Fossils of the Gault Clay
Dr Neale Monks (UK)
The Gault Clay is an Albian (Lower Cretaceous) deposit of blue-grey clay exposed primarily in Southeast England. At the classic exposure at Copt Point, Folkestone, the Gault Clay is sandwiched between the Lower Greensand underneath and the Upper Greensand on top.
It is a stiff clay that preserves fossils extremely well. In particular, aragonite is sometimes preserved on fossils such as bivalves and ammonites, resulting in much more attractive and detailed fossils than those found in limestone or chalk. As well as providing excellent preservation, the Gault Clay is highly fossiliferous at many localities. Unsurprisingly, these two factors have ensured that the Gault Clay has always been extremely popular with fossil collectors.
In terms of what was going on during the Albian, the Gault Clay represents a phase of sea level rise, something known as a “marine transgression”. Compared with the preceding Lower Greensand, this clay was deposited in a deeper, less energetic environment in what is sometimes known as the “Gault Sea”. This was a shallow, warm sea with a muddy bottom. Corals were notably absent, but animals that were adapted to muddy conditions, such as clams and pelican’s foot snails, were common. The sediment particles were finer and less disturbed by water currents and benthic animals (that is, animals living on the sea bottom), resulting in the high-quality fossils so characteristic of this deposit.
At this time, the climate in Britain was warm and the marine fossils found indicate what, by modern standards, would be a subtropical sea, equivalent perhaps to what you might see around Bermuda or Florida. Oddly enough, marine reptile fossils are not common in the Gault Clay, but fish bones and especially sharks’ teeth can sometimes be found. The invertebrate fauna was very diverse, and even casual collectors can expect to find gastropods, bivalves, belemnites and ammonites, and sometimes, if they are lucky, sea urchins, crabs and nautiloids.
Gault Clay exposures
There are Gault Clay exposures at a number of coastal sites, including at Lyme in Dorset and at various places on the Isle of Wight. However, by far the most productive place for collecting fossils from the Gault Clay is along the cliffs at Folkestone, in particular between Copt Point and East Wear Bay.
Compared with many other fossiliferous localities in England, Copt Point is relatively easy to get to. The town itself is about 60 miles from London and, for those travelling by train, the service from London Victoria or Charing Cross is quick and leaves you only a short walk to the beach. Motorists can park at East Wear Road, from where footpaths lead the way down to the exposure itself.
However, as with any coastal exposure, access depends upon the tides and the weather. The paths can be overgrown with vegetation and sticky with mud, and sturdy, sensible footwear is absolutely essential. In wet weather, the clay can become horribly slippery. All things considered, the Gault Clay is not a family-friendly exposure.
For casual collectors, it is safest to walk along the pebbly beach, rummaging through fallen blocks of clay for fossils. This will be especially good after storms, when fresh material has been scoured out of the cliffs and dumped on the beach. While this is not necessarily the best way to find perfect specimens or to observe fossils in clearly recognisable beds, the sheer quantity of fossils present in the Gault Clay means you will not walk away disappointed.
Collecting and preservation
Collecting fossils from the Gault Clay is generally not difficult because the shiny, often pearly-pink appearance of many fossils makes them easy to spot against the dark, green-grey of the clay matrix itself. The Gault Clay isn’t particularly difficult to work with and, while sticky, a bit of work with a trowel and a bottle of water should wash away matrix from most specimens.
The main problem is not so much finding fossils, as ensuring your loot gets home safely. After millions of years of preservation, it would be shame to scratch the aragonite away from a prize ammonite specimen by throwing it carelessly into your rucksack. Depending on your preferences, you can either wrap specimens up in newspaper or put them into plastic specimen bags. The latter approach is particularly good with fossils likely to fall apart in transit, such as gastropods and ammonites. A specimen bag will keep all the bits together until you get home, making it much easier to repair any damage.
Gault Clay fossils are frequently pyritic and consequently prone to decay, especially when exposed to moisture. Pyritic fossils can be immediately recognised by their shiny, slightly golden appearance. The best way to preserve these types of fossil is to keep them in a dry, airtight container, preferably with a desiccant, such as silica gel.
Some collectors like to soak fossils for a week or so in a large basin of water, and then air dry them for a few days afterwards. The water removes the salt from the fossil, reducing the risk of crystals forming inside the fossil, forming cracks. Once the fossil is dry, it can be varnished to keep the fossil dry and prevent pieces from flaking off. While this approach can work well, you do need to be careful because delicate fossils on flaky clay may fall apart when placed in water.
While the Gault Clay is quite uniform in appearance at first glance, with a bit of experience and a good clean section to work with, it is possible to identify a number of distinctive levels, known as “beds”.
Thirteen beds (traditionally referred to as Beds I to XIII) were recognised by L F Spath in his seminal work on the ammonites of the Gault Clay published in several parts through the late 1920s and early 1940s (Spath, L.F., A Monograph of the Ammonoidea of the Gault, 1923-1943, parts 1-16). Over the years, his basic arrangement has been revised and tweaked by a succession of stratigraphers, most notably Ray Casey and Hugh Owen. Geologists describe Beds I to VII as the “Lower Gault” and Beds VIII to XIII as the “Upper Gault”. In terms of time, these two divisions equate to the Middle and Upper Albian and span a period extending from roughly 104 to 96mya.
The Gault Clay runs from Southern England to the northern coastlines of France and Belgium, an area referred to as the “Anglo-Paris Basin”. However, the fossils found within the Gault Clay can be found much further afield. Many of the ammonites found in the Gault Clay in Southern England are found in countries as far away as Mexico, India, Madagascar and Australia.
Within the Gault Clay can be found members of all four of the major cephalopod groups around at the time: nautiloids, belemnites, ammonites and heteromorph ammonites. These are often very well preserved, being three dimensional fossils with much original shell material in place. The ammonites and heteromorphs are especially diverse and, being of tremendous significance as tools for biostratigraphy, have been widely collected and studied by amateur and professional geologists alike.
Among the nautiloids, the most commonly found species are Eutrephoceras bouchardianus, Eutrephoceras clementinus, and Cymatoceras albensis. All three have a globose appearance similar to that of modern nautiluses. Telling small specimens apart is very difficult because the main differences are really only apparent on the living chamber of fully-grown specimens. For example, adult Cymatoceras albensis have very strong ribs on the living chamber, whereas the ribs on the living chamber of Eutrephoceras clementinus are much weaker and concentrated around the umbilicus, and the ribs on Eutrephoceras bouchardianus are even weaker. Eutrephoceras clementinus is limited in range to the bottom part of the Lower Gault (Beds I to IV at Folkestone), but the other two species can be found throughout the Gault Clay.
Only a single species of belemnite occurs commonly in the Gault Clay, Neohibolites minimus. This species is common throughout the Gault Clay and most collectors will get a nice handful of specimens after a few hours of work. They are rather beautiful things, not large, perhaps 5cm or so in length. However, the amber-coloured guard has a very attractive translucency that makes them look like the remains of a modern squid, not something that died a 100 million years ago.
The ammonites of the Gault Clay have been intensively studied and trying to summarise their diversity in a paragraph is impossible. Most of the common species belong to a particular group known as the hoplitids, a name that compares their spiny, heavily-armoured appearance with the famous hoplites soldiers of Ancient Greece.
Many hoplitids have only a limited stratigraphic range, making them important fossils for correlation and biostratigraphy. The Lower Gault is divided into three ammonite zones named after the hoplitid ammonites peculiar to them: Hoplites dentatus (Bed I and part of Bed II); Euhoplites loricatus (the rest of Bed II up to Bed IV); and Euhoplites lautus (Beds V to VII). The Upper Gault is divided into two ammonite zones, named after ammonites that aren’t hoplitids: Mortoniceras inflatum (Beds VIII to XII) and Stoliczkaia dispar (Bed XIII). Neither are likely to be found at Folkestone.
The Gault Clay heteromorphs are important to geologists because they represent a snapshot of the time when they were radiating, and their descendants ultimately included many heteromorph species that were abundant and widely distributed around the world, such as Scaphites and Baculites. However, while we can map out their evolution with some success, actually understanding precisely what they were doing remains a mystery.
Why did these ammonites evolve shells that looked like hockey sticks, paper clips, and corkscrews? Some geologists have speculated that these aberrant ammonites gave up the active life in favour of drifting about like jellyfish instead, catching plankton with their tentacles. There are certainly plenty of ocean squid that do this sort of thing today, notably the squids belonging to the family Cranchiidae. Like many modern squids, it is supposed that heteromorph ammonites might have spent the day in deep water away from predators, swimming upwards into shallow water at night where they could feed safely away from predators that hunted by sight (including, presumably, most of the marine reptiles).
However, not all geologists agree with this interpretation of the fossil evidence and, instead, propose that at least some heteromorphs swam about close to the sea floor, perhaps even crawling about, hunting for benthic prey animals such as shrimps and other shellfish.
As obscure as they might be in terms of ecology, heteromorph ammonites are not rare at Folkestone. Fragments of Hamites and its allies dominate. These may be curved or straight, or more often a combination of the two, and have a concertina-like ribbing. A typical species is Hamites attenuatus, found Beds I and II. In life, its shell consisted of a paper clip-like structure comprised of three parallel shafts. At its widest point, the shell was about 10cm across. Other species you may encounter include Hamites maximus, a species similar in shape to Hamites attenuatus, but reaching a maximum width of nearer 50cm, and Planohamites incurvatus, a species with an open spiral shell shape around 15 to 20cm across.
Near the bottom of the Gault Clay, you should be able to find specimens of Protanisoceras, which are heteromorphs that look a lot like the paper clip-shaped Hamites, but possessed one or two rows of spines along the ventral surface. At the top of the Gault Clay appear Anisoceras and Idiohamites, though these are never common. Like Protanisoceras, these had spines and tended towards having paper clip or hook-shaped shells. They did tend to be quite big though – some fragments of Anisoceras having about the same size and shape as a hockey stick!
Bivalves and gastropods
Cephalopods are not the only molluscs in the Gault Clay. Indeed, among the commonest fossils are the bivalves, Birostrina concentrica and Birostrina sulcata. These are both small examples of a group of now-extinct bivalves known as inoceramids. Later on in the Cretaceous, these would become abundant and important parts of the marine fauna and some species attained massive sizes comparable to the giant clams of today.
However, the species of Birostrina in the Gault Clay are small, a couple of centimetres across at most. At first, the asymmetry between the two valves makes them look a lot like brachiopods, but the absence of an opening for the pedicle (by which the animal attached itself to the sea floor) in the larger valve, and slight spiral twisting apparent on each valve should immediately clear up that confusion. Of the two species, Birostrina concentrica has the widest stratigraphical range and can be found from Beds I to XI. It has a distinctive ornament of concentric rings. Birostrina sulcata, by contrast, is limited to Beds VIII and IX and is ornamented with radial folds instead of concentric rings.
While the inoceramids are long gone, casualties of the great End-Cretaceous mass extinction event, the next most common group of bivalves are still around today and thriving. These are the nut clams, represented in the Gault Clay by species such as Pectinucula pectinata. These oval clams get to a couple of centimetres across at most, but because of the fine preservation offered by the Gault Clay, the delicate ribbing and growth lines on the valves are nicely preserved. Alongside these common bivalves are occasional scallops, file shells, cockles and other more or less familiar types of clam.
Gastropods are another group of molluscs that are abundant in the Gault Clay. Among the most distinctive are various species related to the modern pelican’s foot shell, Aporrhais pespelecani, a species nowadays found, among other places, around most of the British Isles. The most distinctive features of this snail are the spines extending from the aperture and the webbing that connects them. Looking a lot like the foot of a water bird (hence the common name), this bizarre structure functions like a snowshoe, stopping the snail from sinking into soft mud. Presumably, its relatives in the Gault Clay used their elaborate shells for the same thing. If you’re lucky enough to come across an aporrhaid gastropod in the Gault Clay, be careful not to knock away too much of the matrix: these shells are notoriously fragile and easily damaged.
Generally speaking, evidence of the presence of crustaceans is often best revealed by trace fossils, shrimps in particular making very distinctive burrows. But, in the Gault Clay, you can find fossils of the crustaceans themselves, with crabs, lobsters, barnacles and ostracods all being known. They’re not common, but you might find one or two specimens during an afternoon’s work, if you are lucky.
Crabs of various types predominate. Most can be referred to groups still alive today. One of the commonest is Etyus martini, a member of a group of crabs today known as “sponge crabs” on account of their habit of covering their shells with sponges to deter predators. Whether or not Etyus martini did that is unknown, but possible. Crabs of the genus Notopocorystes are also common. Its modern equivalents are the frog crabs, so named for their rotund, frog-like appearance. Most species are accomplished diggers, with shovel-like legs and a round shell that slips into the substrate easily.
Brachiopods and bryozoans are not common in the Gault Clay, presumably because the soft substrate did not provide them with surfaces to attach to. Corals are not common either, probably for the same reason, but occasional specimens of solitary corals can be found. These often resemble small buttons in size and shape. Sea urchins, typically Hemiaster spp, can also be found from time to time, but are not common.
Vertebrate fossils are not commonly found in the Gault Clay, with the exception of sharks’ teeth. Among other species, there are teeth from the so-called ‘Cretaceous Jaws’ Cretoxyrhina, which were sharks of comparable size and habits to the modern great white shark.
Other teeth to be found resemble those from modern dogfish and mackerel sharks. While the muddy nature of the Folkestone exposure of the Gault Clay does not make it the perfect collecting site for the completely inexperienced collector, geologists with a bit more experience will find the combination of good preservation, taxonomic diversity, and paleontological significance very rewarding.
Ammonites and other Cephalopods of the lower Cretaceous Albian (Gault Clay) and Folkestone beds of the South East of England, by David Rayner, Tony Mitchell, Martin Rayner and Frederick Clouter, Medway Lapidary & Mineral Society (2007), 80 pages (Paperback), ISBN: 978-09538243-2-8.
Fossils of Folkestone, Kent, by Philip Hadland, Siri Scientific Press, Manchester (2018), 148 pages (Paperback), ISBN: 978-09-95749-65-8.
Fossils of the Oxford Clay: Guide No 4, edited by David M Martill and John D Hudson, The Palaeontological Association, London (1991), 286 pages (Paperback), ISBN: 090-17024-6-3.