The lost rainforest of the West Cumberland Coalfield (Part 3): Beneath the giants – Stigmaria and the swamp floor

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.

The trunks of Lepidodendron and Sigillaria (Part 2) dominate most people’s mental image of the Carboniferous swamp, but to understand how these forests actually functioned, you need to look beneath the surface, into the layer that supported and anchored the entire ecosystem. In the West Cumberland Coalfield, this means one thing above all – Stigmaria, the distinctive rooting systems of the giant lycopsid trees.

Stigmaria fossils are among the most abundant plant remains in the area. They occur in grey seat-earths, in siderite nodules washed onto the beach, in split fireclay blocks from old spoil tips, and even as large in situ networks exposed in rare natural outcrops. But despite their abundance, these roots are often misunderstood. They are neither taproots nor fibrous roots in the modern sense; and they reveal more about the structure and ecology of the swamp forest than almost any other plant fossil.

What Stigmaria actually was

“Stigmaria” refers to the rooting axis of lycopsid trees, primarily Lepidodendron and Sigillaria. Far from being deep, penetrating roots, they were horizontal, radiating organs that spread outward from the base of the trunk, typically in a four-way pattern (Fig. 3.1).

The key features include.

1. A thick, rigid axis. Usually 10-30cm in diameter, sometimes more. This formed a rigid base capable of supporting a fully grown tree.
2. Spirally arranged rootlet scars. These are the circular or oval marks covering most Stigmaria surfaces (Fig. 3.1). Each scar represents the attachment point of a slender, cylindrical rootlet capable of penetrating soft mud.
3. A cortex designed for wet, anoxic soils. These trees anchored themselves in saturated, oxygen-poor substrates that few modern plants could tolerate. Stigmaria rootlets carried aerenchyma, air-filled tissues that aided gas exchange under waterlogged conditions.
4. Radial architecture. The main axes radiated horizontally from the trunk, forming a stable base in shifting mud. Unlike modern trees, they did not rely on deep rooting for support.

Where Stigmaria is found: the seat-earths

In the West Cumberland Coalfield, Stigmaria is overwhelmingly concentrated in what geologists call seat-earths or underclays, which are the ancient soil horizons beneath each coal seam (Fig. 3.2). These seat-earths represent the surface on which the original forest grew. They consist of:

• mottled or massive grey mudstones;
• root-penetrated fireclays;
• horizons heavily bioturbated by rootlets; and
• occasional sideritic nodules preserving internal casts of the root axes.

In effect, each seat-earth is a fossilised forest floor. Where roots appear in life position as radiating networks around a vertical trunk base, the seat-earth can even show the spacing of the trees, the density of the stand, and the substrate conditions. The West Cumberland Coalfield’s seat-earths, although usually accessible only through spoil material, preserve these networks with exceptional clarity.

Stigmaria in nodules: three-dimensional preservation

One of the distinctive signatures of the West Cumberland Coalfield area is the abundance of siderite nodules containing beautifully preserved Stigmaria casts. These occur because the root axes rotted internally after burial, their cavities filled with iron-rich pore waters, and siderite precipitated rapidly, creating precise internal moulds.

Many shoreline nodules contain Stigmaria with branching patterns intact, rootlet attachment scars preserved in relief, and original cross-sectional profiles unmixed by compaction. This three-dimensional preservation is rare in most coalfields – the Cumbrian coastline is one of the few British districts where nodules routinely capture Stigmaria this cleanly.

What Stigmaria tells us about the swamp environment

Because Stigmaria occurs predominantly in seat-earths, it is one of the most reliable indicators of ancient swamp conditions. Its distribution reveals several key environmental facts.

1. Permanently saturated soils. Lycopsids required near-constant waterlogging. Stigmaria rootlets are adapted to soft, low-oxygen mud. Their widespread presence in the West Cumberland Coalfield confirms a persistently high water table.
2. Stable forest surfaces. The density of Stigmaria networks suggests long-lived peat-forming surfaces, where individual trees stood for decades before drowning or collapse.
3. Limited soil profile development. Seat-earths are typically structureless or only weakly layered. This indicates repeated flooding, poor drainage and soils that had little time to develop, conditions ideally suited to lycopsid forests.
4. Repetition of the swamp cycle. Each seat-earth is buried by a coal seam, then followed by roof shale. The West Cumberland Coalfield contains multiple stacked examples of this pattern, showing that the region hosted a sequence of swamp-pond-floodplain transitions over millions of years (Part 4).

Rooting strategies and forest structure

The radial, shallow rooting of lycopsids shapes how we interpret their forest dynamics.

• Tree spacing. Stigmaria networks often overlap, implying close spacing – sometimes only a few metres apart. This suggests dense stands of lycopsid canopy trees.
• Susceptibility to collapse. Shallow rooting made the trees vulnerable to overturning during floods or subsidence. This is consistent with the sheets of detached bark and toppled trunks preserved in the West Cumberland Coalfield roof shales.
• Rapid recolonisation. Once a forest drowned, new lycopsid sporophytes (or vegetatively reproduced shoots) could colonise quickly, provided the surface was once again waterlogged.

The result was a dynamic ecosystem, but one still dominated by lycopsids, so long as conditions remained consistently wet.

Did Stigmaria form a shared ‘root mat’?

Although Stigmaria systems commonly overlapped in dense lycopsid stands, there is no evidence that the roots of different trees fused into a single, mechanically integrated mat. Individual Stigmaria axes belonged to individual trees, and their rootlets did not graft together or form long-lived structural connections between neighbouring plants.

Instead, the swamp substrate was densely penetrated by thousands of overlapping rootlets from adjacent trees, both living and decaying. This produced a peat and mud surface that was cohesive and resistant to minor disturbance, even though the trees themselves were not physically linked. In effect, the forest floor was stabilised collectively, but not mechanically shared.

This distinction helps explain an important feature of the West Cumberland Coalfield record. When conditions remained stable, dense lycopsid stands could persist for decades. But when flooding, subsidence or water-level rise destabilised the substrate, entire forests could collapse abruptly. The trees did not hold one another upright, and failure was often catastrophic rather than gradual, producing the extensive sheets of fallen bark and toppled trunks preserved in the roof shales.

The West Cumberland Coalfield record: clarity through contrast

In the West Cumberland Coalfield, Stigmaria is most abundant and most informative in grey seat-earth (underclay) blocks beneath coal seams, and siderite nodules preserving axes in 3D. And it occurs more sporadically as reworked fragments in grey roof shales immediately above coal, and red-oxidised mudstones where oxidation has overprinted earlier grey deposits.

This stark distribution helps distinguish the different parts of the depositional cycle. Wherever you find dense Stigmaria, you are looking at the original forest floor rather than the later drowned landscape. Conversely, the red shales represent post-forest oxidation or late flooding events unrelated to the lycopsid soil surface.

A foundation beneath the forest

Without Stigmaria, the lycopsids could not have become the giants that dominated the Carboniferous world. This rooting system, specialised for permanently wet, unstable ground, is one of the defining innovations of the Westphalian swamp flora. Its fossils in the West Cumberland Coalfield offer unparalleled insight into how these trees lived, anchored themselves, competed for space, and eventually succumbed to drowning or collapse.

For anyone reconstructing ancient swamp ecosystems, Stigmaria provides the essential foundation – literally and figuratively. And in the West Cumberland Coalfield, that foundation is preserved with a level of detail matched by only a handful of coalfields anywhere in Europe.

Further reading

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

Falcon-Lang, H.J. (2009). Going underground: in search of Carboniferous coal forests. Geology Today, 25, 181–184.

Gastaldo, R.A. (1986). Implications of lycopod preservation in coals… Palaeogeography, Palaeoclimatology, Palaeoecology

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 Cumberland