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Sunday, 13 November 2016

Mynydd Preseli ice margin

I have been looking through this excellent report on the Teifi catchment, published in 1997.  I'll come back to it with more of the useful material culled from the literature by the authors.  But for the moment, this map caught my eye, since it is right in line with my current thinking on where the ice edge might have been in North Pembrokeshire at the time of the Devensian glacial maximum, around 20,000 - 18,000 years ago.

In line with much of the recent work done by BGS surveyors, they show Welsh ice affecting the upper catchment of the Teifi, as far downstream as Llandysul.  On its western edge, between Llandysul and the Ceredigion coast (which was not there at the time) there was a contact zone with Welsh ice to the east and Irish Sea ice to the west.  Carningli and Dinas Mountain are shown as completely inundated by the Irish Sea ice stream -- and more and more of the field evidence published on this blog now appears to support the idea that the ice edge lay along the northern flank of the Mynydd Preseli ridge, with the Brynberian Valley (including the Rhosyfelin area), Carnedd Meibion Owen and even Cwm Gwaun overridden.  This means that the morainic accumulations at Pont Ceunant, Cilgwyn and Tafarn y Bwlch may owe their origins to temporary stillstands or slight readvances during  a prolonged deglaciation phase.  The thick accumulations of sands and gravels in the Monington - Moylgrove area must also date from this episode of ice melting.

More fieldwork is required.  Watch this space........

R A Waters, J R Davies, D Wilson and J K Prigmore.  1997. A geological background for planning and development in the Afon Teifi catchment.    British Geological Survey Technical Report, WA197I35.102pp.


Dave Maynard said...

Interesting article, I tracked it down on the internet. Not read it all, do you agree with the conclusions?


BRIAN JOHN said...

It's very long, and I haven;t read all of it, but it seems pretty reliable to me....... well researched and referenced.

cysgodycastell said...

What is the Pedran Spillway/Channel? On the images you have posted it is referenced on a couple of occasions and looks like it is situated in the Crymych area.

It is not something I've seen or heard about previously.

BRIAN JOHN said...

Yes, some of the papers mentioning Lake Teifi etc do refer to the Pedran spillway channel -- as part of a sequence of lake overspills, leading eventually to the Gwaun Valley intake. I'm not that convinced that these channels are related at all -- but they have all been messed about with by the ice that advanced across the area AFTER the lakes were formed. I have done a lot of posts about all of this -- you can find the info if you use the search box.