Ceibwr - Geology and Landscape

Dr Brian John takes us on a tour of Ceibwr Bay

Ceibwr is one of the most iconic geological locations in West Wales, with that extraordinary cliffline that stretches away to Pen-yr-Afr and Cemaes Head. But there is a great deal more that deserves our admiration.

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First, the rock strata: Sediments laid down 458–449 million years ago, flushed from a land-mass to the south into a deep ocean basin. It was a dynamic environment. Underwater avalanches formed some thick fine-grained layers (‘turbidite’); slope collapses and sediment fans resulted in shales and sandstones of different textures. Compressed under the weight of successive layers, the muds and sands became rock strata.

Fifty million years or so later, in the Caledonian Orogeny (mountain-building episode), colliding continents from the NNW and SSE closed up that ocean basin and crumpled the sedimentary strata, giving rise to parallel anticlines (upland ridges) and synclines (valleys and lowlands) of impressive proportions.

The Pembrokeshire coastal cliffs of today show us a slice or cross-section through those ancient mountains. Almost all surface traces of the upland landscape have since been eroded away, except for the ridge of Mynydd Preseli and its outliers on Carningli, Pen Caer and the northern part of the St David’s Peninsula. The spectacular synclines and anticlines in the cliffs down towards Pen-yr-Afr are just small details that happen to be visible.

But we can still witness the force of that ancient collision. The folds, faults and brecciated (shattered) zones in the cliff faces are truly spectacular. In places the strata are standing on end, and elsewhere we see pitching anticlines, synclines and other structures that attest to the power of the mountain building processes that operated here.

The “internal organs” exposed in the cliffs around Ceibwr add a special quality to what is already a spectacular cliffline, with caves, stacks, gullies, rockfall scars and tunnels.

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Look carefully and you’ll see that in the cliffs on the east side of the bay, browns and greys predominate, while large portions of the western side are dark grey or black.  This is because the Ceibwr Bay Fault outcrops in the eastern cliffs, exposing rocks that are much younger than those to the west. There is a 600 metre downthrow of the strata, revealing dark Carreg Bica Mudstones to the west and sandstones and mudstones of the Dinas Island Formation to the east and north. The black mudstones are softer and much more susceptible to coastal erosion. This explains why the stacks of Careg Wylan and Careg Yspar are inexorably being denuded by slope collapses and eaten away by the sea.

Ceibwr Bay is really a tidal creek positioned in the mouth of a partly flooded deep valley.  The valley is a “misfit”—far too large to be explained by the work of the small river that is currently in occupation. (It is larger than it appears, because in its lower section—near the beach—the rock valley is plugged with glacial deposits more than 20m thick.)  Its catchment area, just a few kilometres inland, is not high enough nor extensive enough to feed a large river, so the valley as we see it must be an Ice Age relic, formed by glacial meltwater.  

 

 

Like many of the other deep valleys in north Pembrokeshire, it may be composite both in age and origin. The valley might well be millions of years old, modified by meltwater at the end of one, two or even three different glacial episodes. The last of these (referred to as the Devensian) was only about 20,000 years ago. 

 

The most interesting feature in the mouth of the big valley is the existence of a small subsidiary valley which is separated from the main creek by a rock ridge.  This little valley (in which the car parking area is located) has rock walls carrying traces of meltwater flow, and a humped long profile, which means that the meltwater that cut it must have flowed uphill before flowing downhill towards the north.  Meltwater can only flow uphill when it is flowing inside a pipe, under hydrostatic pressure—and this can only happen when there is an extensive ice cover across the landscape.  Comparison with other valleys (including Cwm Gwaun) suggests that this might have happened about 450,000 years ago and again at the end of the last glacial episode.

 

Then it gets even more interesting, since at the outer coast there is a gully with three flooded potholes cut into the floor of this western valley—suggesting another phase of erosion by fast-flowing and turbulent torrents of meltwater.  So how do we sort out the ages of these features?  There is still work to be done, but the answers probably lie in the sediments within the walls of the big valley (at the head of the creek), the smaller western valley, and the gully. 

 

At the side of the road where it goes uphill, on a tight bend, there are two exposures of iron-stained and concreted gravels which appear to be the remnants of a much more extensive gravel deposit which either filled the valley or accumulated against the edge of a melting mass of ice occupying the valley floor.  There are other stained and concreted gravels too, at the northern end of the rocky spur which separates the two larger glacial meltwater channels.  They are underlain by what appears to be a concreted layer of ancient till (which used to be called “boulder clay”) and overlain by a fresh till deposit and a sandy colluvium containing pebbles and broken cobbles. 

So there are traces of TWO glacial episodes. The upper till is similar to the Irish Sea Till which is seen at Newport and Gwbert, and it also plugs the valley at the head of the Ceibwr tidal creek.  There are several discontinuous exposures, and the relationships between the various deposits are complex.  It’s reasonable to assume that the concreted slope deposits, till and gravels are very old (maybe dating from the Anglian glaciation 450,000ya) and that most of them have been eroded away. The fresh deposits (slope breccia, till and colluvium) probably date from the Devensian glacial episode. But we cannot be sure of this until new dating techniques are used on Ceibwr samples, measuring their age by the effects of their exposure to cosmic rays.

One final dilemma: The concreted and fresh deposits also occur in the small gully with iron-stained walls near the outer coast. By implication, this makes the gully very old too, and the two larger meltwater channels even older. There is much still to be discovered here—but my bet is that Ceibwr will turn out to be one of the most important Quaternary sites in Britain.

© 2019 Dr Brian John

Read on, about the geology of the neighbouring landscape - volcanoes and more!

Enquiries, contributions and improvements to anything in these pages:                    0845 519 2791   info@moylgrove.wales

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