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Wednesday, 26 October 2011

The Devensian ice limit in North Pembrokeshire

On 17 April I published this map -- indicating my latest thinking on the extent of the Irish Sea Glacier in North Pembrokeshire.

I have now found four places where there are hummocky deposits which I interpret as morainic and fluvioglacial accumulations of a terminal nature -- in other words they show end moraine or glacier terminus positions.  One is the Pont Ceunant moraine, which I have previously described -- search for the blog entries via the search facility on the blog page.  There are other accumulations at Cilgwyn, around the eastern end of the Gwaun Valley and near the cattle grid on Waun Mawn, near the standing stones described also on this site.  Around Llanychaer in the Gwaun Valley there are other deposits of till and fluvioglacial mounds on the valley floor  suggesting that the valley is old and that the deposits are young.  I have suggested before, in various articles, that the complex of valleys here is indeed very complex, containing the best examples in the British Isles of sub-glacial meltwater channels and also features of many generations -- modified during a number of glacial episodes.  Some sections of the channels are genuinely sub-glacial (dating maybe from the Anglian and Wolstonian glaciations) and other sections may be marginal and sub-marginal, maybe old but freshened up by re-use during the Devensian.
The Gwaun-Jordanston system of meltwater channels in North Pembrokeshire -- a map which I made in 1965 as part of my doctorate research.  Click to enlarge.

It now seems to me that during the Devensian the Irish Sea Glacier pressed inland from Fishguard as far as Llanychaer, effectively blocking the meltwater outlet to the north-west and diverting large quantities of meltwater through the Nantybugail Channel southwards and south-westwards towards Trefgarn Gorge.  After that, as the ice edge retreated, the other channels to the south of Fishguard were also used, one after another.

 Google Earth image of the terrain on the northern flank of Preseli, east of Carn Goedog.  Part of the craggy tor is on the left edge of the photo.  Note the channel-like features on the mountainside -- natural, or made by animal hooves?

Other feature which have a bearing on this new interpretation are the channel-like features which occur on the northern face of Mynydd Preseli to the east of the main road where it goes up towards the summit col, and between Carn Goedog and Carn Alw above the route of the old drovers' road.  For years I have thought of these as made by the hooves of thousands of animals driven eastwards during the days of the cattle drovers, but now I am not so sure.  In some places there are several of these "steps" or channels roughly parallel with one another on the hillside, and I now think they may well be marginal or sub-marginal drainage channels cut along of close to an ice edge.  I hope to do some more work on these channels.

It is also increasingly apparent to me that on the Carningli upland, on the ridge around Carnedd Meibion Owen, and on the main Preseli upland ridge, there are abundant signs of ice-smoothed slabs, erratics, perched blocks and sheets of till which suggest that glacier ice DID actually cover these uplands during the Devensian -- maybe for just a few centuries.  This is where Henry Patton's models of local ice caps around the peripheries of the Welsh ice cap come in very handy -- and I suggest that there were indeed a number of small thin local ice caps over these uplands, maybe sometimes isolated from the larger ice masses to the north and west, and sometimes incorporated into them.  So there would have been an oscillating junction between active ice belonging to the Irish Sea Glacier and sluggish or stagnant ice on these thin local ice caps.  Details still to be worked out.  Watch this space......

Here is a photo of the edge of the ice sheet in North Greenland.  There are four types of "terrain" here.   First, on the left we see the ice sheet edge -- remarkably clean here, because this is polar ice, probably frozen to its bed.  Second, we have small ice caps on the higher land.  They are permanent, in the sense that they are always there, summer and winter, over decades and probably centuries.  Third, there are the areas where there are perennial snow patches interspersed with terrain where the snow melts each summer.  It will not take much of a climatic deterioration, or an increase in snowfall, to incorporate these areas into the local ice caps.  And fourth, there are the snow-free valleys and broader lowlands, where summer melting is sufficient to remove each winter snowfall.  If we were to imagine a steady deterioration of the local climate here these areas would coalesce one by one, until the whole landscape would appear white from above.  It would then be very difficult to fix the actual edge of the ice sheet with any precision.  I imagine something like this scenario for the periphery of the ice sheet in North Pembrokeshire during the Devensian -- and by extension, for the Somerset - Wiltshire area during the Anglian Glaciation around 450,000 years ago.

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