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Thursday 8 December 2022

Ogof Golchfa (Porth Clais) -- a classic Quaternary exposure

Grateful thanks to Dan Soper for placing these new images of Ogof Golchfa (Porth Clais) onto Facebook. They are far better than any of the photos from my own collection, and they show where the key exposures are located, on a little peninsula just to the west of Porth Clais creek and harbour.

This is a far better exposure than any which I examined in 1963 and 1964, so there must have been some coastal erosion.  The top photo shows beautifully the "lower head" resting on raised beach cobbles -- which are in turn resting on a high rock platform (reminiscent of the situation at Abermawr).  We cannot really refer to this "head" deposit as a slope breccia, because many of the fragments are not brecciated or broken by frost action.  It's an unusual deposit, and there must have been a substantial bank of beach pebbles here which were later (at a time of lower sea level and periglacial slope mobility) redistributed or rearranged downslope.  This material is in exactly the same stratigraphic position as the "lower hear" in many other exposures which I have traditionally assigned to the Lower Devensian / Middle Devensian episode of cold climate which preceded the ingress of Irish Sea ice from the NW during the Late Devensian.

Porth Clais Devensian sequence:


The full sequence:

5.  Sandy loam 75 cms
4.  Upper head -- 60 cms
3.  Non-calcareous local till -- up to 2m
2.  Lower head (periglacial slope deposit) of beach pebbles and other debris -- up tp 2m
1.  Raised beach shingle with erratics -- up to 1m

This paper, published in Geological Magazine in 1970, was one of the earliest in which I expounded my theory of Devensian events in West Wales.

The evidence for the late glaciation (LGM) is very strong here, as is the evidence for an early glaciation which introduced erratics into the area which were later incorporated into the raised beach.  The evidence for this early glacial episode is of course even stronger in Whitesands Bay.

When I examined the raised beach here I did not find that any of it was cemented; but Leach recorded a cemented beach here, and a little way to the east of the Ogof Golchfa exposure there does seem to be a quantity of cemented raised beach cobbles in an inaccessible location.

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