The lost rainforest of the West Cumberland Coalfield (Part 4): After the drowning – the roof-shale succession

Jon Trevelyan (UK)

Simplified summary of Britain’s geological history from the Precambrian to the present day. The coloured timeline shows the major geological periods and the illustrations above represent some of the dominant landscapes and environments that characterised each interval. The red line(s) indicates the geological period(s) covered by this article.

Every coal seam in the West Cumberland Coalfield district records the accumulation of peat in a mature lycopsid swamp. But just as revealing as the coal itself is what lies immediately above it – the roof shales. These fine-grained sediments record the moment when the forest drowned, the swamp surface subsided or flooded, and a new ecological phase unfolded above the buried peat. In the West Cumberland Coalfield, roof shales are exceptionally well developed and preserve a full sequence of the events that followed each catastrophic rise in water level.

The result is one of the clearest records in Britain of how a Carboniferous swamp forest died, how the landscape reorganised itself, and how colonising vegetation re-established once the water stabilised. For anyone studying the ecology of the coal forests, the roof-shale succession is not just an accessory to the coal seam – it is the key chapter that explains what happened between one forest and the next.

The drowning of a swamp: how roof shales begin

A peat-forming swamp is a delicate system. A small rise in water level – caused by subsidence, river avulsion or even regional base-level changes – can tip the balance from slow peat accumulation to rapid drowning. When this happens:

oxygen disappears from the surface;
the vegetation dies in place;
trunks and roots rot; and
mud settles onto the abandoned peat.

This transition marks the boundary between the coal and the roof shale.

In the West Cumberland Coalfield, this shift is often abrupt. The base of the roof shale usually consists of dark grey, fine laminae lying directly on the coal or on fireclay seat-earth. In some exposures, fragments of bark, seeds and broken lycopsid cones occur immediately above the seam, representing the last debris shed by the dying forest.

Phase 1: the quiet-water pond

Immediately after drowning, the swamp floor typically becomes a shallow lake. This is the laminated-shale phase, and it is responsible for the West Cumberland Coalfield’s most exquisite plant fossils. The key features of the quiet-water phase are as follows.

Fine lamination. Alternating layers of silt and clay represent regular settling of suspended sediment.
Low-energy conditions. No ripples, cross-bedding or coarse material.
Exceptional fossil preservation. Delicate Pecopteris fronds (Part 5), Neuropteris pinnules (Part 6) and the complete leaves of Calamites (Part 7) are common.
Minimal disturbance. Plants fall gently into an undisturbed water column and settle flat on the mud.

Fig. 4.1. Phase 1 – quiet-water pond (laminated shale phase). Following drowning of the lycopsid swamp, the forest floor became a still, shallow pond. Upright trunks remained standing in low-energy, oxygen-poor water, allowing fine muds to settle and preserving delicate plant remains in laminated shales.

The persistence of these quiet-water conditions is also indicated by the presence of freshwater bivalves (Fig. 4.2), preserved in red and grey mudstones in the West Cumberland Coalfield. These animals lived semi-infaunally in soft pond-floor sediments and imply standing water bodies that remained stable for extended periods.

Fig. 4.2. Non-marine bivalves from the West Cumberland Coalfield. A small cluster of freshwater bivalve shells preserved in red mudstone, showing concentric growth lamellae characteristic of Carboniferous genera such as *Anthracomya* or *Carbonicola*. These bivalves inhabited quiet freshwater ponds and floodplain lakes within the coal-swamp landscape. Their clustered preservation and oxidised matrix suggest burial close to life position under fluctuating wet-dry conditions rather than transport from a marine setting. (*From the author’s collection*.)

This is the environment that produced the West Cumberland Coalfield’s best-preserved leaf impressions and bark sheets. It is also the phase most vulnerable to later alteration, which is why truly fine lamination survives best in the grey shales that escaped oxidation.

Phase 2: the collapse of the forest – logs, bark and debris

As the water deepened, whole stands of lycopsids collapsed. Their trunks, often already hollow, fell into the pond, shedding bark which floated, became waterlogged, and eventually settled.

This creates several fossil signatures.

Slabs of Lepidodendron bark. Large sheets detached from trunks drifted into the pond and came to rest flat on the lake floor (Part 2). These produce the classic diamond-patterned slabs so common in the West Cumberland Coalfield.
Sigillaria fragments. Sigillarian bark, being thinner and less rigid, is preserved less extensively but still occurs as isolated panels (Part 2).
Waterlogged logs. Horizontal compressions of trunks appear in some roof shales, often several centimetres thick.
Disarticulated roots and branches. Pieces of Stigmaria and branch stumps accumulate in the lower roof shales (Part 3).

g. 4.3. Phase 2 – forest collapse and debris accumulation. As water deepened and root systems failed, whole stands of lycopsids collapsed into the pond. Hollow trunks and shed bark floated, became waterlogged, and settled as chaotic log and debris accumulations on the pond floor.

The abundance of such debris shows that drowning was not always gentle; that is, some roof shales indicate catastrophic forest collapse, possibly within days or months of rising water.

Phase 3: the red-shale overprint – oxidation and alteration

Some West Cumberland Coalfield tips preserve two distinct types of roof shale:

grey, finely laminated shales; and
red, iron-oxidised shales.

Both originate from the same depositional environment. The difference lies in later groundwater chemistry.

Red shales formed when:

oxygenated, iron-rich fluids percolated through the sediment long after deposition;
possibly along fracture systems or local channel bodies; and
altering the mudstone and sometimes (but not always) slightly degrading fossil detail.

These red shales often appear in separate spoil heaps because miners stored them apart from the more valuable grey fireclays. Fossils in red shales tend to be robust – bark slabs, larger plant fragments and occasional Sigillaria – whereas delicate foliage is more often preserved in grey shales.

The coexistence of both types is a distinctive feature of the West Cumberland Coalfield area and helps distinguish the primary depositional signal from later diagenetic alteration.

Phase 4: recolonisation – Pecopteris and early colonising plants

As the pond slowly infilled or its water level dropped, new land surfaces emerged. These fresh muds were quickly colonised by marattialean tree ferns, especially species of Pecopteris (Part 5).

Fig. 4.4. Phase 4 – recolonisation of exposed muds. As water levels fell and sediment accumulated, new land surfaces emerged. These unstable muds were rapidly colonised by ferns, especially *Pecopteris*, forming patchy early vegetation among stranded logs and stumps.

Tree ferns arrived first because:

they reproduced rapidly through spores;
they tolerated unstable mud surfaces;
they required less root penetration than seed ferns or lycopsids; and
they formed dense stands on recently exposed substrate.

The West Cumberland Coalfield’s roof shales contain beautifully preserved Pecopteris fronds, often lying intact and sometimes overlapping, indicating calm deposition and rapid burial. This phase marks the beginning of the post-swamp succession, where ferns, seed ferns and calamiteans developed on newly stabilising surfaces.

Phase 5: floodplain development

With further shallowing, the pond became part of a broader floodplain system – channels migrated, levees formed and new vegetation took hold. This stage is represented by:

Calamites stems and Annularia leaves, often aligned by weak currents (Part 7);
seed ferns (Neuropteris, Alethopteris) growing on levees and splays (Part 6); and
Cordaites in slightly drier or brackish niches (Part 7); and
occasional ripple marks and small-scale planar bedding.

Fig. 4.5. Phase 5 – floodplain development and zoned vegetation. With continued infilling, the former pond became part of an organised floodplain. Shallow channels migrated across the surface, sediments differentiated, and vegetation became zoned, marking the transition from drowned swamp to dynamic lowland landscape.

This is the “mixed flora” phase that transitions between deep swamp and drier floodplain environments. The West Cumberland Coalfield’s flora shows this clearly – abundant Neuropteris pinnules and calamite debris occur in the upper roof shales.

Why the West Cumberland Coalfield roof shales are exceptional

Many British coalfields preserve parts of this cycle, but the West Cumberland Coalfield preserves all of it, repeatedly, in a vertically stacked sequence.

Seat-earth → coal → grey roof shale → red-altered roof shale → colonising fern beds → mixed floodplain shales

The following makes the West Cumberland Coalfield among the best-documented coal-forest successions in the UK:

three-dimensional preservation in nodules;
exquisitely flat-lying leaves;
bark slabs in multiple oxidation states; and
repeated cycles of drowning and recovery.

The roof shale as an ecological diary

Each roof shale is effectively a diary entry written by the forest as it died and regenerated:

the grey shales record calm water and delicate preservation;
the coarse debris beds record catastrophic collapse;
the red shales record diagenetic oxygenation;
the fern beds record the first signs of life returning; and
the mixed shales record the maturing floodplain.

Understanding these transitions allows the swamp’s story to be reconstructed with remarkable clarity. And the West Cumberland Coalfield is one of the few places where every chapter of that story is preserved within walking distance of the Solway tide line.

The lost rainforest of the West Cumberland Coalfield (Part 4): After the drowning – the roof-shale succession

The drowning of a swamp: how roof shales begin

Phase 1: the quiet-water pond

Phase 2: the collapse of the forest – logs, bark and debris

Phase 3: the red-shale overprint – oxidation and alteration

Phase 4: recolonisation – Pecopteris and early colonising plants

Phase 5: floodplain development

Why the West Cumberland Coalfield roof shales are exceptional

The roof shale as an ecological diary

Further reading

Like this:

The drowning of a swamp: how roof shales begin

Phase 1: the quiet-water pond

Phase 2: the collapse of the forest – logs, bark and debris

Phase 3: the red-shale overprint – oxidation and alteration

Phase 4: recolonisation – Pecopteris and early colonising plants

Phase 5: floodplain development

Why the West Cumberland Coalfield roof shales are exceptional

The roof shale as an ecological diary

Further reading

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