Location: Wern Ddu Clay Pit (Coed y Werin), Caerphilly, Wales, UK
The site is owned and managed by the Caerphilly Woodlands Trust, and the Wern Ddu Clay Pit is designated as a Geological Site of Special Scientific Interest.
Access: There is no parking at the Clay Pit itself so access to the site is a five minutes’ walk from the Coed Park y Van car park (ST:172-864). The clay pit entrance is at (ST:168-863) where access south on a well-defined access path leads to the clay pit site outlined in red.
The site has been developed over several phases since 2007 and through the help of the British Institute for Geological Conservation (BIGC) formal routes to a series of geological outcrops, industrial and nature stops have been created and accompanied with an audio guide: https://greenspacescaerphilly.co.uk/audio-trails .
Title: Westphalian Coal on the Edge of South Wales Basin - By Andrew Green
The site of the Wern Ddu claypits located on the southeastern edge of the South Wales Basin (c. 2300 km2) has been recognised for its ability to aid the interpretation of a late Carboniferous (Silesian) coal-bearing sequence spanning 312-308Ma (Duckmantian [Westphalian B] – Bolsovian [Westphalian C]). Lithostratigraphically the exposed outcrops (containing 10 coal seams) at this site belong to the middle-upper part of the South Wales Coal Measures Group and the base of the Warwickshire Group (Pennant Sandstone Formation). Wern Ddu is only one of two locations where the famed “black gold” (coal) mined in the Rhondda Valley comes to the surface and can be accessed.
Coal in its simplest classification is defined as any readily combustible rock containing ≥50%wt or ≥70%vol of organic material. In detail, however, coal is a complex material composed of different organic substances called macerals grouped into three major classes based on their optical reflectance, mode of occurrence, and physical appearance:
-Liptinite: Low light reflectance, derived from plant spores, cuticles, resins, and algal bodies
-Vitrinite: Intermediate light reflectance, derived from woody tissues
-Inertinite: High light reflectance, derived from fossil charcoal or decayed material
The primary characteristics of a coal seam are influenced by numerous factors at the point of deposition, e.g. temperature / redox state (aerobic vs. anaerobic) / bacterial activity / pH / Sulphur supply / peat-forming plant communities / nutrient supply. Coal is the end product resulting from physical, chemical and molecular alterations which result from the burial and heating of peat (~11m peat = 1m coal) in a process called “coalification”. During this process, moisture content decreases, carbon ring condensation occurs and optical reflectance increases. Economically the process of coalification also sees the calorific value of the coal increase (quantity of energy/kg) which is expressed commonly as “rank”. Coal rank proceeds from peat through lignite > sub-bituminous coal > bituminous coal > hard coal > anthracite.
Given the depositional and thermal history on the margin of the of the depositional basin, the section at Wern Ddu claypits experienced lower rates of subsidence and lower degrees of thermal transformation from peat to coal. Consequently, the coal seams visible are of low rank: sub-bituminous coal [0.4-0.65%Ro - ~100-120°C]) compared to comparative sections in the basin’s center (see Image 1).
The muds, sands and coal intervals present at Wern Ddu within the Lower and Middle Coal Measures represent deposition amongst a fluvio-deltaic setting with periodic marine flooding resulting from eustatic sea-level rises. The prograding nature of the delta system is seen through a progressive shift from middle delta-plain deposits associated with the productive coal measures to upper delta-plain deposits of the Pennant Sandstone Formation along with a decline in marine bands. The shift in geological setting, noted lithologically, is also accompanied by changes in coal maceral composition, evidencing a climatic shift toward a drier environment.
The cyclic nature of the coal seams and mudstones seen through the Duckmantian [Westphalian B] – Bolsovian [Westphalian C] both at Wern Ddu and throughout the South Wales Basin during the Westphalian (coal → brackish water lacustrine mud → marine flood deposits → non-marine mudstone → silty mudstone-sandstone → seatearth → coal) represent a sequence of cyclothems: a stratigraphic succession representing successive phases of delta building, interspersed with marine incursions.
Stop two on the clay pits audio trail is an excavated amphitheatre with good access and an accompanying information board. The location shows the pits youngest coal seam, the No.2 Rhondda coal seam with the overlying Saron Sandstone. The beds are seen dipping 25° towards the NW in accordance with the orientation of the basin’s synclinal axis. There is a sharp contact between the Saron Sandstone and the No.2 Rhondda coal seam representing a sharp influx of coarse alluvial-dominated immature material from the advancing southern thrusted Variscan front.
The No.2 Rhondda coal seam is fissile in nature and is highly weathered in parts. The coal can be seen to be banded with thin interbeds of soft clay. The coal face in part holds a dull lustre in line with its low sub-bituminous coal rank. Below the coal is soft fine-grained clay-dominated seatearth which commonly would contain signs of root systems associated with the vegetation present at the time, but here this is not evident due to the surface weathering (see Image 2). The site is known to have yielded large fossilised root systems (stigmaria) of common Carboniferous coal forest lycopsid trees.
References:
CLEAL, C.J. & THOMAS, B.A., 1996. British Upper Carboniferous Stratigraphy. Geological Conservation Review Series, No. 11, Chapman and Hall , London, pp. 339.
ALDERTON, D.H.M., Oxtoby, N., Brice, H. and Bevins, R. 2004. The link between fluids and rank variation in the South Wales Coalfield: Evidence from fluid inclusions and stable isotopes. Geofluids, 4,3, pp. 221-236.
