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Saturday, 10 October 2020

The Late Devensian glaciation of West Wales: a reappraisal

What sort of an ice edge was it?

The ice sheet edge in West Greenland.  These images point us to a greater understanding of what happened in West Wales -- but where was the edge located?

During the Late Devensian (Last Glacial Maximum) Pembrokeshire was completely covered by the ice of the Irish Sea Ice Stream, and there was no ice-free enclave in the south of the county.

That has to be the logical conclusion to be drawn from the evidence accumulated over the past few years from both coastal and inland locations.

This is a major shift in my own personal opinion, and it goes against the conclusions of scores of researchers over the last century or so, including Charlesworth, Wirtz, Griffiths, Mitchell and Bowen.  To start with, let's just take a look at a few maps:

Confused?  Well, so is everybody else.  This shows just how confusing the literature is, with the discrepancies in ice edge positions particularly marked in Pembrokeshire.  (Map from the GCR Wales volume, 1989.

The LGM according the Jansson and Glasser, showing roughly the relationships between Welsh ice and Irish Sea ice.

The BRITICE reconstruction showing the assumed Late Devensian ice limit in West Wales and the assumed Pembrokeshire ice-free enclave.  Some of the mapped Quaternary sediments within the "enclave" are shown -- others are inexplicably omitted.

Carr et al (2017) inexplicably show the LGM in St Brides Bay, leaving the southern coast of the bay ice-free, and the outer coasts north and south of Milford Haven also unaffected by glacier ice, in spite of the long-established evidence of Devensian glaciation from sites like Westdale, Mullock Bridge and West Angle.

One of my recent attempts to explain the abundance of apparently fresh till exposures on the South Pembrokeshire coast. 

 The basic idea that has been with us for a very long time is that there is a distinction between "Older Drift"and "Newer Drift" areas, as they were designated by some of the Geological Survey field surveyors and assorted academics.  These terms were used across the British Isles, and they were deemed to be relevant in Pembrokeshire as well; so the belief developed that that Oder Drift area in South Pembrokeshire was different from the north because ice-related deposits were less abundant across the landscape and because they had less surface expression. Charlesworth reinforced this idea in 1929 with his seminal work on the so-called "South Wales End Moraine" associated with lake deposits, meltwater channels and hummocky landforms of both moraine and sands and gravels.  In my own doctorate thesis in 1965 I argued that the "South Wales End Moraine" was of no great significance since the Quaternary sedimentary sequence was not really any different inside and outside the line drawn on a map.  But the idea stuck -- older and degraded deposits outside the limit, and fresher and hummocky deposits inside it.  Almost everybody signed up for it, and instead of questioning it at a fundamental level all of us involved in Quaternary research in West Wales (including me) assumed that there was something in it, and that all we had to do was to refine or correct the position of the line.........

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......)

The distribution of Quaternary deposits across Pembrokeshire, from the BGS geology viewer. Deposits (blue: till and pink: sands and gravels) are found across the county.  The Castlemartin Peninsula is shown as more or less "drift-free" -- but so is most of the St Davids Peninsula, the south Preseli foothills, and the country between Haverfordwest and Milford Haven.

At a higher level of resolution, we see that there are extensive traces of glaciation inland from the inner coast of St Brides Bay.  These are not designated as Devensian by the BGS -- but why not?

2.  It is sometimes stated that the south Pembrokeshire deposits are restricted to interfluves, and that this demonstrates their great age.  I dispute this, and it is not what the geology maps show.  There are well-defined and apparently fresh ice-related deposits in many valleys and low-lying parts of the landscape, including Landshipping and Picton Point, Haverfordwest, around Clunderwen and in the coastal valleys of the Castlemartin Peninsula.

3.  It has never been demonstrated that the south Pembrokeshire deposits are more weathered or rotten than the deposits further north.

4.  The Quaternary stratigraphy associated with the south Pembrokeshire deposits is exactly the same as that of north Pembrokeshire.  I know of no section in which we can see an older till at the base and a fresher till above.  If the south Pembrokeshire deposits really were of Anglian age, almost half a million years old, one might expect a considerable thickness of later deposits (colluvium, slope breccia, and blown sands, for example) to have accumulated on top of them. No such deposits exist.  

5.  In part, the modern acceptance of the idea of a great age for the deposits of south Pembrokeshire is based upon highly suspect dating by DQ Bowen.  He claimed that he had seen an ancient till beneath the raised beach at West Angle, and he also claimed to have observed very ancient glaciofluvial gravels at Llandre; these two sites are now accepted by the BGS as the type localities for the "Penfro Formation" and as proof of an Anglian age for almost all of the ice-related deposits of south Pembrokeshire.  However, as I have explained on this blog, the dating is fundamentally flawed.  No stratigraphy has ever been demonstrated for the Llandre site, and I think DQB completely misunderstood the stratigraphy at West Angle.

6.  It has been assumed by almost everybody (including me) that the Anglian Glaciation was more intensive and more extensive than that of the Late Devensian -- and that it therefore makes sense for the south Pembrokeshire deposits to be old, and for the north Pembrokeshire deposits to be young.  But that idea seems increasingly untenable, with research showing a Late Devensian ice extent very much greater than was believed twenty years ago.  If Late Devensian ice reached the shelf edge in the Celtic Sea, 500 km SW of St George's Channel, and if the ice obeyed the laws of physics, the Irish Sea Ice Stream must have spilled across the whole of Pembrokeshire. 

7.  I'm also increasingly unconvinced by my own hypothesis of an ice lobe pushing eastwards around the south Pembrokeshire coast, past Caldey Island and into Carmarthen Bay, with the cliffline acting as a buffer or obstacle to a landward spread.  I have speculated about a powerful piedmont glacier lobe pushing eastwards onto the Pembrokeshire coast -- but I am coming to the view that the whole of Pembrokeshire was most probably affected by ice from the NW flowing towards the SE. Independent evidence from Ireland and from the offshore zone seems to support this.  In other words, the Irish Sea Ice Stream dominated and resisted any pressure coming from Irish ice which might otherwise have crossed St Georges Channel and affected West Wales.

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:

But then we add in the recent evidence of fresh glacial deposits along the south Pembrokeshire coast, as on this map:

In addition, I'm now convinced that there is till near Ragwen Point and at Amroth -- which means that in all likelihood the WHOLE of the Pembrokeshire coast shows signs of Devensian glaciation.  Bearing in mind all of the above points, I propose that the pattern of glaciation at the time of the LGM was something like this:

So there's the hypothesis.  What we now need is research in the field and and also cosmogenic dating to test whether the hypothesis stands up under scrutiny.....  and if anybody out there has any hard evidence which contradicts my suggestions, please let us know!


PS.  Re my point (10) above, Prof John Hiemstra kindly reminds me that there isn't actually a consensus  in Swansea on the timing of glaciation on the Gower -- with some staff members still of the view that at least some of the sediments are pre-Devensian and that not all of the Gower was ice-covered at the time of the LGM.  Some exciting offshore work is in progress, and it will be interesting to see what that shows up.


PPS.  There was a similar discussion a few years ago with respect to the "Older Drift"and "Younger Drift" of southern Ireland:

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.

This QSR paper is now 8 years old, and I am not sure how the dating of Irish Sea till from SE Ireland fits in with the more recent dating that suggests that deglaciation occurred much earlier than 24,000-21,000 yrs BP.......

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