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It looks as if this map, published a few years ago, is not far wide of the mark -- although I'll reserve judgment on whether the eastern edge of the Irish Sea Glacier impinged on the coasts of Devon and Cornwall. i think that it must have done, in tune with the physics of ice. There are seven new cosmogenic isotope dates from Pembs currently in the publication pipeline. They show as follows:
3 tors on Carningli -- dates all over 100,000 years.
4 igneous rock outcrops in the Dinas - Fishguard - Strumble Head area -- dates between 33,500 and 76,800 years (the last one, near Pwll Arian on the Pen Caer Peninsula, is somewhat anomalous but is explicable by reference to a short-lived ice cover and very little erosion. There might have been slightly more erosion on the other 3 tested sites.)
I should like to see more detail on sampling points and local geomorphology, since some might say that an ice cover around 20,000 years ago, if it was active enough and prolonged enough to leave moulded surfaces and to refashion coastal tors) should leave fresh rock surfaces giving dates of 18,000 years or so. So there are still uncertainties.
However, the clear difference in rock exposures inside and outside a postulated ice limit gives dates on the one hand over 100,000 yrs and on the other around 34,000 years. Sounds reasonable to me -- and this is in tune with some of my earlier posts relating to ice-moulded surfaces and the Pont Ceunant Moraine.
So the ice limit for the Devensian Glaciation in Pembs, as shown on the above map, seems about right. That means we cannot have ice pressing well inland in Somerset (and even reaching Salisbury Plain) in the Devensian Glaciation. Glacier ice does not do contortions like that! That means that the glaciation that carried the bluestones must have been far earlier, when Pembrokeshire was completely covered by ice -- as suggested from other lines of evidence mentioned in earlier posts.
Many thanks to Danny McCarroll for this new info. To be published soon:
Exposure-age constraints on the extent, timing and rate of retreat of the last
Irish Sea ice stream
Danny McCarroll, John O. Stone, Colin K. Ballantyne, James D. Scourse, L. Keith Fifield, David J.A. Evans, John F. Hiemstra
Quaternary Science Reviews xxx (2010) 1-9
4 comments:
Brian, you write
“That means we cannot have ice pressing well inland in Somerset (and even reaching Salisbury Plain) in the Devensian Glaciation. Glacier ice does not do contortions like that!”
Clearly ice glaciers expanding southward in the Celtic Sea will not 'contort' and bend eastward and northward around mountains into Salisbury Plain. We all agree on that. But is that the only way that ice can cover a region? What about 'local ice'? We know that there was a 'Big Freeze' in all of Northern Europe around 9,000 BC that lasted some 1400 years! It's a well established scientific fact. With the wet and misty weather that Salisbury Plain is famous for, don't you feel there was substantial snowfall and ice accumulation over these many years covering this region well into the 5th millennium BC?
Brian, if you decide to end this thread please do it on 'your turn'.
Constantinos
You have a point here, Kostas. Clearly the periglacial zone (beyond an ice edge) will have been subject to permafrost conditions with extensive snowpatches, boggy areas during summer snowmelt, considerable mobility of slope materials in the active layer, and a great deal of freezing and thawing, frost shattering and frost heave (cryoturbation) in the ground. There are plenty of parallels from the Arctic -- eg Arctic Canada, the fringes of the Greenland ice sheet etc. I think I would accept small ice caps on Dartmoor and Exmoor at the height of the Devensian glaciation -- but have to say that the evidence for this is difficult to interpret. There is evidence for permafrost in Southern England in the Older Dryas and Younger Dryas periods, but not for glacier ice.
Brian you write,
“There is evidence for permafrost in Southern England in the Older Dryas and Younger Dryas periods, but not for glacier ice.”
What is 'glacier ice'? Is 'local ice' glacier ice? Would 'local ice' that forms in a region, stays put in the region, does not flow from the region, and over time melts in the region, leave behind the same evidence as 'glacier ice' that flows into a region from elsewhere? Why must all ice sheets originate from the North and flow into a more southerly region?
I apologize for my persistence on this, Brian, but details are important as you know. I am earnest in seeking to understand your objections. But that requires a sustained Q&A to clear up these details.
Constantinos
This isn't the place for explanations about ice and glaciers -- please do a Google search, and you will find huge amounts of info out there.
Glaciers can poerfectly well flow in any compass direction. Ice sheets and ice caps form where there is adequate altitude, adequate precipitation in the form of snow, and an excess of accumulation over ablation over a period of time usually measured in thousands of years. Simple!
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