonged freezing conditions, wont these shallow coastal inlets freeze solid even before the formation of glaciers and the subsequent lowering of the sea levels elsewhere? And once frozen, wont these coastal channels remain frozen for the duration of the glaciation?

Brian, we don't need the Irish Sea Glacier moving into Bristol Channel to have glacier deposits dumped at Caldey Island! A frozen Bristol Channel and ice streams from Wales would do just fine! Just like the ice streams of Antarctica in your post show: frozen coastal inlets surrounded by ocean.

Brian, I make a wager with you staking our intellectual pride! The stones you found in the till you discovered at Caldey Island trace to Wales to the north and east; and not to Ireland to the west.

My hypothesis is consistent with the findings of many animal bone fragments mixed-in with the sediments of caves along the northern coast of Caldey Island. These also came from the mountains of Wales and carried to Caldey Island by meltwater or ice streams. That is why only bits and pieces of bones are found in the caves and not whole skeletal parts, as you would expect to find if whole carcasses were carried into caves by contemporaneous predators to consume in safety.

I can only anticipate your 'summary judgment' and disagreement with all my reasoning. Yet I remain hopeful that you will eventually begin to recognize in all that I am arguing some elements of truth. As you already have (but never acknowledged) with my posted skepticism of the “ice free” area in your original map!

Kostas

BRIAN JOHN said...

Kostas -- perfectly happy to acknowledge your scepticism about that 'ice free" area! However, as I explained,that meant "free of glacier ice" -- not free of snowpatches and perennial ice on water bodies. The difference between me and you is that I'm basing my views (changing views) on reconsiderations of the actual field evidence......

A frozen Bristol Channel? I don't think so. Floating sea ice does not support glaciers, and it does not leave till at assorted altitudes above present sea-level.

There was no such thing as a 'great freeze" which suddenly caused the freezing of the Bristol Channel. The gradual -- intermittent -- colling of the climate in the Devensian took almost 20,000 years -- and sea-level fall was also intermittent. There is no reason at all to doubt that the Bristol Channel was an undulating tundra plain (with rivers crossing it, of course) when glacier ice started to move into the area.

Constantinos Ragazas said...

Brian,

Bristol Channel did not freeze suddenly.

Consider that the depth of Bristol Channel is from “9 m/30 ft to 73 m/240 ft”. I assume the Channel is shallower closer to the easternmost coastline.

So we have a relatively shallow body of water (some 9 meters deep in places) opening up to a much wider and deeper Irish Sea. Under sustained freezing conditions that lead to the formation of glaciers further North, such shallow body of water would freeze first long before the glaciers advance to that point or the sea levels drop.

Thus we would have a frozen Bristol Channel in the shallower easternmost parts before the sea level drop would have drained these areas of water to make them dry tundra.

And as the Irish Sea Piedmont Glacier later advances into the Channel from the west (as your modified map now shows and the evidence you found now confirms) it would join with an already frozen easternmost Channel.

This description is consistent with the photo of Antarctica you posted showing frozen coastal inlets opening up to wider ocean water.

We would have a completely frozen Bristol Channel during the Devensian. All the evidence you presented is consistent with this hypothesis.

Kostas

BRIAN JOHN said...

Enough of this please, Kostas. You completely forget about seasonal rhythms -- even at the peak of a glacial episode. Bodies of ice do not accumulate like this -- the ice you refer to in Antarctica is sea ice or pack ice, floating on sea water and subject to periodic breakup. What you suggest is in defiance of the laws of physics, and is not supported by ANY evidence. I shall block any further comments from you on this. Sorry, but we have done this to death.

Constantinos Ragazas said...

Brian,

You may choose to block this message! I understand!

But I want to let you know, nonetheless, that I do not intent to abuse your hospitality. When I cross the line feel free to call me on it, as you have. But only you can know when that fine line is crossed!

From my vantage, I am only addressing points of reason in a dialectic. Nothing less! Never personal!

Our differences in the “dating continuity” of bone fragments found in crevices and caves in the north and eastern coastline of Caldey Island is a good example of this.

You argue Bristol Channel was mostly a dry tundra with wild life roaming the countryside during the Devensian, thus the bone fragments.

I argue Bristol Channel was a frozen waterway during the Devensian and the bone fragments were carried there by water, mud or ice in some form from Wales.

But once the arguments are made and the two sides can't reconcile, the sides should cease!

I agree!

thanks,

Kostas