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Monday, 7 October 2019

Confusion reigns -- Late Devensian ice limits

This map of Late Devensian ("Newer Drift")  ice limits in South Wales was published in the GCR volume in 1989, and since then, many other limits have been proposed.   Confusion reigns.  I have been  involved in this chaos myself -- most recently in this blog.

In northern and western Pembrokeshire the evidence of relatively recent glaciation is now very strong, and it has been demonstrated that the old lines drawn by researchers prior to 1960 are fundamentally incorrect.    But the really big problem is that the  evidence of glaciation in the "south Pembrokeshire" is very subtle.  There are glacial deposits, and fluvioglacial deposits too -- but they are thin and patchy, and the consensus over the years is that they belong to the "Older Drift."  The Geological Survey still records them on the Geology of Britain viewer as Mid-Quaternary -- ie belonging to the Anglian Glaciation, or maybe even older.  in 1971 I proposed that most of mid-Pembrokeshire was glaciated in the Late Devensian, and that only the Castlemartin Peninsula and the southern coastal strip remained ice-free.  David Bowen, mindful of the considerable expanse of fluvioglacial gravels in the valley of the Western Cleddau as far south as Haverfordwest and Landshipping, suggested an ice spur projecting southwards from the Roch-Trefgarn ridge -- but this proposal has not had much support from anybody else.    But now I'm coming round to it, and I think DQB may be close to the truth.

The other thing that has happened is that I have been back to the southern and western coasts of Pembrokeshire, and am now convinced that all of them, at least as far east as Tenby, were affected by Late Devensian ice.  But how far inland did this ice actually push?

Well, the geology map shows patchy glacial and fluvioglacial sediments all over the place -- no part of Pembrokeshire is "peculiarly free" of these materials.

Above: The geological map of the mid west of Pembrokeshire.  This shows the extent of the sands and gravels on the flanks of the Western Cleddau valley -- while leaving open the possibility that they are all very ancient river terrace remnants that have nothing to do with glacial ice in the vicinity.  They may have been built up during past episodes of large-scale meltwater flow through the Trefgarn Gorge and thence southwards towards Milford Haven.  Work is needed on this.

The map shows that the tills described in my recent posts for the Nolton- Druidston coastal strip extend inland for maybe 500m or so, but no further.  But then another extensive area of till is shown further inland, and another to the NE of the A487 road.  Are these deposits related in age, or could they be much older?  And how many patches of till have been missed completely?

Never fear, dear reader.  I am on the case, and will report back shortly.

I am less and less convinced by the idea of a Late Devensian ice-free enclave........

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