CEIBWR
Geology and Landscape
Ceibwr is one of the most iconic geological locations in West Wales -- on a par with the Green Bridge of Wales, Carn Meini on Mynydd Preseli, and the Sleek Stone at Broad Haven. Here the focus of attention is the extraordinary cliffline that stretches away to the north-east, culminating in Pen-yr-Afr and Cemaes Head -- but there is a great deal more that deserves our admiration.
First, the nature of the rocks. Here, all of the strata are of Upper Ordovician age, and they are all of sedimentary origin. For the most part they are mudstones, shales and sandstones that were laid down in a deep ocean basin around 458 - 449 million years ago. Sediments were flushed down from a land-mass to the south, and they accumulated in deep water in what was a very dynamic environment, with turbidite flows, slope collapses and sediment fan accumulations. There appear to have been a number of cycles of deposition, with muddy materials laid down first, followed by silts and then sandy layers. If you look carefully, you can see that there is a colour difference between the cliffs on the east side of the bay (where browns and greys predominate) and those to the west (where large portions of the cliffs and stacks are dark grey or black in colour). 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 -- and this explains why the stacks of Careg Wylan and Careg Yspar are inexorably being denuded by slope collapses and eaten away by the sea.
But it is the structures in these cliffs that are of greatest interest to geologists. We see the strata with striking clarity, but the folds, faults and brecciated or 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 which attest to the power of the mountain building processes that operated here following the closure of the ocean basin in which the sediments accumulated. What we see today is essentially a slice or cross-section through the landscape which is typical of Mynydd Preseli and the rest of north Pembrokeshire. During the Caledonian Orogeny (mountain-building episode), around 400 million years ago, pressure associated with the movements of the continents was exerted from the NNW and SSE, giving rise to parallel anticlines (upland ridges) and synclines (valleys and lowlands) of impressive proportions. The synclines and anticlines in these cliffs are just the small details. Now, almost all traces of the ancient landscape have been eroded away, except for the upland ridge of Mynydd Preseli and its outliers on Carningli, Pen Caer and the northern part of the St David’s Peninsula. The “internal organs” exposed in the cliffs add a special quality to what is already a spectaculatr cliffline with caves, stacks, gullies, rockfall scars and tunnels. And the cliffs are even more spectacular in the distance, where the “harmonic folds” and faults near Pen-yr-Afr are clearly visible.
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 which 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 or 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 wlls 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.
Stratified and cemented glacio-fluvoal gravels (?) at the roadside, about 10m above the
floor of the channel.
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 stony till and overlain by a fresh till deposit and a sandly colluvium containing pebbles and broken cobbles. The till is similar to the Irish Sea Till which is seen at Newport and Gwbert, and in a thick plug at the head of the Ceibwr tidal creek. There are several discontinuous eposures, and the relationships between the recent 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) and that most of them have been eroded away. The fresher uncemented 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 for the measurement of cosmogenic exposure ages on Ceibwr samples.
In this photo, taken near the tip of the rock spur, we see broken bedrock shales at bottom left, with a concreted bed of stony till resting on it. Above that there is a concreted horizon of finer gravels, with a thin layer of fresh till close to the surface. This is stratigraphically equivalent to the Irish Sea Till exposed at bottom right, overlain by a silty and sandy layer of "rubble drift" and colluvium.
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 be be discovered here -- but my bet is that Ceibwr will turn out to be one of the most important Quaternary sites in Britain.
Brian John
5 January 2019
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The above is all rather generalised -- I went over to take a look at the sediments yesterday, and will do another post......
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