Introduction to the Geology of Tenby

Tenby is situated close to the south-western shore of Carmarthen Bay. The land within the environs of the town and its beaches is made up exclusively of Carboniferous rocks, mainly limestone and millstone grit, with the exception of blown sand and other recent drift deposits. Rocks to the south are older, including Old Red Sandstone part of which forms the “Ridge Conglomerate”. Rocks to the north are younger, and beyond the North Beach trend into the Coal Measures of the now inactive Pembrokeshire coalfield. The whole sequence has undergone extensive folding, faulting and thrusting as a result of the Variscan (Alleghenian in North America, Hercynian or Armorican in Europe) Orogeny, which took place over an extended period between before 350 and possibly later than 280 Million Years ago (Ma) from late Devonian through Carboniferous to early Permian. This orogeny was caused by a complex series of plate collisions as the Rheic ocean closed. In general, it caused the rocks of Pembrokeshire to be compressed northwards, leading to a shortening of strata by some 30% or more in places. Anticlines and synclines, trending east south east to west north west are present both north and south of the town. Unlike the earlier Caledonian Orogeny, no mountain building or metamorphism characterised the Variscan in Pembrokeshire.

The town itself sits astride the eastern end of the Ritec Fault. This fault probably originated in the earlier Caledonian Orogeny, which mostly affected Northern Pembrokeshire. However, if so, it was certainly re-activated in the Variscan, leading to thrusting of older Carboniferous limestone over younger strata. The Ritec fault appears to terminate just east of Tenby and, does so by forming three splays. One of these fractures runs east of the Barrel-post Rock, another trends through the harbour, whilst the most southerly separates St Catherine's Island from the mainland.

Just south of St Catherine's Island, in the Carboniferous Limestone, the axis of the Tenby anticline trends almost east-west. Its core exposes dolomites and oolites which are overlain by calcareous mudstones. Differential erosion has resulted in “caves” being formed in the softer mudstone, bounded by the harder limestone sediments, particularly in the Southern limb. The younger strata of the northern limb runs under the town and Castle Hill, but is poorly exposed and, in any case, is much disturbed by the faulting. However, the older strata are exposed and show, by being almost vertical, that the axis leans northwards. Fossils are observable in situ in the limestones.

North of Castle Hill, the Millstone Grit strata are exposed. This section, North Beach is made very complicated due to thrusting and faulting which has resulted in some difficulty in explaining the succession. Whilst the strata dip and young to the south, in general, the rocks young to the north. This has been explained by sequences of thrusting of blocks of older, southwards-dipping rocks over younger, with the younger blocks being emplaced towards the North. There is some evidence for overturning of strata, whilst small anticline/syncline pairs have been observed. Whilst estimates of the age of rocks are dependant on fossils, they are rare and are not apparent from the beach. In addition, the cliffs in this section have become greatly overgrown and their bases have been obscured by the sea wall and rock armour (rip-rap). The latter, however, offer the opportunity to examine the oldest rock in Pembrokeshire; the pre- Cambrian Johnson Formation, .

The paleogeography and environment of the Tenby region in the Carboniferous is predominantly that of shallow, warm, equatorial marine conditions. This marks a considerable change from the Devonian ORS, characterised by “continental”, land-like, fluvial and deltaic sandstones, due to a subsidence of what was then the Wales-London-Brabant Massif (St George's Land) and subsequent Northwards transgression of the sea. The massive, 800m thick Carboniferous Limestone was deposited conformably onto the ORS, It is highly fossiliferous, reflecting the varied and crowded coralian and brachiopod fauna of the period. At least six cycles of transgression, known as mesothems, have been described and have given names to Stages of the Dinantian. The younger Millstone Grit was deposited by rivers flowing off the Massif once the sea retreated, resulting in deltaic environments and the deposition of coarse sandstones interspersed with mudstones, siltstones and shales.

The main problems interpreting the North Beach Cliff strata are due to the intense folding associated with complex thrusting, the rarity or absence of marker fossils, the lack of exposure of key areas and the obscuration due to vegetation and rock armour. The current state of these cliffs has been described as “... in too precarious a condition to be attacked by wild geologists armed with stout geological hammers”!

Location 1. The most northerly cliff section, just south and west of First Point, exposes the Telpyn Point Sandstone, folded into an asymmetrical syncline with a vertical or partly inverted southern limb. An undulating fault plane traverses the south side of First Point headland, bringing almost overturned beds on the southern side against SW dipping strata on the north. The fault must virtually cut through the centre of an overturned syncline. Ironstone pebble lag marks the erosive base, below which is the First Point Plant Bed in a thin shale which has yielded fossils of Alethopteris, a seed fern of a now extinct group of gymnosperms.

Location 2. The North Cliffs show a highly deformed argillaceous sucession of numerous sequences of mudstones, siltstones and thin sandstones which generally dip to the south. The latter have erosive bases and show some cross-stratification below bioturbated tops which are overlain by mudstones. These “coarsening upwards” sequences are indicative of beach deposition, when lagoons, backed by coastal barrier bars, transgress landwards, whereas the Northern cliffs indicate sedimentation in fluvial channels. These and other structures indicate that the strata are the “right way up”, dipping and younging to the south. Fossils of Sanguinolites and Dunbarella (Bivalves) and Anthracoceras (Goniatite?) have been found in this area.

Location 3. At the southern end of the North Cliffs there is an exposure of a sandstone block which is lithologically similar to the prominent Gosker Rock to its south-east. At one time, it was thought to be inverted, but more recent analysis has shown that this is not so. Both structures are thus related.

The Beach armour quarried blocks come from nearby the town of Johnston (some 20km West south west of Tenby), which gives its name to the possibly intrusive, igneous quartz-diorites of Dimetian age (pre-Cambrian) formation which also includes quartz-albites.

Location 4. Gosker Rock is the most prominent feature of the rocks surrounded by beach sands. It is a southwards-dipping stack of siltstones, coarsening upwards into ripple laminated sandstones. The evidence that it is the “right way up” is provided by load casts and ripple marks. This is similar to the block at Location 3, from which it is displaced by 125m along a dextral wrench fault.

Location 5. The “reefs” in and around Barrel-post Rock show a succession of interbedded fine sandstone, siltstones and mudstones. Limestone “bullions” have been reported, but are unlikely to be visible unless the beach sand has been cleared by storm waves. Two splays of the Ritec Fault pass to the north and south, thrusting a wedge of highly brecciated Carboniferous Limestone into this section. Tenby Harbour has intermittent exposures of black shales bearing goniatites, indicating forced regressive sandbodies, pushed south due to falls in sea level.

Castle Hill, predominantly Carboniferous Limestone, has an exposure of thick-bedded oolite and crinoidal limestone. This dips south under a group of dark, thin bedded fossiliferous limestones with lines of black chert along the upper part of the cliff. The hill affords good views of the Upper Carboniferous of the Pembrokeshire Coalfield Syncline.

Location 6. Southern Castle Hill includes a belt of highly disturbed strata which show shearing and mylonisation with some coal-like material, riddled with red-stained calcite veins. This disturbance has been caused by the Ritec Fault splay which appears to trend along the line of the cliffs in this section.

Location 7. The Castle Beach section is on the northern limb of the Tenby anticline. The overlying High Tor Limestone contains a fossil corallian fauna with crinoids and gastropods. A small but very clear buckle fold and thrust can be seen in the westernmost section.

Location 8. The axis of the anticline intersects the cliffs at Gunfort Gardens. Its core consists of well-jointed dolomitized bioclastic limestones of the Black Rock formation. It is cut by a small thrust.

Location 9. A prominent cave has been formed by differential erosion of the Caswell Bay Mudstones, a succession of of thinly bedded limestones and mudstones. This is a lagoonal and supra-tidal facies of fine grain limestones and dolomites, interbedded with lime mudstone and shales, showing bioturbation. There is some evidence for “chicken wire” anhydrite which is typical of sabkha (supratidal salt flat) environments. The contact with the Gully Oolite is marked by a paleokarst surface showing some veining.

Location 10. 75m south westwards of the cave there is a section showing fossils of the coral Siphonodendron (Lithostrotion) martini. The surface was formed during a shallowing of the shelf sea, possibly including a period of exposure which would explain the erosion seen in these coral fossils.

Location 11. 10m further on , at a set of steps , a vertical bedding plane in the limestone is exposed, showing calcite mineralised slickensides with grooves and ridges. Close by, fossils of the Productids (brachiopods) Composita and Cyrtina can be seen in the limestone just above the base of the Hunts Bay Oolite.

Location 12. 25m southwards in a gully in the cliff face, The Hunts Bay Oolite can be examined, showing a series of shoaling upwards cycles typically 3-5m thick. Each cycle has a sharply defined base overlain by light=grey oolite grainstone with parallel and low angle cross-lamination, capped by thinly-bedded dolomitised limestones. These facies are evidence for a high-energy foreshore grainstone shoal passing upwards into lower-enegy intertidal environments with algal stromatolites.

Location 13. In a further 50m south there is a cleft in the Hunts Bay Oolite, in which there are irregular veins and patches of coarsely crystalline calcite and with limestone fault breccia and other sediments. This indicates the infill of Triassic potholes. On the way to this location, at the top of the steps to the Atlantic Hotel, there is a band of limestone with many Gigantoproductus giganteus.

Leave the beach by the southernmost steps up to the car park and Esplanade.