Fforest Fawr part 11


Hotspots in Fforest Fawr Geopark

  1. Pontneddfechan with: the Waterfall Centre, the Farewell Rock, the walk through ‘Waterfall Country’, the faulted anticline of Bwa Maen, the Neath Disturbance, silica mines, an abandoned gunpowder works, and the Cwm Gwrelych Geo Heritage trail (two miles away).
  2. The Upper Swansea Valley with: the Geology Trail from Craig-y-nos Country Park to the summit of Cribarth which includes fossils, a Variscan anticline and the Swansea Valley Disturbance; Penwyllt with Britain’s deepest cave, limestone quarries and abandoned village and train station. It is also possible to visit The National Show cave Centre for Wales, which is not affiliated with the Geopark.
  3. The Black Mountain near Llanddeusant which includes the Geopark’s wildest scenery (Fig 10), best glacial features and excellent outcrops of the Devonian Brownstones, Plateau Beds, Grey Grits and Carboniferous Limestone, as well as the legend of ‘The Lady of the Lake.’
  4. Carreg Cennen Castle near Llandeilo (Fig 9) with the spectacular castle on top of a limestone cliff, the Carreg Cennen Fault which creates the limestone inlier on which the castle sits.
  5. Pen y Fan and the ridge of the Brecon Beacons. This is the classic Brecon Beacons walk summiting the highest peaks in southern Britain.

6. Brynaman with the Black Mountain Centre, and the nearby Black Mountain quarries (‘Herbert’s Quarry), ‘Rocky Ravines’ and ‘From Cwm to Cwm’ circular walks and the Henllys Vale Geotrail.


Fforest Fawr part 10


The Night’s Sky

In addition to being within the Brecon Beacons National Park, the Fforest Fawr Geopark is also situated within the Brecon Beacons International Dark Skies Reserve. This reserve established in 2013 is one of only five in the world and has a light quality good enough to conduct astronomical research.

Fforest Fawr part 9

Ice Age

During the Ice Ages most of Wales and northern Britain was covered by Ice. These glaciers have sculpted and shaped the beautiful mountains of the Geopark. In the Geopark there are around 30 different cirques cut into the north facing escarpments of the Black Mountain, Fforest Fawr and Brecon Beacons. In these cirques that have been cut out by glacial erosion there are also moraines. Evidence shows that they did not only form at the last glacial maximum about 20,000 years ago but some of them formed in Younger Dryas stadial of just 12,000 years ago.

The best place to see glacial erosion features and moraines is base of the Black Mountain ridge near to Llanddeusant. Here under the peaks of Bannau Sir Gaer, Picws Du, Fan Brycheiniog and Fan Hir are classic cirques but also depositional features that have been a source of confusion for many geologists. There are moraines that  lie in front of headlands rather than cirque mouths (Fig 8), but they could have been formed as medial moraines when the cirque glaciers were joining the larger ice sheet that covered all of Wales. Another less remote place to look at glacial features is Craig Cerrig-gleisiad off the main A470 near to Storey Arms and Pen-y-Fan. The Fforest Fawr Geopark produces a Geotrail for this area titled ‘Glyn Tarell’.

Fforest Fawr part 8

Fforest Fawr_img_12


The dominant structural features in the Geopark are three parallel faults that run through the park from the south west towards the north east. These are named the Carreg Cennen Fault, the Swansea Valley Disturbance and the Neath Disturbance. These faults may well represent ancient lines of weakness from when the landmass of England and Wales was coming together as a series of terranes in the Precambrian. They were active in the Caledonian Orogeny and then again in the later Variscan Orogeny, a mountain building event in the mid to late Carboniferous caused by Laurussia and Gondwana colliding to form the supercontinent Pangaea. Even today as Africa crashes into the Europe to form the Alps it is possible to have small tremors along this fault. In 1999 there was an earthquake measuring 3.5 on the Richter scale centred on Sennybridge. The Variscan Orogeny also had the effect of folding the pre-existing sediments. The best place to see the structural geology of the geopark is Bwa Maen near Pontneddfechan. Here the Neath Valley Disturbance can be seen adjacent to a fold in the Carboniferous Limestone, the two structures together form a faulted anticline (see fig. 7).

Fforest Fawr part 7


Overlying the Carboniferous limestone are three formations of rock that make up the Marros Group. The lower layer of this is the Twrch Sandstone. This is a quartzite and in places is 98% silica. This silica was mined at Pontneddfechan near to the current Waterfall Centre, which functions as the Geopark Visitor Centre, and then crushed to make fire bricks. These bricks were commonly known as Dinas Fire Bricks and were used to line furnaces. They were sold all over the world and even today in Russia fire bricks are still known as Dinas Bricks. Overlying the Twrch Sandstone are the Bishopstone Mudstone and Telpyn Sandstone formations. These alternating layers of mudstone and sandstone form faulted block over which the many waterfalls of ‘Waterfall Country’ flow. These waterfalls on the Mellte and  Hepste rivers near Pontneddfechan form arguably the finest collection of waterfalls in Britain.

Overlying the Marros Group but only jutting into the Geopark on its extreme southern fringe are the South Wales Coal Measures. These coal measures are responsible for the industrialisation and urbanisation of South Wales, to the south of the Brecon Beacons National park and Fforest Fawr Geopark. The lower most part of the Coal Measures is the ‘Farewell Rock’. This was named first by early iron miners who found no more iron nodules below this rock and the name was later adopted by the coal miners who also found no coal beneath this layer. The ‘Farewell Rock’ is beautifully exposed at the Henrhyd Waterfall (fig 6). This waterfall near Coelbren is the highest in South Wales. It is possible with care to walk behind the waterfall and it is the Farewell Rock that forms the hard cap over which the water flows. In the 1830’s Sir William Edmond Logan, after whom Canada’s highest mountain is named, was mapping the South Wales Coalfield, and discovered two fossilised tree trunks in the gorge below the waterfall. These fossils now stand outside Swansea Museum. Another place to see the Farewell Rock is in Pontneddfechan at the start of the waterfall walk behind the Angel pub. Also near to Pontneddfechan is the Cwm Gwrelych Geo Heritage trail. This trail, equipped with audio information points takes you on a guided walked through coal measures in a previously industrialised valley. Henllys Vale near Brynaman is also another good place to see the coal measures exposed . The Geopark has produced a Geotrail for this location. But if you really want to see coal, the best place has to be the National Coal Museum which is part of the Blaenavon World Heritage Site. This is actually outside the Geopark being near to the eastern boundary of the National Park. Here it is possible to descend the Big Pit mine-shaft and also visit a replica mine. It is also possible to visit the ironworks where my great great grandfather Edward Pritchard Martin was the General Manager.

Fforest Fawr part 6

Porth yr Ogof


As the sedimentary basin continued to fill and subside the land eventually fell below sea level once gain and in the early Carboniferous a shallow tropical sea rather like the modern day Bahamas formed over the Geopark region. This has deposited a band of Carboniferous Limestone stretching along the southern margins of the Geopark from Carreg Cennen Castle in the west to Merthyr Tydfil in the east (see fig. 1). In places these rocks contain fossils of the creatures that lived in the  tropical sea such as corals and crinoids (fig 5). A great place to see these fossils is on the geology trail which leads from Craig-y-nos Country Park to the summit of Cribarth Mountain. This mountain and nearby Penwyllt are also an excellent place to see industrial archaeology. The limestone was quarried here and burnt in lime kilns to provide quick lime for agriculture and the iron industry.

The limestones also host Britain’s deepest cave. Ogof Fynnon Ddu is 308 metres deep and boasts 50 km of passageway. The cave is situated under Penwyllt. This cave is for experienced cavers only, but can be explored by becoming a member of the South Wales Caving Club. A more accessible cave is the nearby Dan-yr-ogof  cave. This cave is part of the National Show caves Centre of Wales, and has extensive walk ways where it is possible to see amazing stalactites and stalagmites. Another notable cave in the Geopark is  Porth yr Ogof near to Ystradfellte. It has the largest cave opening in Wales which is 17 m wide and 5 m tall.

Fforest Fawr part 5



The Caledonian Orogeny the name given to the mountain building event that produced the Caledonian Mountains lasted on into the Mid Devonian. The Caledonian Mountains were a huge chain of mountains that were probably Himalayan in height and  stretched from what is now Svalbard down the coasts of East Greenland and Western Norway, through the British Isles on to the Appalachians in North America. Whenever a great chain of mountains is thrust up, it will be eroded down and huge thicknesses of sediment can be deposited in sedimentary basins adjacent to the mountain ranges. The Devonian scene in the Fforest Fawr Geopark would have been something like modern day Pakistan, with a huge mountain chain to the north and an arid plain to the south over which rivers periodically flooded depositing sediment.

Devonian rocks cover more of the Geopark than rocks of any other age, and all the high peaks in the Geopark, the Black Mountain, Fforest Fawr, and the Brecon Beacons (Fig. 2) are made from Devonian Old Red Sandstone. Many of the rocks are red in colour as the name would suggest.  This is due to haematite that formed in these arid conditions. In other places the rocks are green and this is due to a different oxidation state of the iron that may be due to water percolating through the sandstone t a later date. In many places in the Geopark, such as on the slopes of Bannau Sir Gaer on the Black Mountain sedimentary structures such as cross bedding are clearly visible. At a few rare sites such as Heol Senni Quarry fossil fish have also been found. The Devonian is also known for the remains of the planets first forest and in certain places such as near the summit of the Black Mountain, black organic material can be found within the sandstone (Fig 3).

Rocks of the lower Devonian are preserved as the St Maughans, Brownstones and Senni formations. These are all sandstones and mudstone. The middle Devonian however is missing, presumably there was a period of erosion after they were deposited removing them before the Upper Devonian Plateau Beds and Grey Grits were lain down. The Plateau Beds are so called because they cap many of the highest peaks in the Geopark. Figure 4 shows the Plateau Beds lying unconformably on the Brownstones.

The best places to see Devonian rocks in the Geopark are the walks that climb the high mountain peaks. Pen-y-Fan at 886 metres is the highest mountain in southern Britain and the views from the top are magnificent. However it is also a very popular mountain and at certain times of the year can be quiet busy. A wilder and more isolated experience can be had in the west on the peaks of the Black Mountain.

Fforest Fawr part 4


In Llandovery, Wenlock and Ludlow times the Geopark was still under water with sediments being deposited in the Iapetus Ocean. In these oceans lived the marine creatures such as brachiopods which allowed Murchinson to define the period and its stages. However from the mid Silurian onwards the Iapetus was closing. England and Wales part of the ancient micro-continent of Avalonia were colliding with Scotland which was part of the ancient continent of Laurentia. This started to form a huge chain of mountains, the Caledonian Mountains, and uplifted the rocks of the Geopark area out of the sea. By the latest Silurian, the Pridoli, all the sedimentation taking place f England and Wales was terrestrial; this is why a site in the Czech Republic defines this age because it was still marine and consequently preserves marine fossils. The rocks deposited in the Geopark of Pridoli age are the Raglan Mudstones. They are red in colour and are part of the Old Red Sandstone that continued to be deposited throughout the Devonian, the period that succeeds the Silurian.

The global stratotypes of the Telychian and Aeronian are on private land but a great and accessible place within the Geopark to look at Silurian rocks is the Sawdde Gorge. This site is an SSSI due to the geology. Here the transition from the marine conditions in the Ludlow to the terrestrial conditions in the Pridoli can be seen. The Ludlow sediments from the quarry here have also yielded some of the earliest known fossils of a primitive fish.

Fforest Fawr part 3

Llandovery from castle mound


The rocks of Fforest Fawr Geopark are all sedimentary and pile on top of each other like a layer cake. Overlying the Ordovician rocks are rocks of the Silurian Period. These rocks also occur in the North west portion of the Geopark (see fig 1) stretching in a band to the east of the Ordovician rocks from town of Llandovery to near Llandeilo, with a smaller area occurring just to the north of Brecon. As previously discussed the Silurian Period was named by Murchison after the Silures tribe who would have inhabited the Fforest Fawr Geopark during Roman times. Today the Silurian is divided up into 4 epochs: the Llandovery, the Wenlock, the Ludlow and the Pridoli.  These names are internationally recognised as the official names for rocks of these time periods all around the world. The Llandovery is named after the Fforest Fawr Geopark town, the Ludlow and Wenlock are named after places in the nearby English county of Shropshire, and the Pridoli after a site in the Czech Republic. Each epoch is further subdivided into stages and two of the three Llandovery stages the Aeronian and Telychian stages have their global stratotypes within a few miles of  the Geopark boundary. The Aeronian Age lasted from 440.8 million to 438.5 million years ago and is named after Cemcoed-Aeron Farm. The  Telychian lasted from 438.5 million to 433.4 million years ago and is named after the Pen-lan-Telych Farm. Both farms are in the area surrounding Llandovery.

Fforest Fawr part 2

pen y fan

The Cambrian Period was named after the Latin name for Wales, Cambria, by Adam Sedgwick while the Silurian was named by Roderick Murchison in the 1830’s after the Silures tribe who inhabited southern Wales in the Roman times. Originally there was no Ordovician Period, however the Cambrian and Silurian of Sedgwick and Murchinson was found to overlap in places, causing disagreement between the two men. In 1879 Charles Lapworth resolved the dispute by defining the Ordovician. He recognized that the fossil fauna in the disputed layers was different to either the Cambrian or Silurian. Lapworth divided up his Ordovician, which is named after the Ordovices tribe who lived in North Wales and Mid Wales during Roman times, into the following series: Tremadog, Arenig, Llanvirn, Caradog and Ashgill, all named after places in Wales or near the border, where rocks of these ages occur. The Llanvirn is subdivided into the Llandeilian and Abereiddian, with the Llandeilian named after the Fforest Fawr Geopark town, Llandeilo and Abereiddian named after the Pembrokeshire village of Abereiddi. In the UK these divisions  continue to be popularly used, but in some other parts of the world other stages have been used. This led to the standardisation of the Ordovician in 2008 by the International Stratigraphic Committee. The Tremadoc, named after Tremadoc in North Wales, was internationally accepted as the earliest stage of the Ordovician but the Llandeilian is not now accepted as an international time period. However Llandeilo does still possess another claim to geological fame which is that  the very first trilobite described by science was discovered just outside the town. The fossil was collected in 1698 by Reverend Edward Lhwyd and he described as “some kind of flat fish”.

A good place to see Ordovician rocks in the Geopark is the Garn Goch hill fort near the village of Bethlehem. Garn Goch the largest  prehistoric hill fort in South Wales. The rocks here are coarse sandstones lain down in a shallow sea 405 million years ago, and which have since been tightly folded. The Geopark has published a Geotrail that can give you a guided walk around the geology and archaeology.