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.
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.
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