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Tuesday, 8 July 2014

Lundy Island and the LGM dilemma


Herewith some more info from that interesting paper by Chris Rolfe et al -- relating to the glaciation of Lundy Island.  As the authors say, this is a critical location for sorting out the Pleistocene history of the Bristol Channel area -- and it should be possible to tie in the evidence from Lundy with that from the Scilly Isles, where the LGM (Last Glacial Maximum) is now securely dated to the Late Devensian, around 20,000 years ago. Securely dated?  Or so we thought........

The link to the paper:
"Paired 26Al and 10Be exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles"
C.J. Rolfe, P.D. Hughes, C.R. Fenton, C. Schnabel, S. Xu, A.G. Brown
Quaternary Science Reviews 43 (2012) 61e73
https://www.researchgate.net/profile/Christopher_Rolfe/publications

This is the map of glacial and other features -- much more accurate than the old map made by Frank Mitchell in 1968.  Most of the important features are at the northern end of the island.


 The cosmogenic dates are shown on the map, together with the locations from which the rock samples were taken.

This is the Abstract for the paper:
Lundy lies in a strategic geographical position for understanding the glacial history of the British Isles. The island bears evidence of glaciation, largely in the form of ice-moulded bedrock and glacially- transported boulders e an unusual occurrence this far south in the British Isles. Irish Sea ice pene- trated the western Bristol Channel overriding Lundy from the northwest during the last phase of glaciation in this area. The results of paired terrestrial cosmogenic nuclide analyses (26Al/10Be) constrain the timing of this extensive glaciation and provide, for the first time, an age for the exposure of Lundy granite following deglaciation. The results from nine paired samples yield 26Al/10Be exposure ages of 31.4e48.8 ka (10Be) and 31.7e60.0 ka (26Al). This challenges the view that any glaciation this far south must belong to Middle Pleistocene glaciations, such as the Anglian Stage (c. 480e420 ka) and a Devensian age for the last glaciation is consistent with findings from the Isles of Scilly further south. However, the findings suggest early-mid Devensian (marine isotope stage (MIS) 4e3) glaciation of Lundy. It also implies that the island was exposed or covered for a short time by non-erosive cold-based ice at the global Last Glacial Maximum (LGM) during MIS 2 (26e21 ka). The potential exposure of the island throughout MIS 2 contrasts with the evidence from the Isles of Scilly and the Celtic Sea, which were glaciated at the LGM.

Nine locations on ice-scoured surfaces were sampled, with two samples from each site subjected to different cosmogenic analyses, to give 26Al and 10Be exposure ages.  There was a considerable consistency in the results, suggesting exposure ages of between 40,000 yrs BP and 30,000 yrs BP.  Of the 18 analyses completed, only one was anomalous.

The authors consider in detail the possibility that the dates are consistently over-estimating the amount of time that has elapsed since the ice retreat from the summit of the island -- and that the real exposure time is c 20,000 yrs BP, which would be more or less in tune with the results from the Scilly Isles.  But they can find no real reason to disbelieve them, so they stick to the conclusion that Lundy was glaciated during the Early or Middle Devensian, at a time when most other authors have assumed cold but non-glacial conditions in western Britain.  Indeed, that has been my conclusion ever since I worked in Pembrokeshire in the 1960's and 1970's.  The stratigraphy there points to a long period of periglacial / permafrost activity in the Eary and Middle Devensian, followed by a short glacial episode in the Late Devensian, followed by an oscillating but generally warmer climate for the period 20,000 - 10,000 years ago, culminating in the cold snap of the Younger Dryas.

So what is the truth of the matter?  Chris Rolfe and his colleagues now think that a long ice lobe might have crossed the Celtic Sea from Southern Ireland, flowing broadly NW towards SE.  It crossed Lundy and might have reached the coasts of Cornwall and Devon.  But they speculate that it might have missed Pembrokeshire.  This is their map of suggested ice limits:


Faithful readers of this blog will see a strong similarity with this map, which I first published in 2011:


The difference is that Rolfe and colleagues have more or less accepted the flowlines of Scourse, Clark, Evans and others, whereas I have not, since I cannot see the sense of a long surge down the centre of the Celtic Sea when there has been nothing to constrain its lateral expansion.  That is glaciologically inherently unlikely......  so the answer has to be that ice crossed the Celtic Sea from an ice-shed somewhere near the coast of SE Ireland, hitting the coasts of SW England and Wales and leaving traces fairly close to present sea-level at many places along an ice front more than 300 km long.

I cannot see that ice from such a piedmont glacier can have overridden Lundy Island while leaving Pembrokeshire unaffected by ice -- that would imply very limited ice extent on the part of the Irish Sea Glacier and the Welsh Ice Cap. 

For the moment, I'll stick with the tentative suggestion that there might be a consistent error somewhere in the dating of the "glacial event" at Lundy, since an early / Middle Devensian age is not actually supported by glacial stratigraphy.  But the debate is blown wide open, and for this, Chris Rolfe and his colleagues deserve much credit.


4 comments:

Constantinos Ragazas said...

Brian,

If Lundy Island in the middle of the mouth of Bristol Channel was glaciated, wont the Bristol Channel with more shallow waters further inland be frozen? And if Pembrokeshire glacier erratics can be found on Lundy Island, wont glaciers also have transported other Pembrokeshire erratics elsewhere?

More nails into the coffin of the Human Transport Theory and its associated Neolithic “quarries” of bluestones! Ultimately Truth will prevail!

Kostas

BRIAN JOHN said...

Yes, if ice covered Lundy island and pressed against the coasts of Devon and Cornwall, there is a chance of a lake being impounded in the inner parts of the Bristol Channel - Severn estuary. And like most lakes it will have had a frozen surface for part of the year. There has been much speculation about this in the literature. It may be that the Valley of the Rocks and the Fremington till / lake deposits are related in some way.

And yes, there are Pembrokeshire erratics in the Vale of Glamorgan, near Bridgend. Not from eastern Preseli, but probably from the St David's area.

Constantinos Ragazas said...

Brian,

Thanks for your response. You write, ”Yes, if ice covered Lundy island and pressed against the coasts of Devon and Cornwall, there is a chance of a lake being impounded in the inner parts of the Bristol Channel - Severn estuary”.

What area would be thus covered if the impounded lake was say 100m deep? I know there are computer simulations that would easily show the extend of such lake …

Kostas

BRIAN JOHN said...

Not much point in speculating at the moment, because we have no idea where the ice margin was....