It's great to see the publication of two big new articles on the Devensian of Wales which move forward our understanding of both the sequence of events (with new dates) and the nature of deglaciation. This is the first article, which I shall refer to as Glasser et al (2018).
The authors have used luminescence and cosmogenic dating, and have also incorporated information from other studies. The most interesting feature is the conformation (the evidence looks pretty strong) that the maximum extent of the Irish Sea Glacier occurred about 27,000 years ago -- that's a full 7,000 years earlier than I and many others have assumed in the past. the second feature -- suggested by others in the past -- is that the behaviour of the Irish Sea Glacier was out of step with that of the Welsh Ice cap, which seems to have remained in existence as a major feature until it started to retreat around 20,000 years ago. This all has major implications for our understanding of Devensian events in SW Wales, where some territory was affected by Irish Sea Ice, some was apparently ice-free, and some was affected by the ice of the Welsh ice cap. Just one small gripe -- I would have liked a consideration of the role of local ice caps, as suggested by Henry Patton and others -- but that's a small point. Overall, this is an excellent and highly influential paper. Read it!
With respect to the Devensian ice edge in the Preseli district, we have discussed the BGS reconstruction (above) many times, and have decided that it is more or less correct. I still think the Devensian ive overtopped at least part of Carningli - Dinas Mountain, and I think it reached Gernos-fawr, but let's argue about that some other time.......
This is the reconstructed ice edge from Glasser et al for this area. As we can see, it's very similar. the ice edge is dated to about 27,000 yrs BP, and it's suggested that there was still an ice cover on the upland north of the Nevern Valley, bounding the accumulating fluvio-glacial deposits around 26,500 years ago. The sands and gravels in the quarries of Pantgwyn and Trefigin are dated to 26,700 years old.
Late Devensian deglaciation of south-west Wales from luminescence and cosmogenic isotope dating
N. F. GLASSER, J. R. DAVIES, M. J. HAMBREY, B. J. DAVIES, D. M. GHEORGHIU, J. BALFOUR, R. K. SMEDLEY and G. A. T. DULLER
ABSTRACT:
The Welsh Ice Cap was a dynamic component of the last British–Irish Ice Sheet at the Last Glacial Maximum, but there are few chronological constraints on the pace and timing of deglaciation. This paper presents new geomorphological and geochronological evidence that constrains the timing of the separation of the Welsh Ice Cap from the Irish Sea Ice Stream and the subsequent deglaciation of south-west Wales; and allow these to be assessed in the context of late Pleistocene climatic events. Luminescence ages from glacial outwash sediments south of Cardigan demonstrate that the Irish Sea Ice Stream was receding by 26.7 ka. The subsequent recession of the Welsh Ice Cap is documented by cosmogenic ages from landforms and sediments in the Aeron and Teifi valleys and upland areas. Deglaciation of the Cambrian Mountains was underway by 19.6 ka. Cross-valley moraines and associated deglaciation deposits show that minor re-advances interrupted the recession of the Aeron Glacier twice, and the Teifi Glacier on at least 12 occasions. By 14.9 ka, the Aeron valley was probably ice-free, but the northwards withdrawal of the Teifi glacier had halted in the Tregaron area. The final rapid recession of this glacier into the uplands of central Wales was completed during the Windermere Interstadial (13.5 cal ka BP) when, in common with much of the UK, the whole of Wales is believed to have become ice-free. There is no evidence that the Cambrian Mountains contained ice-free enclaves at the Last Glacial Maximum, as has been previously suggested. The new ages presented here support suggestions that there was rapid change in the configuration of the Welsh Ice Cap between 20 and 17 ka as upland areas became exposed and there was increasing topographic control on patterns of ice discharge.
This is one of the "context" maps showing the suggested extent and flowlines of the Irish Sea Ice Stream (ISIS) and the Welsh Ice Cap. What is particularly pleasing is the apparent acceptance that the ice edge was not somewhere offshore to the west of the Dale Peninsula and Milford Haven, but in the vicinity of Caldey Island. I have been arguing for this for years, and have presented evidence on this blog in the form of photos and descriptions of fresh glacial deposits (admittedly very thin and patchy) all along the south Pembrokeshre coast and even on Caldey Island itself. Maybe somebody does pay attention to what appears on this blog.......
But I don't particularly like the ice stream arrows in the outer part of the Bristol Channel and the zone of the NE and SW of the Isles of Scilly. Ice does not flow parallel to an ice edge except in highly constrained environments, as I have repeatedly pointed out. So the ice along this sector must have been flowing towards the ice edge from the NW, not from the N or NNE. More work needed on all of this, chaps.......
With respect to the Devensian ice edge in the Preseli district, we have discussed the BGS reconstruction (above) many times, and have decided that it is more or less correct. I still think the Devensian ive overtopped at least part of Carningli - Dinas Mountain, and I think it reached Gernos-fawr, but let's argue about that some other time.......
This is the reconstructed ice edge from Glasser et al for this area. As we can see, it's very similar. the ice edge is dated to about 27,000 yrs BP, and it's suggested that there was still an ice cover on the upland north of the Nevern Valley, bounding the accumulating fluvio-glacial deposits around 26,500 years ago. The sands and gravels in the quarries of Pantgwyn and Trefigin are dated to 26,700 years old.
Relevance for the two famous sites of Carn Goedog and Rhosyfelin? Well, Carn Goedog is shown as having been glaciated on its lower (northern) flanks, but as having its summit or topmost crags possibly above the ice surface. (The small circles on the map show tors or rock surfaces thought to have been unaffected by Irish Sea ice on this occasion. Cosmogenic dates are clearly needed to decide exactly where the ice edge was.) Rhosyfelin is shown as well within the glaciated area, where we would expect to find till, fluvioglacial deposits and signs of glacial activity on rock surfaces. Exactly what we do find............ But another thing that interests me is the confirmation of a catastrophic ice edge collapse, with the ice edge retreating across about 10 km of countryside in only 500 years. That's pretty dramatic, and the amount of meltwater produced would have been phenomenal. We would expect quite substantial meltwater erosion, including the use of old meltwater channels, maybe the cutting of new ones in softish sediments, and the smoothing and rounding of exposed bedrock surfaces affected by the full force of the torrents. Exactly what we find at Rhosyfelin..........
Below we reproduce the beautiful map which illustrates many of the key points from the article. I will no doubt refer to it again in the future. Click to enlarge.
Below we reproduce the beautiful map which illustrates many of the key points from the article. I will no doubt refer to it again in the future. Click to enlarge.
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Late Devensian deglaciation of south-west Wales from luminescence and cosmogenic isotope dating
N. F. GLASSER, J. R. DAVIES, M. J. HAMBREY, B. J. DAVIES, D. M. GHEORGHIU, J. BALFOUR, R. K. SMEDLEY and G. A. T. DULLER
ABSTRACT:
The Welsh Ice Cap was a dynamic component of the last British–Irish Ice Sheet at the Last Glacial Maximum, but there are few chronological constraints on the pace and timing of deglaciation. This paper presents new geomorphological and geochronological evidence that constrains the timing of the separation of the Welsh Ice Cap from the Irish Sea Ice Stream and the subsequent deglaciation of south-west Wales; and allow these to be assessed in the context of late Pleistocene climatic events. Luminescence ages from glacial outwash sediments south of Cardigan demonstrate that the Irish Sea Ice Stream was receding by 26.7 ka. The subsequent recession of the Welsh Ice Cap is documented by cosmogenic ages from landforms and sediments in the Aeron and Teifi valleys and upland areas. Deglaciation of the Cambrian Mountains was underway by 19.6 ka. Cross-valley moraines and associated deglaciation deposits show that minor re-advances interrupted the recession of the Aeron Glacier twice, and the Teifi Glacier on at least 12 occasions. By 14.9 ka, the Aeron valley was probably ice-free, but the northwards withdrawal of the Teifi glacier had halted in the Tregaron area. The final rapid recession of this glacier into the uplands of central Wales was completed during the Windermere Interstadial (13.5 cal ka BP) when, in common with much of the UK, the whole of Wales is believed to have become ice-free. There is no evidence that the Cambrian Mountains contained ice-free enclaves at the Last Glacial Maximum, as has been previously suggested. The new ages presented here support suggestions that there was rapid change in the configuration of the Welsh Ice Cap between 20 and 17 ka as upland areas became exposed and there was increasing topographic control on patterns of ice discharge.
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