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Monday, 14 February 2011

The Devensian Ice Edge in Pembrokeshire

The map above is based on one drawn by Prof David Bowen which I was previously unconvinced by, but which on consideration I now think is more or less correct.  But it does need some tweaking, and the revised version below is about as accurate as we can get for the time being.

The main difference is that I am convinced that the coast of St Bride's Bay was glaciated, as we can see from glacial deposits at Druidston, little Haven, West Dale etc.  The kame terrace at Mullock Bridge, near Dale, has ice contact materials in it, so the ice must have crossed the mouth of Milford Haven.  It also affected the South Pembrokeshire coast in the area around West Angle Bay -- there are ice contact deposits here, including flowtills.

There are a number of distinct zones shown on the map.

A = the South Wales End Moraine zone, incorporating a number of glacial lakes formed for the most part during the period of ice advance, and with pre-existing (Anglian?) meltwater channels in the Gwaun-Jordanston area re-used by marginal and sub-marginal meltwater streams, sometimes as lake overflows.  The till on the coast is Irish Sea till.  Extensive spreads and hummocks of fluvio-glacial materials, including eskers and kames.  Varved lake deposits in Lakes Teifi and Nevern, and elsewhere, sometimes overlain by glacial materials.

B=  St Davids Peninsula.  Irish Sea till on the north coast and local tills further inland and on the south coast of the peninsula.  Tors and rocky hillocks all glaciated and moulded by overriding ice.

C=  SW coastal areas.  Irish Sea till and local tills in isolated coastal pockets, and some fluvio-glacial materials further inland.

SE Pembrokeshire borders =  Mostly fluvio-glacial materials derived from Welsh Ice which has moved broadly westwards from the ice streams in the Tywi and Taf catchments.  Some mounds, but for the most part the gravels are in valley sandurs.

Ice-free Central Pembrokeshire = The tors of Treffgarne Gorge were unaffected by glacier ice.  Valley sandurs in the Western Cleddau catchment, and the Treffgarne Gorge may have been modified by vast torrents of glacial meltwater flowing south towards Milford Haven.  Great thicknesses of fluvio-glacial gravels -- some terraces up to 30m above the present floodplain of the Western Cleddau river.

That's the best I can do for now -- I think ALL the evidence accords with the glacial limit shown here.

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