"Mission control -- we have a problem..."
A few days ago I did a post suggesting that the LGM ice cover across South Wales was far more extensive that previously suggested by a host of researchers over the past 100 years. I also suggested that the ice-free enclave of south Pembrokeshire, shown on many maps, did not exist, since apparently fresh glacial deposits are just as widespread as they are in North Pembrokeshire. So far so good...
But I have chased after a great many people on this blog for ignoring "inconvenient" evidence -- so I must be honest and admit that the hypothesis is challenged by the evidence of human and other mammal bones found on Caldey Island, at Paviland Cave and elsewhere. The problem is that the dating of the LGM in western Britain is conventionally placed at 25,000 - 20,000 yrs BP -- and if the landscape was covered by glacier ice at that time, how come there are a number of radiocarbon dates for bones (mostly found in caves) suggesting ice-free conditions at exactly the same time period?
This is a rather delicious dilemma. Is my hypothesis shown to be incorrect? Or could the dating of the LGM be faulty? Or are the radiocarbon dates for bone samples incorrect, and subject to adjustment in the light of recent research on methodology and correction factors?
One thing at a time. First, what about the dating of the LGM? Well, it appears that it is being pushed back in time by modern research:
James Scourse et al, Marine Geology, Vol 412, June 2019, pp 53-68
It now appears that the ice reached its maximum extent in the Celtic Sea arena around 27,000 years ago -- somewhat earlier than many of us have assumed in the past. After a rapid initial retreat of the ice edge from the shelf edge dated at around 25,000 years ago, there was a stabilisation of the ice front in St George's Channel which lasted from around 24,000 - 22,000 years ago. On the other hand Jenkins et al (2018) suggest that there was an ice edge running across St George's Channel to the Wexford area at about 25,000 yrs BP. Then the ice edge retreated northwards during a period of catastrophic ice wastage, over a period of c 2,000 years. The Welsh Ice Cap lasted longer, and started its major retreat around 20,000 years ago. So the two ice masses were seriously out of synchroneity -- if the cosmogenic and other dating results are to be believed:
Late Devensian deglaciation of south-west Wales from luminescence and cosmogenic isotope dating
N. F. GLASSER, J. R. DAVIES, M. J. HAMBREY, B. J. DAVIES, D. M. GHEORGHIU, J. BALFOUR, R. K. SMEDLEY and G. A. T. DULLER
JOURNAL OF QUATERNARY SCIENCE (2018)
Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet
Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper and Martyn Stoker
Ogof yr Ychen radiocarbon dating: