Possible isochrones or retreat positions for the Late Devensian ice edge across Pembrokeshire in the period 27,000 - 25,000 yrs BP. Very speculative, but an hypothesis worth testing......... The ice was thicker, and the duration of glaciation greater, in north Pembrokeshire than in the area to the south and south-east of Mynydd Preseli.
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........
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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 https://www.researchgate.net/publication/257110370_Response_of_the_Irish_Ice_Sheet_to_abrupt_climate_change_during_the_last_deglaciation[accessed Oct 22 2020].
Quote:
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.
Observations:
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.
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
https://doi.org/10.1002/jqs.3057
Quote:
"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)."
Observations:
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?
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 https://www.researchgate.net/publication/257110370_Response_of_the_Irish_Ice_Sheet_to_abrupt_climate_change_during_the_last_deglaciation[accessed Oct 22 2020].
Quote:
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.
Observations:
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.
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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
https://doi.org/10.1002/jqs.3057
Quote:
"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)."
Observations:
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.
PS.
The radiocarbon dates and other dates are seriously confusing Scourse et al (2019) in their article "Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the Last Glacial: Timing and maximum extent" claimed that the ISIS reached the shelf edge between 27,000 and 24,000 yrs BP. They have also claimed that the last glaciation of the Isles of Scilly occurred around 25,000 years ago. Well, that cannot have been possible if the ice edge was at the same time retreating from the north coast of Pembrokeshire........
So something is well adrift! On the other hand the approximate chronology suggested in this article does fit with the human occupation of the limestone caves of South Pembrokeshire and the Gower around 24,000 yrs BP.
More dates are needed, together with much more re-calibration of the dates already published.
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