Carboniferous Caerphilly
300 million years ago during the Carboniferous Period the area we now know as South Wales lay somewhere south of the equator and basked in tropical conditions. For the first time in Earth’s long history the land was colonised by plants which formed vast forests around the equator.
These plants flourished, growing and dying quickly taking huge amounts of carbon dioxide from the atmosphere and locking it up as peat. After millions of years the peat was buried, compressed and heated until it became coal. It takes 11 metres of peat to form 1 metre of coal.
Over 350 million years of geological history are revealed in the exposed rocks. You may be able to find fossils that were trapped in the layers of coal, mudstone and ironstone, formed when tropical swamps covered the area in prehistoric times.
During the 1980’s the 23 acre clay pit site was designated as a Geological SSSI because of the exceptional and internationally important Carboniferous, Westphalian A, B and C Coal Measure rocks exposed here.
The geology of the site is considered of great importance being one of only two remaining places where these extensive sequences of Upper Carboniferous strata can be examined and Coed-y-Werin has been designated a Site of Special Scientific Interest (SSSI) because of this.
The coal at this location off The Dram Road is called the Rhondda No. 2 Seam and the coarse orange coloured rock is called Saron Sandstone.
You’ll see that the rocks are at an angle, dipping gently north. In the Neath Valley the same geology can be found gently dipping south.
It was the soft mudstone which lies in between the coal seams that was mined for brick manufacture.
Coal from South Wales was taken all over the world from the ports of Cardiff and Barry.
Wern Ddu shows the only continuous sequence of rocks ranging in age from the early Langsettian to late Bolsovian (300–315 million years old) in the eastern part of the South Wales Coalfield. It is also the only site in the coalfield from where the fossilized remains of plant pollen and spores can still be obtained, which is important for establishing correlations of the strata with sequences in other parts of Europe. This is also helped by a range of other types of fossils, including the remains of various marine and non-marine animals. More detailed information can be found about Coed y Werin’s Wern Ddu Geological site on the following link
https://geoguide.scottishgeologytrust.org/p/gcr11/gcr11_wernddu
Coal Measures: The Story Beneath Wales
The rocks beneath South Wales tell an extraordinary story. Around 320–300 million years ago, during the Carboniferous Period, Wales was a very different place. Instead of rolling hills and valleys, much of the region lay close to the Equator and was covered by vast tropical wetlands.
These swamps supported forests unlike anything seen in Wales today. Giant clubmosses towered above the landscape, while horsetails and tree ferns formed dense jungles along river channels and floodplains. Insects thrived in the warm, oxygen-rich atmosphere, and the remains of these ancient ecosystems would eventually become one of Wales’ most important natural resources.
The geological column shown on the right reveals how this happened.
Each black band represents a coal seam – the compressed remains of ancient forest vegetation. When plants died, they fell into waterlogged swamps where decay was slow. Over thousands of years, thick layers of peat accumulated. Periodically, rivers flooded the area, burying the peat beneath mud, silt and sand. New forests then established on top, beginning the cycle again.
This process repeated many times over millions of years. The result is the sequence of coal seams and sedimentary rocks seen in the South Wales Coalfield today.
This process repeated many times over millions of years. The result is the sequence of coal seams and sedimentary rocks seen in the South Wales Coalfield today.
Geologists can even identify periods when the sea briefly flooded these tropical wetlands. Thin layers known as marine bands contain the fossils of marine animals and act as geological time markers, allowing rock layers to be matched across different parts of Wales.
What makes this story remarkable is that every coal seam once formed part of a living forest. The coal that later powered homes, railways, ironworks and industry across Wales and beyond began as lush tropical vegetation growing in a world almost unrecognisable from the one we know today.
The South Wales Coalfield is therefore more than a record of industrial heritage. It is a record of ancient rainforests, changing climates, rising and falling seas, and the deep geological history that shaped the landscape beneath our feet.
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Coed y Werin (Wern Ddu Claypits): A Geological Site of Educational and Industrial Importance
Article by Andrew Green, Geochemist, IGI Ltd
Situated on the south-eastern rim of the South Wales Coalfield, approximately 1.5 km south-east of Caerphilly, the Caerphilly Woodland Trust site ‘Coed y Werin’ (also known as the Wern Ddu Claypits) is one of the most important geological locations in Wales for understanding the geological history of the Carboniferous Coal Measures. Although the former claypit is now largely vegetated and managed as a woodland, the site remains of outstanding scientific value because it preserves a unique record of environmental change, coal formation and basin development during the late Carboniferous Period, around 315 to 300 million years ago.
Geological Significance
Coed y Werin is remarkable because it contains the only known continuous sequence of strata spanning the upper Langsettian to lower Bolsovian stages in the eastern South Wales Coalfield (1). The historic uninterrupted succession allowed geologists to study how environments changed through time, from coastal and deltaic settings to coal-forming swamp forests and occasional marine incursions.
The exposed sequence records repeated cycles of sandstone, mudstone, ironstone, seat-earth and coal deposition. These sediments provide evidence of the dynamic interactions between rivers, floodplains, peat-forming wetlands and shallow seas that characterised the South Wales Basin during the Carboniferous. The site also illustrates how geological conditions differed near the edge of the basin compared with the more rapidly subsiding central coalfield. The comparatively condensed rock sequence at Wern Ddu is a key indicator of its position on the basin margin.
Of particular importance is the occurrence of plant fossils, non-marine bivalves and exceptionally significant palynomorphs (fossil pollen and spores). Wern Ddu is effectively the only site within the South Wales Coalfield where recoverable Carboniferous palynomorphs have been documented, allowing geologists to correlate South Wales strata with equivalent coal-bearing sequences elsewhere in Britain and Europe.
[1] The recorded ‘continuous sequence’ is not visible today in its entirety. The site now exposes several parts of the succession.
Educational Value
Coed y Werin serves as an exceptional outdoor classroom. For students of geology, geography, environmental science and earth history, the site provides a real-world example of fundamental geological principles.
including:
- Stratigraphic succession and rock correlation.
- Peat accumulation and coal formation.
- Fossil preservation and palaeoecology (past environments with associated organisms).
- Sedimentary environments and sea-level change.
- Basin evolution and tectonic controls on sedimentation.
- Geological mapping and interpretation of former industrial landscapes.
The site is particularly valuable because it demonstrates the connection between fossils and rock layers. Fossil plants, marine faunas and microscopic spores all provide independent evidence that can be combined to reconstruct ancient environments and establish the age of the rocks. This multidisciplinary approach makes Coed y Werin an important teaching locality for both secondary education and university-level earth science programmes.
Furthermore, the site helps communicate broader themes in environmental change. The extensive Carboniferous forests that once occupied this area were responsible for the peat deposits that eventually became coal. Seeing the managed forest around the site provides a tangible link to understanding this process, providing an accessible way for the public to appreciate how Earth’s climate, ecosystems and landscapes have changed over geological time.
Industrial Importance
The geology exposed at Coed y Werin was directly linked to the industrial development of South Wales. The Coal Measures contain numerous economically significant coal seams that contributed to the region’s long history of mining and iron production. The associated clay-rich strata were also worked locally into bricks, giving rise to the claypit from which the site derives its pervious name Wern Ddu Claypits.
The succession includes coal seams that can be correlated with major seams that were and still are being exploited throughout the South Wales Coalfield (Aberpergwm Colliery, Vale of Neath). These records have helped geologists understand seam continuity, thickness variations and the geological controls on coal formation.
The coal is also beneficial as an analogue to those who drill and interact with this rock material in the oil and gas industry. Seeing coal at outcrop provides an opportunity to discuss physical characteristics and mechanical behaviour.
Ironstone bands preserved within the sequence are equally important. During the Industrial Revolution and the nineteenth century, ironstone resources from Coal Measure successions underpinned the growth of South Wales as one of the world’s leading iron-producing regions.
Source Material:
Cleal, C.J. & Thomas, B.A. 1996 British Upper Carboniferous Stratigraphy. Geological Conservation Review Series No. 11
Hartley, A.J. 1993 A depositional model for the Mid-Westphalian A to late Westphalian B Coal Measures of South Wales. Journal of the Geol Soc., V150, p.1121-1136
Scottish Geology Trust – Geoguide: https://geoguide.scottishgeologytrust.org/p/gcr11/gcr11_wernddu
