Glacial features on Carn Goedog

Photo taken from Carn Alw, looking towards the hillside tor of Carn Goedog -- in the middle distance.

I spent a whole afternoon up at Carn Goedog yesterday, enjoying perfect summer weather -- in February.   That's not the way it should be, but it was truly wonderful up there.

I'm working on a new paper about Carn Goedog,  which will not please the megalithic quarrymen, but somebody has to do it........

More in due course on the geology and the archaeology (well, there isn't really any archaeology to talk of at Carn Goedog, but you know what I mean.)  But I made some careful observations on the geomorphology of the tor.

The spotted dolerite outcrops and associated "rock litter" at Carn Goedog are spread across a vast area -- around 60,000 sq m.  The tor is undoubtedly very old, and it has been denuded my many different processes over a vast amount of time -- we are talking of many millions of years.   The most obvious processes that have affected the tor during the last million years are frost-shattering and downslope slumping and collapse -- these processes are still operating, and because there are so many balanced and rocking stones on the north face of the tor one has to move about rather cautiously......  some of them might be referred to as "perched blocks".  But the most recent intensive phase of periglacial activity must have been during the 70,000 years or so of the Devensian cold phase.  (That's a long time, when set aside the mere 20,000 years or so since the cold phase ended.......)  During that time, I should not be surprised if the whole of the Carn Goedog area was buried or "blotted out" by vast snowbanks, maybe for many centuries at a time.  The shady north-facing slopes of mountain ridges  are perfect places for snow accumulation and survival.

What interests me increasingly is this question: to what extent was the tor affected by glacier ice during the Anglian, Devensian and maybe other glaciations as well?

Classic ice-moulded or scoured rock surface on the north flank of Carn Alw.  The ice action reponsible was almost certainly late Devensian.  If the moulding had been  from an earlier glacial episode, this face would by now have been covered with scree.

There are six or seven different locations on the north face of the Carn Goedog tor where we can see ice-moulded bedrock slabs.  These are always north-facing, indicating ice assault from the north.  That figures, since all of the other evidence from North Pembrokeshire points to ice movement involving the Irish Sea Glacier, from the N or NW.  I have shown some images of these undulating moulded and smoothed surfaces here:

https://brian-mountainman.blogspot.com/2015/05/ice-moulded-surfaces-carn-goedog.html

Above:  two locations where undulating ice-moulded slabs can still be seen on the northern flank of the Carn Goedog tor.

The highest of the slabs is very close to the summit of the tor, at about 950 ft (290 m), and this suggests that the whole of the tor was overtopped by ice at some stage.  That of course must have happened during the Anglian glaciation, when the ice may well have been 1,000m thick when it crossed Preseli and flowed off south-eastwards and eastwards towards the Bristol Channel and the Somerset coast.  That's when any erratics from this area must have been entrained and transported. So are the highest slabs on the tor around 250,000 years old?  We need some detailed weathering studies and cosmogenic dating to sort that out.......

But something more fascinating is that the bulk of the visible ice-moulded slabs on the north face of the tor are at or below an altitude of 800 ft (243 m).  It's quite possible that there are others above that altitude that have been buried or obliterated by rock collapse from the myriad of rocky ridges and "mini-summits" on the flank of the tor.  More detailed research may well be rewarding in this respect. But below this approx altitude of 250m it's possible that glacier ice impinging on the face of the tor has removed some boulder debris that might have masked old ice-moulded surfaces and even freshened up the surfaces themselves.

So can Devensian surfaces be differentiated from Anglian surfaces?  Again, cosmogenic dating may be required to sort it all out........

This is not the first time I have speculated about a change in the landscape and the nature of the deposits coinciding with the 250m contour.  I have suggested many times that the ice of the Devensian Irish Sea Glacier pressed against the Mynydd Preseli north face, and here are two  of my maps:

Two suggested ice limits for the Devensian Irish Sea Glacier.  The lower map -- which I now think is most likely to be reliable -- shows the Carnedd Meibion Owen ridge (and its tors) affected by Devensian ice -- but with no ice topping the main upland ridge.

It then starts to gert a bit messy when we bring the Preseli Ice Cap into the frame -- as suggested in earlier posts such as this one:

https://brian-mountainman.blogspot.com/2016/08/the-devensian-preseli-ice-cap.html

As Chris will remember from our rambling up on Banc Llwydlos back in 2016, there are apparently glacial deposits at around 340m -- but which ice body are they related to, and how do they relate to the marginal meltwater channel traces that we see right across the north Preseli landscape?  Work in progress...........

Computer generated modelling of the possible extent of an ephemeral Preseli Ice Cap during the Devensian.  (Thanks to Henry Patton and others.)

But the evidence is accumulating:

1.  The Tafarn y Bwlch moraine -- altitude between 250m and 280m.

https://brian-mountainman.blogspot.com/2015/11/the-tararn-y-bwlch-moraine.html

2.  The morainic accumulation in the col to the west of Carn Goedog -- altitude  broadly similar.

3.  The ice smoothing on the northern face  of Carn Alw and on many other bedrock exposures at and below a similar altitude.

4.  Area of undulating terrain which looks like a moraine -- not far from Hafod Tydfil.  Similar altitude.

5.   Hummocky area between Carn Alw and Carn Goedog, crossed by  deep gullies and by the old drover's route.   Abundant boulders littering the surface.  Altitude c 250m.  Photo below.

My current thesis is therefore that  Carn Goedog and the rest of the north flank of Preseli has been affected at least twice by the Irish Sea Glacier.  During the first episode, around 250,000 years ago, ice flowed across the whole landscape, and even the Preseli summits were deeply submerged.  At that time, there was at least one episode of strong erosion, with the ice creating moulded surfaces on the north face of the tor and also entraining many loose slabs, boulders and elongated "pillars."   Much more recently, around 20,000 years ago, in Late Devensian times, there was a short-lived ice incursion again by the Irish Sea Glacier, during which the ice edge lay at an altitude of approx 250m on the north face of the upland ridge.  There may have been an interaction with a small ice cap centred on Preseli, but the details of this still need to be worked out.