Mike Thorn (UK)
If you ask someone to think of Oxford, they will not usually picture warm tropical beaches and azure coral seas. However, go back to the middle Jurassic and you would be hard pushed to find a dreaming spire or student on a bicycle anywhere. At that time, around 160 million years ago, Oxfordshire lay beneath a shallow, tropical seaway at about the same latitude that the southern Mediterranean occupies today. Over the course of the middle Jurassic, this seaway varied in depth, but remained close to nearby land masses from which a lot of sediment was derived (Fig. 1). A great thickness and variety of limestones, sandstones and clays were deposited over several tens of millions of years.
During the nineteenth and early twentieth centuries, there were many working quarries in the Oxford area, exploiting the abundant clay and limestone for the brick and building stone industries. Quarrymen were frequently paid to look out for fossils and these turned up in abundance, fuelling the academic debates on evolution taking place at the time.
Sadly, many of these quarries have now been filled in and, for the casual fossil hunter in Oxfordshire, it might seem that there are now few opportunities to collect. However, Kirtlington Quarry and Dry Sandford Pit are two old quarries which are open to the public and at which there is still some chance of finding something.
Location: OS Sheet 164, SU 494198
Kirtlington Quarry (Fig. 2) lies close to the banks of the Oxford Canal, just to the west of the village of Kirtlington, a small village north of Oxford. The rocks found at Kirtlington Quarry are older than those found at Dry Sandford Pit, being from the middle to upper Bathonian, between 169 and 175 million years ago.
The whole quarry is a Site of Special Scientific Interest (SSSI) and a nature reserve. Hammering on the rock faces is not allowed and it is not necessary since good quality fossils can be found lying around at the foot of the quarry walls or in the woods leading down to the canal. There are three formations to be found here: the White Limestone, which is the oldest and thickest exposure, the Forest Marble and, at the very top, the Lower Cornbrash.
The White Limestone is a very fine-grained limestone thought to have been produced in very shallow water, possibly only 1m to 2m deep. It is composed largely out of the remains of calcareous algae and distinct beds can be seen. The tops of many of these beds are criss-crossed with the burrows of shrimps and worms, which would have fed off the organic matter trapped in the sediment.
There are many fossils to be found in the white limestone, but the most obvious and numerous are the large brachiopod Epithyris oxonica (Fig. 5.2), which can be found in thick bands at many places. If you wish to collect this brachiopod, the best place to head is into the woods, which lead towards the canal. There are many places where the woodland path cuts through beds containing them and good specimens can be picked up off the ground.
The second-most abundant fossils are those of the mussel Modiolus sp (Fig. 5.3). These are usually found as internal moulds since their shells are composed of aragonite, which dissolves more readily than the calcite shells of the brachiopods. It is possible to find them with their shells, but they are never as well preserved as the brachiopods.
Above the White Limestone lies the Forest Marble. This was formed under deeper water conditions than the White Limestone and contains layers of oolitic limestones, interspersed with thin beds of clay. These clays, known as the Kirtington Mammal Beds, were one of the richest sources of fossil mammal remains from the middle Jurassic found anywhere in the world.
At the opposite end of the scale, the huge bones of the sauropod dinosaur, Cetiosaurus oxonica,were found here in 1870. These can be seen on display at the Oxford University Museum of Natural History. Sadly, since the quarry is no longer worked, a fossil hunter should not get overly excited at the prospect of finding either dinosaur or mammal remains.
The Lower Cornbrash is a rubbly limestone found at the very top of the quarry. It is composed largely of shell fragments and was laid down in deeper water conditions than the White Limestone or the Forest Marble. A wide range of fossils can be found, the most frequent in the author’s experience being the brachiopod Obovothyris magnobovata (Fig. 5.1) and bivalves tentatively identified as belonging to the genus Meleagrinella (Fig. 5.5). It is also possible to find small sea urchins, although the author has not been lucky enough to do so yet.
Dry Sandford Pit
Location: OS Sheet 164, SU 467997
Dry Sandford Pit is a nature reserve owned by the charity BBOWT (Berkshire, Buckinghamshire and Oxfordshire Nature Trusts) and has been designated SSSI status because of its geology. Under no circumstances is hammering on the rock surfaces permitted and neither is it necessary since fossils can be collected from the quarry floor. The rocks at Dry Sandford Pit are a little younger than those at Kirtlington, being from the middle Oxfordian (approximately 160 million years in age). The sediments were laid down close to or on the coastline, with sandbanks and coral reefs just offshore. Although the rocks were laid down in shallow water, the conditions were by no means uniform and this is reflected by the different rock strata visible (Fig. 5).
At the very top is the Coral Rag, there is a rubbly limestone, which is believed to have been formed by localised patch reefs. It is difficult to collect from this now as it is largely overgrown and inaccessible but, occasionally, beautifully preserved radioles (spines) of the sea urchin Plegiocidaris (Fig. 6.7a) fall down to the quarry floor.
Beneath the Coral Rag lie the Urchin Marls in which it is possible to find the small burrowing heart urchin Nucleolites scutatus (Fig. 6.10). These vary in size from one to three centimetres and they are preserved in quite fine detail. Beneath the Urchin Marls are the Upper and Lower Trigonia Beds, separated by a layer of bedded sand. These are named for the numerous shells of the bivalve Trigonia, which can be clearly seen although, sadly, the hardness of the rock makes collecting them almost impossible.
Within the sand and at the base of the Lower Trigonia Bed can be found layers of bivalves. These are mostly made up of the small oyster Nanogyra nana (Fig. 6.8) and the wonderfully crinkly Rastellum gregareum (Lopha gregareum in older books; Fig. 6.11b). The casts of polychaete worms (Fig. 6.3) are also abundant and often show fine detail. Valves of the small scallop Chlamys (Fig. 6.9) are another frequent find and again, these can be preserved in very fine detail. All of the above fossils are preserved with their original shells.
However, in some layers, the original shell material has been dissolved away leaving internal casts. Gastropods – probably of the genus Ampullina (Fig.6.5) are commonly preserved in this fashion, as are numerous different types of bivalves.
Internal moulds of ammonites of the genus Perisphinctes (Fig. 6.7a) turn up rarely, usually as fragments. The very bottom layer of rocks exposed is the Lower Calcareous Grit which can be found down near to the edge of the ponds. There is a layer of Rastellum gregareum (Fig. 6.11b) which is quite prominent. The grit is very tough and should not be hammered.
Oxfordshire might not compare to fossil hunter’s paradises such as the North Yorkshire or Dorset coasts, but Kirtlington Quarry and Dry Sandford Pit are certainly worth a visit if you find yourself in the area. At both sites, it is relatively easy to follow the different rocks strata, even if fossils are now rare. Both are family friendly for those with children.
Finally, nobody interested in fossils should miss out on a visit to the Oxford University Museum of Natural History, which has a vast and fantastic collection on display, including several dinosaurs collected locally.