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Saturday, 10 April 2010

The Last Glaciation of the Bristol Channel




O Cofaigh and Evans, 2007 -- LGM or maximum advance of British-Irish ice into the Celtic Sea:

From the Abstract: "The Irish Sea Till was deposited by the Irish Sea Ice Stream during its last advance into the Celtic Sea. We present 26, stratigraphically well constrained, new AMS radiocarbon dates on glacially transported marine shells from the Irish Sea Till in
southern Ireland, which constrain the maximum age of this advance. The youngest of these dates indicate that the BIIS advanced to its overall maximum limit in the Celtic Sea after 26,000–20,000 14Cyr BP, thus during the last glaciation. The most extensive phase of BIIS growth therefore appears to have occurred during the LGM, at least along the Celtic Sea and Irish margins. These data further demonstrate that the uppermost inland glacial tills, from the area of supposed ‘‘older drift’’ in southern Ireland, a region previously regarded as having been unglaciated during the LGM also date from the last glaciation."

I disagree strongly with this ice margin as it is portrayed in the Bristol Channel and in West Wales. Ice always moves perpendicular to the ice margin except where there is an exceptionally strong physical constraint. So the ice cannot have flowed from NE to SW along the Pembs coast -- it must have come in from the NW, flowing towards SE. That is exactly what the ground evidence says -- when one looks at striations and erratic transport. All of the ice that laid down the latest Irish Sea till deposits around the Pembs coast came in from the NW. And I cannot imagine an ice-free Bristol Channel either -- an ice lobe like this MUST have reached the coasts of Cornwall, Devon and Somerset around 20,000 years ago, even if the ice stream was behaving like a surging glacier and even if it had an exceptionally low long profile (ie a profile very different from the glacier "equilibrium profile".) Colm and David do discuss this as a possibility -- but I think they should have adjusted the ice margin on the map while they were about it!!

What has this got to do with Stonehenge and the bluestone erratics? Not a lot really -- except to flag up the idea of repeated glaciations, with relatively minor differences in ice flow directions.

10 comments:

Kostas said...

You write,
“...an ice lobe like this MUST have reached the coasts of Cornwall, Devon and Somerset around 20,000 years ago ...”

Brian, is it possible than that some 20,000 years ago the Salisbury Plain (or parts of it) may have been under ice too? Is it possible that local conditions (and not glacier advance) may have formed an ice cover? Somewhat like a frozen lake when the temperature is freezing for prolonged time? Under such circumstances, when ice forms locally and does not involve glacier movement and ice flow, will there still be geomorphologic evidence of an ice cover? Especially if this ice sheet is over a very absorbent chalk bedrock and can provide direct drainage of the melting ice water into underground caverns and water deposits? Like a plate holding a sponge with an ice cube on it? When the ice cube melts and the water is absorbed by the sponge, there isn't any evidence left behind of the ice cube being there before.

Brian said...

It's almost certain that at the peak of the last glacial episode (c 20,000 years ago)there would have been permafrost across most of southern England and Wales -- that means deeply and permanently frozen ground with some seasonal thawing close to the ground surface. There would also have been extensive semi-permanent snowpatches, maybe surviving winter and summer on shady north-facing slopes. There might also have been smallish thin ice caps on Exmoor and Dartmoor -- further east there might not have been enough snowfall to maintain such features, even on the Downs, Chilterns and Salisbury Plain. We don't really have enough evidence to tell us exactly what the conditions were -- except for thick deposits of periglacial slope deposits ("head") on some of the clifflines on southern England, and some other periglacial features (eg stone stripes etc) elsewhere.

Brian said...

Sorry -- I should have said that the permafrost would have been well developed in southern England and Wales BEYOND THE GLACIER ICE EDGE.

Kostas said...

Thanks, Brian. I appreciate the info. In your view, when is the latest possible date (closest to the present) for the glacier transport of the bluestones to Salisbury Plain?(I know, “read my book”! When I get my hands on a copy I promise I will read it! For now, forgive me for asking … )

Brian said...

Well, we have had some surprises in recent years -- including the discovery that LGM (Last Glacial Maximum) ice reached the Scilly Islands and flowed across the south coast of Ireland around 20,000 years ago. There are lots of radiocarbon dates now -- much to everybody's surprise. So it is not beyond the bounds of possibility that Devensian ice reached Salisbury Plain about the same time -- 20,000 years ago.

Personally, I would like to see more evidence from Somerset if that is to be confirmed. At the moment the evidence seems to point to a much earlier glaciation.

Kostas said...

Wonderful news, Brian! I am really excited about this latest evidence of glaciation at Salisbury Plain just 20,000 years ago! This is certainly within the limits of possibility that paleolithic men created Stonehenge, not by dragging the stones over land and erecting them upright, but by pushing them over an ice edge! The labor, skill, tools, social organization, etc. needed to do this was minimal! Paleolithic people could have easily done it, just working in very small groups, for no other purpose than to 'have fun' after a long winter, or to 'clear the land' from big stones. I am encouraged … just in case you lost the link to my paper that describes this, including some revisions since the first version you may have read, here is the link to it again,

http://knol.google.com/k/constantinos-ragazas/the-un-henging-of-stonehenge/ql47o1qdr604/16#

Constantinos

Brian said...

Hang on there. I did not say there was EVIDENCE that ice reached Salisbury Plain 20,000 years ago. I said it was a possibility (one would be a fool to say it was impossible) but that I would like to see some evidence to support the idea, particularly from Somerset. And so far, I'm not aware of any....

Kostas said...

Brian,

OK I am not jumping for joy yet. But if there is some plausible possibility that Salisbury Plain was covered by ice 20,000 years ago, then wouldn't the explanatory ability of this to sensibly answer all of the many perplexing enigmas of Stonehenge (and all other stone formations all over Brittany and UK) lead us to “induce” this was so? The many stone alignments can then be useful 'writings' from Paleolithic men giving us valuable records of geological conditions they experienced. It might do some good for Archeologists to consider too!

Kostas said...

Hello Brian,
Since we have the same objective in mind, determining the truth about Stonehenge, do you feel it would be appropriate to post a link to my paper in your web site? I see a real need to have a centralized place on the internet where all 'Stonehenge Thoughts' can easily be found. My paper makes several references to your work and has links to your web sites. Here is the URL to my paper:

http://knol.google.com/k/constantinos-ragazas/the-un-henging-of-stonehenge/ql47o1qdr604/16#

Brian said...

My pleasure! Sadly, with my particulat blogger, URLs don't get converted into hyperlinks. So folks have to copy and paste them -- but at least they are there for those who want to follow them...