Author: Carl Hewlett

In the rural landscape of South Gloucestershire, near the village of its namesake, lies Cromhall Quarry. At first glance, it presents a scene of dramatic, post-industrial beauty: sheer rock faces plunge into the still, crystalline waters of a deep lake, its margins softened by regenerating woodland. It is a place of quiet recreation, where swimmers glide across the surface and divers explore the silent depths. Yet, this tranquil veneer conceals a history of staggering depth and complexity. This is not merely a flooded quarry; it is a place where multiple timelines converge with profound geological and ecological significance.

The chiselled walls of Cromhall are the pages of a stone library, recording not one world, but two. The first is a tale of ancient, tropical seas that existed 350 million years ago. The second, hidden within the first, is a lost world of subtropical islands almost 100 million years later, a terrestrial ecosystem teeming with bizarre reptiles, early dinosaurs, and the ancestors of modern lizards, all preserved with astonishing fidelity. The quarry’s very existence is a paradox: an act of industrial extraction, driven by the needs of the Victorian era, that inadvertently sliced open these prehistoric time capsules, revealing secrets that would rewrite the story of life on Earth.

Today, nature has reclaimed this man-made landscape, forging a new and vibrant ecosystem. The quarry’s artificial cliffs have become a fortress for birds of prey, its clear waters a sanctuary for protected amphibians, and its surrounding woods an echo of the ancient Royal Chase that once covered this land. It has become a microcosm of British natural history, a story of ancient seas, prehistoric life, industrial transformation, and ecological rebirth.

From Carboniferous Seas to Industrial Scars: The Geological Canvas

The story of Cromhall Quarry begins not with the first swing of a pickaxe in the 19th century, but in the depths of the Carboniferous Period, some 350 million years ago. At that time, the land that would become South Gloucestershire lay near the equator, submerged beneath a warm, shallow sea. Over millions of years, the skeletal remains of countless marine creatures (corals, brachiopods, and crinoids) accumulated on the seabed, compacting under their own weight to form the thick layers of what is now known as the Carboniferous Limestone Series.¹

The rock exposures in the quarry today reveal a dynamic past, a record of fluctuating sea levels and shifting environments. The strata of the Hotwells Limestone Group, for which Cromhall is a key locality, show a progression from the open shelf seas that formed the limestone itself, to shallower, marginal conditions that laid down layers of mudstone, siltstone, and sandstone.³ Within these layers, palaeobotanists have discovered fossilised rootlets in ancient soil beds known as seat-earths, tangible evidence that as the seas retreated, these primeval mudflats were colonised by early vegetation, including the giant clubmoss Lepidostrobus.³

For over one hundred million years, these layers of rock lay buried. Then, during the Triassic Period, around 210 million years ago, the world was a vastly different place. The supercontinent of Pangaea was beginning to break apart, and the area around Bristol was a subtropical archipelago of islands set in a shallow sea.⁵ The Carboniferous limestone that had formed on the ancient seabed now constituted the bedrock of these islands. Over millennia, acidic rainwater dissolved the limestone, creating a classic karst landscape riddled with a complex network of caves, deep fissures, and vertical cracks.²

It was these fissures that would become Cromhall’s greatest secret. As animals lived and died on these Triassic islands, their remains (bones, teeth, and entire skeletons) were washed into this subterranean network. The crevices acted as natural pitfall traps, accumulating a dense and diverse collection of the local fauna, which was then sealed by reddish, iron-rich sediments known as the Magnesian Conglomerate.⁴ This process created a series of extraordinary geological time capsules, preserving a complete terrestrial ecosystem within the fabric of a much older marine one. The quarry is, therefore, a physical cross-section through these two distinct epochs, a place where the fossils of 350-million-year-old sea creatures can be found in the bedrock that encases the 210-million-year-old remains of land-dwelling reptiles.

This astonishing window into the past remained sealed until 1840, when quarrying began at Cromhall.⁴ The target was not the fossil-rich fissures, which were then unknown, but a specific formation known as the Upper Cromhall Sandstone. This brightly coloured red to yellow quartzitic sandstone was a valuable resource for construction, and for over 150 years, the quarry was worked for this economic purpose.³ It is a profound irony that the industrial activity that scarred the landscape was the very process that revealed its priceless scientific treasures. Without the relentless excavation of the sandstone and limestone, the Triassic fissure fills would have remained undiscovered, and a crucial chapter in the history of life would have remained unread.

A Window to a Lost World: The Triassic Fossil Treasury

The unassuming quarry at Cromhall, also known as Slickstones Quarry, is today recognised by palaeontologists as a site of global significance.⁸ Since the first fossils were identified here in 1938, it has yielded an astonishingly diverse fauna from the Late Triassic Period, providing one of the most detailed snapshots of terrestrial life during the pivotal era when the first dinosaurs were beginning their ascent.¹

The Fissure Fill Phenomenon: A Prehistoric Time Capsule

The exceptional nature of Cromhall’s fossil record lies in its unique mode of preservation. Unlike traditional fossil beds where remains are found in sequential sedimentary layers, the treasures of Cromhall are locked within the “fissure fills” that penetrate the older Carboniferous limestone.⁶ These ancient cave systems acted as perfect traps, accumulating and protecting the remains of the small animals that inhabited the Triassic islands some 210 million years ago.¹

The compact nature of these fissures dictated the type of fossils preserved. This is not a site for the colossal skeletons of sauropods; indeed, many of the fossils are microscopic, requiring meticulous laboratory work to identify.¹ However, what the fossils lack in size, they make up for in abundance, diversity, and often exquisite preservation, offering a high-resolution view of a miniaturised island ecosystem.¹¹ The quarry has produced over thirty different species, including a rich menagerie of reptiles, dinosaurs, and early mammal relatives, some of which are found nowhere else on Earth.¹

Meet the Inhabitants: A Menagerie of Triassic Reptiles

The fauna of Triassic Cromhall was dominated by a remarkable variety of reptiles, showcasing a period of intense evolutionary experimentation. The most common fossils belong to the sphenodontids, a group of reptiles whose only living relative is the Tuara of New Zealand. Multiple species have been found, with the genera Clevosaurus and Planocephalosaurus being particularly abundant, providing a wealth of information on the anatomy and evolution of this ancient lineage.¹

Perhaps the most spectacular of the Cromhall reptiles is Kuehneosaurus, a creature that possessed elongated ribs forming a wing-like structure, allowing it to glide between the trees of its island home.¹ Its discovery highlights the diverse ways in which reptiles adapted to different ecological niches long before the reign of the dinosaurs was fully established.

The site’s importance is continually reinforced by new discoveries derived from specimens collected decades ago, a testament to the painstaking work of palaeontologists and the power of new analytical techniques. Two recent identifications have fundamentally altered our understanding of reptile evolution:

Threordatoth chasmatos

Described in 2024, this small, lizard-like creature was one of the last surviving members of a group called the procolophonids.⁵ Resembling a modern horned lizard with bony spikes on its head,
Threordatoth possessed a highly unusual dental arrangement. Its name, derived from Old English, means “three-pointed teeth,” a reference to the unique tricuspid teeth in its jaw.¹¹ This, combined with a flexible jaw structure where the two halves were likely connected by ligaments, suggests a highly specialised diet, perhaps for processing tough plants or insects in the harsh, high-carbon-dioxide environment of the Late Triassic.⁵ Another related procolophonid with similarly unusual teeth, Hwiccewyrm, has also been identified from the quarry.⁴

Cryptovaranoides microlanius

The most significant discovery from Cromhall to date came from a fossil originally unearthed in 1953 but only fully analysed and identified in the 2020s.⁴ This tiny specimen has been confirmed as the world’s oldest known modern-type lizard. This single fossil pushes back the evolutionary origins of all lizards and snakes (the group known as Squamata) by a staggering 35 million years, placing their emergence deep in the Triassic Period.¹⁵ This discovery fundamentally reshapes the evolutionary tree of modern reptiles and cements Cromhall’s status as a site of world-changing palaeontological finds.

The faunal list extends even further, including the oldest pterosaur (flying reptile) remains found in Britain, a terrestrial crocodile relative named Terrestrisuchus, and enigmatic, long-necked drepanosaurids, painting a picture of a complex and crowded reptilian community.¹

Species Name	Classification	Scientific Importance
Cryptovaranoides microlanius	Squamata (Lizard)	Confirmed as the world’s oldest known modern-type lizard, pushing back the origins of the group by 35 million years.15
Agnosphitys cromhallensis	Dinosauria (Dinosaur)	A small, carnivorous dinosaur named after the village of Cromhall, highlighting the site’s local and scientific identity.1
Kuehneosaurus	Kuehneosauridae (Gliding Reptile)	A remarkable example of a Triassic gliding reptile, showcasing the diversity of reptile forms during this period.1
Threordatoth chasmatos	Procolophonidae (Parareptile)	A recently discovered, late-surviving procolophonid with unique three-pointed teeth, indicating a specialised ecological niche.5
Clevosaurus	Sphenodontia (Rhynchocephalian)	One of the most abundant fossil reptiles found at the site, providing rich data on this group of Tuara relatives.1
Unnamed Pterosaur	Pterosauria (Flying Reptile)	Represents the oldest known pterosaur remains discovered in Britain.1

More Than Just Reptiles: Dinosaurs, Mammals, and Derived Sharks

While reptiles dominate the fossil record, Cromhall has also provided crucial evidence of other animal groups. The site has yielded remains of a theropod dinosaur, and most notably, a small carnivorous dinosaur was named Agnosphitys cromhallensis in honour of the village.¹ Far from a terrifying predator, Agnosphitys was a diminutive creature, measuring only about two feet in length and likely feeding on insects and other small prey.¹ Its presence demonstrates that even as the first dinosaurs appeared, they occupied a wide range of ecological roles, many of them far from the apex of the food chain.

The fissures also contain the tiny teeth and jaw fragments of early mammaliamorphs, or mammal relatives. These finds are part of a wider regional fossil fauna that includes landmark genera such as Morganucodon and Kuehneotherium, which are vital to understanding the transition from cynodont reptiles to true mammals.⁶

In a fascinating geological twist, the Triassic deposits are also seasoned with much older fossils. Researchers have identified teeth from Carboniferous sharks, some 350 million years old, mixed in with the 210-million-year-old reptile bones.² These are “derived fossils,” weathered out of the surrounding limestone walls of the fissures by erosion and redeposited alongside the Triassic remains. This discovery serves as a geological detective story. It provides direct physical evidence of the erosion process that formed the fissures and simultaneously confirms the fully terrestrial nature of the Triassic ecosystem preserved within them, as these ancient shark teeth are the only marine fossils present.²

Taken together, the fossils of Cromhall do more than just list species. They allow for the detailed reconstruction of a complete, miniaturised island ecosystem. The diversity of forms; insect-eating dinosaurs, gliding reptiles, specialised plant-eaters like Threordatoth, and abundant sphenodontids, allows scientists to piece together food webs and understand how different species partitioned resources in a constrained environment. Furthermore, the fact that these animals thrived in a hot, high-CO₂ world provides a valuable prehistoric analogue for researchers studying how modern ecosystems might respond to anthropogenic climate change.⁵

Life on the Ledge and in The Lake: The Modern Wildlife of Cromhall

Long after the last Triassic reptile was entombed and the final truckload of stone was hauled away, a new chapter began at Cromhall Quarry. As human activity ceased, nature began the slow, patient process of reclamation. The stark, industrial landscape, a void carved from the earth, has transformed into a mosaic of novel habitats, attracting a unique assemblage of species.

Today, the quarry is a perfect case study in ecological succession on a post-industrial site, a place where the ‘blank canvas’ left by industry has been colonised by wildlife, creating an ecosystem of significant conservation value.

The Apex Predator’s Return: Peregrine Falcons of the Quarry Face

The sheer, vertical rock faces left by decades of blasting are an inhospitable environment for most species, but for the Peregrine Falcon (Falco peregrinus), they are a fortress. Quarries across the UK have become crucial habitats for these magnificent birds of prey, providing the high, inaccessible ledges they favour for nesting.¹⁸ These artificial cliffs mimic the sea cliffs and mountain crags that are their natural nesting sites, offering superb protection from ground predators like foxes.²⁰

From these high vantage points, peregrines can survey the surrounding countryside, launching into their famous high-speed stoops to hunt pigeons and other medium-sized birds over the open fields surrounding the quarry.²¹ The combination of secure nesting sites and ample prey makes quarries like Cromhall prime real estate for these apex predators. The presence of peregrines, often accompanied by the deep, cronking calls of Ravens (Corvus corax) which also favour quarry faces, is a powerful symbol of nature’s ability to adapt and thrive in landscapes dramatically altered by human hands.¹⁸

An Amphibian Haven: The Protected Great Crested Newt

While the cliffs host avian predators, the flooded depths of the quarry provide a sanctuary for a much smaller, but no less significant, resident: the Great Crested Newt (Triturus cristatus). When quarrying operations ended in 1998, the vast pit naturally filled with groundwater, creating a deep, clear lake.⁴ The water, filtered through the surrounding limestone, is of exceptional quality, creating ideal conditions for aquatic life.²⁵

This artificial lake has become a vital habitat for the Great Crested Newt, a species given strict protection under both UK and European law.²⁷ Its presence is so important that it forms one of the two pillars of the quarry’s designation as a Site of Special Scientific Interest (SSSI), alongside the geological treasures.⁴ For this amphibian, the quarry provides the perfect combination of deep, open water for foraging and terrestrial habitat around the margins for hibernation and shelter. Divers and open water swimmers at The Lake are sometimes treated to the rare sight of these striking amphibians, often called “warty newts,” moving gracefully through the underwater world.²⁸ The thriving population at Cromhall underscores the immense, and often unexpected, conservation value that post-industrial sites can acquire.

Waterside Predators: Herons, Egrets, and Cormorants

The Lake’s clear waters and healthy ecosystem also attract a variety of fish-eating birds. Grey Herons, patient hunters of the water’s edge, have been spotted at similar quarry sites in the region.²⁶ The striking white plumage of egrets, now an increasingly common sight in the wider area, can also be seen against the dark quarry walls.²⁹ The deep water is a perfect hunting ground for the Cormorant, an expert diving bird that pursues its prey underwater.³⁰ The regular presence of these piscivorous birds at The Lake is a strong indicator of a healthy fish population and adds another layer to the quarry’s rich biodiversity.

Echoes in the Woods: The Woodpeckers of Cromhall

The ecological revival of Cromhall is not confined to its cliffs and water. The land surrounding the quarry, once disturbed by industrial infrastructure, has gradually reverted to scrub and woodland, blending into a landscape that was historically part of a Royal Hunting Chase.³¹ This regenerating woodland provides a crucial habitat for several bird species, most notably woodpeckers.

South Gloucestershire is fortunate to host all three of Britain’s native woodpecker species.³² The most conspicuous is the Green Woodpecker (Picus viridis), whose loud, laughing “yaffle” call is a characteristic sound of the English countryside. Rather than drumming on trees, this species spends much of its time on the ground, probing lawns and parkland for its favourite food: ants.³⁴

The Great Spotted Woodpecker (Dendrocopos major) is the classic drummer, using its powerful bill to excavate nest holes in dead trees and to extract beetle larvae from beneath the bark.³⁵ The third, and much rarer, species is the Lesser Spotted Woodpecker (Dendrocopos minor). This sparrow-sized bird is a species of high conservation concern, and its survival depends on extensive areas of mature, deciduous woodland with plenty of standing deadwood.³⁶

While the Green Woodpecker is often seen among the grasses close to the treeline at Cromhall Quarry, bird reports from the immediate area confirm the presence of Great Spotted Woodpeckers too.³⁸ Crucially, the nearby Lower Woods Nature Reserve, one of the largest ancient woodlands in the region, is recognised as a key site for the elusive Lesser Spotted Woodpecker.³⁶ This proximity suggests that the regenerating habitats around Cromhall Quarry could play a valuable role as foraging territory for these birds, demonstrating that the quarry does not exist in isolation.

A Connected Landscape: The Severn Flyway and Local Habitats

The ecological value of Cromhall Quarry is magnified by its strategic position within a wider network of habitats. The quarry lies just a few miles from the Severn Estuary, a vast wetland of international importance, designated as a Ramsar site and a Special Protection Area for its birdlife.⁶³ This estuary is a vital migration route, a crucial ‘rest-stop’ and feeding station for tens of thousands of birds that travel from as far as Siberia, Russia, and Greenland to spend the winter in the UK’s milder climate.⁶⁴

A cornerstone of this avian superhighway is the world-renowned WWT Slimbridge Wetland Centre, located just 9.5 miles from the quarry.⁶⁷ Slimbridge is famous for the spectacular flocks of wintering wildfowl it attracts, including thousands of Bewick’s swans, white-fronted geese, wigeon, pintail, and numerous species of wader.⁶⁸ Given that birds are highly mobile, the deep, open water of Cromhall Quarry provides an attractive satellite habitat within this busy landscape. It almost certainly serves as a valuable resting and foraging site for birds associated with the wider estuary and Slimbridge, contributing to the diversity of waterfowl that visit the quarry throughout the year.

This connection to the wider landscape is also evident in other species. Red Kites, once persecuted to near-extinction, are now a common sight, soaring over the quarry with their distinctive forked tails.³⁸ Bats, too, find the combination of woodland, open water, and sheltered cliff faces an ideal environment for both roosting and hunting insects.

This local biodiversity has been significantly enhanced by the creation of the nearby Cromhall Wetland. This innovative project, managed by Wessex Water, uses a series of constructed ponds and marshes to naturally filter water from the local recycling centre before it enters the Tortworth Brook.⁴⁰ An ecological survey of this new wetland revealed a dramatic 111% increase in biodiversity value, having attracted over 22 species of birds including two red-listed species of high conservation concern and six different species of bat in a single night.⁴² This neighbouring habitat acts as a rich source of wildlife that can utilise the resources of the quarry, further cementing Cromhall’s role as a cornerstone of a thriving landscape-scale ecological network.

The Human Element: A Site of Industry, Recreation, and Conservation

The story of Cromhall Quarry is inextricably linked with human activity. Its very form is a testament to the industrial might of the 19th and 20th centuries, and its modern identity has been shaped by closure, reinvention, and the complex challenge of balancing competing human interests. The site itself is part of the historic Tortworth Estate, which encompasses quarrying activities in the area, and is now leased and managed by South West Maritime Academy.⁴⁴ This relationship underpins its current use, as it has journeyed from a place of pure extraction to a multi-faceted asset for recreation, education, and conservation, all while navigating the legal and ethical responsibilities of its protected status.

A History of Extraction: From Victorian Quarrying to Modern Reserves

The industrial life of Cromhall Quarry spanned more than 150 years. From its opening in 1840 until operations ceased in 1998, it was a site of labour and production, yielding vast quantities of sandstone and limestone for construction across the region.⁴ However, the cessation of work in 1998 did not mark the end of its industrial potential.

Behind the sheer rock walls lie significant permitted reserves of limestone.⁴⁸ The quarry is officially considered “mothballed,” not permanently closed and the Tortworth Estate, who own the site, have indicated they intend to recommence working of the reserves contained there.⁴⁸ South Gloucestershire’s Local Plan acknowledges that as other active quarries in the region approach the end of their permitted reserves, expected between 2032 and 2040, Cromhall Quarry may be called upon to play a future role in maintaining the region’s supply of aggregates.⁴⁸

The Lake: A New Era of Diving, Swimming, and Maritime Safety Training

Following its closure in the late 1990s, the quarry was reborn. The deep pit, filled with naturally filtered groundwater, transformed into a 2.7-hectare lake of remarkable clarity, with a maximum depth of around 17 metres.⁴ Its potential for recreation was quickly recognised, and for 14 years it was successfully run as the Cromhall Dive Centre by Simon Chen and Maggie Alger.⁴ It became a popular and accessible site for diver training, known for its easy beach-style entry and submerged attractions.⁵¹

In 2020, a combination of severe flooding from winter storms and the nationwide lockdown imposed by the COVID-19 pandemic forced the dive centre to close permanently.²⁴ However, the site was swiftly given a new lease of life. In July 2020, South West Maritime Academy (SWMA), a specialist provider of safety and survival training, took over the site, securing a lease that included the water body.⁴⁷ SWMA, which had previously used the site for training, was then able to establish it as its permanent base for a wide range of practical, hands-on courses.⁴

South West Maritime Academy utilises the unique environment of the quarry for a wide range of maritime safety training courses. The deep, open water is ideal for simulating sea survival scenarios, rescue boat operations, and underwater training, while the surrounding land is used for fire training and other emergency drills.⁴ Recognising the site’s value to the local community, SWMA also reopened the site on selected days for public recreational use on selected days and now operates as, ‘The Lake’.⁵³ It is once again a thriving hub for open-water swimming, scuba diving, and paddleboarding, operating on a flexible pay-as-you-go basis that makes it accessible to all.²⁵

A Delicate Balance: Managing a Site of Special Scientific Interest (SSSI)

First notified in 1966, the designation for ‘Slickstones Quarry’ (Ref: 1002898) provides national-level legal protection for its two most important features: its outstanding Triassic reptilian fossils and its significant population of Great Crested Newts.⁴

This designation necessitates a delicate and continuous balancing act. South West Maritime Academy facilitate commercial training and public recreation without compromising the very features that make the site scientifically and biologically precious. This stewardship is reflected in the The Lake’s operational rules. For example, divers are explicitly instructed to maintain a safe distance from the quarry walls and to avoid dislodging any rocks, a measure that directly protects the fossil-bearing geological exposures from damage or erosion.⁵⁰ Similarly, safety protocols for swimmers indirectly help to minimise disturbance to wildlife.⁶²

The current arrangement represents a form of symbiosis. The commercial activities of South West Maritime Academy provide the site with a constant human presence, security, and the funding required for its upkeep. This active management acts as a powerful form of stewardship, protecting the site from neglect, vandalism, or unauthorised and potentially damaging activities. In this way, the economic life of the quarry directly supports the preservation of its natural heritage, creating a functional, if complex, harmony between its competing identities.

Cromhall’s Legacy and Future: A Microcosm of Conservation

Cromhall Quarry is far more than a simple hole in the ground. It is a landscape of profoundly layered meanings, a place where the geological past, the industrial present, and the ecological future are in constant dialogue. It is a geological library holding the 350-million-year-old story of ancient seas; a prehistoric cemetery containing the unique remains of a lost island world; a regenerated ecosystem providing a sanctuary for protected species; and a valued community asset for recreation and specialist training.

Cromhall Quarry’s journey encapsulates a remarkable transformation. It began as an act of industrial consumption, a landscape sacrificed for its material resources. Through the serendipity of geology and the patient resilience of nature, it has become a site celebrated not for what can be taken from it, but for what it holds within: irreplaceable scientific knowledge and a vibrant, living biodiversity.

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Working here at The Lake, nestled within the Cromhall Quarry, we’re constantly reminded that we’re standing on a treasure trove of prehistoric secrets. Recently, another incredible discovery has been added to that list: a brand-new species of Triassic reptile!

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