england fossils geology palaeontology

The lost rainforest of the West Cumberland Coalfield (Part 1): A window into the Carboniferous tropics

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Jon Trevelyan (UK)

Today, the coast from near Maryport, south down to Whitehaven is a quiet edge of the Solway Firth, marked by overgrown farmland and the remnants of old collieries. Yet the rocks beneath preserve something far more remarkable – the record of one of the most extensive tropical wetlands in Earth’s history.

This is the first of eleven articles on the palaeobotany and geology of the West Cumberland Coalfield, together with a comparison with the fossils of the Somerset Coalfield. Around 313-310 million years ago, within the Westphalian interval of the European coalfield succession, which is broadly equivalent to the late Moscovian to earliest Kasimovianstages of the global Pennsylvanian (Figs. 1.1 and 1.2), the West Cumberland Coalfield area lay close to the equator. It formed part of a vast lowland rainforest-swamp system that stretched across what is now Britain and mainland Europe.

Fig. 1.1. The Carboniferous Period, based on the International Commission on Stratigraphy (ICS).
Fig. 1.2. proportional comparison of Upper Carboniferous international (ICS) stages and traditional UK stages. International stages (left) are shown to scale, while the UK Westphalian (right) is subdivided into Langsettian (A), Duckmantian (B), Bolsovian (C) and Asturian (D). The diagram highlights the lack of one-to-one correspondence between UK and international stage boundaries.

These forests (Fig. 1.9), dominated not by flowering plants but by giant lycopsids, horsetails and early seed ferns, produced the coal seams mined in the West Cumberland Coalfield. The associated roof shales, seat-earths and ironstone nodules have made the West Cumberland Coalfield one of the most informative windows into the ecology and environmental dynamics of the Carboniferous coal forests. Few places in Britain demonstrate the full developmental sequence of a swamp forest as clearly as this area.

A tropical wetland along the Carboniferous equator

Palaeogeographic reconstructions place northern England at an approximate 5-10° south latitude during the Westphalian. The climate was consistently warm, humid and dominated by high rainfall. There was negligible seasonality, with no frost, minimal temperature variation, and an almost permanently high water table.

This climatic stability created ideal conditions for peat formation, and repeated subsidence of the West Cumberland basin allowed thick peat layers to accumulate. Over geological time, these became the coal seams, with each being the compressed remnant of a mature swamp forest. The sediments deposited above these seams, known as roof shales, preserve the plants that colonised the landscape immediately after the peat-forming forest drowned.

It is these roof shales, as well as the underlying seat-earths, together with associated grey and red mudstones (Fig. 1.3), that provide the extraordinary level of palaeobotanical detail found in the West Cumberland Coalfield.

Fig. 1.3. A large slab preserving the external bark of a Carboniferous lepidodendrid tree (Lepidodendron) from the West Cumberland Coalfield, showing a dense pattern of overlapping, lozenge-shaped leaf cushions arranged in oblique rows. Although the surface is distorted by post-burial collapse and compaction, the underlying cushion geometry remains clearly recognisable and distinguishes Lepidodendron from other arborescent lycopsids. The vivid red coloration reflects iron-rich diagenesis within the enclosing mudstone rather than original bark colour. Such slabs represent fragments of large trunks shed or buried on the swamp forest floor during flooding and rapid sedimentation. (From the author’s collection.)

Why the West Cumberland Coalfield area preserves so much

The richness of the West Cumberland Coalfield flora is not a matter of chance alone. Several geological factors combine to produce exceptional preservation.

  • A subsiding foreland basin. Gentle, long-term subsidence allowed repeated formation of low-lying wetlands, each ultimately generating a coal seam. The repetition of this cycle created a vertically stacked record of swamp development and collapse.
  • Fine-grained, quiet-water roof shales. After each swamp drowned, a shallow pond or lake developed. Suspended silts and clays settled into thin laminae, preserving delicate foliage, such as Pecopteris (Part 5), Neuropteris (Part 6) and fine Calamites leaves (Part 7). These laminated shales are particularly common in the West Cumberland Coalfield coal seams.
  • Iron-rich pore fluids. During early diagenesis, iron in groundwater precipitated as siderite, filling cavities inside rotting trunks and roots. These siderite nodules preserve spectacular internal casts of Lepidodendron, Sigillaria (Part 2) and Stigmaria (Part 3) – some of the finest anywhere in Britain (1. 4).
Fig. 1.4. Two beach-collected ironstone nodules recording the characteristic diamond-shaped leaf-scar pattern of arborescent lycopsid (Lepidodendron) bark. The nodular cementation has captured the external bark surface in three-dimensional relief, preserving individual leaf scars and their spacing with exceptional clarity. Such nodules formed early during burial within waterlogged swamp or floodplain sediments, providing durable snapshots of the Carboniferous rainforest trees that dominated the landscape. (From the collection of Duncan White.)
  • Mining and coastal erosion. Industrial extraction in the nineteenth and twentieth centuries brought large quantities of roof shale and fireclay to the surface. Modern coastal erosion continues to release nodules and plant impressions from old spoil material. As a result, the West Cumberland Coalfield provides an unusually accessible fossil record.

What the fossil evidence reveals about the forest

The ancient vegetation was structurally complex but taxonomically narrow by modern standards. It was dominated by three or four major groups.

Lycopsids (Lepidodendron and Sigillaria)

These were the canopy-forming trees, often 30-40m tall. Lepidodendron trunks carried diamond-shaped leaf-cushions, while Sigillaria displayed vertical rows of oval leaf scars. Both had shallow, radial rooting systems (Stigmaria; Fig. 1.5) and occupied permanently waterlogged environments. Their dominance reflects the ever-wet conditions of the early Westphalian (Part 2).

Fig. 1.5. Stigmaria root preserved as impressions in oxidised red shale from the West Cumberland Coalfield, showing regularly spaced circular rootlet scars. The fine-grained host sediment and lack of abrasion indicate burial close to the original forest floor, recording in situ root networks beneath Carboniferous coal-swamp vegetation rather than transported material. (From the author’s collection.)

Tree ferns (Pecopteris and its relatives)

Following the drowning of a swamp, tree ferns rapidly colonised exposed mud along pond margins. The exquisitely preserved fronds in the West Cumberland Coalfield roof shales, often of the Pecopteris plumosa type (Part 5 and Fig. 1.6), represent early stages of recolonisation after peat formation ceased.

Fig. 1.6. A well-preserved Carboniferous tree-fern frond (Pecopteris) from the West Cumberland Coalfield showing regularly spaced pinnules with a strong central midrib, preserved as a flattened impression in fine-grained shale deposited on a coal-swamp floodplain. (From the author’s collection.)

Seed ferns (Neuropteris, Alethopteris and Odontopteris)

These plants proliferated on slightly elevated ground such as levees, splays and the margins of floodplain lakes (Part 6). Their pinnules (Fig. 1.7) appear in a range of sizes and shapes depending on frond position. Their diversity increases in the Duckmantian-Bolsovian (Fig. 1.2), reflecting subtle increases in habitat diversity.

Fig. 1.7. Neuropteris pinnules from the Radstock Coalfield (Somerset), preserved in fine-grained Carboniferous mudstones. Both specimens show the characteristic elongate, lens-shaped outline and smooth margins typical of Neuropteris seed-ferns. Subtle differences in relief and surface texture reflect variation in preservation and compaction rather than botanical differences. Comparable pinnules occur abundantly in the West Cumberland Coalfield, making these Radstock examples representative of the foliage types found there. (From the author’s collection.)

Calamitean horsetails, sphenopsids and Cordaites

These added further structural complexity (Fig. 1.8), but the forest remained dominated by lycopsids throughout much of the West Cumberland Coalfield succession (Part 7).

Fig. 1.8. A Cordaites leaf impression from the West Cumberland Coalfield preserved as a carbonaceous compression in fine-grained shale, showing strong, closely spaced parallel venation typical of cordaitalean gymnosperms. The width, rigidity and simple venation distinguish Cordaites from fern or seed-fern foliage and reflect its adaptation to better-drained substrates within the Carboniferous lowland mosaic. Such leaves are common in roof shales and record the presence of tall, woody trees growing alongside – and often slightly beyond – the core lycopsid swamp forests. (From the author’s collection.)

A repeating ecological cycle recorded in stone

Fig. 1.9. A dense, peat-forming swamp dominated by arborescent lycopsids, such as Lepidodendron and Sigillaria. Tall, unbranched trunks rise from waterlogged seat-earth soils, with abundant broken stumps reflecting flooding, subsidence and forest turnover typical of British Coal Measures environments. A giant dragonfly (Meganeura-type) highlights the distinctive insect fauna of these humid, low-oxygen forests, whose accumulated plant material would later form coal seams.

The West Cumberland Coalfield succession documents multiple repetitions of a classic Westphalian environmental cycle (see also Part 4).

  1. Mature lycopsid swamp. Thick peat accumulates beneath a canopy of Lepidodendron and Sigillaria (Fig, 1.9).
  2. Swamp drowning event. Water levels rise due to subsidence or flooding. Peat formation ceases. The forest dies upright and then collapses (Fig. 1.10).
  3. Shallow lacustrine phase. Mud settles in quiet water. Bark fragments, logs and fallen fronds accumulate on the lake floor. Laminated roof shales form.
  4. Colonisation by tree ferns. As water shallows, Pecopteris and other marattialean ferns rapidly occupy exposed substrates.
  5. Seed-fern and calamite development. As the surface stabilises, Neuropteris and calamitean horsetails establish along levees and channel margins.
  6. Return of swamp conditions. Rising water tables favour lycopsids, restarting the peat-forming cycle.

Different spoil heaps around the West Cumberland Coalfield represent different stages of this cycle, which is why the flora appears so varied from one heap to another.

Fig. 1.10. The aftermath of forest failure following flooding and subsidence, in which stands of arborescent lycopsids have been killed and reduced to a waterlogged stump field. In contrast to the scene of a living swamp forest in Fig. 1.9, the canopy has vanished, leaving truncated trunks, exposed root bases and shallow peat-stained pools. Such collapse phases were common in British Coal Measures environments and mark the transition from forested mire to open, stagnant wetlands preceding burial and coal formation.

A unique window into a vanished ecosystem

The West Cumberland Coalfield provides one of the clearest records in Britain of how a tropical coal forest functioned. Few other parts of the country preserve the transitions between swamp, shallow pond, colonising fernland and floodplain vegetation preserved so comprehensively. The combination of excellent fossil preservation, multiple environmental horizons, and accessible spoil material gives an almost continuous ecological narrative.

These forests were unlike anything alive today – immense, humid and dominated by groups of plants now reduced to small relics or extinct altogether. Yet the processes that structured them, that is, flooding, sedimentation, succession and competition for light, are strikingly familiar. The West Cumberland Coalfield allows us to view this ancient world not as an abstraction, but as a coherent, functioning ecosystem preserved in remarkable detail.

Further reading

Benton, M.J. & Harper, D.A.T. (2020). Introduction to Paleobiology and the Fossil Record (2nd ed.). Wiley-Blackwell

Cleal, C.J. & Thomas, B.A. (2009). Introduction to Plant Fossils. Cambridge University Press.

DiMichele, W.A. & Falcon-Lang, H.J. (2011). Pennsylvanian ‘fossil forests’ in growth position…. Journal of the Geological Society, London, 168, 585–605

Other parts in this series
The lost rainforest of the West Cumberland Coalfield (Part 1): A window into the Carboniferous tropics
The lost rainforest of the West Cumberland Coalfield (Part 2): A forest of giants – Lepidodendron and Sigillaria
The lost rainforest of the West Cumberland Coalfield (Part 3): Beneath the giants – Stigmaria and the swamp floor
The lost rainforest of the West Cumberland Coalfield (Part 4): After the drowning – the roof shale succession
The lost rainforest of the West Cumberland Coalfield (Part 5): the ferns return – Pecopteris and the first colonisers of the drowned swamp
The lost rainforest of the West Cumberland Coalfield (Part 6): the seed-fern story – Neuropteris, Odontopteris and Alethopteris
The West Cumberland Coalfield’s lost rainforest (Part 7): rivers, reeds and dry patches – Calamites and Cordaites on the Carboniferous floodplain
The lost rainforest of the West Cumberland Coalfield (Part 8): From Langsettian to Bolsovian – dating the West Cumberland Coalfield flora
The lost rainforest of the West Cumberland Coalfield (Part 9): Life on the equator – climate and environment in Westphalian Cumbria
The lost rainforest of the West Cumberland Coalfield (Part 10): Why the West Cumberland Coalfield? Geology, tectonics and preservation
The lost rainforest of the West Cumberland Coalfield (Part 11): Two coalfields, two records: reconstructing the Carboniferous forest at Radstock and Maryport  

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