David Bone (UK)
“I have been greatly disappointed … [owing to] sand, sometimes two to three feet in thickness, or the tide not leaving the shore sufficiently exposed; so that a stranger might conclude that there were no fossils to be procured at Bracklesham”.
The Sussex geologist, Frederick Dixon, writing about Bracklesham in 1850 warned readers with these words and it is no different today. Exposures of the richly fossiliferous Palaeogene sediments, which comprise the Bracklesham Group (Eocene), come and go unpredictably with the tides and weather. On a good day, extensive shell beds, around 46 million years old, cover the beach and sharks’ teeth may be found by the hundred. On a bad day, Dixon’s quote is all too true.
Bracklesham Bay is located seven miles south of Chichester in West Sussex, on the south coast of England (Fig. 1), at the eastern end of the syncline known as the Hampshire Basin. To the north, beyond Chichester, the ground rises to the Cretaceous chalk hills of the South Downs, while, to the south, across the waters of the Solent, the Isle of Wight stretches across the horizon. It is often said that if you can see the Isle of Wight, it is going to rain. If you can’t see it, then it is raining. This is a fair warning to anybody planning a trip here – this balmy stretch of coast, even on a sunny day, takes the full brunt of south-westerly gales and temperatures can fall dramatically in the evening around low tide (Fig. 2).
The Palaeogene geology is only exposed around the coast as, inland, much younger Quaternary deposits blanket the West Sussex coastal plain right up to the foot of the South Downs. There are no cliffs (very safe for young children) and loose fossils can be collected almost as soon as the tide falls sufficiently (Fig. 3), although exposure of the fossil beds depend on having a good low tide and no cover of beach sand (Fig. 4). The best low tides usually occur around 6am and 6pm, plus or minus an hour or so. Therefore, trips are best planned for late afternoon. When conditions are good, the site can produce some fantastic finds.
The Bracklesham Group sediments are predominantly soft, silty clays and sands that were deposited between approximately 50 and 45Ma (the Turritella Bed has been radiometrically dated at 46.4 ± 1.5myrs). This was a time of minor sea level changes, with coastal sediments alternating with shallow marine sediments. It is divided into four ‘Formations’.
The Wittering and Marsh Farm Formations are characterised by coastal sediments (including marsh, lagoon, tidal flats and tidal channel deposits). The Earnley and Selsey Formations are characterised by sediments of shallow marine origin. These are the most fossiliferous parts of the succession. Reconstructions of the palaeogeography indicate a large, shallow bay, with land some 50km to the west, drained by one or more fairly substantial rivers and covered by lush vegetation, reminiscent of today’s tropical rain forests. Mangrove palms fringed the coastal areas and offshore beds of sea-grass thrived for at least some of the time. The sea was no more than 10m to 30m deep, with a water temperature of around 18°C. This higher sea temperature was partly due to a warmer world climate, as well as continental drift, because, at this time, southern England lay at a latitude of around 40ºN.
The beds dip to the south and strike E-W, providing a full sequence, from the underlying London Clay Formation in the northwest at the entrance to Chichester Harbour, to the very youngest beds at the tip of Selsey Bill in the southeast. Much of this coastline has been designated a Site of Special Scientific Interest (SSSI) for its Palaeogene geology. The most fossiliferous parts of the sequence occur around Earnley and Medmerry, which are best reached from Bracklesham and Selsey, respectively. I usually recommend Bracklesham for a first visit, as the preservation of the fossils is generally better and they can be found washed up on the beach, even when there are no exposures. Around Medmerry, collecting is generally dependent on actual exposures being present.
From Bracklesham car park (charges apply), walk eastwards along the beach towards Selsey (this is to the left as you face out to sea). Immediately start looking for fossils – they are often more common than modern seashells. They may be washed up along the foot of the shingle – large sharks teeth can be found here, but they are frequently broken by wave action on the pebbles. I prefer to search further down the beach towards low tide, particularly where there are drifts of fine broken shell debris. These are good places to look for smaller fossils, including sharks’ teeth, and all you need is a collecting bag. About 1km further along the beach, there may be exposures of the more fossil-rich beds (Fig. 5), although they are often concealed beneath the beach sand.
There are three fossil-rich beds that the average visitor to Bracklesham is likely to encounter, named the Cardita Bed, the Turritella Bed and the Palate Bed. These are numbered E3, E4 and E5, respectively (after Earnley Formation) and are named after the most common fossil in each bed. These three beds are the principal source of the fossils being washed up on to the beach from exposures further offshore. The easiest fossils to find are small Turritella gastropod shells (now correctly called Ispharina and Haustator, but most people still call them Turritella), up to 5cm long and representing several species – look closely at the shape and form of the spiral.
Sometimes, the shells have small holes drilled into them by small predatory gastropods. Much larger specimens can be found in some of the younger beds, towards Selsey. Also common are the bivalve shells, Venericor planicosta (often known under its former name of Cardita, as in the name of the bed). Exposures of the Cardita Bed usually show a mix of current-drifted single valves and double-valve specimens, preserved in the position in which they died, around 46Ma (Fig. 6).
A little further along the beach, the Turritella Bed is similarly packed with fossil Turritella shells (Fig. 7).
Often overlooked, but probably the commonest fossil on the beach, are the Nummulites laevigatus (named from the Latin for “small coin”). These coin-sized fossils are the shells of foraminifera – single-celled organisms that are normally microscopic in size and build themselves elaborate shells of calcium carbonate (Fig. 8).
Living species of foraminifera seldom exceed 2mm, but Nummulites laevigatus is a giant form that, at Bracklesham, reaches a diameter of up to 18mm. Further along the beach from Bracklesham towards Selsey is the Nummulites laevigatus Bed (E7), which contains millions of these fossils, as well as corals and a variety of shells and sharks teeth. Another shell that frequently survives being washed out onto the beach is the bivalve, Venericardia carinata. Its smaller size and sharper profiled ribs contrasts with the flatter ribs of Venericor planicosta. Small, brown oyster shells, Cubitostrea elegans, are also quite common. In the Cardita Bed, these oysters can be found in colonies, where they have grown on top of each other.
In total, there are many hundreds of different species of fossil shells to be found, although many do not survive erosion by the sea and can only be collected carefully from exposures of the beds. Most Bracklesham fossil hunters want to find a shark’s tooth, especially a large one over 5cm in length, but smaller specimens are far more common (Fig. 9).
There are at least 70 different species (including rays) to be found, as well as other vertebrate fossils, such as turtles, crocodiles and sea snakes, as well as rare bones of birds and mammals. All these fossils tend to appear very dark, almost black, against the wet beach sand. In life, most of these would have been white, but have become stained black during the millions of years of burial in the sediments. Until they get to recognise sharks’ teeth, many people also pick up small black flints, bits of seaweed and fragments of fossil wood (lignite). However, the teeth are always there, somewhere, as sharks shed their teeth regularly and grow new ones. A single tooth may be in use for as little as six weeks and, in its lifetime, a shark may produce 20,000 teeth.
Among the more common fossil vertebrates are the rays (particularly eagle rays). These leave two types of fossil remains – teeth and spines. Their teeth are unlike those of sharks, because the rays feed on shellfish, which they crush between tooth plates or palates (Fig. 10).
They are smooth on the crushing surface and have small ridges on the opposite surface (the root of the tooth). As fossils, the separate teeth are far more common than the complete palate. These give their name to the Palate Bed, although they are not as common as the name might suggest. The rays also have tail spines, which are normally up to 30mm long, with fine barbs running down each edge. Many different types can be found.
Most beach-collected fossils require little or no treatment, other than a gentle scrub and soaking to remove the sea salt. Delicate specimens may be hardened with a dilute solution of PVA. Sharks teeth and other vertebrate material will need no extra treatment to preserve them. Shell collecting from the fossil beds usually needs to be done very carefully, as they are can be extremely delicate (except for the Venericor planicosta). A pointed trowel and collecting tray is essential, plus lots of gentle cleaning afterwards.
Any description of the geology of Bracklesham Bay and Selsey should not be without mention of the younger, Quaternary stream channels that cut thorough the foreshore outcrop. Now filled with muddy alluvium, these channels variably date from the Pleistocene (400,000 and 200,000 years ago) and Holocene (less than 10,000 years ago). Bones and teeth of mammoth, elephant and other large Pleistocene mammals are occasionally found, washed up on the beaches around Bracklesham Bay.
Alluvium-filled channels of Holocene age, between Earnley and Medmerry, contain abundant shells of the bivalve Scrobicularia plana, while pieces of tree trunk, branches, twigs and roots suggest that the area was wooded at the time. Pottery fragments and worked flints have also been found, suggesting a Bronze Age date. Animal bones, mostly of domesticated animals such as cattle, horse and dog, may date from farming in recent times, before the land was lost to the sea by coastal erosion.
The Selsey peninsula used to be one of the fastest eroding coastlines in the country – in one area, the coastline retreated over 250m between 1909 and 1950, peaking at 8m a year. Since the 1930s, sea defence works have greatly slowed the rate of erosion, but sea levels are rising. This is principally due to climate change with melting of the ice caps and thermal expansion of the oceans, but another contributing factor to local sea level rise is sinking of the land. Since the ice caps over the north of Britain melted at the end of the last Ice Age, land in the north has slowly risen due to the removal of the weight of ice. The south of Britain has, in turn (like a seesaw) moved downwards. Sea level is currently expected to rise by 1m over the next 100 years, bringing with it increased storm surges (due to changes in weather patterns) and bigger waves (due to increase in near-shore water depth).
The existing sea defences are not sustainable under these conditions and longer-term plans are now being made for a managed coastal retreat. The shingle bank, protecting the low-lying farmland and Earnley marshes inshore of Bracklesham Bay, will be breached and allowed to flood on high tides. A new seawall to be constructed in the next few years will separate the new salt marsh from the surrounding land. The existing beach will be allowed to adjust naturally to the new coastal alignment. What effect this will have on the geological exposures and the future of the SSSI is unknown. Further information on Bracklesham fossil hunts and other aspects of the local geology can be obtained from the author at: www.westsussexgeology.co.uk or by purchasing a copy of the guide, Fossil hunting at Bracklesham & Selsey (published by Limanda Publishing), which I discussed in Issue 21 of this magazine.