Porthcawl Rocks

 

Porthcawl Rocks

Carboniferous limestone showing level strata

Porthcawl town lies on  a bedrock of carboniferous limestone. This hard, grey rock was formed when the land here was a shallow tropical sea, around 350 million years ago. In this sea coral reefs formed, and the remains of microscopic marine animals collected on the sea floor. These deposits accumulated over millions of years, building up as the  sea level rose or the land sank, to form deposits hundreds of metres thick. The deposits were compressed and gradually hardened to become rocks. Movements of the earth’s crust later pushed this rock to the surface and folded and twisted the layers of rock. Over time, later rocks which had been laid down on top of the limestone were removed by erosion, resulting in what we see today, a hard, resistant rock in distinct layers or strata, which , around Porthcawl, are relatively flat.  In other areas nearby, the rocks have been twisted by earth movements so that they are often steeply slanted or folded. 

Steeply tilted carboniferous limestone strata, Gower peninsiula

Evidence of the origin of the limestone can be seen in the many fossils to be found embedded in the rocks. Walking along the flatter areas of rock when the tide is out will reveal  fossils of Brachiopods (a primitive kind of shellfish with two shells) and Gastropods (snail-like molluscs), and several kinds of corals.

                 

    Brachiopod fossil                                                             Gastropod fossil

 

               

    Fossil coral                                                                    Fossil coral

In places, the hummocky surface of a fossilised coral reef can be seen.

    Fossilised coral reef

Over millions of years, the limestone has been subjected to enormous pressures, which have resulted in breaks, or faults, in the rock, which have been eroded by the sea to create inlets and caves.

                        Fault in limestone

 Smaller cracks in the rock have become filled with calcite, which is calcium carbonate, dissolved from the limestone and re-deposited in the cracks over millions of years.  

    Calcite veins

During periods of volcanic activity, some cracks were infilled with hot liquid rock from deep down in the earths crust. These veins are of quartz and other minerals, including metal ores such as lead sulphide or galena, a shiny grey mineral. 

                                    Quartz vein with galena

About a mile to the west of the town along the coast is  Pink Bay, so called because of the pink colour of the rocks. The rock here is different, consisting of fragments of broken rock, mainly limestone, embedded in a reddish matrix. 

     Triassic conglomerate, Pink Bay

This rock is much younger than the limestone, dating from the Triassic era, about 90 million years ago. It was formed when the area was a hot, dry desert, with higher land to the north. Occasional floods carried broken rock from the mountains and deposited the stones, mixed with  red, iron-rich sand and mud, which eventually became fossilised into a ‘conglomerate’, lying on top of the limestone bedrock (PHOTO 13).

There is a gap of some 240 million years between the deposition of the limestone rock, and the Triassic rock which rests directly on top of  the limestone. Accumulated deposits on top of the limestone have all been removed by erosion in the intervening millenia, so that the relatively newer Triassic rock lies on top of the much older limestone.

Limestone has been widely used as a building material, and there are large working quarries nearby, where the stone is crushed and used mainly as road building material.

Limestone is mainly composed of calcium carbonate, and is used to make lime, which is used to reduce acidity on agricultural land, and also as a mortar in building work. Limestone was an important export in the 19^th century, much of it taken across the Bristol Channel to Somerset and Devon. Small boats were hauled upon the beach at Newton, where they unloaded the ‘ballast’, stones taken from their own beaches which were essential to keep the empty boats stable in the rough seas of the channel. Thousands of these ‘foreign’ stones can be found on the western end of Newton beach, large rounded stones of a hard greenish or reddish coloured sandstone, easily distinguished from the surrounding limestone. 

     Ballast stones, Newton Bay

To the East and West of the town lie two large areas of sand dunes, both of which are important Nature Reserves.

To the east of the town lies Newton Bay, with its long sandy beach. Looking inland from the top of the dunes, you can see that the land is relatively flat until you reach a line of hills. About 50,000 years ago , during an ‘interglacial’ period , the ice caps melted and the sea level was higher than it is now. The line of hills was then the shoreline.

  Then, during the last period of glaciation, the sea level fell by about 80 metres  and the bay , and the whole of the Bristol Channel, became a forested plain, with the River Severn running through the middle of it. About 10,000 years ago, as the ice  caps once again melted, the sea level rose again, flooding the forests and  pushing millions of tons of sand and gravel from the sea bed into the bay, creating the sand dunes which we see today. On very low tides, when the beach sand has been scoured away by storms, you can sometimes find ‘fossil forest’ deposits of peat, with the remains of tree stumps from the forest which once filled the bay and most of the Bristol Channel. 

    Fossil forest exposed on a Welsh beach

 A similar situation can be seen at Kenfig Burrows, where the remains of Kenfig Castle can be seen emerging from the sand.

     Remains of Kenfig Castle

 A particularly windy period in the 15^th Century is thought to have pushed  sand from the shoreline further inland, covering settlements here, and in other parts of Wales,  forcing whole  villages to be moved inland. The story goes that Pyle church was built with stones taken from the original church at Kenfig when the sand encroached. It is known as the ‘upside-down’ church, as the smaller stones which would have been in the upper parts of the original church were used in the base of the new church.

 Land Use

The soil which has formed over thousands of year on top of the limestone rock, is  generally fertile and well drained, and is used to grow a variety of crops including wheat, oil seed rape and maize. Other areas, with slightly less fertile soils, grow grass and are used for grazing by sheep and cattle. The fields are generally bordered by old dry stone walls, though in most cases the walls have been neglected, resulting in the remnants of the walls being colonised by bushes and trees, creating a hedge. 

    Hedge growing on wall

Some areas, with thinner, or less well-drained soils have remained as common land. These areas would have been much larger in the past, but in the 18^th and 19^th Centuries, the better common land was ‘enclosed’ for agriculture. Locks Common and Stormy Down are remaining fragments of these larger areas. All common land is owned by someone, but traditionally local ‘commoners’ had the right to graze animals, collect firewood, and dig turf. In the case of Lock’s Common, grazing rights have been lost, and management of the common has been taken over by the local Council. it is now an open area much used by local people for recreation.

Other nearby areas of  common land, such as Stormy Down, have been taken over by bracken and scrub, as grazing by commoners has ceased and no other management has taken place.