The famous red and white cliffs that can be seen when crossing the River Severn contain a highly productive bone bed at the top from the Rhaetian Penarth series. This bed is full of teeth, and reptile and fish remains, and is the most productive Triassic site in the UK.
DIRECTIONS
♦ Aust is located on the eastern side of the Severn estuary, close to the eastern end of the Severn Bridge.
♦ Access to the cliffs from Junction 1 of the M48 is through a steel gate, over a stile onto a concrete causeway. Parking is on the B4461 Aust Wharf Road at Old Passage.
♦ Walk along Passage Road towards the bridge, until you get to the beach.
♦ Parking area: BS35 4BG (see Google Maps)
♦ What3Words: South side of bridge: ///obstruct.vets.outsiders
♦ What3Words: North side of bridge: ///wing.umbrellas.tame
PROFILE INFO
FIND FREQUENCY: ♦♦♦♦♦ – Fossils are regularly found at Aust and just a small amount of the famous bone bed can yield a high number of teeth, coprolites and bone fragments. And, even if you cannot find any of these blocks, there are plenty of Carboniferous molluscs to search for. The blocks of Aust bone bed are highly collected, so are usually quickly broken down by collectors. However, the broken down smaller pieces of bone bed can usually be broken down further.
CHILDREN: ♦♦♦♦ – Aust is suitable for children, provided they are supervised by an adult. Keep to the southern side of the bridge and do not walk round to the northern side. And keep away from the cliffs and mudflats.
ACCESS: ♦♦♦♦ – Aust is easy to access, providing that you stick to the southern side of the bridge. The northern side is much more difficult to access and should only be visited by experienced collectors, wearing appropriate footwear. There are no toilets near Aust.
TYPE: – Fossils at Aust are found on the foreshore from the beds at the top of the cliff. The rest of the cliff section is unfossiliferous, so you will need to search the fallen blocks on the foreshore and in the areas of shingle.
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FOSSIL HUNTING
There are cliffs at Aust both on the north and south side of the bridge. When you reach the foreshore, you are on the southern side, which is currently being washed out more than the northern side. To access the northern side, you will need to wait until the tide is low enough to walk around the concrete bridge support. Take extreme care if accessing this area, as it can be slippery with mudflats. Return before the tide turns to avoid being cut off.
Look out for shells, and loose bones, teeth and bone fragments in the areas of shingle. Occasionally, you can come across large lumps of the bone bed, although lumps of the bone bed are usually quite small because collectors have already split them down. You only need a small amount of the bed to get some finds. Common fossils are shark and fish teeth, and fish scales and coprolites. Fragments of bone are also very common and often are from ichthyosaurs and plesiosaurs, but identifiable bones are less common. You can also find fossil clams and oysters. However, it is a very popular location for collecting fossils, so competition means that the bone bed blocks are becoming harder to find. However, in recent times, the south side of the bridge is regularly washing out, with fresh cliff falls, giving rise to a good amount of material to search through.
The Red Marls are unfossilferous, so there is no point in searching within this layer. The bone bed comes from the top of the cliff at the base of the Lower Lias and is unreachable. Therefore, finding fossils at this location is dependent on cliff falls.
GEOLOGY
This Middle to Late Triassic and Early Jurassic coastal section lies downstream (south) of the Severn Bridge and is instantly recognisable for its bold colour contrast: red-brown mudstones low on the cliff, passing up into pale green-grey beds and then darker grey strata, before finishing with lighter, more limestone-rich beds at the top. Together, these layers capture a major environmental shift from arid continental mudflats, through marginal marine conditions, into fully marine seas at the start of the Jurassic.
At the base of the cliffs, the Mercia Mudstone Group is represented by the Branscombe Mudstone Formation(around 221–206 million years old). These are typically brick-red to reddish-brown mudstones and siltstones, often weathering into steep faces and producing abundant slumped debris. The red colour reflects oxidation during deposition in hot, generally dry conditions on broad coastal plains and sabkha-like mudflats. In the field, the Branscombe Mudstone is usually massive and blocky, with subtle banding, occasional green mottling, and a tendency to break down into sticky mud after rain. Because this unit is comparatively weak and impermeable, it often forms the unstable foundation for the higher, more fractured beds above.
Up-section, the colour shifts into greenish-grey and grey mudstones of the Blue Anchor Formation (also Late Triassic, broadly 221–206 million years old). This change is one of the most useful field markers at the site: it reflects a move away from the most arid conditions into wetter coastal flats and increasingly brackish, marginal-marine settings. The Blue Anchor beds can show a more obviously striped appearance than the Branscombe Mudstone, with repeated alternations of green-grey and paler bands. Thin, harder ribs may locally stand out as small ledges, and the unit often weathers into a stepped profile where those firmer layers resist erosion slightly more than the surrounding mudstones. Gypsum can occur within this interval, consistent with evaporation and fluctuating salinity on coastal flats.
Above the Blue Anchor Formation, the cliff section enters the Penarth Group (latest Triassic / Rhaetian, ~210–206 million years old), where the exposure becomes more distinctly grey and commonly more bedded. The Penarth Group here includes the Westbury Formation and the Cotham Member. In practical field terms, this interval marks the transition into conditions dominated by shallow seas, with repeated changes in oxygenation and energy levels. The Westbury beds are typically darker grey at the base of the group, while higher parts can become paler and more thinly bedded, giving the “white-and-grey” banding that makes the upper cliff so striking compared with the red beds below. This part of the sequence tends to fracture into more tabular blocks, so it often produces a distinctive style of cliff fall compared with the softer mudstones beneath.
At the top of the cliff, the section passes into the Blue Lias Formation of the Early Jurassic. Here the rocks typically become more regularly bedded, with a stronger limestone–mudstone rhythm than the units below, and they often weather into a series of small ledges and steps. In the landscape, these beds help form the lighter, tougher cap to the cliff profile and can shed blocks that accumulate on the slope and foreshore.
This is a detailed stratigraphic breakdown of the Upper Triassic to Lower Jurassic succession at Aust Cliff, Gloucestershire. The section is one of the most important Rhaetian localities in Britain, exposing red Mercia mudstones, the Blue Anchor Formation, the famous Aust Bone Bed at the base of the Penarth Group, the Cotham beds above, and the lowest Jurassic Pre-planorbis beds at the cliff top.
MERCIA MUDSTONE GROUP
Twyning Mudstone Formation (Upper Triassic — Norian To Rhaetian)
Upper Twyning Red Mudstone Cycles
Bed AC1 — Red Dolomitic And Calcareous Mudstones And Siltstones
The lower part of Aust Cliff is formed by red, dominantly argillaceous mudstones and siltstones with a strong dolomitic component. These beds were deposited in dominantly terrestrial sabkha, playa and saline mudflat conditions and represent the final stage of long-lived red-bed deposition before the marine Rhaetian transgression. At Aust they form the lower two-thirds of the cliff and are one of the best visible red Mercia Mudstone successions in southern Britain.
Bed AC2 — Gypsum-Nodule Cycles, Alabaster Veins And Halite-Pseudomorph Beds
Within the upper Twyning Mudstone are repeated sedimentary cycles in which red mudstones with gypsum nodules and vertically oriented gypsum stringers are followed by more variegated mudstones and then by laminated greenish silts and mudstones. Long V-shaped fissures filled with gypsum may extend downward from horizontal gypsum beds, and the laminated units may show halite pseudomorphs and ripple marks on bedding planes. These cycles record alternating arid evaporation, groundwater sulphate growth, occasional flooding and shallow ephemeral water bodies on a coastal plain.
Total Visible Thickness Of Upper Twyning Mudstone Formation At Aust Cliff: Approximately 30–35 Metres
Blue Anchor Formation (Upper Triassic — Rhaetian)
Grey-Green Transitional Marine Beds
Bed AC3 — Grey-Green Gypsiferous And Dolomitic Clays With Sandstone
The Blue Anchor Formation at Aust forms a lighter-coloured unit between the red Mercia mudstones below and the dark Penarth shales above. It consists mainly of green and grey mudstones, siltstones and minor sandstones, with gypsum and local dolomitic cement still present. The formation records increasing marine influence but is not yet fully open marine; it represents tidal-flat, shallow-lagoon and marginal-marine environments preceding the main Rhaetian transgression. Marine fossils are sporadic, and pale blue celestite may occur on joint planes.
Bed AC4 — Upper Blue Anchor Erosion Surface
The top of the Blue Anchor Formation is an uneven, locally burrowed and eroded surface on which the Aust Bone Bed rests. Blue Anchor mudstone clasts within the bone bed show that the uppermost sediment was reworked while still soft, emphasizing the erosive nature of the transgressive contact. This surface is one of the most important stratigraphic breaks in the Aust succession.
Total Visible Thickness Of Blue Anchor Formation At Aust Cliff: Several Metres, With Nearby Continuous Sections Showing About 3 Metres
PENARTH GROUP
Westbury Formation (Upper Triassic — Rhaetian)
Basal Bone-Bed And Black-Shale Interval
Bed AC5 — Aust Bone Bed
The Aust Bone Bed forms the base of the Westbury Formation and is the single most famous bed in the section. Unlike Blue Anchor and some other west Somerset localities, where several bone beds may occur, Aust Cliff has only one principal Penarth Group bone bed. It is a discontinuous coarse-grained transgressive lag made up of grit and breccio-conglomerate with quartz grains, potassium feldspar, phosphate nodules, vertebrate material, a fine shelly matrix and calcitic plus pyritic cement. Large clasts are commonly derived from the underlying Blue Anchor Formation and show signs of soft-sediment deformation before final burial. Grading within the bed suggests an arrested turbulent sediment flow, with dense apatite concentrated near the base and larger but less dense mudstone rip-up clasts toward the top.
Bed AC5a — Basal Phosphatic Lag
The lowest part of the bone bed is a coarse phosphatic lag rich in durable vertebrate material, quartz grains and coprolites. Many elements are abraded, rounded and polished, indicating strong reworking during marine flooding of the Blue Anchor surface.
Bed AC5b — Main Vertebrate Conglomerate
The main body of the bone bed contains the densest concentration of teeth, scales, denticles and small bones. This is the most productive microvertebrate layer and the main source of sieved Aust Cliff fish and shark material.
Bed AC5c — Upper Rip-Up Clast And Shelly Matrix Layer
The top of the bone bed is richer in soft Blue Anchor mudstone clasts and finer shelly matrix. It marks the waning stage of the initial transgressive event, when reworking remained active but fine marine sediment was beginning to accumulate more continuously.
Fossil Content
The Aust Bone Bed has yielded one of the most diverse Late Triassic vertebrate assemblages in Europe. Dominant microvertebrates include the sharks Lissodus minimus and Rhomphaiodon minor, together with osteichthyans such as Gyrolepis albertii and Severnichthys acuminatus. Scarcer taxa include Pseudodalatias barnstonensis, Pseudocetorhinus pickfordi, Hybodus cloacinus, Parascylloides turnerae, Sargodon tomicus and unidentified lepidotids. Larger fossils include ichthyosaur and plesiosaur bones, the choristodere Pachystropheus, shark fin spines, coprolites and rare dinosaurian material, including records of possible theropod remains. Classic nineteenth-century accounts also listed Acrodus, Nemacanthus and other fish remains from the bone bed fauna.
Interpretation
The bone bed is a high-energy marine transgressive lag laid down on the flooded top of the Blue Anchor Formation. It concentrated vertebrate material derived from multiple shallow-marine and nearshore environments into a single condensed horizon. Aust is especially notable because osteichthyan teeth are unusually abundant compared with some other British Rhaetian bone beds, even though Lissodus and Rhomphaiodon still dominate numerically among the sharks.
Bed AC6 — Basal Sandstone
A sandstone directly above the Aust Bone Bed, marking the start of more sustained marine sedimentation in the Westbury Formation. It is less fossil-rich than the bone bed itself but still contains transported shell and vertebrate debris and provides the first true clastic marine cover above the condensed lag.
Bed AC7 — Shales With Isolated Sandstone Ripples And Trace Fossils
Dark shales above the basal sandstone with isolated sandstone ripple beds and trace fossils. These beds record a quieter but still shallow marine setting, with intermittent traction-current events on an otherwise muddy sea floor. They form the lower black-shale phase of the Westbury Formation at Aust.
Bed AC8 — Black Fissile Shales With Blue Anchor Pellets
Black fissile shale with pellets of Blue Anchor Formation mudstone, calcareous sandstone pellets and rare vertebrate fossils at the base. Selenite rosettes are common. These beds indicate continued low-energy deposition under relatively low oxygen, but still with some reworked material derived from the substrate below.
Bed AC9 — Lower Pecten Bed
A dark grey, shelly, sandy biosparite with quartz pebbles at the base. This is the lower of the two named Pecten beds in the Aust Westbury Formation and represents a higher-energy shell-rich pulse within the otherwise shale-dominated succession. It is a useful internal marker and a key invertebrate-bearing bed.
Bed AC10 — Dark Shale With Sand Lenticles
Dark shale with lenticular sand bodies in the lower part, lying between the two Pecten limestones. These beds indicate fluctuating energy conditions and repeated minor scouring or sand influx into a muddy shallow-marine shelf setting.
Bed AC11 — Upper Pecten Bed
A dark grey hard shelly limestone, generally more indurated than the Lower Pecten Bed. It marks another shell-rich event and is one of the strongest upper Westbury markers at Aust. The combined Pecten-bed interval is important because it preserves the main invertebrate fauna of the Westbury Formation above the transgressive bone bed.
Bed AC12 — Upper Dark Greenish Shales
Dark greenish shales forming the top of the Westbury Formation. These beds are less spectacular than the bone bed and Pecten limestones below but complete the marine black-shale succession before the switch into the paler, more argillaceous limestones and shales of the Cotham Member above.
Typical Westbury Invertebrates
Rhaetavicula contorta, Pleurophorus elongatus, Chlamys valoniensis, ophiuroids and the inarticulate brachiopod Orbicula townshendi. Vertebrate fossils are commonly concentrated in the harder beds and are much rarer in the intervening shales.
Total Thickness Of Westbury Formation At Aust Cliff: Approximately 3.5 Metres In Nearby Continuous Sections, With Comparable Thickness At The Cliff
Lilstock Formation (Upper Triassic — Rhaetian)
Cotham Member
Bed AC13 — Lower Cotham Limestones And Grey Shales
The Cotham Member at Aust consists of paler alternating limestones and shales than the Westbury Formation below. Its lower part includes unfossiliferous or sparsely fossiliferous limestones and grey shales that record a shift away from the dark, more open-marine Westbury facies into a shallower, more restricted setting. These beds are commonly fine-grained, argillaceous and comparatively pale.
Bed AC14 — Ostracod Limestones And Plant-Bearing Beds
Argillaceous limestones and interbedded grey shales containing ostracods, occasional plant debris and, locally, insects. These beds represent low-energy lagoonal or intertidal waters with fluctuating salinity and frequent still-water conditions. The Cotham Member at Aust is notable for preserving the liverwort Naiadita lanceolata, ostracods and the branchiopod crustacean Euestheria minuta.
Bed AC15 — Cotham Marble
The top limestone of the Cotham Member, often used historically as an ornamental stone. At Aust it occurs in two classic forms: Landscape Marble, in which microbial or algal growth creates the appearance of hedges, trees and sky in section, and Crazy Marble, in which angular flakes of limestone are set in a finer matrix. These two facies formed in different ways, the first reflecting algal-mat development in very shallow intertidal conditions and the second representing channel or runnel infill by brecciated sediment.
Total Thickness Of Cotham Member At Aust Cliff: Probably No More Than About 2 Metres
Langport Member
Bed AC16 — Langport Member / White Lias (Not Recognised At Aust Cliff)
Unlike Blue Anchor and many west Somerset sections, the Langport Member is not recognised at Aust Cliff. The top of the Cotham Member is eroded and separated from the overlying Jurassic by a disconformity. This means Aust does not preserve the full White Lias limestone development seen farther south and west, making the Aust Triassic–Jurassic boundary section notably condensed compared with Blue Anchor or St Audrie’s Bay.
LIAS GROUP
Blue Lias Formation (Lower Jurassic — Earliest Hettangian)
Pre-planorbis Beds
Bed AC17 — Basal Lias Conglomerate
The base of the Lias Group at Aust is marked by a discontinuous conglomerate containing limestone clasts in a dark matrix. This rests disconformably on the eroded top of the Cotham Member and records another break in sedimentation at the Triassic–Jurassic boundary interval. It is the local boundary bed below the Pre-planorbis limestones.
Bed AC18 — Brown Flaggy Pre-planorbis Limestones
Brown, flaggy limestones forming the lowest Lias beds visible at Aust Cliff, near the vegetation line at the top of the cliff. These beds represent the earliest Jurassic shallow-marine carbonate phase following the boundary break and correspond to the Pre-planorbis Beds of older usage. They are the highest beds usually visible at Aust and are only thinly exposed, but they demonstrate that fully marine Lias deposition was established immediately above the condensed Penarth Group succession.
Ammonite Significance
The lowest Lias beds at Aust belong to the basal Hettangian interval below or at the base of the Psiloceras planorbis succession in the wider Bristol Channel–Severn region.
Depositional Environment
Aust Cliff records a complete late Triassic to earliest Jurassic environmental shift. The Twyning Mudstone Formation represents arid sabkha and playa conditions on a saline coastal plain; the Blue Anchor Formation reflects increasingly shallow marine and tidal-flat influence; the Aust Bone Bed marks the main Rhaetian marine transgression as a coarse vertebrate lag; the Westbury Formation above records shallow marine black-shale and shell-bed deposition; the Cotham Member represents a return to very shallow restricted lagoonal or intertidal carbonate-mud conditions; and the Pre-planorbis Blue Lias at the top records the onset of open marine Jurassic sedimentation.
Total Thickness Covered Here: Approximately 40–45 Metres Of Upper Triassic To Earliest Jurassic Stratigraphy At Aust Cliff
References
Hamilton, F.W. (1977). The Mercia Mudstone and Penarth groups of the Severn Estuary area.
Curtis, C.D. (1982). Cyclic sedimentation in the Mercia Mudstone Group at Aust Cliff.
Storrs, G.W. (1993, 1994) on the Aust Bone Bed and Rhaetian vertebrates.
Cross, S.R.R. et al. (2018). Microvertebrates from the basal Rhaetian Bone Bed at Aust Cliff.
British Geological Survey Lexicon: Twyning Mudstone Formation, Blue Anchor Formation, Westbury Formation, Cotham Member and Blue Lias Formation.
Geological Conservation Review account for Aust Cliff.
ACID PREP
The Aust bone bed (often referred to as part of the Rhaetian bone-bed horizon within the Westbury Formation) is typically a thin, locally patchy layer formed as a high-energy lag deposit when the sea advanced over older Triassic mudstones. Rather than preserving fossils neatly “in place,” it commonly concentrates tough, durable material: vertebrate teeth and bone fragments, fish debris (scales, denticles), phosphatic pebbles, small clasts ripped up from underlying mudstones, and occasional shell or limestone fragments.
That mix is exactly why acid-based preparation can be useful for this bed: the cement and matrix are often carbonate-rich, while many of the collectible microfossils (especially the smaller vertebrate elements) are more phosphate-rich and can survive careful, controlled chemical prep better than the surrounding carbonate.
What acid prep is best for (and what it isn’t)
Acid prep is usually most effective here when your aim is bulk recovery—freeing lots of small, isolated fossils for sorting—rather than producing a single “display specimen.” Bone-bed material is commonly reworked and fragmentary, so the payoff is often in the microfossils you can’t easily extract by hand.
A practical, non-technical workflow overview
It helps to frame acid prep as “lab-style processing” rather than a quick trick.
- Start by understanding your matrix. A small test on non-valuable matrix can tell you whether the rock is likely to respond (carbonate-rich material reacts readily; clay-rich material may not).
- Work in controlled cycles. Short exposures with long rinses between them are generally safer than prolonged soaking, because they reduce the chance of softening or etching fossil surfaces.
- Rinsing matters as much as the acid. Bone-bed residues can trap dissolved salts; if those salts crystallise during drying they can crack or flake delicate pieces. Thorough rinsing and patience reduces this risk.
- Expect a lot of “non-fossil” residue. Because the bed is a lag deposit, you’ll often process plenty of pebbles and mudstone clasts along with the fossils. The real results come from sieving and careful picking under magnification.
- Fragile pieces may need stabilising. Small bones and teeth can be porous or fractured; some collectors use suitable consolidants (applied sparingly) to help pieces survive handling and drying.
Common pitfalls with Blue Anchor bone-bed material
- Over-prepping: Rushing the process can leave fossils chalky, etched, or weakened—even if they don’t fully dissolve.
- Hidden weak spots: Some fossils look solid until they dry, then split along old cracks. Gentle handling and slow drying help.
- Iron sulphides (pyrite): Dark, organic-rich horizons can contain minerals that cause long-term deterioration if stored damp or sealed without care. Keeping material dry and well-ventilated after proper rinsing is important.
Safety note
Even “mild” acids are hazardous. If you choose to do chemical preparation, treat it like proper chemical handling: use appropriate eye/skin protection, good ventilation, acid-safe containers, and follow local rules for neutralisation and disposal. If you’re not set up for safe chemical work, stick to mechanical preparation and selective collecting.
SAFETY
Common sense when collecting at all locations should be used and knowledge of tide times is essential. There are several dangers at Aust and these become more apparent if you plan to collect from the northern side of the bridge. To get there, you will need to walk round a pillar of the bridge when the tide is retreating. However, this is slippery and often has dangerous mudflats. You can also easily get cut off by the tide if you do not return before the tide starts to come in.
Keep away from the cliff as this can fall or crumble at any time, and keep away from the mud on the foreshore which can be dangerous. And beware of the Severn Bore (a large wave that happens several times a month and is created as the tide flows up the river into an increasingly narrow space). Times for this are advertised at: www.severn-bore.co.uk.
EQUIPMENT
Fossils can only be collected from the foreshore, especially along the tide line. However, any large lumps of the bone bed will require strong tools to split them up. A good eye is often all you need, but a geological hammer, chisels and eye protection may be necessary for the larger lumps.
CLEANING AND TREATING
Begin by removing any loose sediment very carefully using a soft toothbrush. Take your time, as many fossils—particularly pyritic specimens—are fragile and easily damaged. Once cleaned, fossils should be desalinated by soaking them in fresh water for at least 24 hours to remove residual salt. After soaking, allow specimens to dry naturally at room temperature. Do not dry them on radiators or other heat sources, as rapid drying can cause cracking or long-term damage.
Once fully dry, we recommend sealing fossils with Paraloid B-72, dissolved in acetone. This is a museum-grade consolidant that is widely available in pre-mixed bottles. Paraloid B-72 is stable, long-lasting, and does not yellow or react chemically over time. Importantly, it is also fully reversible, making it suitable for scientifically important or display-quality specimens.
ARTICLES
- New Zealand rock hunting
- The happy sound of hammer on rock: A story of migratory Isle of Wight stonepeckers
- Marvellous microfossils (Part 2): The fascination of microfossils from the Gault of Folkestone
- The dinosaur footprints of Whitby (Part 3): A brief look at the six footprint groupings
- The dinosaur footprints of Whitby (Part 2): Problems matching footprints to dinosaurs
- The dinosaur footprints of Whitby (Part 1): Introduction
- Fossils of the Gault Clay
- Seeing into the ‘Stone Age’: The stone tools of early man
- Fossil collecting at Bracklesham, West Sussex
ACCESS RIGHTS
Although not an official right of way to the beach, the council and National Grid allow the use of the raised road, for walking but not for vehicles. An information board explaining this is at the end of this raised road.
This site is a site of special scientific interest (SSSI). This means you can visit the site, but hammering the bedrock is not permitted. For full information about the reasons for the status of the site and restrictions, download the PDF from Natural England.
It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions‘
LINKS
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