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Thursday, 22 October 2020

Chronology of the Late Devensian Irish Sea - Celtic Sea ice Stream

Possible retreat positions of the Late Devensian ice edge across Pembrokeshire in the period 27,000 - 25,000 yrs BP, shown in black.  The ice was thicker, and the duration of glaciation greater, in north Pembrokeshire than in the area to the south of Mynydd Preseli.

I have referred to these publications before, but having looked at them again I'm coming to the view that  the LGM in West Wales was around 27,000 years ago, at which time the whole of the landscape was inundated by ice.  That's when the South Pembrokeshire deposits were laid down -- thin and patchy maybe, but in the same stratigraphic sequence as the deposits of north Pembrokeshire. The ice thickness across south Pembrokeshire was thin and maybe (at least for part of the time) cold-based.

Then there was a rapid deglaciation, with a major stillstand or readvance which affected north Pembrokeshire around 25,000 years ago -- and giving rise to many of the features which Charlesworth described as components of the "South Wale End Moraine."  Simultaneously there may have been an expansion of Welsh ice across parts of Gower, west Carmarthenshire and Ceredigion.  There may even have been some reorganization of deposits laid down at the time of the LGM.  And ice may have survived or even expanded in the uplands in the form of a Preseli ice cap. 

After that, the ice edge retreated rapidly across Cardigan Bay and cleared Llyn around 23,000 years ago.    there were also a number of ice edge oscillations associated with the Welsh Ice Cap to the east and the Irish ice cap to the west.  After a total ice melt, there might have been a regeneration of small glaciers (including one in Cwm Cerwyn) in the Younger Dryas.

So the deposits of south Pembrokeshire ARE older than those of north Pembrokeshire -- albeit by just a couple of thousand years.  The ice edge across south Pembrokeshire probably retreated westwards or north-westwards.

And how does the evidence of the bone caves and the radiocarbon dating record slot into this scenario?   As they say, time will tell........ 


Response of the Irish Ice Sheet to abrupt climate change during the last deglaciation. March 2012
Quaternary Science Reviews 35:100–115
DOI: 10.1016/j.quascirev.2012.01.001
Jorie Clark, A. Marshall McCabe, David Bowen, Peter U. Clark[accessed Oct 22 2020].

The timing of advance to the LGM limit in the Irish Sea basin and onto the southern continental shelf is not well constrained. On the Irish Sea coast, a 14C age on reworked shells from till on the Ards Peninsula suggests ice advance across the site sometime after 28.6 cal ka (Hill and Prior, 1968). Otherwise, limiting 14C ages areonly available to constrain advance of the southern ice-sheet margin onto the continental shelf sometime after 24.2 cal ka (O’Cofaigh and Evans, 2007). Trimlines in the Wicklow Mountains 120 km to the north suggest the LGM IIS surface elevation over the central Irish Sea coast was 600 m (Ballantyne et al., 2007). Corresponding isostatic depression is indicated by fossiliferous raised marine deposits, ice-contact deltas and glaciomarine morainal banks along the Irish Sea coast that record high relative sea level associated with the northward retreat from the LGMmargin (Eyles and McCabe, 1989). The marine limit that formed at 30 m asl at Kilkeel following retreat of LGM ice in the Irish Sea, for example, suggests isostatic depression of 160 m (Clark et al.,2004), consistent with the LGM ice loading reconstructed by Ballantyne et al. (2007). A calibrated 14C age from the base of a marine core east of Killard Point (Kershaw, 1986) suggests that deglaciation of the Irish Sea began 23.3 ka (Fig. 4d). This is earlier relative to coastal sites which remained ice covered until 20 ka (see below), suggesting that a deep reentrant initially developed in the core of the Irish Sea with the ice margin remaining on the basin margins. It is likely that the ice margin stabilized at this point as it retreated onto the coast, and may have formed reequilibration moraines, perhaps such asthe Bride moraine on the Isle of Man. This early deglaciation of the Irish Sea combined with the limiting ages that suggest ice advance across the southern Irish coast 24.2 cal ka (O’Cofaigh and Evans,2007) indicates that the ice margin in the Irish Sea advanced and retreated in 1 kyr (Fig. 4d). The timing of this fluctuation corresponds to a large peak in IRD flux in cores to the south and southwest of the Irish Sea (Scourse et al., 2009) as well as to the northwest of Scotland (Knutz et al., 2007) (Fig. 4g). The IRD includes a source from the British and Irish ice sheets as well as from the Laurentide Ice Sheet associated with Heinrich event 2 (H2) (Fig. 4h), reinforcing the hypothesis byMcCabe and Clark (1998) and Bowen et al. (2002) that the IIS was particularly sensitive to climate change at the time of Heinrich events. Subsequent deglaciation as a deep reentrant may have developed by way of a calving bay (Eyles and McCabe, 1989). Additional dating is required to evaluate this hypothesis. Along the western IIS margin, McCabe et al. (2007a) concluded from the stratigraphy at Glenulra that following a short-lived advance that overrode the site at 28 cal ka B.P. the site remained unglaciated during the LGM and subsequent deglaciation.

Much of the evidence in this paper comes from mainland Ireland, where there was a complex relationship between the Irish Ice Cap and the “British and Irish Ice Sheet / Celtic Ice Sheet” which was supplied for the most part from the mountainous areas on the western flanks of Scotland and NW England. Nonetheless, since the coast of SE Ireland is only 60 miles from Pembrokeshire there must have been a degree of synchroneity on the two sides of St Georges Channel. It’s suggested by the dating (up to the year 2012) that a buildup of ice was going on for at least 3,000 years prior to 27,000 yrs BP, which was approximately the date at which the ice edge in the Celtic Sea reached the shelf edge. At that time the ice surface elevation may have been around 600m over the Central Irish Sea coast. 

Was there a massive surge, and did the ice stream in the Celtic Sea really have the shape of a long thin lobe?  I have my doubts on both of those matters; and as I have said often enough on this blog, narrow long lobes do not make sense in glaciological terms in unconstrained situations.  The jury is still out on the lobe's dimensions and on its ice surface gradient.

 The ice edge may have been at its LGM limit only for a short period of time, after which there was a rapid (catastrophic?) deglaciation, with the edge retreating c 500 km over the course of 2,000 years — that’s 4 km per year -- a phenomenal rate of retreat. The deglaciation of the Irish Sea Basin (ie north of the Llyn Peninsula) occurred after 23,000 yrs BP, and was much more patchy and discontinuous.


In this map the "Older Drift" terrain to the south of the South Ireland End Moraine is deemed to belong to the Late Devensian glaciation -- but the "end moraine" may represent a significant readvance or stillstand during overall ice wastage.

Ice margin oscillations during deglaciation of the northern Irish Sea Basin
R. C. Chiverrell R. K. Smedley D. Small C. K. Ballantyne M. J. Burke et al
Jnl of Quaternary Science
First published: 31 July 2018

"Retreat of ice margins across the Llŷn Peninsula have been dated to between 23.9 ± 1.6 and 21.1 ± 0.6 ka (Smedley et al., 2017a). The Celtic Sea advance of the ISIS has been suggested to have been a rapid and short‐lived event (Chiverrell et al., 2013) and was followed by rapid retreat (Smedley et al., 2017a; Small et al., 2018). Advance and rapid retreat of this nature is likely to have been accompanied by significant drawdown of the ice stream surface and was invoked to explain changes in the retreat of the western lateral margin of the ISIS (Small et al., 2018). The ages for ice thinning in the mountains of the Isle of Man and Cumbria are older than previously published ages in the range 18–16 ka for ice‐free conditions in the Cumbrian Mountains (Ballantyne et al., 2009; Ballantyne, 2010; Wilson et al., 2013; Wilson and Lord, 2014), where a locally nourished ice field persisted after deglaciation of the NISB. They are fairly similar, however, to surface exposure ages indicating the timing of emergence of higher ground in SE Ireland (∼24–21 ka), the Wicklow Mountains of eastern Ireland (∼22–21 ka) and North Wales (∼20–19 ka) (Ballantyne et al., 2006; Glasser et al., 2012; Ballantyne and Stone, 2015; Hughes et al., 2016)."

The evidence is stacking up for a retreating ice margin at the northern end of Cardigan Bay — in the vicinity of the Llyn Peninsula between 24,000 and 21,000 yrs BP. Thinning of ice and emergence of high ground in the period 22,000 - 19,000 yrs BP seem to support this. After that, oscillating retreat, with multiple short-lived advance-retreat cycles across the Northern Irish Sea Basin — and also a change from a powerful ISIS to glaciation dominated by ice from the Lake District and Southern Uplands — a transfer from a regional to a more local style of glaciation, especially in the eastern part of the NISB (as in Svalbard). Further west, there was a deeper channel and so ice-calving was going on.

So it makes sense for there to be an ice margin in St George’s Channel around 26,000 - 25,000 yrs BP. Was there a major advance at the time?

Ice directions and well-established ice margin positions in the southern Irish Sea / St Georges Channel area.  Adapted from a map by Jenkins et al, 2018.  The ice edge position shown by the white line across Pembrokeshire may have been approximately right at one stage, but this was not the position at the LGM.  At the LGM (approx 27,000 yrs BP?) the WHOLE of Pembrokeshire was probably ice covered.

Wednesday, 21 October 2020

The Late Devensian ice edge and the inconvenient bones

"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:

Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent
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
ISSN 0267-8179.
DOI: 10.1002/jqs.3061 

There are a host of issues here, still to be resolved.  The Celtic Sea ice edge at c 25,000 yrs BP is variously placed at the shelf edge and in St George's Channel -- one postulated position is almost 500 km from the other!  No matter how catastrophic the collapse of the ISIS (Irish Sea Ice Stream) was, somebody has the dating all wrong.  Also, we have problems of the ice build-up, which is always much slower than ice wastage.  So when was west Wales first affected by the ice of the ISIS?  It must have been well before 30,000 yrs BP.  So should we start thinking about Early and Middle Devensian ice edge positions?  (Of course, there is a school of thought that says Lundy was ice-covered during an Early Devensian glacial episode.)  And what about the binge-purge scenario postulated by Alun Hubbard and his colleagues some years ago?  Was there a massive surge, as suggested by James Scourse and others?  And if so, what was the footprint of the ISIS at the time?  Many questions and no adequate answers..........

Quaternary Science Reviews Volume 28, Issues 7-8, April 2009, Pages 758-776
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

On the evidence currently available, it seems most likely that the land surface of Pembrokeshire was covered by ice for around 5000 years (c 30,000 yrs BP - 25,000 yrs BP) and maybe longer, and that there was then a catastrophic ice edge retreat with a transformation from glacial to tundra conditions.

Source:  A Mid-Upper Palaeolithic human humerus from Eel Point, South Wales, UK.  
Rick J Schultinga, Erik Trinkaus, Tom Higham, Robert Hedges, Michael Richards, Bernice Cardy,  Journal of Human Evolution, 2005

On to the animals and the bones. 

I have done a number of posts on this before, focussing on Caldey Island and Paviland.  

Ogof yr Ychen radiocarbon dating:

Sadly, most of the research on animal and human bone samples from the limestone caves is now rather old, meaning that the radiocarbon dates cited by Schulting, David and others need to be recalibrated.  In Chapter 1 of Vol 1 of the Pembrokeshire County History (2016), Elizabeth Walker seeks to unravel the assorted bits of evidence relating to the animal occupation of caves and the human occupation as well.    She suggests that there was human use of the Coygan Cave near Laugharne between 66,000 yrs BP and 38,000 yrs BP, and that mammoth, woolly rhinoceros, reindeer, bison and brown bear all roamed about in a tundra landscape at the time.  Evidence from Hoyle's Mouth near Tenby, suggests human presence around 32,000 yrs BP, and animal remains have been dated to the period 35,000 - 29,000 yrs BP.   The Paviland Cave evidence suggests human occupation between 28,000 and 21,000 yrs BP, and the famous radiocarbon date on the "Red Lady" skeleton is now put at 24,490 yrs BP.  The radiocarbon date from a human bone found at Eel Point on Caldey Island was dated by Schulting et al as 24,470 yrs BP, again suggesting the presence of human hunters around Carmarthen Bay at the peak of the last glaciation locally, when ice cover must have been very extensive. There are other animal remains from caves dated around 22,350 yrs BP (our "rhino date" from Ogof yr Ychen) and 22,800 yrs BP (barnacle geese bones from Little Hoyle cave) -- but the general assumption among archaeologists is that there was a "human settlement hiatus" between 24,000 and 12,600 yrs BP all around the South Wales coasts.

The Eel Point human humerus, dated as 24,470 yrs BP.  But it might not actually have come from Eel Point......

A plot, by Schulting et al (2005) of the radiocarbon dated bone samples from South Wales in the period 17,000 - 30,000 yrs BP.

This is all very confusing, made even more confusing by the current opinion that the "Eel Point human bone" did not come from Eel Point at all, and the assumption that nearly all of these radiocarbon dates need recalibration -- which tends to push dates back in time by highly variable amounts.  Also, some of the studies in which these dates are published do not accurately show the cave stratigraphy which might enable us to identify episodes of environmental change.  Which bone caves, for example, were covered by Devensian ice and which ones were not? 

There is no problem at all over the ability of both grazing and carnivorous animals  and human beings to exist and indeed thrive very close to an active ice edge -- and indeed at the present day musk-oxen, Arctic hares, wolves, foxes and reindeer move about freely very close to the edge of the Greenland ice sheet and even cross glaciers when necessary. 

Muskoxen and morainic ridge, close to Russell Glacier, NE Greenland.

But there are substantial dating issues here, with uncertainties and debates around the precise dating of glacial events and around the precise timing of animal and human occupation of this glaciated landscape.

Tuesday, 20 October 2020

Carn Goedog and the moon


I thought I'd share this one -- not that I'm all that keen on lunar alignments and so forth.  It's just a nice photo -- probably photoshopped, but who cares?
Not sure who took it.  Location -- difficult to sort out, but I think it's taken at Carn Goedog, looking east towards Foel Drygarn......

Sunday, 18 October 2020

Jacking and ripping -- new thoughts on glacial erosion?

This is an interesting article which purports to have "discovered" a new process of glacial erosion during studies undertaken in Sweden.  Adrian Hall et al suggest that late in the last glacial cycle in Scandinavia, conditions existed on the ice sheet bed  in which "groundwater overpressures" were capable of exploiting bedrock fractures and lifting the surface rock mass, with "fracture dilation" and the creation in some cases of "fracture caves". The authors use the term "hydraulic jacking".   This facilitated bedrock disruption associated with reduced frictional resistance along fractures.  Then came subglacial ripping and displacement, transport and then emplacement of multiple large blocks, left after ice melting as vast spreads of large sharp-edge rock masses dragged up to 100m from their places of origin.

That all sounds very interesting, but I am not sure how well that all fits with the laws of physics.  Is it really possible for "groundwater overpressures" to be so high that they can force water into fractures with so much force that the surface rock mass, and the overlying glacier ice burden, can physically be lifted, allowing sediment to then accumulate in the cavities thus created?  The authors propose that this only happens when an ice sheet is in an advanced state of decay, and after considerable ice thinning,  but I have my doubts........

The authors are justified in drawing attention to the vast boulder spreads that cover much of Sweden, but their map of course incorporates many areas of ground moraine, ice-edge morainic accumulations and many glaciofluvial deposits as well.  Yes, there was warm-based ice flow and high meltwater fluxes, but that does not mean groundwater pressures were capable of lifting millions of tonnes of rock / ice overburden.  Also, I have a problem with the sediment fills of opened fractures, which the authors suggest as some sort of proof of the powerful hydraulic pressures required to "lift" masses of bedrock.  If the water pressures were high enough to lift millions of tonnes of overburden, there is no way that sediments could have survived without being expelled or flushed out.  Also, there is no adequate explanation of how "ripping" or block removal is supposed to work;  plucking or dragging of bedrock slabs from the lee side of roches moutonnees is generally -- and quite sensibly -- assumed to be associated with freezing conditions on the glacier bed.  How does that equate with the large volumes of meltwater and groundwater thought to be flowing through fractures, caves and conduits on the glacier bed?  

The authors show in two diagrams how hydraulic jacking, rock disintegration and ripping are assumed to operate on roches moutonnees and on flat-slab hills -- but I am not convinced.  Some of their examples come from the east coast of Sweden, and I'm familiar with evidence of roche moutonnee disintegration on Rödlöga Storskär -- but I think that the jointed bedrock there has simply disintegrated because of high compression under thick compressing ice, pressure release under thinning ice, and conventional freezing-on and block removal afterwards.  In the examples I'm familiar with, I see no need for a new theory involving high volumes of meltwater under extreme pressure.

Roches moutonnees in the process of breaking up on the island of Rödlöga Storskär in the Stockholm Archipelago.  I do not think meltwater at high pressure is needed to explain the fractures, caves, chasms and block removal that is much in evidence.

But it's always good to see new theories, and we'll see what response this paper gets from other specialists in ice physics.

Adrian M. Hall, Maarten Krabbendam, Mikis van Boeckel, Bradley W. Goodfellow, Clas Hättestrand, Jakob Heyman, Romesh N. Palamakumbura, Arjen P. Stroeven & Jens-Ove Näslund (2020) Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion, Geografiska Annaler: Series A, Physical Geography, 

In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultant boulder spreads are covers of large, angular boulders, many with glacial transport distances of 1–100 m. Boulder spreads occur alongside partly disintegrated roches moutonnées and associated fracture caves, and are associated with disrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1–4 m. This compares with 0.6–1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Hence glacial ripping is a highly effective process of glacial erosion.

PS.  It would be fun, wouldn't it, if this is just an elaborate joke published on April Fools Day?  I can't get it out of my head that there was this fellow called Jack the Ripper, whose name is too close for comfort to the new process being proposed........  Only joking -- or am I?!!

Saturday, 17 October 2020

Quaternary sediments at Marloes


Annotated BGS map for the Marloes Bay area

Here we go again.  Yet another important coastal site displaying abundant till, glaciofluvial sands and gravels, and slope breccia accumulations that appear to date (yet again) to the Devensian.  I can find no reason why they should be considered older -- and they tie in very nicely to the exposures at St Bride's, Westdale Bay and Mullock Bridge.

Today my wife and I walked along the coast path and checked that the sediments shown on the geological map do indeed outctrop along the cliffline.  Indeed they do, in abundance -- but the mapping of some areas as sands and gravels and other areas as till is pretty arbitrary.  In reality, the sediments are very mixed up with the deposits grading into one another laterally.  We see nothing here that looks like the clay-rich blue-grey Irish Sea till of the North Pembs coast --  but that's not surprising since the ice here must have come from the NW, flowing across land and generating what I have called elsewhere a "land facies" of the till which is an exact stratigraphic equivalent.  I have referred to the sticky clay till with shell fragments and lignite as the "marine facies" -- formed where the ice has entrained sea-floor sediments before crossing the coast (which of course was not there at the time).

Glaciofluvial sands and gravels exposed on the clifftop near Raggle Rocks

Gravelly till on the clifftop west of Raggle Rocks.  This till is seen along the whole of the clifftop between Gateholm and Hooper;s Point -- a distance of c 2 km.  It is always underlain by coarse slope breccia and sand and silt layers; and it is overlain by sandy loam and colluvium, incorporating modern soil.

The focus of attention at Marloes must be the stream valley which is followed by the footpath called Sandy Lane.  It's remarkably similar to the stream cutting at Druidston -- but there the valley's "drift fill" comprises Irish Sea till for the most part.

Bing satellite image of the stream exit, showing the locations of the two key exposures.

The "Sandy Lane Valley" seen from the west.  Note that behind the cliffline there is a reverse slope with stream cuttings exposing up to 20m of slope breccia and till

Western exposure

If you walk down to the beach via Sandy Lane, this exposure is on the clifftop to the right of the concrete steps.  One can see a broken section of sediments c 4-5m thick, in an inaccessible position.  the sequence is as follows:

4. Sandy loam with modern soil -- c 1 m thick
3. Diamicton with a large variety of stone types, colours and sizes and incorporating much broken bedrock material,  interbedded with irregular layers of sand, silt and gravel.  These layers are discontinuous. Thickness c 1.5m.
2. Slope breccia made of Silurian sandstone and shale fragments and incorporating some sandy horizons.  Several different facies with irregular junctions.  Direction of downslope debris travel uncertain, but there may be signs of debris flows?  In the eastern part of the exposure, the sandy layers are concentrated near the base.  Greatest thickness c 3.5m.
1.Very coarse rockfall debris c 50 cm thick, in contact with a steeply sloping sandstone rockface.

Western part of the western exposure, showing slope breccia overlain by interbedded till and sandy and gravelly layers (lighter colouring).  Brownish sandy loam near the surface.

Eastern part of the same exposure, showing c 3.5 m of accumulated slope breccia interspersed with irregular and discontinuous sandy and silty layers.  Total thickness c 3.5m.

Interpretation -- a long period of slope breccia accumulation with climatic oscillations.  Prevailing climate -- probably periglacial.  Some episodes of sand and silt accumulation (as colluvium or hillwash, or maybe as blown sand?).  Early / Middle Devensian?  Then a relatively short-lived glacial incursion by ice that has travelled across the Dale Peninsula from the NW.  Accumulation of meltout till or flowtill with interspersed layers of sands and gravels in a chaotic ice wastage environment.  Late Devensian?  Finally accumulation of Holocene sandy loams and blown sand in an interglacial environment.

Eastern Exposure

This exposure can be seen if you turn left on reaching the beach via the concrete steps.  It's quite spectacular, and the sediments rest on a very prominent but broken-up rock (raised beach) platform c 5m above the beach.  You can climb up onto the platform if you take care. Once there, the Quaternary sequence can be examined closely without danger to life and limb.

This is a seriously interesting exposure.  I have not seen anything quite like it anywhere else on the Pembrokeshire coast.  

The essential features are as follows:

6. Modern soil c 20 cm
5. Sandy loam c 80 cm -- discontinuous
4. Blown sand? c 30 cm
3. Diamicton with varied stone types and sizes in a sandy and gravelly matrix and interbedded discontinuous sand and silty layers. Complex internal structures, assumed glaciotectonic in origin. Greatest thickness c 3m, but to right of section, above slope breccia less than 1m thick.
2. Silt and clay band, discontinuous and irregular, c 10 cm.
1. Flaky sandstone slope breccia with some larger fragments. c 80 cm thick -- but thickens to right of section to more than 2m thickness.
Rock platform eroded across steeply dipping Silurian sandstones.

Interpretation -- The diamicton has to be interpreted as a meltout till or flowtill, resting on a thin silt-clay (basal ice?) layer and interspersed with apparently water-lain sandy and silty lenses.  These are similar to the features seen at Abermawr, in the upper till layer above the Irish Sea till.  

The silt and clay band which lies between the slope breccia and the overlying gravelly till.

Interbedding of silty and sandy layers with horizons of gravelly till with abundant stone inclusions mostly of local origin

Interpretation and stratigraphic relationships around the vertical contact between tectonized till to the left and block slope breccia to the right. 

Sediments at the top of the section, accumulated following ice wastage.  

Because of the vertical contact between gravelly till and relatively undisturbed slope breccia, it is assumed that overriding glacier ice has eroded away most -- but not all -- of the slope breccia in this relatively low-lying enviromnent.  The structures, and the complex and variable stratigraphic sequence, suggests that there was a complex environment here, as ice descended from the Dale Peninsula plateau down towards the base of the cliffline, with much removal of preexisting sediments.  Later, there was chaotic ice wastage and an accumulation of sediments that were saturated and highly mobile.  In the modern jargon, this was a "paraglacial" environment in which sediments were mobilized and rearranged.   In that respect, the environment was very similar to that of Westdale, less than 2 km away, to the SE.

The only other place where till is seen at a low level, at the base of the Marloes cliffs, is in the little valley called Mathew's Slade.  It's a typical till for this area, but the lower part of the valley is filled with landslide material and fallen blocks of bedrock, and I suspect that the till has been carried downslope on a "raft" of landslide debris:

I was intrigued to find that none of the sediments examined is cemented, and even the lowest layers in the sequence are easy to excavate with a trowel.  To me, this suggests a recent (Devensian) age for all of the sediments currently exposed.

Is the raised beach visible hereabouts?  It does not appear to be present on the rock platform which I examined, but adjacent to the path at the top of the concrete steps, there is a poor exposure at the base of a grassy slope in which a number of large well-rounded boulders appear to be in situ.   This might be an exposure of the beach, at about the same level as the rock platform at the mouth of the valley -- but I did not have the time for excavations to check this out.

I'd very much welcome other opinions on the Marloes exposures from somebody with the time and resources to undertake detailed study........

Monday, 12 October 2020

Mountain summits with blockfields were not necessarily nunataks

Top two photos: Brecon Beacons.  Lower photo:  Foel Drigarn and the upland ridge of Mynydd Preseli. During the LGM did these uplands support extensive icefields of thin cold-based ice which acted in a protective capacity?

For many years it has been assumed that felsenmeer (or blockfields of shattered rock) on mountain summits in glaciated areas were probably nunataks in the last glacial episode.  The line of reasoning has always been that if glaciated rock slabs, erratics and till are concentrated on the lower mountain slopes, then those are reliable signs of glaciation -- usually running up to a discernible "trim line".  Above the trimline, the reasoning is that periglacial conditions must have existed, with ongoing rock shattering, solifluction and ground ice processes operating over much longer periods of time than beneath the trim line.  That all sounded logical enough -- although in North Wales and other areas the nunatak debate has been quite an active one, with some experts believing that the ice of the Welsh ice cap MUST have covered all of the summits during the LGM, if the ice had been obedient enough to obey the laws of physics.

Well, this 2004 article is very interesting, since it shows (quite convincingly, I think) that trimlines on mountain sides may simply show the positions at which the thermal conditions of the ice mass changed from cold or polar ice at higher altitudes (related in part to limited ice thickness and limited movement or activity) to warm-based ice lower down, with enhanced ice flow, landscape modification, erratic entrainment, and till deposition.  

This does not show that there were no nunataks -- and clearly they must have existed at various stages of the glacial cycle -- but it does show that the presence of blockfields and the lack of a till cover cannot be used as reliable indicators of ice-free status.

As I have suggested at various times, the summits of the uplands of mid Wales, the Brecon Beacons, the Black Mountains and Mynydd Preseli may have supported local ice caps which were quite thin, where the ice acted in a protective capacity while erosion was concentrated along flowlines within the main valleys and landscape depressions.  This of course  conforms with glaciological theory, and it is built into all of the modelling work done by Henry Patton, Alun Hubbard and others over the last 20 years.

Here is the article:

Felsenmeer persistence under non-erosive ice in the Torngat and Kaumajet mountains, Quebec and Labrador, as determined by soil weathering and cosmogenic nuclide exposure dating

Geneviève C. Marquette, James T. Gray, John C. Gosse, François Courchesne, Lisa Stockli, Gwen Macpherson, and Robert Finkel

Can. J. Earth Sci. 41: 19–38 (2004) doi: 10.1139/E03-072


Soil analyses and terrestrial cosmogenic nuclide exposure dating are combined and a conceptual model proposed to explain altitudinal weathering contrasts in high-latitude highlands. We show that summits in the Torngat and Kaumajet mountains were covered by ice during the Last Glacial Maximum, and that their felsenmeer cover probably survived multiple glaciation events. For similar lithologies, soils on felsenmeer covered summits are signigicantly more weathered than those below the felsenmeer limit, displaying higher concentrations of crystalline iron, amorphous aluminium, and silicium extracted with oxalate. Secondary minerals such as gibbsite and kaolinite occur in felsenmeer soils, whereas those formed in till lacked these secondary minerals. 10Be and 26Al exposure ages for nine of ten samples, from high-elevation tors and autochthonous felsenmeer blocks, range from 73 ± 6 to 157 ± 15 ka. By contrast, ages of 11.4 ± 1.0 and 11.7 ± 1.0 ka are measured for bedrock in the much lower Saglek zone, indicating extensive (>3 m) glacial erosion of this zone during Late Wisconsinan glaciation. 26Al/10Be ratios demonstrate that exposure of the high-elevation surfaces was interrupted during at least one cosmic ray shielding event by either ice or till cover. In either case, Late Wisconsinan glaciers could not have extensively eroded these surfaces. Five erratics dated above the Saglek zone, including one in the felsenmeer zone, have exposure ages ranging from 11.6 ± 1.0 to 13.6 ± 0.7 ka. This indicates that valley and high-elevation ice persisted through the Younger Dryas Chron and provides further evidence that the highlands were not nunataks during the Late Wisconsinan period.

Sunday, 11 October 2020

Devensian (?) till at Locks Common, Porthcawl


Many thanks to Greg Nuttgens for drawing this to my attention.  And thanks to him for the photos.  On Locks Common, Porthcawl, on the west-facing coast, there are remnants of an old raised beach platform with exposures of what must be a till with a reddish colour and a highly varied content of stone inclusions. Many different shapes and colours -- and origins.  The grid reference is SS805774.  I assume from the appearance of the deposits that they are uncemented.

We should not be surprised by this -- till patches are shown on the geological map, and it's clear that the extensive blown sands in the area overlie glacial sediments (both till and sands and gravels) which extend well inland.

It will be good to get to this site and take a look -- just to see what the full stratigraphy may be. 

This confirms in my mind that the Devensian ice edge as portrayed in much of the literature is incorrect, and that it lay to the south of Porthcawl .......