Discover Bristol and Explore the Fossils and Geodiversity of Aust Cliff
- Wayne Munday
- 12 minutes ago
- 4 min read
Sip back and discover the outskirts of Bristol and explore the fossils and geodiversity of Aust Cliff located on the eastern bank of the River Severn framed by the Severn Bridge or Prince of Wales Bridge. This is one of the UK’s most significant Rhaetian fossil sites dating to between 208.5 - 201.3 million years ago known as the final stage of the Late Triassic Epoch and the Triassic Period directly preceding the Jurassic Period. Designated a Site of Special Scientific Interest (SSSI), Aust Cliff tells a story from over 200 million years ago when desert sands transformed into shallow seas teeming with fish and where marine reptiles such as ichthyosaurs and plesiosaurs patrolled the depths and early dinosaurs were washout to sea to be buried by sediment to be preserved as fossils.
Aust Cliff holds a near continuous geological record of rock layers from the Late Triassic to the Early Jurassic. These cliffs have been world-renowned since the early 1800’s for its Rhaetic Bone Bed a narrow but incredibly fossil rich layer marking the end of the Triassic Period. Aust Cliffs still today continue to yield fossil finds along the foreshore as the cliff is continuously being eroded.
The Severn Estuary and Bristol Channel are known for their powerful ebb tides driven by a combination of one of the world's highest tidal ranges up to 14 meters and the estuary's funnel-shaped geography. As the tide recedes, strong ebb currents flow seaward, dramatically influencing the local ecosystem. These tides stir up and transport large amounts of sediment, shaping intertidal mudflats and salt marshes that support diverse bird and marine life. They also affect salinity levels as freshwater from the River Severn mixes with incoming seawater.
Aust Cliff has been recognised as an interesting fossils site since the 1820s by geologists William Buckland and William Conybeare for exposing the Triassic–Jurassic boundary and its fossil-rich Rhaetian bone bed containing fish and marine reptile remains.
In the 1830s, Swiss naturalist Louis Agassiz studied fossil fish from Aust Cliff and identified 17 taxa, 8 of which remain valid today. His research was featured in key works like "Recherches sur les Poissons Fossiles" that highlighted the Rhaetian bone bed at Aust Cliff. Agassiz described genera like Ceratodus, Gyrolepis, Severnichthys, Sargodon, and Lepidotes living alongside both large reptiles and micro vertebrates. His pioneering work on fossil fish significantly advanced the understanding of Rhaetian marine life and remains influential in palaeontology.
Aust Cliff’s Triassic Rhaetian bone bed yields a wide range of fossils, including teeth and bones from fish and marine reptiles like ichthyosaurs and plesiosaurs, as well as vertebrae, jaw fragments, shark teeth, coprolites, and microfossils. Collectors frequently find bony fish remains, placoid scales, gill rakers, cephalopod hooklets, and maybe the occasional rare shark species such as Parascylloides turnerae along with fossil clams, oysters and reptile limb fragments.
Aust Cliff tells a story of when the desert met the sea. At the base of the cliff, you'll find the vivid, rust-red rocks of the Mercia Mudstone Group, laid down over 240 million years ago during the Late Triassic Period. These sedimentary layers were a vast, sun-scorched desert plain once crisscrossed by salt lakes a barren and inhospitable world where fossils are incredibly scarce.
Above the desert lies the soft green-grey mudstone of the Blue Anchor Formation, a crucial turning point in Aust Cliff’s geological record. These layers capture a time when arid landscapes gave way to tidal flats and saline lagoons, gradually succumbing to the encroaching sea. This transition reflects the immense climatic and sea-level shifts that reshaped the region during the Late Triassic.
Next comes the real highlight for fossil hunters and the Penarth Group, a formation from the Rhaetian Stage and includes the Westbury Formation and the Cotham Member, home to the famous Rhaetic Bone Bed. Here, in a thin but incredibly rich seam of sediment, lie the fossilised remains of prehistoric marine reptiles, fish, and other long-extinct creatures.
Crowning the top of Aust Cliff is the iconic Blue Lias Formation made up of alternating bands of limestone and shale deposited during the Early Jurassic Period. These rocks were formed in warm, shallow seas that once covered much of Southern England.
The fossil bed at Aust Cliff formed around 205 million years ago during a major marine transgression at the end of the Triassic Period. As sea levels rose, the area transitioned from desert to tidal flats and eventually to a shallow sea. This environmental shift led to the deposition of a concentrated bone bed, where remains of land and marine life including bones, teeth, scales, and coprolites were swept in, transported, and buried. Over time, these remains fossilised through mineral replacement. Today, Aust Cliff is a key site for understanding Late Triassic marine ecosystems and the transition into the Jurassic.
