Early Eocene London Clay deposits at High Ongar, Essex (Part 1)

Bob Williams (UK)

I first encountered the geological deposit known as the “London Clay” when I accompanied a friend to an exposure of the stuff. He told me that it was good for collecting fossils. It was and I was taken aback by the quality and quantity of fossil material. However, I knew nothing at all about the geological details of the sediment. However, like all keen amateurs, I wanted to know more about the deposit.

To the uninitiated, the name “London Clay” suggests a single, uniform deposit. However, in truth, it does not fit that description. The name is given to a sedimentary deposit that contains at least five different and distinctive horizons (referred to as Divisions A to E). They were laid down in early Eocene times (50 to 54Ma) in conditions that were particular to slightly different environments or habitats (I use the terms interchangeably in this article). In a non-scientific way, the London Clay environments can be compared to the environments found in an ocean such as the Indian Ocean.

Fig. 1. Estimated position of the clay exposures at High Ongar Essex and nearby Aveley, showing the London Clay sedimentary deposits.

In broad terms, it is possible to describe the Indian Ocean as having warm, marine waters, being subject to tropical or sub-tropical climates and containing particular life forms. However, a variety of individual habitats can also be found in the Indian Ocean. There are shallow waters, deep waters, coastal waters, reef systems, trench systems, rocky substrates, soft substrates, cooler areas, warmer areas and so on.

Each habitat possesses its own physical characteristics and contains its own fauna. Some of these will eventually be represented as fossils in the resulting geology. So, for “Indian Ocean”, substitute “London Clay” and, hopefully, things will start to become clear. Each different habitat in the London Clay sea is represented by each of the named horizons, with each horizon being distinguishable from the others, but still forming part of the “London Clay”.

The horizons are further subdivided to represent habitat changes. For example, Division B is divided into Division B1 and Division B2. These may be distinguished by their faunal content that, in turn, indicates a change of environmental conditions allowing different life forms to exist.

The London Clay at Ongar

This article is in two parts. Over the course of this part, one of the things I will endeavour to show is how, considering the crustacean fauna in relation to each other and in relation to other faunas, different animal types allows insight into the habitats that might have existed at the time of deposition of the London Clay.

While undertaking research work at the ARC landﬁll site at Aveley in Essex (TQ 556809), I became aware of an exposure of London Clay nearby at the LECA aggregate works and landﬁll site at High Ongar in Essex (TL 562024). The site revealed a signiﬁcant exposure of London Clay that prompted me to look for fossils. As a result, I am now in a position to compare this site to others in the Eastern London Basin and to place it in an approximate stratigraphic position.

Fig. 2. Ariel view of High Ongar, Essex.

Fig. 3. Clay exposures at High Ongar, Essex.

Fig. 4. Old photograph showing clay exposure at High Ongar Essex, date unknown.

Some 18 miles north of the River Thames and 15 miles due east of London is the village of High Ongar in Essex. Located in the southeast corner of the village were the industrial premises of the LECA works and the Greenways landfill site. Part of these premises consisted of an excavation pit where clay was dug for use in the manufacture of aggregate. As the clay was excavated, it was sorted and any bulky materials removed. It was this excavation and sorting that revealed fossils suitable for collection.

Fig. 5. Locality map showing site locations and the positions of the Tertiary sediment basins that exist in the London and Hampshire regions of the UK.

The clay was exposed in a pit lying to the east of the main buildings. It was dug and extracted by means of a large, caterpillar-tracked, single bucket excavator. It was then placed on a single-track conveyor belt and transported to the nearby buildings.

In the pit, the clay was not exposed in neat terraces with horizontal and vertical faces, as it was at Aveley. Instead, the pit had sloping worksurfaces down which freshly excavated clay boulders could tumble. At times, this made it very difficult to identify which level a particular boulder had come from. Specimens were usually collected from the active excavation pit. In addition, site workers occasionally put specimens aside for me but, as a result of that kindness, I had little idea where these specimens had come from.

Much of my research was carried out between 1991 and 1998. The works have closed and the site is no longer accessible. However, when in use, it may have been the second largest, man-made inland exposure of London Clay found in the UK. (The biggest was probably at Aveley, where there was another landfill site, again operated by Greenways (Williams, 2002).

Stratigraphy and palaeontology

London Clay is a marine deposit that is well known for its fossils. It is the most important member of the Lower Eocene (Ypresian) strata in Southern England and is deposited over a wide area of the London and Hampshire Basins. There is now an established lithostratigraphic framework for the London Clay (King, 1981) based around ﬁve informal divisions (cyclotherms) labelled A to E (with A being the oldest). These, in turn, are divided into lithological members. For example, Division A1 in London and Essex described as a silty clay with ash bands, whereas Division A2 is a silty clay with silt streaks and lenticular accumulations of vegetable debris.

Broadly, the London Clay is thought to have been laid down in a marine environment inﬂuenced by a tropical or sub-tropical climate. Water depth is thought to have averaged about 200m, but, obviously, would have varied locally. It also depended on transgressive and regressive cycles, of which there are thought to have been ﬁve (giving rise to the A to E classification).

The London Clay from the Ongar pit consists of divisions from the top of the Isselicrinus Zone at the base of Division B2 to the top of Division C (King, 1981). The site has been investigated before and earlier reports include Cooper (1972) and Kirby (1974).

Visually, the clay was an homogenously textured, stiff, blue-grey deposit, weathering to a brown colour. Very little could be seen in the way of bedding. However, it was evident that there were layers of concretions in the deposit. These consisted of large, calcareous septarian concretions. Also present were smaller, softer, phosphatic nodules that yielded the bulk of the fossils.

One rare and important fossil and mineral specimen I found was a broken septarian nodule in which were exposed three examples of the crinoid Isselicrinus subbasaltiformis. The rarity of this fossil at this location meant that I donated it to the London Natural History Museum (specimen no. EE6650). The specimens were well-preserved stem sections. Their presence was taken to indicate close proximity to the Division B1/B2 junction that is marked by a distinct bed in which specimens of this crinoid species are elsewhere abundant (hence, the name “Isselicrinus Zone”). As no specimens of this species have been found in levels higher than the base of the pit, it is reasonable to assume that the main Isselicrinus Zone is located beneath the base of the pit.

Many of the fossils I found at Ongar were crustacea. This taxon of animals consists of arthropods with hard shells (mostly marine) and includes lobsters, crabs and shrimps. When I reviewed this crustacean fauna, the presence of the species Hoploparia gammaroides and Linuparus scyllariformis (which have both been found at Aveley in distinct beds about 3m and 5m respectively above the Isselicrinus Zone) appeared to conﬁrm that Division B2 was one of the horizons exposed at Ongar.

This was reinforced by the presence of the crab species Dromilites bucklandi and Cyclocorystes pulchellus, both believed to be Division B2 species. As at Aveley, the presence of the crab species Zanthopsis leachi, which was undoubtedly the most abundant fossil species I found at Ongar (and which occurred at higher levels in the pit at Aveley), indicated that Division C was also exposed. In addition, as at Aveley, fossils of the rare glypheid lobster, Trachysoma scabrum,were found. In my experience, the total absence of species such as Dromilites lamarckii (syn. Basinotopus lamarckii), Mithracia libinoides and Glyphithyreus wetherelli suggests that material from Division D and above was not present.

Many specimens of Zanthopsis leachi were also collected, representing a wide range of sizes, as there were fragmentary remains of some very large individuals, as well as some quite obviously juvenile individuals. Such an extensive range of sizes suggests that this horizon represented an area inhabited by members of the species at all stages of growth in the lifecycle and was not an area inhabited solely by animals of one particular age, for example, a nursery area.

Comparisons with modern species and their habitats

When looking at the crustacean fauna of any modern marine environment, it is apparent that particular families of crustacea favour particular types of habitat, just as individual species show a preference for particular habitats. However, when considering palaeohabitats, individual factors limiting the habitat range of an individual species may not be apparent.

Therefore, it is useful to consider the habitat range occupied by a particular crustacean family as a whole. This often shows a similarity of ranges occupied by species within the family. However, for fossil crustacea, such information can really only be obtained by the study of surviving and related species that, arguably, have similar environmental tolerances to their fossil relatives. It may then become possible to assemble a general picture of the environment in which a particular family may have lived.

Inevitably, this will not be an exact description. However, it provides a starting point from which a narrower and more precise picture can be developed, as more information comes to light. Such other information may come from the presence of other fossils that, when considered with information gathered by studying fossil crustacea, may indicate the existence of a particular habitat that favoured the co-existence of all the animal types under consideration.

For example, a particular family of modern crustacea may survive only in a particular range of water-depths or on a particular substrate (coral, rock, mud, sand and so on). Therefore, the presence or absence of that family as fossils in any deposit and/or of any other families of animals that favour a similar habitat will give some indication of the environmental conditions that may have existed at that location or very nearby at the time of deposition.

However, ﬁnding a solitary fossil specimen in a deposit does not prove that that species actually inhabited that depositional environment. The fossil may be the remains of an individual that was simply washed into the environment in which sediments were accumulating from another habitat nearby. On the other hand, the more common a fossil species is in a deposit, the more likely it is to have been an inhabitant of the depositional environment in which it was found.

I will now discuss certain modern animals that give some clues as to the habitats that may have existed at the time of deposition of the London Clay at Ongar.

Dromidae (sponge crabs)

Members of this crab family are commonly called sponge crabs and the family includes the fossil species Dromilites bucklandi referred to above. Today, Dromids are mostly found in water-depths of less than 100m and usually less than 70m to 75m. (However, as in most modern animal families, there will be the odd species that is adapted to live outside these parameters.) These crabs have a habit, from which the common name has been derived, of cutting and trimming pieces of living sponge that are then attached to the carapace to form a living camouﬂage. Other items may also be used such as pieces of soft coral, shells and Ascidians (sea-squirts). Living Dromids are also found on rocky substrate.

These features provide clues as to the range of habitats that may have been occupied by their fossil relatives at the time Division B2 was deposited. This habitat is likely to have consisted of relatively shallow water in which many sponges (or similar) lived providing camouflage for these crabs. It was also likely to have consisted of a harder surface, as opposed to muddy or sandy conditions. Where are such habitats found? Obvious examples include coral reefs and coastal habitats just below the inter-tidal shoreline.

Xanthidae (mud crabs)

Modern members of the crab family, Xanthidae, have a similar shallow-water depth range of up to 20m. The fossil species Zanthopsis leachi referred to above is a member of this family. These crabs show a preference for rocky or ﬁrmer substrates that provide holes and crevices into which individuals venture for shelter or protection. Again, such habitats are found in reef environments and surviving Xanthid species are often shallow reef-dwellers or live in a shoreline habitat.

Fig. 10. *Xanthopsis leachi* is a synonym for *Zanthopsis leachi*

Conclusion

The above examples provide useful clues as to the habitat that the fossil crab species found at Ongar may have occupied. In the next part of this article, I will provide more examples and will continue to discuss what this says about the environments that existed at the time the deposits at Ongar and at other sites were laid down. I will also try to explain how it is possible to locate the London Clay at Ongar in the stratigraphic column.

Acknowledgements

The author wishes to convey thanks to staﬀ members of the LECA company, who allowed unrestricted access to the excavation pit to permit collecting to take place. Thanks are also due to Essex Rock and Mineral Society member, Derek Breden, for initially ringing this site to the attention of the author and for supplying the author with a ﬁne collection of fossil material. Finally, once again, the author wishes to dedicate this paper to his family who have continued to endure his periodic absences from the family environment during the undertaking of research work and the preparation of this text.

References

Cooper. J. (1972) Report of Field Meeting to High Ongar, Essex. Tertiary Research Group;

King C. (1981) The Stratigraphy of the London Clay and associated deposits. Tertiary Research special paper;

Kirby. R. I. (1974) Report of project meeting and field meeting to Aveley, Essex. Tertiary Times. 2, pp53-67.

Williams, R.J. (2002) Observations on the London Clay excavation at Aveley, Essex. Tertiary Times. 2, pp 53-67.