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Sunday, 13 February 2011

The North Pembrokeshire Devensian Trimline

I've been pondering on this matter lately, after another walk on Carningli yesterday.  I'm still investigating where the trimline might be along the north Pembrokeshire coast which shows us the maximum elevation of Devensian glacier ice coming in from Cardigan Bay and St George's Channel.

 The zone referred to as being occupied by the "The South Wales End Moraine" -- characterized by extensive glacial deposits, fluvio-glacial landforms and deposits, and pro-glacial lakes. Click to enlarge.

Many years ago Charlesworth identified the "South Wales End Moraine" as marking the maximum extent of glacier ice during the last glacial episode.  It consists of a scattered assemblage of mounds and terraces of fluvio-glacial material, located for the most part between Fishguard and Cardigan.  I spent years studying this "moraine" and it's not really a line at all, since the sands and gravels (including some patches of ice-contact materials) are in a band of country some 15 km long and 8 km wide.  I have always thought that the ice progressed well beyond this "zone of deposits" and that it should therefore not be referred to as an end moraine.  But how much further?  Danny McCarroll and others think that the north coast was glaciated because the tors and craggy hills on Pencaer and along the coast between Fishguard and Dinas are rounded or ice moulded, whereas  those further inland, on Dinas Mountain and on the Carningli upland, are frequently delicate, with steep crags and balanced boulders etc.  They appear to have been affected by frost action but not by overriding ice.

So somewhere there is a trimline...........  as I have suggested, the ice appears to have affected Carningli up to an altitude of c 300m, possibly leaving behind the Pont Ceunant moraine after a retreat downslope on the north-facing mountainside.  I have found more evidence of overriding ice almost as high as the summit crags.


Evidence of ice action on Carningli.  Top and bottom photos:  ice smoothed undulating rock slabs near the summit.  Middle photo: smoothed rock slabs with crescentic gouges, on the south side of the mountain near the western summit.  If these really are Devensian features, there must have been ice at least 30 metres thick.....

Other evidence comes in the form of perched blocks.  There are two spectacular ones in the area -- one on Carn Edward (alt 280m), to the west of Carningli, and the other on one of the four tors which are called Carnedd Meibion Owen (alt 240m).  These blocks are extremely unlikely to have been emplaced by periglacial action or any other mode of tor formation;  they must have been stranded or let down by melting glacier ice.


Yet more evidence coming into the frame is that relating to the pro-glacial lakes referred to as Llyn Teifi and Llyn Nevern -- occupying the lower parts of those two valley systems.  Charlesworth (see above maps) was the first one to refer to these lakes, in 1929 -- but he thought that they formed during ice retreat or deglaciation, which is when we would expect vast quantities of meltwater to be pouring off the wasting ice.  New work, however, suggests that the lakes were impounded by ADVANCING glacier ice, since lake deposits including fine silts and clays with varved sequences are overlain by coarser stream sediments and even by till.  So the glacier coming in from the coast rode over these lake sediments and passed further inland.  Etienne and others (2006) think that the ice eventually flowed right over the Carningli upland and blocked off the whole of Cwm Gwaun, with lakes flowing over a succession of cols to cut overflow channels in the vicinity of the ice edge.  They think that the overflowing water utilized the older and larger channels of the Gwaun-Jordanston channel system -- which were inherited from earlier glacial episodes.  I think I would agree with much of that, although I won't be entirely convinced about all those lakes until we have varved sediments to look at.

So if the ice margin pressed as far inland as Eglwyswrw and Brynberian, with meltwater impounded up against the north face of Mynydd Presely, we can set the altitude of the ice edge at somewhere around 200m.  Now that is very important indeed, as we shall see.......

http://brian-mountainman.blogspot.com/2010/06/new-dates-shed-light-on-old-ice-limits.html

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