Devensian Welsh Ice in North Pembrokeshire

Had a Eureka moment the other day, when I was sitting on top of Carningli, drinking my cocoa and gazing northwards, taking in the fantastic view of Cardigan Bay -- I could see Aberystwyth, Plynlimon, Cader Idris, Snowdonia, Llyn and Bardsey.  Most impressive......

Anyway, I realized that the experts have got it all wrong, and that during the Devensian NE Pembrokeshire was not so much affected by Irish Sea Ice as Welsh Ice.  The most up-to-date publication to portray the position of the Devensian ice edge is that of Etienne et al in 2006.  Their map is shown above.  We are talking about an ice edge position here at around 20,000 years ago.

We will forgive the authors of the article for showing the ice edge as a smooth line -- they were generalising.  But there is no way that the actual ice edge will ever have looked like this. it will have been much more closely adapted to the lie of the land, with fingers into the lowlands and embayments where the higher land acted to divert ice to right or left.  Another summary of this ice edge position is here, showing the local topography rather more clearly:

That's one of my maps, made while I was pondering on the southernmost extent of the Irish Sea Ice and coming to the conclusion that there are so many ice-smoothed slabs and perched blocks on Carningli and Dinas Mountain that the ice must have flowed across the whole of that upland area and reached the spectacular meltwater channel (a very old one) called Cwm Gwaun.  You will also notice that the lime on the map by Etienne et al is about a mile further south than mine.  You will also notice that on both maps there is an assumption that the ice has come in from the N or NNW.

But I have been very worried by the morainic accumulations at Cilgwyn and Waun Mawn, and have been wondering how Irish Sea ice managed to bend around the eastern flanks of Carningli and Carnedd Meibion Owen in order to leave these deposits where they are, when it melted.  Ice can do extraordinary things, but if it can, it prefers to do things the simple way.

So I have got it wrong.  The ice has actually crossed NE Pembs from the N or NNE, meaning that it was propelled by the Welsh Ice Cap rather than by the Irish Sea Ice Sheet.  This is my new scenario:

Suggestions for ice movement directions at the peak of the Devensian Glacial episode in NE Pembrokeshire.  On the map, the blue arrows represent Welsh ice and the orange arrows represent ice from the Irish Sea Glacier.

There is a great deal of evidence in support of this scenario:

1.  Strong evidence of ice streaming on the ice-smoothed slabs on the NE-facing edge of the Carningli upland, overlooking Newport.  This evidence is best explained by ice coming from the NE.
2.  The morainic accumulations at Cilgwyn and Waun Mawn are best explained by ice pressing in across the Brynberian lowlands from the NE.
3.  The evidence of ice smoothing and perched blocks at Carnedd Meibion Owen is best expolained by ice movement from the NE.
4.  The enigmatic accumulations of fluvioglacial sands and gravels in the Monington area (near Moylgrove) are best explained as accumulations on a wasting ice contact edge, between Irish Sea ice to the west and Welsh ice to the east.

Note the "excrescences" or rough areas on this image of the Monington area, just to the NW of the centre of the image and also to the SW of the centre.  These are the accumulations of sands and gravels, exploited today in two gravel pits.  Note that elsewhere the land surface appears smooth. (Thanks to Henry Patton)

5.  Till deposits exposed on the beach at Newport since recent storms have considerable quantities of red, purple and pink sandstone clasts in them -- suggesting strongly a North Wales origin.  There are also strange metamorphic rocks in the Nevern estuary on the foreshore (samples sent to Rob Ixer) which I strongly suspect as having a N Wales origin.

6.  There are faint traces of marginal channels on the north face of Preseli which suggest an ice gradient sloping westwards.  Again this suggests ice coming from the NE.

This does not alter the fact that there might have been a later slight advance of Irish Sea Ice across the Pembrokeshire coast in this area  since there is heavy clay till at Gwbert, Parrog and other places which appears to be a very typical Irish Sea till.  There are also striae on the coastal rocks at Gwbert and Parrog which show that ice has come across the coast from the N and NNW.

This is just a start.  I have to give this much more thought.......

The relevance of all this as far as archaeology is concerned?  Quite considerable, since both Rhosyfelin and Carn Meini  (and Preseli generally) are in the frame here.  If we can tidy up on the details of chronology and the precise sequence of geomorphological episodes in this region, that will help us greatly in determining what happened at these sites which certain people are pleased to call "quarries."  By the way, Glacial Lake Brynberian is still in the frame here, as seen in the map at the top of the page.  (It's the lake shown as overflowing via the Rhosddu Channel at 220m asl.  I'm not convinced about that overflow, and that needs to be looked at..........)