Storm damage at Mewslade Bay

Mewslade Bay is one of my favourite beaches on the south Gower Coast. Well-known for its dramatic rock formations and beautiful sandy shore. It has taken a pounding during recent storms. The result of the relentless attacks by wind and waves is a complete transformation – the sand has disappeared! The pictures shown above, revealling the new topography of Mewslade, were sent to me by kind friends from the nearby village of Middleton. They regularly walk their dogs on the beach and captured these shots on an i-pad yesterday afternoon. It seems that this vanishing sand event is a not a unique occurrence. It has happened before but I am not certain how long ago. The good news is that on that occasion the beach eventually recovered and the sand came back.

The images below I took myself. They show how that same part of the shore at Mewslade Bay –  the part which is now totally denuded of sand to reveal the underlying wave-cut platform – looked prior to the storms, with deep sand extending up the beach as far as the fault gully that connects it with the dry valley beyond. I am told that an estimated depth of 2 – 3 feet of sand has been washed away and that rocks shattered by the waves lie all around. Many of these rocks no doubt derived from the fault breccia that surrounds the gully. I wish I could be there myself to record the details of this storm-driven revelation of the underlying geology.

It is interesting to note that the sand moves around this whole area on both a short-term and long-term basis – eroding in one area and being deposited in another (May, V. J. 2007). This can be seen (and I have been recording it over recent years) in the way that beach topography changes with deeper or shallower levels of sand, maybe even between tides, as seen from the relationship between fixed objects and the changing levels of sand at Rhossili and Whiteford Sands; also, for instance, by the uncovering of once submerged and buried forest timbers, ancient peat layers, and glacial deposits (like at Broughton Bay). Sand dunes are cut away (for example, at Llangennith and Whiteford Point) and sand banks accumulate.

The sand, however, is a finite resource. The purely mineral part at least is not being formed to any significant degree at the present. The sand deposits from around the Gower Peninsula were left there by melting ice sheets and are a product of glaciation. While glaciers and ice sheets moved down towards the coast, rocks were picked up by the passage of the ice over the ground, and these were responsible for grinding to minute fragments the rock over which they traversed. When the ice started to melt and retreat, the rocks and sand were dumped. This is how the sand arrived in Gower.

There are concerns about the way this valuable resource of sand is being exploited. There are worldwide fears about the impact of removing too much sand from an un-renewable resource. A documentary film has recently been released called Sand Wars – drawing attention to this potential problem.

Click here for more posts about Mewslade Bay on Jessica’s Nature Blog.

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Rocks on the west side of Broughton Bay – Part 1

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

These rocks are not pretty but they are interesting! This is the project I am working on at the moment. I am trying to understand the rocks on the west side of Broughton Bay on the Gower Peninsula. I know that they are Hunts Bay Oolite Subgroup of the Carboniferous Limestone from looking at the geology map – but that is just the beginning of the understanding.

Not all the rock looks the same. There seem to be two main parts. The upper part of the cliff looks a bit of a mess, a bit of a jumble, coarse sediments and lots of fractures, ?faults, and various lenses and bands of material. Sometimes it looks like some sort of breccia. It is mostly examples of the appearance of the rock in the upper part of the cliff that are featured in this post. By comparison, the lower part of the cliff tends to be a different colour, is more solid, massive, and continuous, with fewer fractures, and containing obvious fossils.

In this and subsequent posts I am just putting down my thoughts as I read more about the subject. I have already found a couple of good references to help me. What I really need to do is visit the “type location” for this particular rock type, from which this particular subgroup of the Carboniferous Limestone was first properly identified and described. Perhaps the next time I am in Gower. Maybe it will all become a lot clearer then. Meanwhile, I will continue my attempts to identify the components of the rock and understand their origins.

First of all, it should be noted that in the photographs shown here, the orange, yellow, and black patches are modern lichens living on the surface of the rock and are nothing to do with the rock itself. The white colour on the rock close-ups is usually crystalline calcite. The top picture in this post shows the general location where the following photographs were taken, from just before Twlc Point to Foxhole Point on the west side of Broughton Bay. The boulders on the beach at the foot of the low cliff are covered with living barnacles attached to mussels, and by green seaweed.

The Hunts Bay Oolite Subgroup (HBO) is the most recent nomenclature for this type of rock but it was first named and defined as the Hunts Bay Group by Wilson et al (1990). The type section is the cliffs and foreshore reefs between Bacon Hole and Pwlldu Head, Southgate, Gower, South Wales (SS 5604 8674 – 5728 8632).

This is a work in progress and  I will amend and annotate the text and pictures as I learn more, as well as adding new posts on the subject! If you, as a reader and expert geologist, know that I as an amateur have made a mistake in my understanding about these rocks, please do drop me a line and put me right.

I am currently reading:

Waters C N, Waters R A, Barclay, W J, and Davies J R (2009) A lithostratigraphical framework for the Carboniferous successions of southern Great Britain (onshore), British Geological Survey Research Report RR/09/01, NERC, Keyworth, Nottingham.

Willoughby, C (1996) Environments of Deposition in the Carboniferous Limestone of South  East Gower, Bsc Geology Thesis, University of Wales, Aberystwyth.

Reference

Wilson, D, Davies, J R, Fletcher, C J N, and Smith M. (1990) Geology of the South Wales Coalfield, Part VI, the country around Bridgend. Memoir of the British Geological Survey, Sheets 261 and 262 (England and Wales).

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

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Triassic remnants at Fall Bay, Gower

Red Triassic rock remnant with calcite crystal in faulted Carboniferous limestone.

Triassic rocks are mostly absent from the Gower Peninsula because they were worn away in ancient times. However, isolated pods and thin veins of brick-red sandstone and clay survive as remnants from the Triassic period at Fall Bay within the grey Carboniferous limestone. The limestone has been cut by many faults and fractures, and these massive cracks are often filled with coarsely crystalline iron-stained calcite which, in turn, surrounds the remnants of the now missing Triassic cover.

The Triassic sediments are thought to have ‘migrated’ downwards into the older strata below at the time of the earth movements that led to the faulting, and subsequently had been incorporated in the rocks from these earlier periods. In places, chunks of broken limestone from the faulting are included within the red clays.

These photographs show some of the red Triassic sediments, especially the large example which can be found at the base of the steep steps leading to the beach from the lime kiln at the top of the dry valley above. I searched for the Fall Bay Triassic remnants after reading about them in George (2008). The Fall Bay Triassic remnants are less well known than the small outcrop of Triassic red breccia and conglomerate at Port Eynon that is discussed in Bridges (1997). More details of the Permian-Triassic rocks in South Wales can be found in Howells (2007).

REFERENCES

Bridges E. M. (1997) Classic Landforms of the Gower Coast, Series Editors Rodney Castleden and Christopher Green, Geographical Association & The British Geomorphological Research Group, p 7, ISBN 1-899085-50-5.

George G.T. (2008) The Geology of South Wales: A Field Guide, gareth@geoserv.co.uk, p 77, ISBN 978-0-9559371-0-1.

Howells M. F. (2007) British Regional Geology: Wales, British Geological Survey, Keyworth, pp156 – 163, ISBN 0-85272-584-9.

Rocks at Caswell Bay

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Caswell Bay in Gower features a classic sequence of different rock types within the broader category of Carboniferous Limestone. As you walk in an easterly direction from the café and car park at the top of the beach, towards the sea with the rock outcrops on your left, you walk past a series of spectacular rock formations with marked stratification and jointing, weathering and erosion patterns, faults, thrusts and folds. A repeated sequence of Caswell Bay Mudstone, Caninia Oolite, Laminosa Dolomite, Crinoidal Limetones, and Seminula Oolite.

It is not a straightforward series because of the synclinal and anticlinal folding and thrusts – so I am still trying to fathom out which rock is which! Nevertheless, artistically and photographically there was much to enjoy and this Posting presents a range of the natural patterns and structures in the limestone. Some of the more interesting rock patterns have been photographed close-up and were shown in an earlier Posting Caswell Rock Patterns & Textures.

One of the sources of information I am using to try and understand the geology at Caswell Bay and to identify the rocks that I am photographing is the on-line Geological Society Field Guide to Caswell Bay.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

Carboniferous Limestone formations at Caswell Bay, Gower, South Wales, UK.

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Rusty Pebbles at Whiteford

Rusty Pebbles at Whiteford (1) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Vast swathes of pebbles on the beach at Whiteford in Gower are coloured orange – or at least they were the last time I looked. (The beach sediments there are very mobile so it cannot be guaranteed that you will see exactly the same thing on each visit). These coloured pebbles are found in a band stretching from the base of the sand dunes at the eastern end of the beach towards the disused Victorian Whiteford Lighthouse.

The pebbles seem to be coated in rust rather than rusty because of their intrinsic composition. I guess the first couple of times that I noticed the orange pebbles I vaguely thought that they were stained by rust emanating from the decomposing remains of the old iron causeway that linked the lighthouse to the shore. You can often find pieces of the iron framework of the walkway – sometimes supports still in situ and other times single pieces of the structure lying free.

However, lately, I have been discovering more and more about the Quaternary geology of the Gower – a relatively recent geological period dating from about 2.5 million years ago to the present. This includes the Pleistocene with a variety of glacial, peri-glacial and inter-glacial deposits; and the recent Holocene (from 11,800 years ago) with peat and submerged forests, marsh, dune, beach and alluvial deposits. As I read more, I am gradually reaching something of an understanding about some of the natural phenomena that I observe and photograph on Gower beaches. So I now tentatively consider that the rusty pebbles are not related to the dilapidation of the old lighthouse but are the result of a much older natural geological process.

I have already mentioned in Jessica’s Nature Blog the remains of the submerged forest at Broughton Bay which lies to the west and adjacent to Whiteford Sands. These ancient tree trunks are embedded in peat deposits. While I was reading George (2008), I learnt that the peat decomposes to form a hard ferruginous layer called an iron-pan or hardpan. This has led me to wonder if the iron compounds that coat the pebbles at Whiteford are derived from an iron pan layer.

Supporting evidence for this idea comes from the presence of ancient tree trunks emerging from black peat deposits close to the rusty pebbles – similar to those stumps found at Broughton. The old waterlogged wood is also stained with rust – as you will see from the photographs below. Additionally, slightly higher on the beach, closer to the dunes, the shore is strewn with pebbles around which orange-coloured watery ‘tears’ rise to the surface and weep across the surface of the sand – making me think they might originate from a concealed ferruginous hardpan below.

Then again, I suppose the rust could come from buried decomposing munitions as the beach was used for firing practice in the Second World War!

Reference:

George, Gareth T. (2008) The Geology of South Wales – A Field Guide, G.T.George at gareth@geoserve.co.uk , ISBN 978-0-9559371-0-1, p 70.

Rusty Pebbles at Whiteford (2) - View looking across to Llanmadoc Hill showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (3) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (4) - Pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer.

Rusty Pebbles at Whiteford (5) - View looking across towards Whiteford Lighthouse showing pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are covered with a rusty deposit thought to derive from the break up of an iron-pan associated with a Holocene peat layer. Ancient waterlogged wood from the submerged forest is also visible.

Rusty Pebbles at Whiteford (6) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (7) - Ancient iron-stained log embedded in peat from a submerged post-glacial forest - associated with pebbles on the beach at Whiteford Sands, Gower, South Wales, some of which are also covered with a rusty deposit thought to derive from the break-up of an iron-pan associated with the disintegration of the Holocene peat layer.

Rusty Pebbles at Whiteford (8) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

Rusty Pebbles at Whiteford (9) - Pebbles scattered on the surface of the sand with 'tears' of rusty water, possibly rising from a buried Holocene iron-pan layer below the sand, weeping across the beach.

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Ancient Gower Shells & Raised Beaches

Ancient Gower Shells & Raised Beaches (1) - Limpet shells from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Britain began to warm up after the first stage of glaciation,  between 130,000 and 125,000 years ago, and rising seas caused by the melting of the ice sheets carved wave-cut platforms in coastal bedrocks, and deposited new beaches. In Britain this time is known as the Ipswichian Interglacial period – in geological terms a relatively recent phase of the Pleistocene xx. Subsequently, following various geological stages of ice formation and recession, sea levels dropped again to current day levels – leaving these earlier beaches up to 6 metres above present Chart Datum.

Remnants of these raised beaches have been described from around the coastline of the Gower Peninsula in South Wales. Most of the readily available academic texts refer to the sites at Foxhole, Langland, and at Fall Bay. Despite the focus on a few locations which demonstrate the phenomenon particularly well, there would have to be evidence of this sea level change all around the Gower Peninsula – and I discovered for myself an apparently hitherto un-described location for these ancient shell-bearing deposits right on the tip of the Rhossili Headland where it drops down to the Worms Head Causeway.

Here I found ancient limpet shells (Patella spp.) and winkle shells (Littorina spp.), embedded with minute shelly fragments and pebbles, in a matrix of reddish, iron-stained sediment consolidated by crystalline calcium carbonate. These deposits were located in cracks and crevices of the jagged limestone bed-rock projecting upwards at steep angles from the shore. More significantly, however, the shells were found in distinct, horizontally-aligned strata clearly visible at the base of the softer recent sediments leading down to the present-day beach.

The layers of ancient beach material originally would have spread out and covered all the bedrock which juts up from the upper and middle shore. It has now been mainly washed away but remains in a few places, almost like a coating of conglomerated cement with hard-core, attached to the Carboniferous limestone outcropping on the higher shore. The horizontal layers of raised beach deposits, seen in vertical section at the base of the headland, are often undercut by extreme high-tide wave action and consequently overhang slightly. Above the raised beach levels are solifluction deposits and red soil.

For more information about the raised beaches of Gower, have a look at:

George, G.T. (2008) The Geology of South Wales – A Field Guide, published by gareth@geoserve.co.uk, ISBN 978-0-9559371-0-1, pp 66-89.

Mullard, Jonathan (2006) Gower, Collins New Naturalist Series, Harper Collins Publishers, London, ISBN-13 978-0-00-716067-6, ISBN-10 0-00-716066-6, pp 44-50.

PS Don’t forget that you can click on individual photographs to enlarge them and read a detailed description of the image.

Ancient Gower Shells & Raised Beaches (2) - Limpet shells from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (3) - Limpet (Patella spp.), winkle (Littorina spp.), and other shell fragments shells from 125,000 to 130,000 years ago, in iron-stained stalagmitic calcium carbonate matrix, in  raised beach deposits, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (4) - Winkle and limpet shells cemented in a hardened matrix with pebbles and shell fragments, in part of a raised beach, dating from the Ipswichian Interglacial 125,000 to 130,000 years ago in the Pleistocene - found between Carboniferous limestone layers at Worms Head Causeway, Rhossili, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (5) - Limpet shells from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (6) - Limpet shells from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (7) - Winkle shells (Littorina spp.) from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (8) - Winkle shell (Littorina spp.) from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (9) - Winkle shells (Littorina spp.) from 125,000 to 130,000 years ago, in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (10) - Shell fragments from 125,000 to 130,000 years ago, embedded in an iron-stained calcite cement (seen here as a network of crystalline strands), in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (11) - Shell fragments and gravel  from 125,000 to 130,000 years ago, embedded in an iron-stained calcite cement (seen here in close-up as a network of crystalline strands), in raised beach deposits dating from the Ipswichian Interglacial Period, near Worms Head Causeway, Rhossili Bay, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (12) - Shells and pebbles in a raised beach naturally cemented together by rusty-coloured calicite cement. Dating from the Ipswichian Interglacial Period 125,000 to 130,000 years ago. Photographed on the tip of the Rhossili Headland where it drops down to the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (13) - Over-hanging layers of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, on the landward edge upper shore of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (14) - Over-hanging layers of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, seen on the landward (Rhossili) edge of the upper shore of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (15) - Layer of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, forming a carpeting layer cemented to wave-cut bed-rock, seen on the landward (Rhossili) edge of the upper shore of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (16) - Layer of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, forming a carpeting layer cemented to wave-cut bed-rock. A chunk of the deposit has become detached.  Vertical section of raised beach deposits, solifluction debris, and red soil derived from Old Red Devonian sandstone seen in background. Photographed on the landward (Rhossili) edge of the upper shore of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (17) - Layer of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, forming a carpeting layer cemented to wave-cut bed-rock. Another layer can be seen overlying it and extending seawards, while the vertical section of raised beach deposits, solifluction debris, and red soil derived from Old Red Devonian sandstone is visible  in background - with the lowest layer undercut and overhanging the beach. Photographed on the landward (Rhossili) edge of the upper shore of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (18) - Over-lapping and protruding layers of ancient raised beach deposits with pebbles and marine shells, formed during an interglacial period 125,000 to 130,000 years ago, with solifluction debris, and red soil derived from Old Red Devonian sandstone also visible, with the lowest layer undercut by high tide wave action and overhanging Carboniferous Limestone bed-rock below. Photographed on the Rhossili Headland edge of the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (19) - Deep red soil (derived from Old Red Devonian rocks) obscuring the raised beach deposits at the base of the Rhossili Headland, adjacent to the Worms Head Causeway, Gower, South Wales.

Ancient Gower Shells & Raised Beaches (20) - The jagged rows of Carboniferous Limestone, jutting up from the upper shore of the Worms Head Causeway near the Rhossili Headland, would all at one time have been overlain by ancient raised beach deposits with pebbles and shells. These have been mostly eroded away by wave action. At the present time, remnants of these old beach layers still remain at the top of the shore.

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‘Black Lias’ at Clements’ Quarry

'Black Lias' in Clements Quarry 1 -  Exposure of 'black lias' strata, (more correctly named Oystermouth Formation and formerly known as Upper Limestone Shales) from the Carboniferous limestone rocks in the Gower Peninsula, near Swansea, South Wales. Site of the old Clements Quarry where Mumbles Marble was extracted. Now the long-term car park in Oystermouth. P1270583aBlog1

Surprisingly, the long-term car park at Oystermouth near Mumbles on Swansea Bay is a also a good location for seeing the topmost strata of the Carboniferous Limestone in Gower – the Oystermouth Formation (which used to be called the Upper Limestone Shales). Locally it has been referred to as ‘Black Lias’ but this is a bit misleading.  The use of the term ‘Black Lias’ to describe this type of rock is just a colloquial or common name – based presumably on its superficially similar appearance to the true Lias rocks – as seen on the Dorset coast near Lyme Regis for example.

The Oystermouth Formation, as it is correctly called, or ‘Black Lias’, dates from the Carboniferous Period (353 to 290 million years ago). So it is much older than the Blue and White Lias rocks that date from the much later Jurassic Period (205 – 146 million years ago). However, there was a commonality in the environmental conditions prevailing at the time of the deposition of all three: the ‘Black’, Blue and White Lias sediments.

In the Jurassic Period, the climate was uniformly warm and humid with widespread shallow-water marine conditions but also with localised estuarine and deltaic phases. Different kinds of limestones were formed in the shallow seas and alternated periodically with thick mudstone sequences –  typical of much of the early Jurassic Lias layers – due to the input of clay and mud materials from the land (terrigenous sediments).

In the Carboniferous period, the climate was equatorial for Britain most of the time, again with intermittant deltaic phases where sediments flowed downstream in rivers to accumulate in shallow coastal waters and swamps. The variations in climatic and environmental conditions during both the Carboniferous and Jurassic Periods resulted at times in an alternation of harder limestone rocks more typical of fully marine deposition – with softer muddier organically influenced rocks more characteristic of shallower marine or brackish water situations where debris and sediments from the land were additional components.

The rocks formed during these varying conditions, transitions between fully marine and brackish environments, is reflected in the rock layers exposed at Clements’ Quarry in Oystermouth. Harder, paler, high calcium content, impure limestones are sandwiched (interbedded) between layers of darker, softer, muddier, finely-stratified calcareous shales. The limestone layers are 20 – 50 cm thick. The shale layers are 5 – 15 cm thick. The cut faces of the quarry look striped.

The rock layers are special with regard to some of the fossil brachiopods, rare corals, and trilobites that are found in them. Two of the fossils are actually named after this site where they were first recorded and described: the coral Amplexizaphrentis oystermouthensis and the brachiopod Spirifer oystermouthensis.

It was the limestone  rather than the shale that was considered to have a commercial value. The quarried limestone  with its white calcite streaks and veins could be polished to look like marble – but of course, being sedimentary and not metamorphic was not geologically like the real thing. More like the polished fossiliferous ‘Purbeck Marble’ that is so frequently found in decorative features of buildings. Sometimes slabs were quarried that were stained by iron oxides to a liver colour. Occasionally, fossils would be included in the polished surface. The stone was marketed as ‘Mumbles Marble’,  also known as Swansea or Cambrian Marble.

Click here for more information about Mumbles Marble.

Useful References to Gower geology include:

George G. T. (2008)  The Geology of South Wales – A Field Guide, published by the Author, ISBN 978 0 9559371 0 1 (E-mail gareth@geoserv.co.uk) pp 87-88.

George T. N. (1970) British Regional Geology – South Wales, Natural Environment Research Council Institute of Geological Sciences, HMSO, London, pp 67 – 68

Owen T. R. & Rhodes F. H. T. (1960) Geologists’ Association Guides No. 17: Geology around University Towns: Swansea, South Wales, Benham & Company Ltd, Colchester, p 14.

'Black Lias' at Clements Quarry 2 - Alternating limestone and shale layers of the Oystermouth Formation (also known as the Carboniferous Upper Limestone Shales), at Clements Quarry near Mumbles, Gower, South Wales. P1270606aBlog2

'Black Lias' at Clements Quarry 3 - Alternating limestone and shale layers of the Oystermouth Formation (also known as the Carboniferous Upper Limestone Shales) at Clements Quarry near Mumbles, Gower, South Wales. P1270585aBlog3

'Black Lias' at Clements Quarry 4 -  Alternating bands of limestone and finer-bedded shales, coated with algae and lichen, from the Upper Carboniferous Oystermouth Formation in Clements Quarry at Oystermouth, near Mumbles, Gower, South Wales. P1270591aBlog4

'Black Lias' at Clements Quarry 4 -  Alternating bands of limestone and finer-bedded shales from the Upper Carboniferous Oystermouth Formation in Clements Quarry at Oystermouth, near Mumbles, Gower, South Wales. P1270599aBlog5

'Black Lias' at Clements Quarry 6 - Looking up to the top of the quarried face of Oystermouth Formation Carboniferous rocks at Clements Quarry, Oystermouth, near Mumbles, Gower, South Wales. P1270603aBlog6

'Black Lias' at Clements Quarry 6 - The long-term car park at Oystermouth is located in the disused Clements Quarry with the Upper Carboniferous Oystermouth Formation of alternating bands of limestone and shales, where Mumbles Marble used to be quarried, , at Oystermouth, near Mumbles, Gower, South Wales. P1270576aBlog7

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