Then people became preoccupied with dating. Which glaciations were responsible for the Older Drift deposits and the Newer Drift deposits and landforms? Anglian? Wolstonian? Devensian? Gradually a consensus emerged -- that the Older Drift deposits were of Anglian age (approx 450,000 years old) and that the Newer Drift deposits dated from the Late Devensian (approx 20,000 years ago). Some people (including me) have wondered whether there may be Wolstonian traces in the landscape, and others have argued that there was an extensive ice cover in the Early Devensian as well. But the idea of the Devensian ice-free enclave has survived, without ever being subjected to careful scrutiny.
So let's do some scrutiny.
1. It's assumed that glacial and glaciofluvial deposits are less abundant in South Pembrokeshire than they are north of the LGM line. Well, that's broadly the case, but the evidence has been distorted -- for example in the BRITICE representation where only some deposits are shown in the "Devensian ice-free enclave" on a map that purports to be definitive. If you check out the distribution of ice-related deposits on the BGS geology viewer, and then check back to the observations in the Memoirs, you find that there is no part of Pembrokeshire that is free of erratics and glacial deposits and no particular logic to the designation of some of them to an ancient glaciation and others to a more recent one. (The labelling of the various deposits as either Devensian or "mid-Pleistocene" by the BGS is erratic in the extreme......)
8. I am mindful of the views of Danny McCarroll, Francis Synge and others in claiming that the rocky summits along the north Pembrokeshire coast are heavily denuded by overriding ice whereas the tors of Preseli, the summit of Carningli and the delicate crag of Maiden Castle (Trefgarn Gorge) are not. They have deduced therefore that Devensian ice affected some hills, but not others. (James Scourse and others have used the same argument in relation to the granite tors of the Isles of Scilly, claiming that "tor delicacy" is an indicator of ice-free status during the whole of the Devensian.) This is a reasonable hypothesis, but past workers have failed to recognize just how many ice-moulded rock surfaces there are in the areas they have deemed to be "ice free" -- and there is now abundant evidence for the survival of quite delicate tor-like features in heavily glaciated terrain in Antarctica, Greenland, Scotland and elsewhere. On balance, I think that the crags of Maiden Castle and Lion Rock must have been ice-covered at the time of the LGM.
9. So what are we to make of the moraines and trimline traces that I have described (in previous posts) from the northern flank of Mynydd Preseli? I'm now inclined to think that they are traces of stillstands or minor readvances of the ice edge during the progress of ice wastage. This might resolve some of the issues surrounding "conflicting" or inconclusive field evidence.
10. One key area of direct relevance to the Quaternary history of Pembrokeshire is Gower, which has been studied intensively by senior staff members from the Geography Dept at Swansea University. There has been considerable confusion over the dating of -- for example -- the Paviland Moraine and the Rotherslade deposits. In many past reconstructions the Devensian LGM limit is shown running across the Gower Peninsula. But in the 2015 "Quaternary of Gower" Field Guide there was a distinct shift towards an acceptance that the whole of the Gower was submerged beneath Devensian ice.
11. We need a comprehensive programme of cosmogenic dating across Pembrokeshire, and thus far we have just seven dates derived from 36Cl analyses. In 2019 Prof Danny McCarroll and his colleagues published dates for seven rock surfaces in north Pembrokeshire, four from inside a putative Devensian ice limit (near the coast) and three outside it, from parts of the Carningli upland. The "coastal" dates were between 33,000 and 76,000 yrs BP, and the Carningli dates were between 107,000 and 152,000 yrs BP. This seems to confirm a difference in age, but the authors accept that all of the dates are anachronistic and thus erroneous, involving nuclide inheritance on rock surfaces that were not heavily eroded by ice. There may also have been intermittent rock sheltering, and since the authors only gave six-figure grid references for each sampled site, it is impossible to check where those sites are, and what the local site conditions might have been. For the time being, the dates must be set aside as irrelevant.
In previous posts I have played around with the idea of a much diminished "ice-free enclave", as in this post from 2017:
Late Pleistocene chronostratigraphy and ice sheet limits, southern Ireland
Colm Ó Cofaigh et al
Quaternary Science Reviews, Volume 44, 21 June 2012, Pages 160-179
The morpho-stratigraphic subdivision of the surficial glacial drifts of Ireland into ‘Older Drift’ and ‘Younger Drift’ is a long-standing convention in Irish Quaternary studies. Across southern Ireland a broad swath of terrain has traditionally been interpreted as Munsterian (penultimate glaciation) in age and large end moraine complexes bordering this zone of ‘Older Drift’ such as the ‘South Ireland End Moraine’ have long been regarded as marking the limit of the Late Midlandian (last glaciation) ice sheet. Sedimentary sequences exposed along the south coast of Ireland provide a window into the stratigraphy of the ‘Older Drift’ and have been studied for over a century. The present paper supports a fundamental revision of the traditional interpretation of ice sheet limits in southern Ireland and argues for an extensive last Irish Ice Sheet which covered much of the area of the Older Drift at the Last Glacial Maximum. The basis for this revision is threefold. Firstly, nineteen new optically stimulated luminescence (OSL) dates on the Courtmacsherry Raised Beach and overlying shallow marine sands demonstrate that on the south coast the beach and shallow marine sands formed during marine isotope stages 4–3. Secondly, AMS radiocarbon dates on reworked shells from the ‘Irish Sea Till’, demonstrate that this till and overlying ‘inland’ tills from central and SW Ireland were formed after ∼24 cal ka BP. Thirdly, new OSL dates of 24–21 ka BP from deglacial outwash overlying the Irish Sea Till. Collectively these data are consistent with a last glaciation age for the glacigenic sequence along the south coast of Ireland, thus supporting a fundamental revision of the age of the Older (Munsterian) Drifts of southern Ireland. Most of southern Ireland was glaciated during the LGM and the moraine belts which have traditionally been interpreted as marking the last glacial limit such as the South Ireland End Moraine are reinterpreted here as recessional features formed during ice sheet retreat.