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Sunday, 1 November 2015

The Tafarn y Bwlch Moraine

This gets more and more interesting -- all part of the ongoing fieldwork designed to find out what went on in the area around Brynberian during the Devensian.  A couple of years ago I published this map:

You can click to enlarge -- I have put on it the locations of Rhosyfelin and other key sites and settlements.  I have also devoted some space to the interesting moraine at Gernos-fawr in this 2013 post:

After mooching about recently in the area around Tafarn y Bwlch I took the chance today to have a wander about -- over 20 deg on a November day, not a cloud in the sky, and not a breath of wind.  And no rugby World Cup matches on the telly.......

I'm now quite convinced that there is a massive terminal moraine here, just to the south of the proposed new Visitor Centre.  Here is an old OS map of the area:

On the map, you can see the position of the col between the Brynberian and Cwm Gwaun catchments (purple rectangle) just to the west of the old farm and near a pair of standing stones.  Note the contours.  To the west of that, the ground (very soggy) slopes gradually down towards the position of the Gernos morainic accumulations, and all of the stream cuttings show that this is a till plain littered with sub-angular and sub-rounded boulders, some of them over 3m in diameter.  I had expected to see here an extensive sheet of fluvio-glacial materials -- an outwash plain or sandur -- but that is not what I found.   There is continuous till at the surface all the way from Tafarn-y-bwlch to Gernos, and then further down the valley towards Gelli-fawr -- a distance of around 2 km.

On this annotated satellite image I have marked the main features.  The black dot marks the position of the col.  The red lines show the position of a series of meandering and interconnected channels which I interpret as ice-marginal channels, since they do not run directly downslope but diagonally.  There is another set of channels (shown in green) just to the west, but their orientations are difficult to explain.  Could they have something to do with meltwater escaping from a melting Preseli ice cap?  The channels marked in blue run more or less directly downslope, and may have formed after the melting of glacier ice from this area.

To the south of Tafarn-y-bwlch, the land surface rises gradually and when we pass the edge of the cultivated land the slope steepens considerably.  On the 1:25000 map it is referred to as "Banc Llwydlos".  The bank,  about 50m high, is too steep and prominent to have been cut by river action or spring sapping, and the only reasonable interpretation has to be that it is an ice-contact slope associated with a lobe of ice sitting on Brynberian Moor.  This is exactly what is shown on the landform / ice limit map shown above.  So we should not be surprised.  I explored along this slope today, and there are boulder accumulations all over the place -- I hesitate to call them "erratics" because most of them are rather local dolerites, gabbros and ashes.  These are typical:

I estimate that the morainic accumulation here is at least 40m thick -- and that must represent a very considerable stillstand of the ice margin.  Of course, this lobe of ice must also have been responsible for the till that we now find on Brynberian Moor and at Rhosyfelin.

More work needs to be done on tracing the Late Devensian ice edge on the north face of Mynydd Preseli.

Finally, it's worth mentioning that there is no trace of a meltwater overspill on the col at Tafarn-y-bwlch.   There is no channel which might be associated with a "Lake Brynberian" -- and I am more and more convinced that there never was a sizeable meltwater lake here either before or after the LGM.


Dave Maynard said...

Could your red and green meandering squiggles be the remains of earlier trackways terraced into the hillside, before the route was settled into the present road? Perhaps some of the green ones as they are close to, and visible from the road. I have walked across here once and the trackway visible on the satellite image is pretty clear.


BRIAN JOHN said...

Yes, the trackway above the marked channels is the old roadway that ran across from Tafarn y Bwlch towards Rosebush. It's very clear on the satellite image. The channels a bit lower down the hillside are the problematical ones. They don't seem to be man-made -- but they need more investigation. It's possible, for example, that they are marginal meltwater channels which have also been deepened by use by animals being driven over the mountain. But in general, the principle was that the herders kept their animals away from the routes used by horses and coaches etc. The drover's routes on Preseli are in quite different places. An interesting little dilemma!

Hugh Thomas said...

It is avery intetesting area to explore, Banc Llwydlos iron age settlement is in your ariel photo too. The longest blue representing a stream has a crossing point of pathways converging at one point. Crossing the stream from left to right you will see the site within the next 50 yards give or take . Also in the image is a site I have come to call "fake henge " , not a henge but a semi circular collection of large boulders that realky do catch the eye once spotted , I believe they are randomly placed but perhaps yhe glacier was using a bit of artistic liscence.... ;)

BRIAN JOHN said...

Yes Hugh, I have spotted assorted interesting little details on the satellite images -- there are some on the crest of the moraine too. With such abundant boulders lying about on the surface, and reasonably dry conditions underfoot, I am not at all surprised that Neolithic / Bronze Age inhabitants should have used some of them for circles, walls and maybe other things too.....