Plants, fish and floodplains: The Heol Senni story

Jon Trevelyan (UK)

High in the central Brecon Beacons, not far from the quiet village of Heol Senni, a small disused quarry cuts into green-grey sandstone, not far from the quiet village of Heol Senni, lies a small disused quarry cut into green-grey sandstone. To most passers-by, it is an unassuming feature – a slice of rock along the hillside with ruined quarry buildings, streaked with moss and fern. Yet, within those thin beds of rock, lies one of the most eloquent records of life’s first steps onto land: a fossil flora that captures the dawn of terrestrial ecosystems, and a single, irreplaceable fossil fish that swam among the earliest green riverbanks.

This is Heol Senni Quarry, type locality of the jawless fish Althaspis senniensis and one of the most important Welsh localities for the Early Devonian plant Gosslingia breconensis. The quarry’s significance lies not only in what it contains, but in what it represents, that is, a world poised between barren continents and the first true landscapes of life.

A quarry in the Old Red Sandstone

The Heol Senni Quarry exposes the Senni Formation, a sequence within the Lower Old Red Sandstone. The rocks here were laid down about 410 million years ago, during the Pragian-Emsian stages of the Early Devonian. At that time, what is now Wales lay close to the equator, part of the Old Red Sandstone continent (Laurussia) – a hot, semi-arid land crossed by seasonal rivers and ephemeral floodplains. Rocks formed in this environment can still be seen in many parts of rocks that can be found in many places across the UK, including the distinctive red rocks of the south Devon coast.

The Senni Formation is made up of greenish-grey sandstones and siltstones, often displaying ripple marks, desiccation cracks and soil-like horizons. These features show that the sediments were deposited by rivers that migrated across broad plains. Some channels were shallow and short-lived, while others persisted long enough to cut terraces and lay down metre-thick packages of cross-bedded sand. The colour, a subdued olive-green, comes from reduced iron minerals formed under low-oxygen conditions, a sign that some parts of the floodplain were periodically waterlogged.

The source of the Senni sediments

The sediments that make up the Senni Formation were not formed locally but were carried into the basin from distant uplands. During the late Silurian and earliest Devonian, the collision of continents closed the ancient Iapetus Ocean and raised the Caledonian mountain belt across what is now Scotland, Scandinavia and parts of North America. These mountains, probably comparable in scale to the great mountain belts of today when first formed, began to erode rapidly under the action of rivers and seasonal storms. Vast quantities of sand, silt and mineral fragments were transported southwards across the Old Red Sandstone continent.

Among this debris were flakes of mica derived from metamorphic rocks deep within the former mountain roots. Because mica splits into thin plates, it travels easily in suspension and settles only in very quiet water. The glittering flecks visible in the Senni siltstones therefore represent tiny fragments of those long-vanished Caledonian mountains, carried by Devonian rivers and deposited on floodplains where early plants such as Gosslingia grew.

And it was in this restless setting of rivers, levees and muddy flats that both the plant Gosslingia breconensis and the fish Althaspis senniensis lived and died.

The plant: Gosslingia breconensis – discovery and description

Gosslingia breconensis was described in detail by Dianne Edwards in a landmark paper in the Philosophical Transactions of the Royal Society (1970) (Fig. 1). Her work at several Welsh localities, including Heol Senni and the nearby Storey Arms (Craig-y-Fro) Quarry, helped illuminate one of the most critical evolutionary transitions in Earth history – the greening of the continents.

The fossils occur as flattened, pyritised axes, preserved in fine siltstone. Many are branched in a characteristic pattern – a forking in which one branch continues the main line of growth while the other diverges slightly. This pseudo-dichotomous branching (that is, it appears to be divided into two equal parts but is not actually a true dichotomy) is a hallmark of Gosslingia (Fig. 2). The axes bear terminal sporangia, which are small, pear-shaped capsules where spores formed. Under magnification, their walls sometimes show a line of weakness indicating where they split open to release their spores.

Structure and significance

Internally, Gosslingia was more complex than earlier land plants like Cooksonia (see Fig. 3 below).

It contained a central strand of conducting tissue, a primitive vascular system capable of transporting water and nutrients. Although it lacked true roots and leaves, this conducting tissue represented a major step towards the sophisticated vascular networks of later plants. Its sporangia also display a more organised attachment pattern, suggesting the beginnings of differentiation between sterile and fertile parts of the shoot, a developmental innovation that would later lead to specialised organs.

In short, Gosslingia breconensis sits at a pivotal point in evolution. It is more advanced than the earliest leafless rhyniophytes, yet still far simpler than the trimerophytes and lycophytes that followed. It is among the earliest plants that can truly be called vascular.

The floodplain pioneer

Equally important is where Gosslingia grew. The Senni Beds were not swampy deltas but semi-arid floodplains. Between floods, the mud dried and cracked. When rains returned, the water cut shallow channels that filled and emptied again and again. Gosslingia thrived in this unstable environment, on levees and abandoned channels, where the soil was damp but not permanently wet.

To survive, such plants needed adaptations for water conservation: a waxy cuticle, stomata for controlled gas exchange, and internal tissues strong enough to resist collapse when water was scarce. The presence of Gosslingia at Heol Senni shows that by the Early Devonian, plants had already evolved these key features. They were no longer tied to continuously wet habitats – they had truly colonised the land.

Preservation in pyrite

One of the remarkable aspects of Gosslingia from Heol Senni is the mode of preservation. The plant axes are often replaced or coated with fine crystals of pyrite (iron sulphide). This type of fossilisation, called pyritisation, requires anoxic (oxygen-poor) conditions and sulphate-rich porewaters, that is, conditions that probably developed after the plants were buried by flood sediments.

The result is astonishingly detailed fossils, in which cellular patterns and delicate branching are retained. Later work by Edwards and Kenrick (1986) on pyritised material showed the cellular organisation of the xylem and cortex in exquisite detail – evidence not only of anatomy but of the geochemical pathways that created such fossils.

The fish: Althaspis senniensis – a single specimen

If Gosslingia represents life’s conquest of the land, Althaspis senniensis represents vertebrates’ first real foothold in fresh water. Described by R S Miles in 1968, this small armoured fish is known from just one specimen, found at Heol Senni Quarry and nowhere else. That uniqueness makes the site a geological treasure.

The fossil was preserved almost complete, lying within a fine, laminated siltstone bed – the infill of an abandoned channel or flood pool. Its bony plates remained articulated, suggesting the fish was buried where it died, and not transported far by currents.

Anatomy and lifestyle

Althaspis was a heterostracan, one of the jawless fishes that dominated Devonian seas and rivers. It was perhaps 15-20cm long, with a flattened head shield and a tapering tail. The front of the body was encased in bony armour; the rest was soft. Without jaws, it probably fed by filtering or suction, drawing in small particles from the sediment. Fig. 5 represents a speculative representation of how it may have looked.

Most heterostracans were marine or marginal marine, but Althaspis senniensis lived in fresh water. Its presence in the Senni Beds proves that vertebrates had already begun to exploit inland habitats, a change that paralleled the spread of vegetation on land. The fish and the plants were, in ecological terms, pioneers of the same transformation.

Death in the flood

The manner of preservation suggests a short, violent event. A sudden flood may have swept through a shallow pool, burying the fish in a blanket of silt. The same flood carried stems of Gosslingia and other plants from the riverbanks. When the water receded, a layer of mud sealed them both providing a snapshot of life and death on an early Devonian floodplain.

A growing community: other plants

Heol Senni is not just the story of one plant and one fish. The rocks preserve traces of a developing ecosystem.

Alongside Gosslingia breconensis, the quarry and its surrounding exposures have yielded remains of other early vascular plants such as Drepanophycus, Zosterophyllum, Uskiella, Tarella and Sennicaulis. Together, these represent several lineages – rhyniophytes, zosterophylls and primitive lycophytes – all experimenting with different ways of building and reproducing on land.

In addition, the siltstones are rich in miospores, the microscopic pollen-like spores that form the backbone of Devonian biostratigraphy. Species of Emphanisporites and Apiculiretusispora among others confirm the Pragian-Emsian age of the beds and provide insight into the vegetation beyond the immediate site.

The diversity of both macro- and microflora shows that the Heol Senni environment was not barren scrub but a patchwork of low vegetation, perhaps forming the earliest analogue of a riparian community.

Sedimentary traces

The sediments themselves tell part of the story. Small-scale ripple marks show the direction of currents, and desiccation cracks show drying between floods. There are thin horizons of red and purple mudstone – palaeosols – which represent ancient soils forming under oxidising conditions. The alternation of green and red beds records the waxing and waning of water across the floodplain.

In some layers, delicate plant fragments are scattered through cross-bedded sand, evidence that floods ripped vegetation from the banks. In quieter lenses, complete stems lie parallel, suggesting gentle settling in still water. It is in one of these calm layers that Althaspis was found.

A slab of mica and plant debris

Among the most instructive rocks at Heol Senni are slabs of finely laminated siltstone in which tiny black flecks lie scattered among glittering mica flakes. At first glance these beds appear unremarkable: thin grey layers that break into flat plates and sparkle faintly in the light, sometimes with black specks of organic matter (Fig. 6), lying between the massive slabs of sandstone of the Senni Formation that dominate the quarry (Fig. 7). Yet they capture, in miniature, the working of an Early Devonian floodplain.

The mica flakes themselves are clues to a far larger geological story. Mica is a sheet-like mineral formed deep within the Earth during the metamorphism of older rocks. During the late Silurian and early Devonian, the collision of continents that closed the Iapetus Ocean created the great Caledonian mountain belt. As these mountains rose and began to erode, rivers carried vast quantities of sand, silt and mineral fragments southwards into the Old Red Sandstone basin. The glittering flakes in the Senni beds are therefore fragments of those vanished mountains, transported hundreds of kilometres by Devonian rivers before settling into quiet water.

The black patches scattered through the laminae tell a second story. These are fragments of early land plants, washed from the riverbanks and trapped in shallow floodplain ponds. Plants such as Gosslingia, Zosterophyllum and their relatives were small and delicate, often only a few centimetres tall. During floods they were easily broken and carried away as plant litter. When the water slowed, fine silt settled together with these fragments, preserving them as thin carbon films within the sediment.

The structure of these slabs reflects the settling of different particles from suspension. Sand grains dropped out first as floodwaters slowed. Finer silt followed, forming the thin laminae. The mica flakes, light and plate-shaped, drifted longest in the water column before settling gently onto the surface of the mud. Between them lie the scattered fragments of plants that once grew along the Devonian riverbanks.

Such slabs therefore record a single quiet episode in the life of the floodplain. A flood had carried sediment and plant debris into a shallow pool or abandoned channel. As the water stilled, the sediment settled layer by layer, sealing the organic fragments beneath a thin blanket of silt. In time, the mud hardened to stone, preserving a snapshot of a landscape in which the first terrestrial vegetation was beginning to take hold.

Seen in this way, the mica-rich slabs are not merely background sediment. They are the physical record of the processes that shaped the earliest plant-dominated floodplains: erosion of distant mountains, transport by rivers, and the quiet settling of sediment in the pools where life’s first land plants grew.

Plants, fish and feedbacks

The coexistence of Gosslingia and Althaspis is more than a coincidence of fossilisation. It represents a biological feedback loop that reshaped the planet.

Stabilising the landscape: Before plants colonised the land, rivers were braided and unstable, their channels wandering freely across barren sediment. With the arrival of rooted vegetation – even the simple rhizoids of Gosslingia and its kin – riverbanks began to stabilise. Plants trapped silt and organic matter, building up levees and floodplains. The result was the first meandering river systems, a new kind of fluvial architecture that persists today.

Creating freshwater habitats: As rivers became more stable, they began to support permanent pools and channels, ideal for early freshwater fish. In this way, the spread of plants indirectly opened new ecological niches for vertebrates like Althaspis.

In turn, the fish contributed to nutrient cycling within these systems, their decay and waste enriching the sediments, which was a small but significant step toward self-sustaining terrestrial ecosystems.

Changing the atmosphere: On a global scale, the colonisation of land by plants like Gosslingia altered the carbon cycle. Rooting and weathering drew down atmospheric carbon dioxide, while increased organic burial began to raise oxygen levels. The green floodplains of the Early Devonian helped set the stage for later evolutionary radiations, including insects, forests and eventually terrestrial vertebrates.

Reading the rocks

Standing in Heol Senni Quarry today, it is not difficult to see traces of that ancient world. The sandstone beds dip gently into the hillside, their surfaces etched with ripple marks and faint impressions of stems. In the quarry wall itself the story can be read on a larger scale: thick sandstone beds form resistant ledges (Fig. 9), while thinner siltstone layers weather back into recesses, revealing the alternating river-channel sands and quieter floodplain deposits that built up the Senni Formation. Tiny pyritic flecks sparkle where Gosslingia once stood upright. Each layer, only a few millimetres thick, is a record of one brief pulse of sediment – a single flood, perhaps, or a quiet summer of dust and still water.

The fossil of Althaspis senniensis is obviously now safely in a museum collection. Even so, to stand where it was found is to stand at a turning point in natural history – where water first met land in a biological sense, and where vertebrates and plants began to shape each other’s worlds.

Other fossils of note

Although Althaspis senniensis remains unique to this quarry, the broader Senni Formation of South Wales has yielded a modest fauna. Rare remains of other early jawless fish – such as Rhinopteraspis dunensis (Fig. 10) – occur in nearby outcrops, as do trace fossils of invertebrate burrows and resting marks. None are abundant, but together they hint at a sparse, low-diversity biota characteristic of early non-marine environments.

The plants are far more common and diverse. Fossil fragments of Sennicaulis hippocrepiformis, Tarella trowenii and Uskiella spargens (Fig. 11) have been reported from similar beds in the region. These species, although modest in appearance, represent a wide evolutionary experiment in plant architecture with creeping, branching and upright forms all testing how best to live on land.

Heol Senni in context

The Early Devonian of Wales has produced several internationally important fossil localities, each illustrating a different part of the emerging terrestrial ecosystem. The Storey Arms (Craig-y-Fro) Quarry, not far from Heol Senni, yielded some of the best Gosslingia material studied by Edwards. The Llanover Quarry, near Abergavenny, is renowned for its Pragian flora, including Cooksonia (Fig. 12) and other rhyniophytes.

However, Heol Senni is distinctive because it brings together both plant and vertebrate evidence in one stratigraphic horizon. It is both a palaeobotanical and palaeontological type locality – a rare overlap of two Geological Conservation Review (GCR) categories: Palaeozoic Palaeobotany and Fossil Fishes of Great Britain.

A window into an evolving world

To appreciate Heol Senni, one has to imagine a landscape stripped of all familiar vegetation. There were no trees, no flowers, no grass. The tallest plants barely reached 20cm in height. Yet from such beginnings came the soils and forests of later ages.

The floodplains of the Senni Beds were experiment stations of evolution. Plants were learning how to pump water and stand upright; fish were learning to breathe and feed in fresh water. Both were finding ways to live in a world that changed from wet to dry and back again.

Their brief coexistence in this one quarry captures a moment when life’s frontier moved inland – when the biological transformation of the continents truly began.

The quarry itself

Although the geological story of Heol Senni stretches back more than 400 million years, the quarry itself is a comparatively recent feature of the landscape. The sandstone exposed here belongs to the Senni Formation of the Lower Old Red Sandstone, a rock that has long been valued locally as a building material. The quarry was probably first opened during the nineteenth century, when many small rural quarries supplied stone for nearby farms, walls and bridges. The Senni sandstones are hard and durable, although often interbedded with finer siltstones, making them suitable mainly for rubble masonry and local construction. The surviving structures suggest that the quarry later experienced a brief period of mechanised working (Fig. 14). The large concrete buildings below the quarry face appear to be foundations for crushing machinery installed during the twentieth century. Such plants were commonly used to break sandstone into graded aggregate for road construction and other civil engineering works. The heavy poured concrete blocks would have supported vibrating crushers and screens, while the open bays probably served as loading areas where crushed stone could be collected. The quarry face itself lies some distance upslope from these buildings, indicating that rock was extracted above and then transported downhill for processing. This arrangement allowed gravity to assist in moving the stone, reducing the effort required to feed the crusher. Despite this mechanisation, the quarry was never a large industrial operation. Access to the site is along a narrow track from the Senni valley, suggesting that production was limited and probably served local needs rather than distant markets (Fig. 15). Once improved transport allowed cheaper aggregate to be brought in from larger quarries, the site fell out of use. Today, the quarry’s industrial phase has faded into ruin. Moss grows over the concrete foundations and the rusted ironwork of the machinery has long since vanished. What remains is a quiet hillside exposure whose true significance lies not in the stone that was once extracted, but in the ancient plants and fish preserved within it.

Visiting the site

Although the Heol Senni Quarry is disused and not formally open to the public, it remains a point of geological interest. The beds are easily visible from the surrounding area, and similar exposures occur throughout the central Brecon Beacons. Visitors are reminded that the site is protected under the Geological Conservation Review; collecting is not permitted (and the quarry should therefore be afforded the utmost respect).

A visit, however, needs no hammer. Simply tracing the bedding planes with your eye reveals the structures of ancient rivers, the ripples of vanished currents, and perhaps the shimmer of a pyritised stem. Knowing the story of Gosslingia and Althaspis turns the quarry from a patch of rock into a window on deep time.

Final thoughts

Heol Senni Quarry, modest and half-hidden among the hills, records one of the pivotal transitions in the history of life. In its thin layers of sandstone lie the remains of plants that learned to live on dry land and of a fish that ventured into fresh water. Together they tell a story not of sudden transformation, but of slow mutual shaping – of rivers that grew quieter under the first greenery, and of animals that followed the water inland.

The fossils of Heol Senni speak softly, in pyrite and stone, but what they say is immense: that the world we know – with forests, soils and freshwater life – began on floodplains like these, in ancient Wales more than 400 million years ago.

Further reading

Edwards, D. (1970). Further observations on the Lower Devonian plant Gosslingia breconensis Heard. Philosophical Transactions of the Royal Society B 258: 225–243.

Edwards, D. and Kenrick, P. (1986). The anatomy of Lower Devonian Gosslingia breconensis based on pyritized axes. Botanical Journal of the Linnean Society 93: 283–308.

Friend, P.F. (1979). The Old Red Sandstone basins of Wales and the Welsh Borderland. In The Devonian System of the British Isles (Geological Society Special Report).

Gibling, M. (2021). River Planet: Rivers from Deep Time to the Modern Crisis, Dunedin Academic Press, Edinburgh, 222 pages (hardback), ISBN: 978-1-7804609-9-4

JNCC Geological Conservation Review. Heol Senni Quarry. In Fossil Fishes of Great Britain and Palaeozoic Palaeobotany of Great Britain volumes.

Miles, R.S. (1968). The Old Red Sandstone Fishes of the Welsh Borderland. Palaeontographical Society Monograph.

National Museum Wales Field Guides: Lower Old Red Sandstone Plants of South Wales.