Canada Tips: Reconstructing a Carboniferous forest from a handful of fossils

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.

A few slabs of black shale from a South Wales spoil tip may look unremarkable at first glance, but with the right approach, they can reveal the structure of a 310-million-year-old coal-swamp forest. By combining plant fossils with sedimentary clues, collectors can reconstruct the ecosystem that once grew above the coal seams.

Introduction

Fragments of dark shale scattered across a spoil tip may appear to be little more than industrial waste. Yet split one of these slabs open – or simply turn it over – and a very different picture emerges. Delicate leaf impressions, fragments of bark and sections of fossilised roots lie preserved within the rock, silent evidence of a vanished landscape.

These fossils come from the Upper Carboniferous Coal Measures of South Wales, deposited around 310 million years ago, when Britain lay close to the equator. At that time, the region was covered by vast tropical wetlands. Dense forests of giant plants grew across waterlogged plains, periodically buried by mud and sand, as rivers shifted across the landscape. Over time, the accumulated peat from these swamps was transformed into the coal seams that would eventually fuel the Industrial Revolution.

A visit in March 2026 to one of these spoil tips (see box: The Canada Tips) yielded a small but revealing collection of plant fossils preserved in black shale. Among the specimens were fragments of ribbed bark belonging to the giant lycopsid tree Sigillaria (Figs. 1, 2 and 5), pieces of its distinctive rooting system, known as Stigmaria (Figs. 3, 4 and 7), strap-like leaves from the primitive tree Cordaites (Figs. 12 and 6), and delicate pinnules of seed ferns such as Neuropteris (Fig. 6).

Individually, these fossils represent only fragments of plants. Yet taken together, they provide enough clues to begin reconstructing the structure of the forest in which they once grew.

The Canada Tips

The fossils described here were collected from a spoil heap near Pwll Du on the eastern side of the South Wales Coalfield (Fig. 9 (top)). Locally, this area is known as the “Canada Tips”, a name dating from the Second World War. During the early 1940s, large-scale opencast mining was carried out in this upland area to help maintain Britain’s wartime coal supplies. Canadian military engineering units were involved in the operation, and the extensive spoil heaps left behind became known locally as the Canada Tips. Today, these spoil heaps form part of the Blaenavon Industrial Landscape, a UNESCO World Heritage Site that preserves many traces of the region’s coal and iron industries. Although the original opencast workings have long since been filled or eroded, the tips still contain abundant fragments of the Carboniferous rocks that once formed the roof of the coal seams (Fig. 9). For fossil collectors, this disturbed ground provides an unusual opportunity. The spoil exposes pieces of the ancient swamp deposits that would otherwise remain buried underground, allowing plant fossils from the Coal Measures to be collected and studied more than 300 million years after the forests themselves disappeared.

The clues in the fossils

Most of the plant fossils occur as flattened impressions within dark, fissile shale – the typical preservation style of muds that accumulated above coal seams. Such deposits formed when quiet water covered the peat swamp and buried fallen plant material beneath fine sediment. This type of preservation is typical of transported “roof-shale” assemblages, in which plant material from different parts of the landscape is mixed together during burial.

The most common fossils in the assemblage are fragments of bark belonging to Sigillaria (Figs. 1, 2 and 5), one of the characteristic trees of the late Carboniferous coal forests. These distinctive lycopsid trees grew as tall, columnar trunks with relatively little branching, their surfaces marked by vertical ribs and rows of leaf scars. When living, they formed dense stands within the waterlogged swamps that produced many of the coal seams of Britain.

Associated with these bark fragments are numerous pieces of Stigmaria, the specialised rooting system of sigillarian trees (Figs. 3, 4 and 7). These consisted of four main axes radiating outward from the base of the trunk and spreading horizontally through the peat. Their presence indicates that the vegetation represented in the shale grew directly within saturated swamp sediments, rather than on well-drained ground.

One notable feature of the assemblage is the apparent absence of fossils belonging to Lepidodendron, another well-known member of the coal-swamp flora. Lepidodendron commonly occurs alongside Sigillaria in many Carboniferous plant assemblages. Its absence here may indicate that the vegetation represented in this deposit was dominated by sigillarian trees, although collecting bias and preservation effects cannot be ruled out. Such local variations in plant communities were common within Carboniferous wetlands, where small differences in water depth, sediment supply and substrate stability could favour different groups of plants.

In addition to the swamp trees themselves, the shale yielded several fragments of strap-shaped leaves belonging to Cordaites (Figs. 6 and 12), a primitive gymnosperm that formed tall trees along the margins of Carboniferous wetlands. (Strictly speaking, Cordaites is a form-genus used for the characteristic leaves, although the name is commonly applied to the plant as a whole.)

Scattered among these larger plant remains are delicate pinnules from seed ferns, including forms attributable to Neuropteris (Fig. 6). These plants, which resembled modern tree ferns, probably formed part of the understory vegetation of the coal-swamp forest.

Taken together, these fossils represent several different layers of the ancient vegetation. Sigillaria formed the dominant canopy of the swamp itself, supported by their Stigmaria roots. Seed ferns occupied the shaded understory, while cordaite trees probably grew on slightly firmer, better-drained substrates such as levees, peat islands or channel margins.

Taken on their own, these fossils provide a snapshot of the plants present. However, to understand how these plants were arranged across the landscape – and how they came to be preserved – it is necessary to consider the sediments in which they occur.

Reading the rocks

The fossils themselves provide the first clues as to the nature of the ancient forest, but the surrounding rocks add another important layer of evidence. The plant impressions occur mainly in black, finely laminated shale – a type of sediment that typically forms when very fine mud settles out of still or slow-moving water.

Such conditions are typical of coal-swamp environments. When a peat swamp became flooded, mud carried by nearby rivers spread across the surface and buried the vegetation growing there. Leaves, bark and other plant fragments became trapped within the mud and were preserved as flattened impressions when the sediment eventually hardened into shale.

At the spoil tip, the fossiliferous shale occurs alongside large blocks of fine-grained sandstone (Fig. 8). These sandstone fragments are often many inches across and display irregular patches of rusty iron staining on their weathered surfaces. In hand specimen, the rock appears massive, with no obvious sedimentary structures visible.

The iron staining probably formed when iron-bearing minerals within the sandstone oxidised after exposure to air and water. In many Coal Measures rocks, iron originally occurs as siderite (iron carbonate). When such rocks are exposed on spoil heaps, the siderite alters to iron oxides, producing the characteristic reddish-brown staining seen on many fragments.

An interesting feature of the spoil heap is that these sandstone blocks are particularly abundant at one end of the tip, whereas the plant fossils occur mainly within the black shale fragments (Fig. 9). This uneven distribution suggests that the spoil was derived from several beds within the same sedimentary sequence, rather than from a single uniform layer.

In the original Carboniferous landscape, such an association of shale, sandstone and coal was very common. Peat accumulated in waterlogged swamps where plant growth exceeded decay. Periodically, nearby rivers flooded the swamp plain. Fine mud settled across the peat surface to form the shale beds that now preserve the plant fossils, while sand carried by stronger currents was deposited in nearby channels or flood deposits.

These alternating layers record a dynamic environment in which forests grew on low, poorly drained floodplains influenced by nearby rivers.

Rebuilding the Carboniferous forest

The fossils and sediments together allow a reconstruction of the ancient environment (Fig. 10).

The most conspicuous plants in the assemblage are fragments of bark belonging to Sigillaria (Fig. 11). These giant lycopsid trees formed tall, columnar trunks that could reach heights of 20m or more. Their distinctive ribbed bark represents the outer surface of the trunk beneath the leafy crown.

At the base of these trees grew the remarkable root system known as Stigmaria. Unlike the deep branching roots of most modern trees, Stigmaria spread horizontally through the peat as four main axes radiating from the trunk. Numerous small rootlets emerged from these axes, penetrating the surrounding sediment and anchoring the tree in the soft swamp substrate.

Beneath the canopy of sigillarian trees, other plants occupied the forest floor. Seed ferns such as Neuropteris (Fig. 6) probably formed part of the dense understory vegetation that filled the spaces between the larger trees.

Fragments of strap-shaped leaves belonging to Cordaites (Fig. 12) indicate the presence of another type of tree within the broader landscape. These gymnospermous plants are thought to have favoured slightly firmer ground than the waterlogged swamps dominated by lycopsids, growing on peat islands or along the margins of channels crossing the floodplain.

When these elements are combined, a coherent picture of the ancient forest emerges. The wettest parts of the floodplain supported dense stands of Sigillaria, their trunks rising vertically from the peat and forming the dominant canopy of the swamp. Beneath them, seed ferns and other low plants covered the forest floor, while the spreading Stigmaria roots anchored the trees within the saturated sediment.

Nearby, slightly higher ground supported cordaite trees, whose long strap-like leaves contrasted with the crown foliage of the lycopsids. Periodic flooding from neighbouring rivers buried parts of the forest beneath mud and sand, preserving the fragments of vegetation that now appear as fossils within the shale.

A forest beneath the spoil tips

Today, the fragments of shale and sandstone scattered across the spoil heaps represent the disturbed remnants of these ancient deposits. Yet, hidden within this industrial landscape, are pieces of a far older story.

Professional geologists reconstruct ancient environments using detailed stratigraphic studies, laboratory analyses and large collections of specimens. Nevertheless, the fundamental principles of geological interpretation remain surprisingly simple. Careful observation of rocks and fossils – and an understanding of how modern environments work – can reveal a great deal about the past.

In this case, a handful of fossils is enough to outline the structure of a Carboniferous ecosystem. Bark fragments reveal the dominant trees of the swamp. Root fossils show how those trees were anchored in the peat. Leaves record the plants that occupied the forest floor and the margins of the wetland. The surrounding rocks reveal the flooding events that buried and preserved the vegetation.

The spoil heaps therefore preserve more than just the history of coal mining. Scattered among the fragments of shale are the remains of a Carboniferous forest that once covered the floodplains of South Wales. With a little patience and observation, it is still possible to glimpse that vanished landscape in the rocks beneath our feet.