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.
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.
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‘
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