A few fossils at Mewslade Bay

In the way that some people have their own personal trainer or financial adviser, what I really need is my own personal geologist! I am learning by looking, consulting geology maps, reading books, consulting academic treatises, and visiting museums but translating all the information into an interpretation and understanding of the rocks and fossils that I see on the shorelines, is a difficult task and I know it is open to error. Particularly since the way the strata are categorised and named has changed over time.

 In 1970 when the 3rd edition of the British Regional Geology Series for South Wales was published, the stratigraphical succession of the Carboniferous comprised the Cleistopora Zone (K), Zaphrentis Zone (Z), Lower Caninia Zone (C1), Upper Caninia Zone (C2S1), Seminula Zone (S2) and Dibunophyllum Zone (D). These zones have been reclassified and renamed so that in the current version of the British Regional Geology Series for Wales (2007) the Carboniferous is divided up into the Avon Group, Black Rock Limestone Group, Gully Oolite Formation, Caswell Bay Mudstone Formation, High Tor Limestone, Hunts Bay Oolite Formation, Oxwich Head Limestone Formation and Oystermouth Formation. Cross-referencing the information from old and new sources is not always straightforward.

I am enjoying what I do, and learning a lot as I go along, with the posts I write being part of the learning process. However, it would be really great one day to go out in the field with a professional geologist, who has consummate knowledge of the areas I study, to see the rocks and fossils from an expert perspective to confirm my identifications and ideas.

With all that in mind, I have been closely examining the rocks that have been recently exposed by the removal of overlying sand at Mewslade Bay in Gower – before they disappear from view again – the sand is rapidly returning after the extraordinary winter clear-out of all loose sediments. The temporary exposure of the hidden rock surfaces, in combination with an elemental scouring that has virtually scrubbed the rocks clean of encrusting organisms, and has revealed a number of fossils that I had never noticed before.

These fossils seem to be mostly gastropod molluscs and corals with a few bivalves or brachiopods. They are not the perfect fossils that you see on display in museums; and you cannot pick them up and take them home in your pocket. They are generally small and are embedded in the rocks; and what you see at the surface is a part of the fossil from all sorts of odd angles, so they are not always easy to recognise for what they are. They all seem to have had the original hard parts replaced by crystalline calcite which is normally white but often here tinted pink or red. The colour is mostly likely to be caused by staining from iron. There are also numerous, seemingly random, streaks of calcite in the rocks, some of them extending outwards from the fossils or traversing them.

Putting a specific name to the different types of fossils is somewhat dependent on knowing the type of rock in which they are embedded. Usually, somewhere, there is an academic work listing all the fossils that are found in a particular rock type. But here it gets complicated. I know that the rocks on this part of the Gower Peninsula are Carboniferous Limestone. However, the Carboniferous Limestone is composed of many named strata or layers laid down at different times according to the varying conditions prevailing. The Carboniferous Period started 359.2 +  2.5 million years ago; and ended 299 + 0.8 mya. The Carboniferous has two stages, and the earlier Dinantian stage (to which these Mewslade rocks belong) finished 318 mya.

The rocks were laid down in a warm tropical sea subject to rises and falls in sea level, that periodically resulted in exposure of the surface rocks to aerial weathering and increased input of  terrestrial materials. The fossils are the remains of the animals that were living, dying and being buried in the seabed sediments of that tropical sea. The subsequently uplifted rock strata have been affected by earth movements that have caused tilting, fracturing, and faulting. It is possible to have more than one kind of strata on display in the same area.

I know from looking at the literature and maps, that the high cliffs surrounding Mewslade Bay are High Tor Limestone Formation (HTL). In the geological succession High Tor Limestone has Gully Oolite Formation (GO) beneath it and Hunts Bay Oolite Subgroup (HBO) above. I noticed a narrow band of what looked like mudstone, low down, and sandwiched between two thick layers of rock that looked slightly different composition and colour. So that set me thinking – while at first I believed the fossils were embedded in HTL, I am now thinking maybe they belong to the Gully Oolite. The Gully Oolite exhibits a palaeo-karst surface elsewhere in Gower (as at Caswell Bay), where the mudstones (Caswell Bay Mudstones) that separate HTL from GO are particularly well developed. Surely the fantastical erosional hollows that characterise Mewslade Bay are also part of this palaeo-karst surface – and the fossils illustrated here are in that particular sculptured rock surface.

Given the uncertainty hanging over the exact identification of the rocks in which the fossils were found, I hesitate to attach names to them. Here anyway is a selection of the fossils I found, which have their own abstract beauty as well as their intrinsic value in aiding our understanding of the environments of the deep past. I will update when I find out more information. As usual, feedback and corrective information is welcome in solving the mystery of the Mewslade fossils.

REFERENCES

George, G. T. (2008) The Geology of South Wales: A Field Guide, published by gareth@geoserve.co.uk.

George, T. N. British Regional Geology: South Wales (1970) Institute of Geological Sciences, Natural Envoronment Research Council, HMSO, 3rd Edition, SBN 11 880084 1.

Howells, M. F. (2007) British Regional Geology: Wales, British Geological Survey, NERC, ISBN 978 085272584 9.

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