Fossils at Prissen’s Tor

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

I was sitting on a rock ledge at Prissen’s Tor eating my picnic lunch when I noticed that I was sitting on lots of small fossils. Prissen’s Tor is one of a number of rock outcrops at Broughton Bay on the north shore of the Gower Peninsula. The rocks are all made of Carboniferous Limestone but each outcrop was laid down at a different stage during that period and has its own characteristic composition, texture, fossils, and name.

This post is about Prissen’s Tor on the north side of the beach at Broughton (Grid Ref. SN 425937) – to the right as you face the sea. It is composed of that part of the Carboniferous Limestone known as High Tor Limestone (HTL) which was laid down after the Black Rock Limestone, and after the Gully Oolite, but before the Hunts Bay Oolite that outcrops at Twlc Point on the south side of Broughton Bay (Grid Ref. SN 415931).

The spectacular cliffs along the south shore of Gower, featured in earlier posts about Mewslade Bay, are also comprised of High Tor Limestone. HTL additionally includes the Caswell Bay Mudstones found in their type exposure at Caswell Bay on the south shore of the Gower Peninsula.

High Tor Limestone strata vary in thickness from place to place in South Wales – between 100 and 150 feet thick – and make up a massive cliff-forming unit of Arundian age. Based on a study of 44 localities within the HTL in South Wales, Beus (1984) says that invertebrate marine fossils within it occur mainly in distinct mollusc or coral-brachiopod associations reflecting the original habitat communities and particular environmental conditions.

Beus says that of the two recognised lithofacies in the HTL, the main one is known as the “standard” facies and is composed of crinoidal bioclastic limestone. This richly fossiliferous crinoidal limestone is anything from thin to thick bedded and generally forms blocky and resistant ledges or massive cliffs. These bioclastic limestones are composed mainly of whole shells or shell fragments of brachiopods, bryozoans, corals, molluscs, foraminifera and crinoidal plates together with limestone pellets set in starry calcite or in some cases micrite matrix. The “standard” facies seems to compare with the character of the rock at Prissen’s Tor.

In this post, photographs 1 – 6 show longitudinal- and cross-sections through fossilised solitary corals, with the internal divisions clearly visible. Photograph 7 shows a cross-section through the two valves of a shell which is probably a brachiopod but could possibly be a bivalve mollusc. Photographs 8 – 12 mostly show fossilised pieces of crinoid stems. These are made up of numerous articulating segments in life, and here occur both as individual plates or segments, or chains of segments. Crinoids, also known as Sea Lilies, are related to the Echinoderms like Brittle Stars, Sea Urchins, and Starfish. They are a group that has survived to the present day, and though rare, live on deep sea beds.

The remaining images 13 – 16 give views across Broughton Bay to indicate the location of Prissen’s Tor.

REFERENCES

Beus, S. S. (1984) Fossil Associations of the High Tor Limestone (Lower Carboniferous) of South Wales, Journal of Palaeontology, Volume 58, No. 3, pp 651-667.

Howells, M. F. (2007) British Regional Geology: Wales, British Geological Survey, Natural Environment Research Council, Chapter 7, p 112-153, ISBN 978-085272584-9.

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Coral fossils in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil seashell (either brachiopod or bivalve) in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil crinoid stem segments in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil crinoid stem segments in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil crinoid stem segments in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil crinoid stem segments in High Tor Limestone at Prissen's Tor on the Gower Peninsula

Fossil crinoid stem segments in High Tor Limestone at Prissen's Tor on the Gower Peninsula

View of Prissen's Tor on the Gower Peninsula

View looking north-east across Broughton Bay towards Prissen's Tor on the Gower Peninsula

View looking north-east across Broughton Bay towards Prissen's Tor on the Gower Peninsula

View looking north-east across Broughton Bay towards Prissen's Tor on the Gower Peninsula

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Storm Beach Boulders & Coral Fossils at Broughton [4]

Close-up detail of coral fossils in Carboniferous Limestone boulder on the Gower Peninsula

This is the fourth in a series of photographs documenting boulders with colonial coral fossils seen on the beach at Broughton Bay on the Gower Peninsula in South Wales. See the earlier posts:

Fossil Coral at Broughton Bay

Storm Beach Boulders & Coral Fossils at Broughton Bay [2]

Storm Beach Boulders & Coral Fossils at Broughton Bay [3]

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Storm Beach Boulders & Coral Fossils at Broughton [3]

Storm beach boulder with coral at Broughton Bay

This is the third in a series of posts documenting boulders with colonial coral fossils seen on the beach at Broughton Bay on the Gower Peninsula in South Wales. See the earlier posts:

Fossil Coral at Broughton Bay

Storm Beach Boulders & Coral Fossils at Broughton Bay [2]

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Storm Beach Boulders & Coral Fossils at Broughton [2]

Close-up detail of coral fossils in Carboniferous Limestone boulder on the Gower Peninsula

This is the second in a series of posts about coral fossils in Carboniferous Limestone at Broughton Bay on the Gower Peninsula.  See the earlier post Fossil Coral at Broughton Bay for more details.

These fossils belong to a group of colonial corals of the lithostrotionid type, probably Lithostrotion junceum.

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Rip-Rap and Storm Beach Rocks at Broughton Bay

Detail of a boulder on the beach used as a defence against coastal erosion

Storms battering the coast can drive substantial boulders up-shore where they remain at the top of the beach, frequently above extreme high tide level. By this natural phenomenon, a strong barrier is incidentally formed, that protects the upper shore, the base of dune systems, and cliff strata from the extremes of erosion by undercutting and removal of sediments by the sea.

The knowledge that loose arrangements of weighty boulders, with plenty of space between the individual rocks, can absorb and dissipate the energy of the waves, is used in the construction of artificial sea defence systems such as rip-rap for reducing or preventing coastal erosion.

There is evidence of both kinds of barrier, the accidental and the artificial, at Broughton Bay on the Gower Peninsula in South Wales. The rocks are mostly local with the majority comprised of local Carboniferous Limestone – with the occasional boulder of Old Red Sandstone conglomerate or individual mass of brick-wall debris. Many boulders are beginning to be colonised by lichens of various kinds and colours; and the rip-rap offers the opportunity to get up close to the rocks to study their natural textures and fracture patterns.

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Brachiopod Fossils in Hunts Bay Oolite at Broughton Bay

Brachiopod fossils embedded in the Hunts Bay Oolite Subgroup strata of the Carboniferous Limestone

The rocks higher up the exposed face on the west side of Broughton Bay are characterised by their churned up and complex structure that could be evidence for storm events breaking up newly-formed sediments in the deep past. I was unable to recognise any fossils embedded in those particular rocks. See the earlier posts:

Rocks on the west side of Broughton Bay – Part 1

Rocks on the west side of Broughton Bay – Part 2

Rocks on the west side of Broughton Bay – Part 3

However, the lower levels of the exposed Hunts Bay Oolite Subgroup rocks on the west side of Broughton Bay have an entirely different appearance, being a great deal more uniform in colour, texture, and structure. They represent a distinct phase of rock formation. Fossils are present. These fossils represent shoreline accumulations of empty shells on the edge of a shallow lagoon where calcareous deposits gradually built up around them without disturbance by extreme weather events.

The fossils are those of large brachiopods, which are similar to bivalved molluscs but differ from them in possessing a form of internal skeleton. The shells are a couple of inches in diameter and are most often seen side-on in groups that have settled one within the other. The shells frequently occur in layers. In the photographs shown here, the curved edges of the shells are clearly visible. Sometimes the shells themselves have been preserved (perhaps by permineralisation) and in other places it may be just the moulds of the shells or the casts of the moulds. From the angles visible, I have not been able to specifically identify them and put a Latin name to them yet.

In nearby caves, and elsewhere among storm boulders on the beach, fossil corals can also be found. These were featured in the earlier post Fossil coral at Broughton Bay.

The whole of Broughton Bay has a fascinating geological history which I am gradually getting to understand. Relatively recent aspects of this geology include the emergent tree stumps of a Submerged forest at Broughton Bay and also the recent exposures of an iron pan derived from decomposition of old peat beds.

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Rocks on the west side of Broughton Bay – Part 3

Rock textures and fracture patterns in Gower Carboniferous Limestone

Some more images showing the mixed-up nature of the upper rocks in the exposure of Hunts Bay Oolite Subgroup of the Carboniferous Limestone in the cliffs at the west end of Broughton Bay on the Gower Peninsula – illustrating the irregular fracture patterns, rough textures and combinations of rock type within the formation. I am considering whether these messed up rocks represent a storm event way back at the time they were forming?

According to Willoughby, C (1996) Environments of Deposition in the Carboniferous Limestone of South  East Gower:

In the sequence of the Hunts Bay Oolite there is also further evidence of storm events with the appearance of bends and lenses of coarse laminated packstones*. These are associated with breccias* comprised of angular fragments of coarse oolites in the packstone matrix, indicating hardground formation before these have been ripped up in storm events and rapidly redeposited within reworked packstones. In places the junctions between these two lithologies are gradational, probably indicating that the sea floor at this time was uncemented and soft allowing mixing to occur.

I visualise that scenario as resulting in something that looks like pack ice,  where a solid sheet of ice has been broken up by some force of nature into many angular fragments that then refreeze into a solid form with the pieces ‘cemented’ together by newly formed sea ice.

Definitions

* Breccia is coarse, clastic, sedimentary rock, the constituent clasts* of which are angular. Breccia literally means ‘rubble’ and implies a rock deposited very close to the source area.

* Clasts are particles of broken-down rock. These fragments may vary in size from boulders to silt-sized grains, and are invariably the products of erosion followed by deposition in a new setting.

Packstone is defined by the Dunham Classification as a limestone characterised by a grain-supported texture, together with a lime-mud matrix.

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

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