Ragwen Point, near Pendine. Blue-grey diamicton made predominantly of clay and silt, with small fragments of shales and mudstones and sandstone blocks derived from pre-existing slope breccia. It's at beach level, and it is clearly in a secondary position, having flowed / slid downslope from its original position. Interpreted as a local Late Devensian till laid down by Irish Sea ice flowing west to east along the old cliffline at Ragwen Point.
Multiple opinions about the extent of Late Devensian ice across South Wales
Having been collection field evidence from here and there over past weeks, I am now convinced that I have got it all wrong in the past with regard to the LGM in west Wales. Everybody else has got it wrong too. A reminder of the multitude of suggestions in the specialist literature, arising out of a too-easy acceptance of the Older Drift - Younger Drift ruling hypothesis and out of the sheer difficulty of interpreting the field evidence in central and southern Pembrokeshire. The evidence is not that easy to interpret in north Pembrokeshire either!!
Having argued for years in favour of a substantial unglaciated enclave across much of southern Pembrokeshire, I am having to confront the fact that every time I visit the South Pembs coast, I come away with evidence of quite recent glaciation, or at the very least, evidence that shows the same stratigraphy -- and hence the same sequence of events -- as the coasts of north Pembrokeshire and St Bride's Bay.
South Pembrokeshire
So now we have evidence of sticky blue clay with faceted and abraded erratic pebbles beneath the offshore peat beds and submerged forest at Freshwater West, Amroth, Marros and Pendine. There are blocky slope breccia materials in the clay as well, but I cannot think of any better explanation for it, other than to say that it is a till carried and deposited by glacier ice travelling from the west towards the east. We have fresh clifftop till at scores of locations along the south Pembrokeshire cliffline. The erratics at Flimston, Loveston and many other sites on the south Pembrokeshire coastal plateau are suggestive of ice that has travelled overland broadly from the NW towards the SE (but admittedly these erratics might have been carried and emplaced in an earlier glaciation). We have the red uncemented till at Ballum's Bay on Caldey Island which I insist must be of LGM age. (In this I differ from Prof John Hiemstra and colleagues at Swansea University, who claim that it is a redistributed ancient till.) Inland, we have apparently fresh fluvio-glacial gravels at Picton Point, at the confluence of the two Cleddau branches, and sticky clay till at Landshipping. We have scattered till patches and fluvio-glacial materials in many locations across south Pembrokeshire, as confirmed by the field geologists of the Geological Survey. And my recent discovery of sticky blue-grey till in the exposures at Ragwen Point again suggests eastwards-travelling ice, and pushes the Late Devensian ice limit much further east than any other worker has ever suggested.
To summarise the Pembrokeshire evidence:
1. There are far too many spreads of glacial and fluvio-glacial deposits in those parts of Pembs deemed to be outside the LGM limit. Some of them, at least, appear quite fresh, and some even have surface expression. They are by mo means all "degraded or denuded" and are by no means restricted to hilltops or interfluves, as claimed by some.
2. The glacial deposits in the valleys and on the clifftops of South Pembrokeshire are so abundant and so fresh that they have to be Devensian. The exposures are itemised on this blog. I now think that there are glacial deposits around Marros and Pendine as well; and these are best explained by ice pushing in from the west and the north, and not from the south.
3. The glacial and fluvioglacial deposits around Picton Point and Landshipping, right in the centre of the county at the confluence of the two Cleddau rivers, cannot adequately be explained without invoking a complete ice cover across Pembrokeshire. I have another report of fresh till near Haverfordwest.
4. Over and again I have described on this blog glacial deposits and apparent trimlines at a variety of altitudes on the northern side of Preseli, up to an altitude of 340m. It looks to me as if that was the altitude of the highest Devensian ice surface associated with Irish Sea ice. If the ice covered Carn Ingli and pressed onto the northern flank of the Preseli ridge at that altitude, it would make no sense for the ice edge to be at or near present sea level around the mid and south Pembrokeshire coast. Ice must have filled the Cwm Gwaun depression, and indeed I have described apparent morainic features above and on the south side of the meltwater channel, and at Cilgwyn and Tafarn y Bwlch. Nor does it make any sense for an ice edge to be parked around the 200m contour in the vicinity of Wolfscastle and Letterston, as suggested by the BGS. If the ice was coming in from the NW, it MUST have inundated the whole of Pembrokeshire to the south of Preseli, according to the laws of ice physics. The ice edge might have been slightly lower on the southern flank of the Preseli ridge. I now think that the assorted morainic ridges and glaciofluvial mounds in and around the Preseli uplands may be associated with retreat stages or short-lived readvances -- still to be defined.
None of this eliminates the possibility that there are Anglian or Wolstonian glacial and meltwater deposits in the area previously designated as an ice-free enclave. As indicated on this blog, I think there are ancient glacial deposits at Ceibwr, Witches Cauldron and Black Mixen, Lydstep; and Prof Danny McCarroll and I are rather convinced that glaciofluvial gravels exposed at Llangolman may well be very old indeed.
So now we have evidence of sticky blue clay with faceted and abraded erratic pebbles beneath the offshore peat beds and submerged forest at Freshwater West, Amroth, Marros and Pendine. There are blocky slope breccia materials in the clay as well, but I cannot think of any better explanation for it, other than to say that it is a till carried and deposited by glacier ice travelling from the west towards the east. We have fresh clifftop till at scores of locations along the south Pembrokeshire cliffline. The erratics at Flimston, Loveston and many other sites on the south Pembrokeshire coastal plateau are suggestive of ice that has travelled overland broadly from the NW towards the SE (but admittedly these erratics might have been carried and emplaced in an earlier glaciation). We have the red uncemented till at Ballum's Bay on Caldey Island which I insist must be of LGM age. (In this I differ from Prof John Hiemstra and colleagues at Swansea University, who claim that it is a redistributed ancient till.) Inland, we have apparently fresh fluvio-glacial gravels at Picton Point, at the confluence of the two Cleddau branches, and sticky clay till at Landshipping. We have scattered till patches and fluvio-glacial materials in many locations across south Pembrokeshire, as confirmed by the field geologists of the Geological Survey. And my recent discovery of sticky blue-grey till in the exposures at Ragwen Point again suggests eastwards-travelling ice, and pushes the Late Devensian ice limit much further east than any other worker has ever suggested.
To summarise the Pembrokeshire evidence:
1. There are far too many spreads of glacial and fluvio-glacial deposits in those parts of Pembs deemed to be outside the LGM limit. Some of them, at least, appear quite fresh, and some even have surface expression. They are by mo means all "degraded or denuded" and are by no means restricted to hilltops or interfluves, as claimed by some.
2. The glacial deposits in the valleys and on the clifftops of South Pembrokeshire are so abundant and so fresh that they have to be Devensian. The exposures are itemised on this blog. I now think that there are glacial deposits around Marros and Pendine as well; and these are best explained by ice pushing in from the west and the north, and not from the south.
3. The glacial and fluvioglacial deposits around Picton Point and Landshipping, right in the centre of the county at the confluence of the two Cleddau rivers, cannot adequately be explained without invoking a complete ice cover across Pembrokeshire. I have another report of fresh till near Haverfordwest.
4. Over and again I have described on this blog glacial deposits and apparent trimlines at a variety of altitudes on the northern side of Preseli, up to an altitude of 340m. It looks to me as if that was the altitude of the highest Devensian ice surface associated with Irish Sea ice. If the ice covered Carn Ingli and pressed onto the northern flank of the Preseli ridge at that altitude, it would make no sense for the ice edge to be at or near present sea level around the mid and south Pembrokeshire coast. Ice must have filled the Cwm Gwaun depression, and indeed I have described apparent morainic features above and on the south side of the meltwater channel, and at Cilgwyn and Tafarn y Bwlch. Nor does it make any sense for an ice edge to be parked around the 200m contour in the vicinity of Wolfscastle and Letterston, as suggested by the BGS. If the ice was coming in from the NW, it MUST have inundated the whole of Pembrokeshire to the south of Preseli, according to the laws of ice physics. The ice edge might have been slightly lower on the southern flank of the Preseli ridge. I now think that the assorted morainic ridges and glaciofluvial mounds in and around the Preseli uplands may be associated with retreat stages or short-lived readvances -- still to be defined.
None of this eliminates the possibility that there are Anglian or Wolstonian glacial and meltwater deposits in the area previously designated as an ice-free enclave. As indicated on this blog, I think there are ancient glacial deposits at Ceibwr, Witches Cauldron and Black Mixen, Lydstep; and Prof Danny McCarroll and I are rather convinced that glaciofluvial gravels exposed at Llangolman may well be very old indeed.
https://brian-mountainman.blogspot.com/2019/07/caldey-island-till-must-be-late.html
The South Pembrokeshire Quaternary regional stratigraphy (based on my own observations and those of Dixon and Leach) is as follows:
8. Sandy loam and blown sand
7. Upper head (uncemented)
6. Fluvioglacial sands and gravels -- traces
5. Till from Dewisland (Devensian) glaciation -- many coastal exposures
4c. Lower head (cemented in some localities)
4b. Cemented sands (sandrock)
4a. Head incorporating raised beach cobbles (cemented)
3. Cemented raised beach
2. Older glacial deposits -- mostly destroyed, but exposed at Lydstep (Black Mixen)
1. Raised beach platform (complex modifications over several interglacials?)
The Gower Peninsula
The South Pembrokeshire Quaternary regional stratigraphy (based on my own observations and those of Dixon and Leach) is as follows:
8. Sandy loam and blown sand
7. Upper head (uncemented)
6. Fluvioglacial sands and gravels -- traces
5. Till from Dewisland (Devensian) glaciation -- many coastal exposures
4c. Lower head (cemented in some localities)
4b. Cemented sands (sandrock)
4a. Head incorporating raised beach cobbles (cemented)
3. Cemented raised beach
2. Older glacial deposits -- mostly destroyed, but exposed at Lydstep (Black Mixen)
1. Raised beach platform (complex modifications over several interglacials?)
The Gower Peninsula
What of the other evidence from Carmarthen Bay and the Gower? There seems to be an emerging consensus:
https://brian-mountainman.blogspot.com/2018/11/the-glaciations-of-gower.html
1. There are not multiple interglacial raised beaches on Gower, as suggested by DQ Bowen, but one raised beach which is sometimes cemented and sometimes not, and which occurs at a range of different altitudes depending on precise locations on peninsulas, in bayheads etc. The beach always contains well rounded pebbles and in some places it is very rich in shell fragments and even complete shells. It's dated to the last (Ipswichian) interglacial.
2. There is plenty of evidence of a prolonged periglacial episode following the formation of the interglacial raised beach, since here and there we see many metres of pseudo-stratified slope deposits or "head" above the raised beach and beneath the glacial and fluvioglacial materials. (This is exactly the sequence we see in Pembrokeshire.) Sometimes the head is cemented, and sometimes not. The slope breccia must have accumulated during the Early and Middle Devensian.
3. The glacial sediments exposed in the cliffs at Rotherslade, Horton and elsewhere are not ancient deposits moved into place by periglacial and slope deposits, but fresh materials emplaced at or very near a Late Devensian ice edge, with chaotic and even catastrophic flowing and emplacement of till and mixed fluvioglacial materials at a time of rapid ice wastage.
4. The Paviland Moraine near Horton, which was interpreted by DQ Bowen as well beyond the Devensian ice margin (and hence a very ancient glacial feature) contains material which is no more ancient than any of the other glacial and fluvioglacial materials exposed in the cliffs. It should therefore be re-interpreted as a Late Devensian feature -- deposited maybe during a long ice edge stillstand at this position, on a ridge not far from the southernmost tip of Gower.
As far as the east coast of Swansea Bay and the Vale of Glamorgan are concerned, there has been a consensus over many years that there was a Late Devensian ice edge near Porthcawl. It has been assumed that this ice edge was associated with Welsh ice flowing southwards from the Welsh Ice Cap; but this has been called into question by Wilson et al in the new Geological Survey Memoir following a re-examination of the evidence from Ewenny and Pencoed, and the erratics of the Storrie Collection.
1. There are not multiple interglacial raised beaches on Gower, as suggested by DQ Bowen, but one raised beach which is sometimes cemented and sometimes not, and which occurs at a range of different altitudes depending on precise locations on peninsulas, in bayheads etc. The beach always contains well rounded pebbles and in some places it is very rich in shell fragments and even complete shells. It's dated to the last (Ipswichian) interglacial.
2. There is plenty of evidence of a prolonged periglacial episode following the formation of the interglacial raised beach, since here and there we see many metres of pseudo-stratified slope deposits or "head" above the raised beach and beneath the glacial and fluvioglacial materials. (This is exactly the sequence we see in Pembrokeshire.) Sometimes the head is cemented, and sometimes not. The slope breccia must have accumulated during the Early and Middle Devensian.
3. The glacial sediments exposed in the cliffs at Rotherslade, Horton and elsewhere are not ancient deposits moved into place by periglacial and slope deposits, but fresh materials emplaced at or very near a Late Devensian ice edge, with chaotic and even catastrophic flowing and emplacement of till and mixed fluvioglacial materials at a time of rapid ice wastage.
4. The Paviland Moraine near Horton, which was interpreted by DQ Bowen as well beyond the Devensian ice margin (and hence a very ancient glacial feature) contains material which is no more ancient than any of the other glacial and fluvioglacial materials exposed in the cliffs. It should therefore be re-interpreted as a Late Devensian feature -- deposited maybe during a long ice edge stillstand at this position, on a ridge not far from the southernmost tip of Gower.
Traces of terminal moraine loops in Swansea Bay, interpreted as evidence of very powerful ice from confluent valley glaciers or ice streams pushing out into the Bristol Channel lowlands during the Late Devensian
The West Glamorgan coast
The authors of the new Memoir suggest that the deposits around Ewenny might well be Late Devensian rather than Anglian in age -- and if this is accepted, then the famous erratics from the west might have been emplaced by the eastward-flowing ice of the Irish Sea Glacier. The recent discovery of a giant erratic at Limeslade, near Mumbles, and the discoveries of granite and other far-travelled erratics on the south coast of Gower tends to reinforce the idea that the ice of the Devensian Irish Sea Glacier was powerful enough to intermittently affect the south Wales coast in multiple locations. How the junction between Welsh ice and Irish Sea ice oscillated over time still has to be worked out...........
Discussion: was there an ice-free enclave?
I have published versions of this map several times. If we now accept that there are fresh glacial deposits in the Amroth - Marros - Pendine area, we are left with in ice-free enclave (completely surrounded by glacier ice) which is glaciologically vanishingly unlikely. this is not, after all, an area of complex high relief --mid and south Pembrokeshire is an undulating lowland with incised river valleys that slopes gradually southwards from the Mynydd Preseli foothills. Ice must maintain a sloping profile in an area like this from source area towards snout. For Irish Sea ice to have affected the coast of Carmarthen Bay, the WHOLE of Pembrokeshire must have been inundated.
Note the suggested ice-free enclave. I no longer think
that this idea is tenable......
There are still many questions to be answered, but it seems to me that the time is ripe for a reappraisal of how extensive the ice cover of South Wales was in Late Devensian time, and which areas might have been affected by Irish Sea ice. My main suggestion (as of today) is that there was no ice-free enclave in Pembrokeshire. There is just not enough evidence to support it.
By removing the ice-free enclave and accepting that all of Pembrokeshire was covered by glacier ice (which may not have been very thick) we come to a map that looks something like this:
One recent attempt to portray the extent of Late Devensian glacier ice across South Wales. New evidence suggests that this meeds to be modified further.....
So, to move on.......
The map below represents my latest hypothesis. It seems to me to be based on quite sound evidence. I hope that others may now be willing to join the discussion!
Suggested Late Devensian relationship between Irish Sea ice and Welsh ice. In the contact zone between the two ice masses there must have been a southwards diversion of ice streams. There must also have been considerable shifts in the ice contact position, associated with sharp changes in the ice movement directions. Most of the South Pembrokeshire coast is shown as being affected by Irish Sea ice, which must at some stages have flowed more or less west to east. The ice cover over central and south Pembrokeshire might have been rather thin. Swansea Bay was affected mostly by a powerful ice stream made up of ice flowing in the Nedd and Tawe valleys -- but at one time Irish Sea Ice might have dominated, possibly reaching Pencoed and Porthcawl. Gower is shown as completely submerged beneath Welsh ice, but at some stages the south coast might have been affected by Irish Sea Ice. Mynydd Presely might have had its own local ice cap in the waxing and waning phases of the glaciation. In Carmarthen Bay there was a conflict between Irish Sea ice coming from the west and Welsh ice flowing via the Tywi and other valleys.
1 comment:
Interesting, thought provoking and comprehensive post,thanks!
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