Geology of the Fforest Fawr Geopark and Blaenavon WHS day tour

ITINERARY

Abergavenny: The tour starts at 9am at either Abergavenny railway station or Byefield Lane Car Park which has free parking. Abergavenny started life as the Roman fort Gobannium then, after the Norman conquest, it became a medieval walled town within the Welsh Marches, and has a stone built castle. We drive by both the castle and site of the Gobannium as we leave the town. Soon after leaving the town, we enter the National Park and follow the Usk valley eastwards. We pass through the picturesque town of Crickhowell which lies under Table Mountain, a little while later pass the historically important Tretower castle, all the while having magnificent view of the Black Mountains. Soon after the village of Bwlch the Brecon Beacons come into view and a little while later we make our first stop.

View of the Brecon Beacons, Llanhamlach: We stop at a layby for impressive views of the north face of the Brecon Beacons. The highest peak Pen-y-Fan (886m) is the highest peak in southern Britain. The majority of the mountain is made from Lower Old Red Sandstone, which is Lower Devonian in age, but the upper part is capped with Upper Old Red Sandstone, which is Upper Devonian in age, and there is an unconformity with the Middle Devonian being absent. The beautiful shape of the mountain is due to glacial erosion which occurred in the last ice age.

Brecon: Having entered the Geopark on our drive from the previous stop, we drive through the historic town centre of Brecon and, as we pass over The Usk Bridge, we get views of the cathedral, the castle and The Castle Hotel. The latter is notable because Sir Roderick Impey Murchison stayed here when he undertook his pioneering geological work in south Wales which led to the establishment of the Silurian and Devonian periods.

Twyn y Gaer Iron Age fort: A 20-minute walk takes us through the ramparts and natural gradient to the summit of Twyn y Gaer Iron Age fort, dating to around 2,000 years ago. From here we can enjoy commanding views over the whole area. Firstly, looking to the north, we see the hills of mid-Wales that are made up of Silurian-aged rocks and covered by impressive moraines. To the north-east, 17 miles away, is a site known as Cavasham Ferry where Murchison exclaimed his ‘First True Siluria’; it was here that he first saw marine rocks lying beneath the Old Red Sandstone, and he realised that there must be a Silurian period. To the west, also 17 miles away, is the type-area of the Llandovery epoch of the Silurian period, the name of which is used the world over to describe this period of geological time. If we look to the south, we can see all four mountain ranges of the National Park, and this is part of Murchison’s type Old Red Sandstone area which he later attributed to the Devonian period.

National Park Visitor Centre: A short stop at the Visitor Centre gives us a chance to use the facilities and to view a 3-D model of the National Park’s mountain ranges, with further geological explanation from your guide, James.

Craig Cerrig Gleisiad: One of the most dramatic features of the Geopark is the towering mountainous scenery that has been created by glacial erosion and one of the best places to see this is right here at Craig Cerrig Gelisiad. The name of this site translates as ‘the salmon-coloured rocks’. Ice in the last ice age eroded a large, circular bowl, known as a cirque or cwm, here. Subsequently, during an interglacial, there was a massive landslip which resulted in a large portion of cliff falling away. Then, in a later glacial readvance, some of this landslip material was reworked into moraines. We walk a short distance into this cwm, climbing on to the landslip, to get impressive views of the towering wall of the cwm.

Craig y Fro: A short distance along the road from the previous stop is a quarry adjacent to the main road where a fossilized Lower Devonian river channel exposes how the Old Red Sandstone was formed. Early Devonian fossil land plant remains have been found in this quarry and were the first recorded in southern Britain. They are one of the best-preserved Devonian floral assemblages in Britain. Four plant bearing horizons have been recorded, with the lowest at approx. 7m above road level. Sir David Attenborough filmed here for his ‘Life On Earth’ television series.

Upper Old Red Sandstone outcrop: We make a brief stop in a layby to see an exposure of the Upper Old Red Sandstone which is the same as that outcropping on the summit of Pen-y-Fan. It is different to the Lower Old Red Sandstone which has been seen at the last three stops.

Cwm Crew moraine view: Another layby stop grants up great views of the Cwm Crew moraine which was created by an ice age glacier that flowed down from the top of Pen-y-Fan.

Porth yr Ogof Cave: Our journey south through the Geopark now leaves the Devonian strata and enters the overlying Carboniferous strata. The rocks of the Lower Carboniferous are limestones and dolomites, and host north-west Europe’s most extensive cave system that runs, intermittently, along the southern margin of the National Park. Porth yr Ogof is the largest cave opening in the National Park and a popular destination for cavers. We stop here to observe the opening of the cave and to discuss cave formation.

Dinas Rock & Bwa Maen fold: There is much to see at the next stop which is the structural geology highlight of the Geopark. Bwa Maen, which means ‘the stone bow’, is a faulted anticline, in other words, it is a large fold that lies adjacent to a fault. This fault is, in fact, a large structural feature called The Neath Valley Disturbance which runs for many miles and is probably an older Precambrian fault that was reactivated in the Variscan orogeny. This area was also the site of a former silica mine, abandoned in 1964. As we approach Bwa Maen, we walk along the path of a former tramway which served the mine and to one side, we can see a former mine entrance.  At the far end of the track is Sychryd waterfall. Dinas Rock, where we park, is interesting because there was once an Iron Age fort on its summit and it also has a cave in it where, according to legend, King Arthur’s army is sleeping until Britain needs them again.

Sgwd Gwladys waterfall: The Geopark is home to the largest collection of waterfalls in Great Britain. It is possible to do an all-day walk which takes in fifteen of these waterfalls. However, as part of this day tour, we will just walk for approximately an hour in one direction to Sgwd Gwladys waterfall, one of the waterfalls that you can walk behind. The walk to the waterfall starts at a location where the ‘farewell rock’ is exposed. This is the lowest unit of the Upper Carboniferous South Wales Coal Measures and was so-called as there was no iron or coal to be found beneath it. Our walk then proceeds through the Middle Carboniferous strata which outcrop beneath the farewell rock and there are a number of sites of geological interest, including several stops with fossil tree remains and the entrance to a former silica mine. The waterfall itself flows over a harder band of sandstone that contains the fossils of tree roots. We then retrace our steps before moving on to the next stop.

Trevithick Monument, Merthyr Tydfil: We travel along the newly upgraded Heads Of The Valleys road, which has numerous road cuts that expose interesting geology, to reach Merthyr Tydfil which lies just outside the Geopark boundary. At one time, Merthyr Tydfil was home to the world’s four largest iron works, a product of the local geology – coal, ironstone and limestone all outcrop in its vicinity. The town boasts the world’s oldest metal railway bridge (closed and under restoration at the time of writing) and the site of the world’s first train journey. As we drive through Merthyr, we stop to view the Trevithick Monument which commemorates this latter event in 1804.

Brynmawr Sections: Located on the southern boundary of the National Park, the Brynmawr Sections are a geological SSSI and here we can see a thin coal seam and ironstones outcropping in situ.

Blaenavon Ironworks: Our route then takes us into the Blaenavon Industrial Landscape World Heritage Site. As we drive through the area, we see the remains of opencast iron mines and glimpse the headgear of the Big Pit National Coal Museum of Wales, before pausing outside the gates of Blaenavon Ironworks. It was here, in 1878, that the problem of the removal of phosphorous from the Bessemer process was finally resolved, having a huge effect on the rapid development of the steel industries of Europe and America.

The Carboniferous System and Garn Ddyrys Forge: The type area of the Carboniferous is the coal fields of England and Wales, but there is no real single type location. However, in 1822, William Phillips defined the Carboniferous System as consisting of Old Red Sandstone (now known to be Silurian and Devonian), overlain by Mountain Limestone, overlain by Millstone Grit, capped by coal measures. In this view, we can see all four units, effectively giving us a view of a type area of the Carboniferous. Adjacent to this stop are the remains of Garn Ddyrys, an iron foundry that operated from about 1817 to 1860. Pig iron was brought to the foundry from Blaenavon Ironworks along Hill’s Tramway and it was used to make rails. By the 1840s there were about 450 people living around the Garn Ddyrys Forge.

On our return to Abergavenny, we cross the Monmouthshire & Brecon Canal, with a glimpse of Govilon Wharf. In the nineteenth century, this was the main route for taking iron out of the industrial area (now the World Heritage Site) and down to the south Wales coast for onward transport. We aim to arrive back in Abergavenny for 6:00pm.

GeoWorld Travel's Director James Cresswell is an accredited ambassador to the Fforest fawr Geopark