We live in the southern part of the Cilgwyn Amphitheatre, not far from the "entrance" to the Gwm Gwaun meltwater channel, and having made a few more observations in recent days i am now convinced that this was a location in which a large ice lobe from the north melted away in rather spectacular fashion. The lobe is shown on the BGS geology map for the Fishguard area; I think is is pretty reliable, although the line does not necessarily show the MAXIMUM extent of Late Devensian ice.
The map shows Devensian ice pushing some way into the Gwaun Valley, and I don't have a problem with that. But there must have been a major ice edge stillstand or retreat stage in the area around Tyriet (Bluestone Brewery), Ysgarwen, Brynglas and Penybont, since there is distinct hummocky topography here, with thousands of erratic boulders scattered across the ground surface and incorporated into hedge banks. The soil is generally clay-rich, and there are areas of waterlogging. Occasionallyv we see exposures of stony till.
To the north-east of this moraine (which I have called the Cilgwyn Moraine) there are several distinct mounds of sand and gravel which have been referred to as remnants of a kame terrace -- but I do not see a consistent surface level, so they are best referred to kames, probably formed in a series of independent and ephemeral dead ice hollows that were partly filled with meltwater. The fields where these kames occur are very dry. I have not seen any clean exposures which might reveal bedding structures and water flow directions.
This area of sandy and gravelly mounds extends northwards past Fachongle Isaf towards Brithdir Mawr, but in and around the incised valley of the Clydach river there are several areas of waterlogging where till is exposed at the surface. In the field south of Coed y Pwll there is a sharp break between a dry sandy slope (to the south) and a wet area with waterlogging to the north. On both sides of the road to the south of Ty Rhos there is an undulating till surface -- this is the Coed y Pwll till sheet which has been shown in a water supply borehole to be around 29m thick and which extends to the east of the Clydach river for more than 3 km. This is classified as Irish Sea till by the BG, and I agree with them -- but because the exposures at Coed y Pwll (adjacent to the pond and the exposed erratic boulder) are several km inland from Newport Bay there appear to be no shell fragments. The till is very similar in texture to that exposed on the Parrog and at Abermawr; as mentioned in my earlier post, the clay is so clean that it has been exploited in a series of connected claypits that are now lost in the wooded area to the north and west of the cottage.
On the rising slope to the north of Dolbont, on land belonging to Llystyn Farm, the soils become sandier again, and there are several distinct mounds of sand and gravel that can be picked up on the satellite imagery. Again I interpret these as signs of catastrophic ice wastage at the end of the last glacial episode.
All in all, the assemblage of landforms and sediments in the amphitheatre of Cilgwyn indicate that a lobe of ice was present here, probably wasting away over several centuries in somewhat chaotic circumstances. The mosaic of different deposits suggests that "anything could happen, and probably did........" The signs are subtle, but the landscape here is a classic of its kind.
How the meltwater channels in Tycanol Wood, and the features in Pentre Ifan Wood, relate to these sedimentary features is still to be worked out....... but these recent observations show that Glacial Lake Nevern never did exist, and that meltwaters from this ice wastage environment drained away northwards, and not through the Gwaun Valley. But we knew that already.
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