Rocks on the West Side of Three Cliffs Bay

The rocks on the west side of Three Cliffs (Threecliff) Bay on Gower in South Wales are made up predominantly of Lower Carboniferous Limestone. Although there are some Devonian rocks higher up the valley, these are mostly obscured and hard to spot. The starting point for the images shown in this post is the south face of the large vegetated dune that juts out into eastwards into the bay and causes the Pennard Pill river to be diverted in a great meander loop in order to reach the sea.

At the foot of the dune (SS 535 881), the northern end of the western cliffs emerge. There is a very small exposure of Avon Group strata, comprising grey-green finely bedded shales and mudstones which used to be known as the Lower Limestone Shales. Most of this early part of the Carboniferous sequence is hidden from view by the sand deposits but it extends westwards into the area called Stonefields.

On top of the Avon Group shales lies a sequence of Pembroke Group strata starting with the Black Rock Limestone Subgroup, then the Gully Oolite Formation, Caswell Bay Mudstone Formation, High Tor Limestone Formation, and finally the Hunts Bay Oolite Subgroup. The strata, though horizontal when first laid down, have been steeply tilted by subsequent earth movements to 60 – 80 degrees south. Looking at the exposures of rock in the face of the cliffs, the rocks become increasingly younger to your left (southwards) and older to your right (northwards).

A basic description of these rocks can be found in Barclay (2011) and George (2008) and the geological map for the Swansea area (Sheet 247). The rock layers reflect not only the conditions under which they were laid down initially but also the effects of great pressures that resulted in fracturing and faulting at later dates.The first of the sequence of rock layers belongs to the Black Rock Limestone Subgroup that, if I may quote from Gareth T. George’s excellent field guide, “comprise alternating bioclastic limestones and bioturbated lime mudstones and shales, which are succeeded by thicker-bedded bioclastic packstones with graded bedding and sets of hummocky cross-stratification (HCS) ” etc.

Stylolitic seams are said to be common. These occur where pressure within the rock causes some minerals to dissolve and seems to result in specific types of irregular white lines of calcite within limestone, and also irregular crystalline textures along some flat surfaces. This phenomenon may be responsible for a number of the features shown in some of the images in the gallery below.

REFERENCES

Barclay, W. J. (2011) Geology of the Swansea District – a brief explanation of the geological map Sheet 247 Swansea. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 247 Swansea (England and Wales). British Geological Survey, Keyworth, Nottingham, NERC, ISBN 978-085272581-8, pp 1 – 9.

George, G. T. (2008) The Geology of South Wales – A Field Guide. Published by gareth@geoserv.co.uk, ISBN 978-0-9559371-0-1, pp 77 – 82.