Rocks at Caswell Bay

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.

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Rusty Pebbles at Whiteford

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.

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Ancient Gower Shells & Raised Beaches

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.

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‘Black Lias’ at Clements’ Quarry

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.

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Submerged forest at Broughton Bay

The ragged tree stumps and roots, strewn over the seashore at Broughton Bay on the north coast of the Gower Peninsula, are the remains of a birch tundra woodland that once covered the ancient land surface. They lie in position, just as they were growing before they were inundated. Ten thousand years ago in the Pleistocene Period, a large river, fed by tributaries such as the Loughor, occupied what is now the Bristol Channel with its Atlantic waters. The last extension of the ice sheets in this area, during the late Devensian Period, had been about 8,500 years earlier. As the ice receded up into the valleys of South Wales, the climate had warmed up and allowed vegetation to flourish. The sea level at that time was about 22.5 metres lower than it is at the present.

By the beginning of the Neolithic Period 5,700 years ago, however, the sea level began to rise because of the increasing volume of global meltwater and  its accompanying land subsidence. The forests and peat bogs of the coastal margins were submerged and buried in sediment…..until the 1980s when the remains began to reappear on Gower shores as the surface sediments began to erode away. Now, large expanses of Broughton beach have been stripped of sand showing the strata and entrapped woodland beneath.

Wood from these ancient forests is visible on the seashores of  Swansea Bay and Port Eynon on the south Gower coast as well. Large blocks of peat dating from this time also wash up on the sand at Whiteford – the next bay to Broughton. The plant species already recorded include silver birch, hazel, alder, elder, deergrass, rushes, irises and spurges. As I understand it, no full investigation of this palaeo-environment has yet been conducted. I hope that full attention can soon be given to this valuable evidence before the rapid rate of erosion destroys all that is readily accessible between tides. 

 

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Red sediment seepage at Rhossili

 

The terrible wet weather of last weekend provided an opportunity to witness close up the process which so significantly contributes to the wonderful coloured patterns in the sand at Rhossili, Gower.  These patterns have been illustrated in earlier postings, and were very much in evidence following the recent stormy conditions.

Basically, the flat platform that runs along the foot of Rhossili Down contains red soil derived from Old Red Devonian Sandstone in the hill. The red soil and lumps of stone result from the shattering of this rock in icy conditions associated with glacial and periglacial events - eventually being washed downhill and seawards.

At first it would have formed a massive tongue of sediments reaching out into the bay. With time, the outward edge of this material has been eroded back by the sea. Today, it remains as a long narrow terrace with a soft, crumbling cliff edge.

In the top photograph you can see this cliff face with a face of eroding red soil sandwiched between the green turf above and the wind-blown sand below. The pebbles and cobbles below the sand would also have fallen from the cliff. During and after heavy rainfall, water seeps down through the sediments and carries some of these red pigments with it as it spills out onto the beach. The pigments are also washed down in streams. Here, the red slurry is forming dendritic patterns as the seepage runs over the dry beach sand.

The red particles normally mix evenly with the yellow sand grains and the black-stained sediments from below the surface. However, during the weather and wave-driven processes that stir up and redistribute the beach sands, the different grain sizes and colours can separate out again. Then they form an endless-seeming variety of designs in two or three dimensions.

 

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Frost-damaged rocks at Whiteford?

In the previous post I mentioned the glacial till exposure near Whiteford Point, Gower. According to expert accounts, some of these rocks show frost damage caused by the ice sheet and the icy conditions 17,000 years ago. The photographs in this post illustrate what I believe to be some of these frost-damaged rocks. The stones in the first two pictures, exhibit multiple parallel splitting. The third image shows a stone, maybe of a different type, with a single crack. It is easy to imagine how the smaller pieces of stone that cleave from these rocks end up as the large, flat,  smoothed pebbles so typical of the beach further along to the north east. The final picture shows some of these pebbles with my walking pole to give scale.

The earlier related post is Pebbles, spits & banks at Whiteford (2).

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Pebbles, spits & banks at Whiteford (2)

At the remotest end of the Whiteford National Nature Reserve is a tranquil place where wild ponies and sheep graze. Here, you can find clues to the geological past of the area and observe evidence of current geological processes that will change and shape its future appearance.

Beneath the 3 Km length of the dune-covered Whiteford Burrows, lies a spit of  glacial till deposited by the ice sheet that once occupied the Loughor valley in the late Devensian period of 17000 years ago. These boulders, cobbles, and gravels were scooped up by the scouring action of the thick and heavy layer of ice as it extended downwards and outwards towards the sea. They can be seen outcropping between Whiteford Point and the old derelict Whiteford Lighthouse.

Despite subsequent sea level rises and falls, the deposit remains more or less intact. In some rocks there is evidence of ancient frost damage * from the ice sheet. Such damage facilitates the eventual break up of the rock into smaller, often flat, pieces. Once loosened, these smaller pieces are rolled around by the tides – thus acquiring rounded edges.

These pebbles have washed out from the original galacial deposit to form the base of a further spit that extends eastwards from the Point. Most of the time the pebbles here are covered by sand but they are sometimes visible as a band of stones at the top of the tidal reach especially when stormy waves have licked the sand away.

The pebbles within these exposures (of  the spit’s foundation) frequently appear to be layered or orientated in curving lines – reflecting the direction of the currents that carried them into position. In one particular place there is an additional recurved spit of pebbles, never covered by sand, pointing inland rather than offshore.  This is near to a branch of the Burry Pill stream that drains Landimore saltmarsh.

I am not certain what accounts for this structure because it looks as if  some sustained power would have been needed to create it. The sea does regularly cover the marshes but mostly creeps slowly up as the incoming tide combines with the outgoing estuary waters to raise the water level. I wonder whether the recurved pebble spit forms in extreme storm conditions when the sea takes a short cut across the main sand-covered spit. There is certainly a lower area of damper ground that could provide such a route.

* See the following post for more about frost-damaged rocks at Whiteford.

If you would like to read in some more detail about the geology of  this area, I would recommend two publications:

Classic Land Forms of the Gower Coast by E. M. Bridges (1997) published by the British Geomorphological Research Group and The Geographical Association. ISBN 1 899085 50 5

Carmarthen Bay by V. J. May (2007) Vol. 28: Coastal Geomorphology of Great Britain. Chapter 11: Coastal assemblage GCR sites. Site: CARMARTHEN BAY (GCR ID: 2102). Extracted from the Geological Conservation Review. 

 

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Pebbles at Whiteford (1)

At the eastern-most end of Whiteford National Nature Reserve are beds, banks and spits of pebbles – overlain by and intermixed with wind-blown sand and tidal drifted seashells. Here the three main types of rock represented by the pebbles are Carboniferous limestone, Old Devonian red sandstone, and Millstone Grit – all part of the local Gower geology.

However, this was an area affected by glaciation – with the ice sheet terminating in this vicinity. Many different kinds of rock were brought to this place by the ice as it moved southward across the British Isles – scouring the geology in the process. Huge boulders, smaller rocks and stones, and finer sands and gravel were deposited in Gower and in this location when the ice eventually melted and the ice sheet receded. So, amongst the Whiteford pebbles are stranger stones from further afield.

One of the characteristics of the Whiteford pebbles at the eastern end of the NNR is that many are smoothed by wear but mostly flattened, disc-like, rather than rounded and sub-spherical. In many places, they approximate in shape to the very large, thick, heavy, old empty oyster shells that are scattered in their midst.

 

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Red mud-slips at Rhossili

22,000 years ago a massive ice sheet covered most of Gower in South Wales – but the hill known as Rhossili Down escaped the clutches of the ice. However, deep down, the ground was permanently frozen (the permafrost); while at the surface, the repeated alternation of freezing and thawing cracked and crumbled the rocks.

This gave rise to jagged shards of stone as well as to finer sediments. In periods of warmer weather this debris was washed down the scarp of Rhossili Down and formed an apron of material at its base that extended out into the sea.

This soft deposit of solifluction debris has been gradually worn back by the tides and can be seen as the low flat platform or bench on which the white-walled Old Rectory now stands. This is shown in the photograph above. The old Medieval village of Rhossili was also built here.

The platform has a steep scarp slope down to the beach. It is still subject to erosion today by under-cutting waves, down-cutting streams, trampling cattle, and clambering holidaymakers. Looking face-on to the platform, as in the photograph above, you can see how the soil is slipping down at intervals along its length. 

In the picture above, of one recent mud-slip or land slide, entire clumps of turf now rest on the sandy shore and the underlying rich red soil is revealed. The colour is due to its origin as Old Red Devonian Sandstone. This is in contrast to the grey rocks of Carboniferous Limestone that form the headlands of Worms Head and Burry Holm which are the outer arms of Rhossili Bay.

In the picture below, cattle graze on the solifluction bench, its slope, and the beach at its base.

For more information about the geology of Gower (and other matters), there are two really excellent books:

Bridges, E. M. (1997) Classic Land Forms of the Gower Coast, Series Editors Rodney Castleden and Christopher Green, The Geographical Association, ISBN 1 899085 50 5.

Mullard, J. (2006) Gower, Collins New Naturalist Library, ISBN 13 978 0 00 716066 6.

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