The remarkable persistence of Devensian chronology; and more about Abermawr

 

Well, this is fun.  In the light of my recent posts about Devensian glacial chronology, I was struck by the very familiar look of that old climatic oscillation diagram from 50 years ago -- based on the work of West,  Godwin, Epstein, Zagwijn, Peaepe and others in the period 1950 - 1970.  Their pioneering recreations of paleoclimates, based on pollen analyses, oxygen isotope studies, molluscan faunas, varves, radiocarbon dating, dendrochronology etc, were surprisingly accurate -- and they have stood the test of time.

I thought it would be quite entertaining to take part of that brilliant Sidestone Press diagram, flip it onto its side, turn it inside out, and then to set it alongside the diagram based on Richard West and others. Bingo --  an extraordinary match, after five decades of increasingly sophisticated research.  Now of course we have all the modern deep sea and ice core research, computer modelling, marine isotope stages and so forth -- but we can see that the founding fathers were pretty well spot on, not just with the big or high amplitude oscillations but with the minor ones as well, in the period we refer to as the Early and Middle Devensian.

Just a reminder:

MIS1 -- the Holocene, the last 12,000 years, starting at the end of the Younger Dryas stage.

MIS2 -- the Late Devensian or Last Glacial Maximum, lasting from c 31,000 BP to 12,000 BP. The peak in the British Isles was around 26,000 years ago.

MIS3 -- c 70,000 BP to 31,000 BP -- encompassing the Early and Middle Devensian

MIS4 -- around 71,000 BP, the onset of the Devensian / Weichselian / Würm / Wisconsin glacial stage

MIS5 -- subdivided into 5 sub-stages, starting 130,000 BP and ending c 71,000 BP. Eemian / Iswichian Interglacial. Warm peak around 109,000 BP, but considerable oscillations.

This sequence is under constant review, and some versions may be substantially different from the Wikipedia version.

Anyway, it's good to know that here, at least, we have an hypothesis or a hypothetical time-line that has been scrutinized continuously for over 50 years and altered in its details by ongoing research, but which has in its broader outlines survived the test of time.  Karl Popper would have been pleased.........

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From a previous post:

https://brian-mountainman.blogspot.com/2014/10/abermawr-one-of-top-50-quaternary-sites.html

ABERMAWR

This is the most comprehensive exposure of Late Pleistocene deposits in West Wales. There are exposures at both ends of the bay. Storms have revealed an Ipswichian raised beach on a rock platform remnant, and above that there is a sequence of periglacial deposits made up of angular bedrock fragments, but incorporating far-travelled erratics. Above that is a clay-rich Irish Sea till of Late Devensian age and containing striated clasts, fragments of carbonized wood and sea shells. The main components of the till are sea-floor deposits, dredged up by glacier ice moving across the old coastline and later laid down by lodgement and shearing. There are also flow-tills, and the glacial deposits are capped by fluvio-glacial materials, an upper head (referred to in the past as “rubble-drift”), sandy loam and modern soil. The deposits represent a complete advance/retreat cycle close to a glacier margin. In the upper head there are fossil ice-wedges and involutions of Late Glacial age. Beneath the storm beach there are peat beds and remnants of the “submerged forest”, and these organic-rich sediments can be examined in the marsh on the landward side of the storm ridge. There is a continuous stratigraphic record here, probably stretching back c 100,000 years.

Rijsdijk, K and McCarroll, D. 2001. Abermawr, in The Quaternary of West Wales Field Guide, QRA, pp 32 - 38.

John, BS 1970. Pembrokeshire, in Lewis, CA (ed) The Glaciations of Wales and Adjoining Regions, pp 229-265

From Kenneth Rijsdijk's detailed studies at this site, it is clear that the stratigraphy covers a whole glacial cycle -- representing an Early and Middle Devensian ice-free phase, a Late Devensian glacial incursion followed by catastrophic ice melting, and then a colder episode of slope breccia accumulation, as I outlined it in 1965.

There was no Early of Middle Devensian glacial incursion from the north, and therefore it is vanishingly unlikely that glacier ice can have overtopped Lundy Island at this time.  The cosmogenic dating exercise that suggests this must be faulty.

Pseudo-stratified accumulations of slope breccia at Abermawr, probably laid down on an undulating rock surface with remnants of the Ipswichian raised beach occasionally exposed.  Variations in the character of the slope breccia may be related to climatic oscillations and the waxing and waning of permafrost conditions.  These are Early and Middle Devensian deposits. They are overlain by Irish Sea till and meltout deposits related to the LGM glacial incursion.