New article: Was there a Late Devensian ice-free corridor in Pembrokeshire?

 

Citation:
Brian John. 2023. Was there a Late Devensian ice-free corridor in Pembrokeshire? Quaternary Newsletter 158, pp 5-16.

https://www.qra.org.uk/uploads/documents/QN%20158.%20Feb%202023.pdf

COVER PHOTOGRAPH

Flimston Churchyard and its erratic boulders. Some are used as grave headstones. The churchyard is on the limestone coast of South Pembrokeshire, and the erratics are mostly igneous, probably from the St David’s Peninsula. How and when were they transported? (Photo: Louise Trotter)

2 Quaternary Newsletter Vol. 158, February 2023


The Quaternary Research Association has just published my new article on the problem of the "south Pembrokeshire ice free enclave" -- as the lead article in Vol 158 of Quaternary Newsletter. It's good to see it in print at last! It has nothing whatsoever to do with Stonehenge or the bluestones, except maybe in flagging up the great extent of Late Devensian ice in western Britain -- making new evidence available as a supplement to much else that has been published in recent years.

It will be interesting to see what response there is from other specialists in this field -- I'm not really getting after anybody else here, and over the years I have dithered as much as everybody else in trying to understand what the field evidence is telling us.

The plan is that the article should be open access, although for the moment it may be available just to QRA members. Please let me know if you are not able to get at it.

I will in any case try to make the article available on Researchgate and Academia in the near future.

Ice really does flow....

 This is a fabulous image -- Fedchenko Glacier in Tajikistan.  The biggest glacier outside of the polar regions -- 70 km long.

This isn't too bad either -- Bering Glacier in Alaska:

Below:  Barnard Glacier, Alaska (a photo much used in text books)

And finally Corridoren Gletscher, Greenland -- a photo that makes me feel slightly giddy, 

every time I look at it......

Bluestone Country

 

There was a sad event last night -- the winding up dinner of the Preseli Tourist Association, after 43 years of promoting NE Pembrokeshire as a tourist destination and trying to sell the unique character of the landscape and its history.  Over the years we have had tremendous support from the Welsh Govt and from the local authorities, and as a membership-based organization we have supported those involved in the tourist trade, held literally hundreds of talks, site visits and specialist meetings (and convivial social gatherings!), lobbied government,  designed out own promotional campaigns, and produced our own leaflets, maps and web sites. We have never employed anybody -- everything has been funded through membership subs and small grants for specific projects.

But times have changed. When we started, Preseli District Council was our local government admin area, with its HQ in Haverfordwest.  Then came Dyfed CC -- it came and it went away again.  Patterns of holidaymaking have changed, people now get their information via the web, and Visit Wales and Visit Pembrokeshire use all the latest marketing techniques to compete with other tourism areas to attract future visitors. Small "regional" or "sub-regional" marketing exercises don't really have a role any more.  So effectively the PTA has become redundant, having done its job over many years -- just like Eco Centre Wales, which I founded back in 1980 and which promoted energy conservation and renewable energy for 35 years until it was wound up in 2015.

Bluestone Country was our brand -- based of course on the idea that this where the Stonehenge bluestones came from and covering the area across which the Fishguard Volcanic Group rocks are exposed at the surface.  Of course the landscape is unique too -- dominated by Mynydd Preseli and rougher and stonier than the landscapes to the south and west.  Over the years PTA produced a number of leaflets and maps extolling the virtues of the area.

Now, of course the branding label has been stolen from us -- we have the huge Bluestone National Park holiday resort in mid-Pembrokeshire, Bluestone Brewery and assorted other bits of branding as well, not to mention Australia and the USA, which have their own "bluestone branded areas".  Such is life......

Ironically, the Bluestone resort is not in the National Park, and neither is it in an area where bluestone rocks occur -- except as glacial erratics.....

The great stone circles of Weeley and Waun Mawn

 

Anything Weeley can do, we can do better.........

Periglacial Britain

I have mentioned this article briefly in the past, but it now seems more relevant than ever as we try to reconstruct the conditions that might have been experienced in Great Britain before, during and after the LGM.  This is a vast paper -- almost 100 pages long -- and it contains a wealth of information as well as a comprehensive reference list.  I intend to take a more detailed look at it as I try and work out what happened during the growth phase of the Preseli Ice Cap and the Welsh Ice cap and when they were transformed from being independent ice caps into ice domes surrounded by, and maybe partly overwhelmed by, the fast streaming ice associated with the Irish Sea Ice Stream.....

Chapter 5 Periglacial and permafrost ground models for Great Britain 

J. B. Murton & C. K. Ballantyne

https://www.lyellcollection.org/doi/10.1144/EGSP28.5

From: GRIFFITHS, J. S. & MARTIN, C. J. (eds) 2017. Engineering Geology and Geomorphology of Glaciated and Periglaciated Terrains – Engineering Group Working Party Report. Geological Society, London, Engineering Geology Special Publications, 28, 501–597

https://doi.org/10.1144/EGSP28.5

Abstract: 

Periglacial environments are characterized by cold-climate non-glacial conditions and ground freezing. The coldest periglacial environments in Pleistocene Britain were underlain by permafrost (ground that remains at or below 0°C for two years or more), while many glaciated areas experienced paraglacial modification as the landscape adjusted to non-glacial conditions. The growth and melt of ground ice, supplemented by temperature-induced ground deformation, leads to periglacial disturbance and drives the periglacial debris system. Ice segregation can fracture porous bedrock and sediment, and produce an ice-rich brecciated layer in the upper metres of permafrost. This layer is vulnerable to melting and thaw consolidation, which can release debris into the active layer and, in undrained conditions, result in elevated porewater pressures and sediment deformation. Thus, an important difference arises between ground that is frost-susceptible, and hence prone to ice segregation, and ground that is not. Mass-movement, fluvial and aeolian processes operating under periglacial conditions have also contributed to reworking sediment under cold-climate conditions and the evolution of periglacial landscapes. A fundamental distinction exists between lowland landscapes, which have evolved under periglacial conditions throughout much of the Quaternary, and upland periglacial landscapes, which have largely evolved over the past c. 19 ka following retreat and downwastage of the last British–Irish Ice Sheet.

Periglacial landsystems provide a conceptual framework to interpret the imprint of periglacial processes on the British landscape, and to predict the engineering properties of the ground. Landsystems are distinguished according to topography, relief and the presence or absence of a sediment mantle. Four landsystems characterize both lowland and upland periglacial terrains: plateau landsystems, sediment-mantled hillslope landsystems, rock-slope landsystems, and slope-foot landsystems. Two additional landsystems are also identified in lowland terrains, where thick sequences of periglacial deposits are common: valley landsystems and buried landsystems. Finally, submerged landsystems (which may contain more than one of the above) exist on the continental shelf offshore of Great Britain. Individual landsystems contain a rich variety of periglacial, permafrost and paraglacial landforms, sediments and sedimentary structures. Key periglacial lowland land-systems are summarized using ground models for limestone plateau-clay-vale terrain and caprock-mudstone valley terrain. Upland periglacial landsystems are synthesized through ground models of relict and active periglacial landforms, supplemented by maps of upland periglacial features developed on bedrock of differing lithology.

This diagram has some relevance for our understanding of what went on 

on Salisbury Plain.....

Have some of the Penblewyn erratics come from Caerfai?

Red Cambrian sandstones on the beach at Caerfai, near St David's.  These red and purple sandstones outcrop in just one small area on the south side of the St David's Peninsula

I have been looking again at those red sandstone boulders and cobbles found in the roadworks near Penblewyn, near Narberth.  There are quite a lot of them in the glacial deposits -- mostly quite small, less than 50 cm in diameter.  They are sub-rounded, but they do have glacial facets on them, and some are clearly striated.  HH Thomas (yes, he of the bluestones) recorded other red sandstone boulders in the area, and assumed they had come from the ORS red marl beds -- which are exposed around 3 km to the south of the find site.

This is what I said in another post:

The biggest puzzle relating to the glacial deposits at Penblewyn is the relative abundance of boulders and cobbles of red marls -- coloured bright red and pink. What on earth are they doing here, 3 km to the north of the ORS outcrop? Are they derived from ancient river gravels that have been carried northwards from the outcrop and then incorporated into glacial deposits? That would be vanishingly unlikely, because there is no reason to assume that local drainage ever flowed northwards from the outrop, across the Afon Marlais valley (Lampeter Vale) and up the south-facing slope of the ridge. I checked the old Geological Survey Memoir for the Country around Haverfordwest (1914) and found that similar deposits from nearby are described on page 221: "The presence of red marl from the Old Red Sandstone in this drift.........shows that the transportive agency had a certain amount of northerly direction." The author? None other than our old friend HH Thomas. He was confused, and so am I........

So it is vanishingly unlikely that these red sandstone clasts can have been carried northwards by ice or any other transport medium -- so the conclusion is that they are not from the ORS at all, but from the Cambrian sandstones at or near Caerfai.  That would involve ice transport from the WNW -- within the range of erratic transport directions already established by Griffiths and others.

The only alternative would be that the boulders and clasts have come from some other ORS outcrop in Western Britain -- but such outcrops are rare, and they may not even have the same red and purple colouring........

All in all, I incline to the view that the red, pink and purple clasts found in glacial deposits in the Penblewyn - Llanddewi Velfrey area have come from the St David's Peninsula -- but I doubt that they were freshly entrained from bedrock outcrops during the Devensian glaciation.  I am pretty sure they have come from an ancient till deposit somewhere to the west or north.  The boulder surfaces are heavily weathered or rotten, and they still have residues of iron oxide / manganese oxide cement, providing a rather intriguing kaleidoscope of colours!  Patches of concreted gravelly debris still remain, especially in weathering pits on boulder surfaces.  And on the larger boulder collected there are faint traces of striations which cut through the veneer, as seen on the smaller cobble featured in earlier posts:

https://brian-mountainman.blogspot.com/2023/01/jagged-pits-on-ice-moulded-surfaces.html

https://brian-mountainman.blogspot.com/2022/09/the-penblewyn-ors-erratics-interesting.html

That means the boulders and cobbles have been moved in TWO (at least) glacial episodes -- and there must be other ancient till deposits to match those describes at Black Mixen (Lydstep), Witches Cauldron and Ceibwr.  Where are they?  The hunt starts now........

This is a rather battered (and far travelled?) small boulder, faceted, striated and heavily abraded, and once clearly incorporated in a solidly cemented ancient till at a site still be be discovered.

Surface pitting on the boulder, with black cemented gravelly debris in the pits.

There are hairline cracks all over the boulder, so it is something of a miracle that it is still intact.  

Here we can see that in certain light conditions the colour shades towards purple, 

like some of the outcrops near Caerfai.

Postulated ice movement direction for direct clast transport from Caerfai to Penblewyn  -- 

but it might not have been this simple.

 

Ice cap growth on coastal lowlands

 It's always interesting, in glacial geomorphology, when the evidence just doesn't fit your hypothesis and you have to come up with something quite counter-intuitive in order to explain the features on the ground. When my old friend David Sugden and I were working in the South Shetland Islands in 1965-66 we assumed at the outset that all of the major ice streams of the Quaternary must have flowed northwards -- ie away from the West Antarctic Ice Sheet and towards the deeper waters of the Drake Passage, that must have remained largely ice-free. But we kept on encountering major landforms which did not fit the hypothesis, and conversations with Capt John Frost of the RRS "Shackleton" about the presence of deep channels -- in places where one might not expect them -- eventually led us to conclude that the axis of an enlarged South Shetlands Ice cap had not run along the island chain itself, but had been out to sea, over the shallow submerged platform with its skerries and shoals which was very dangerous for shipping.

Bathymetric map of the South Shetlands island chain, showing the shallow platform 

to the north and west.

At the time nobody knew very much about eustatic and isostatic interactions, but we did know that sea-level had been low during the big glacial episodes, and if there had been a sea-level drop of c 120m at a time of extensive and intensive glaciation, then the coastal platform would have been exposed, and available for ice cap growth.  This was the map which we put into the scientific domain:

https://www.researchgate.net/publication/262414644_The_ages_of_glacier_fluctuations_in_the_South_Shetland_Islands_Antarctica

We postulated in a number of articles that as sea-level fell with the onset of a big glacial episode, more and more of the coastal shelf would have been exposed, leading to a northwards migration of the ice cap axis, in response to high precipitation rates to the north and west of the island ice caps, which must already have been in existence.  We thought that there might have been an intervening ice-shelf phase, but we had no way of determining that from the evidence as we saw it.

We suggested that the big glaciation, with ice streams flowing southwards and south-eastwards into the Bransfield Strait via the sounds between the islands,  was not the Devensian / Weichselian / Wisconsin glaciation but the preceding one -- or two, or three........

https://www.researchgate.net/publication/262414489_Evidence_from_the_South_Shetland_Islands_towards_a_glacial_history_of_West_Antarctica

Much ice has flowed and melted since 1966, and although our ideas have stood the test of time there has been a huge amount of research in the South Shetlands and on the Antarctic Peninsula, and it's now apparent that the "offset South Shetlands Ice Cap" which we described has existed on multiple occasions, including -- most recently -- the Weichselian.  So the outlet glacier troughs have been occupied by streaming ice over and again, maybe getting deeper with each successive glacial episode.  Bethan Davies has written about the most recent deglacial phases around the Bransfield Strait. Some of her maps seem to show an LGM ice cap axis over the island chain, with a grounding line on the shelf, and some show cold-based ice grounded between the South Shetlands and South Orkneys.  I must check up on what the latest thinking might be.......

https://www.antarcticglaciers.org/glacial-geology/antarctic-ice-sheet/icesheet_evolution/

https://www.sciencedirect.com/science/article/abs/pii/S027737911100326X

There is now a big literature.  

What interests me about all of this is the extent to which we can extrapolate for our studies of "the South Pembrokeshire problem" -- was the area glaciated during the Devensian, or was it not?

If an ice cap could thicken and grow, and if its axis could migrate windwards across a low-lying coastal platform in the South Shetland Islands within the time-frame of a glacial episode, why not in Pembrokeshire too?  I shall ponder further.....

A slice of one of the maps from Bethan Davies and colleagues, 2012.