The geology of Essex

Robert J Williams (UK)

To the north-east of London lies the county of Essex, which extends out to the east coast of England. Although not noted for its geology, it does have quite a bit to offer the amateur geologist. In addition, there is a significant body of published geological research on the area and, in this article, I will discuss some of the geological work (focusing, in particular, on the London Clay) that amateur palaeontologists and collectors have undertaken in recent times to show that amateurs really can make a difference to the earth sciences.

Admittedly, Essex does not have an abundance of fossils or geologically rich, ancient sediments from the Palaeozoic and Mesozoic, like elsewhere in the UK. Notable exceptions include exposures of Mesozoic chalk from the Cretaceous, which are about 80myrs old. However, Cenozoic sediments can prove just as interesting and worthy of investigation, particularly those dating from the Pleistocene (the ‘Ice Age’), which occur at ground level exposures across most of Essex. In addition, Jurassic fossils can be found in deposits of ‘boulder clay’, which constitute the most extensive of these Pleistocene deposits.

Essex chalk

The oldest sediments at the surface in Essex are deposits of chalk laid down during the late Cretaceous, about 80 million years ago (Figs. 3 rand 4). These deposits have also been exposed in commercial quarries around the Grays area, in the south of the county, and there is a small exposure near Saffron Walden. Chalk fossils typically include brachiopods and echinoids. Flint nodules are also found in the chalk, flint consisting of the mineral quartz. (Flint was of great importance to early man, who used it to make tools such as hand axes, hammer stones and scrapers, these all frequently being found in the Essex countryside.)

The London Clay and earlier deposits

The main deposit overlying the chalk is the London Clay, but small exposures of earlier sediments, which were laid down after the chalk, are sometimes found exposed in the south of the county. These include the Oldhaven Beds, which contain large numbers of fossil molluscs.

The London Clay was laid down over a period of four to five million years, commencing about 55 million years ago. It is a seemingly homogeneous, dark clay that can actually be separated into five divisions, which geologists refer to by the letters A to E (King, 1981). The fossil evidence indicates that the London Clay was laid down in tropical marine conditions. It was deposited in a syncline referred to as the ‘London Basin’, which was created by mid-Tertiary tectonic folding. The London Clay is succeeded by a series of younger Eocene deposits, these being the Bagshot Sands, Bracklesham Beds and Barton Beds, but only the Bagshot Sands are exposed in Essex.

London Clay has been, and is, exposed at a number of sites in Essex, with the deposits achieving a maximum thickness of about 150m. The exposures that have received the most attention, particularly since the 1960s, have been in active quarries where the clay has been excavated for bricks and cement. Unfortunately, these pits saw later use as landfill sites, which were gradually backfilled, completely concealing the clay.

Two such pits were at Aveley, 5km north of the River Thames, and at High Ongar, 25km further north. Both pits received considerable attention from amateur palaeontologists, as the result of which a number of technical papers were published, notably by Kirby in 1974 and Cooper, also in 1974. Other sites of a similar nature included South Ockendon, near Aveley (George, W. & Vincent, S. 1978) and Brentwood (Kirby, R. 1975).

I was introduced to the High Ongar site by a friend in the early 1990s and collected fossils from there over a period of time, the details of which were published in Issues 8 and 9 of Deposits (see Early Eocene London Clay deposits at High Ongar, Essex, parts 1 & 2). As a result of these visits, a site worker informed me of another nearby exposure at Aveley. When I first went there, the site was in the process of being converted into a landfill. However, there was an exposure of London Clay to a depth of 30 to 35m, which yielded a rich collection of fossils.

When I researched the site, it became apparent that only a limited amount of research work had been carried out there. Therefore, I took the opportunity to investigate the site thoroughly and published a paper in 2002 (Williams, 2002). I was also lucky enough to find a new species of fossil crab. This has now been examined, described and named, and I was delighted to discover that the name it was given was Londinimola williamsi (ref. BMNH IC453).

At the end of the last century, the Aveley site ceased to be a landfill, having been backfilled to capacity. It has also been fully landscaped. A similar fate befell the site at High Ongar. However, many species of crabs and lobsters were recovered from both sites (see box: Crustaceans found at the sites discussed in this article), these being the most frequently found fossils in the London Clay.

However, a number of sharks’ teeth, fish bones and other fossils, both vertebrate and invertebrate, have been found. These included a specimen recovered at Aveley initially thought to be a small set of jaws from a turtle (upper and lower jaws together), but was subsequently identified by the Natural History Museum in London as the ‘beak’ (jaw set) of a ‘Parrot’ fish, a rare find from this deposit.

While many of the London Clay sites in Essex have disappeared, significant new exposures were temporarily exposed by roadworks and excavations made when the M11 and M25 motorways were constructed. Plans for the future widening of the M25 motorway will doubtless reveal more exposures, which will hopefully attract the attention of researchers once more.

Much work is currently being undertaken by amateur palaeontologists on riverside exposures of London Clay in Essex. This includes research by two members of the Essex Rock and Mineral Society (www.erms.org): Jeff Saward, who specialises in crustacea, and Rick Johnson, who specialises in sharks’ teeth. Although riverside exposures are ideal for constantly presenting a researcher with fresh sediment, working time at the two rivers Jeff and Rick are researching (the rivers Crouch and Blackwater) is restricted as the rivers are tidal.

However, they have managed to find two new species of fossil crab (Sharnia burnhamensis and Penticarcinus maylandiensis) and one new species of fossil lobster (which is currently being described and named). They have also extended the chronostratigraphic range of a number of other species. The sites being investigated on the River Blackwater include Creeksea, Maylandsea (Fig. 6) , Osea, Steeple and Stansgate, with Burnham and Butts Cliff situated along the River Crouch (Fig. 7).

For the amateur geologist, two further exposures of London Clay can be visited, one at Roper’s Farm, Barling, near Southend (this is another landfill site) and another at Walton-on-the-Naze. These latter deposits lie beneath the well-known Red Crag (Fig. 8) sediments and expose sediments from Division A of the London Clay. Access is simply a matter of walking along the beach in a southerly direction and examining the clay deposits exposed on the shore.

Minerals in the London Clay

As well as fossils, there are some interesting minerals found in the London Clay. Large, hard calcareous concretions, known as septarian nodules, are common. These are often found in a fractured condition, having broken along internal fracture planes that are lined with a yellow coloured deposit of the mineral calcite (‘septae’, hence the name ‘septarian’) (Fig. 9).

Occasionally, pale coloured rosettes of the mineral barite (Figs. 9 and 10) are present and blades of the mineral selenite (gypsum) (Fig. 11) can also be found, if superficial weathering has occurred.

An interesting property of these latter two minerals is their ability to fluoresce when illuminated with ultra-violet light. Long and short wave ultra-violet lamps can be bought and they can open up a completely new world of study and interest for mineral collectors. As a bonus, some of the barite rosettes will continue to glow in the dark, a property known as ‘phosphorescence’, once the ultra-violet light source has been extinguished.

Another common mineral encountered in the London Clay is iron pyrite (Fig. 12). This can occur as discrete nodules (often burrow fillings), small distinct aggregates with characteristic cubic crystal formations and also as an infilling of fossils.

Pleistocene deposits

There are no younger sediments than those from the Eocene exposed in Essex until Pleistocene deposits are encountered. These include the two million-year-old Red Crag sediments mentioned above. The best exposures are in the coastal cliffs of Walton-on-the-Naze (Fig. 13) , but also occasionally appear in small, inland quarries. A walk along the beach at Walton takes you past orange/red coloured sandy cliffs, which are packed full of fossil shells (gastropods and bivalves, some of which species are alive today). The cliffs here are very easily eroded, resulting in a constantly changing shape to the coastline. The fossils are not fully mineralised and can be very delicate, so care should be taken when collecting. The site is also an SSSI, so hammering at the cliffs is not permitted.

Gastropod species found in the Red Crag include Neptunea contraria (the ‘left handed’ whelk, which still lives in the Mediterranean), Nucella tetragona, Hinia granulata, Searlesia costifera, Uzita reticosa, whelks (Buccinum undatum), the dog winkle (Nucella incrassata) and a small cowrie shell (Trivia coccinelloides). The bivalve fauna includes Dosinia exoleta, Glycymeris glycymeris, Nucula laevigata, Astarte obliquata, Cerastoderma parkinsoni, the razor clam (Ensis ensis) and the mussel (Mytilus edulis). There is also a small, irregular echinoid (Echinocyamus pusillus).

Vertebrate fossils can also be found, including teeth of the sharks Isurus hastalis and Carcharocles megalodon (the latter of which have probably eroded out of long-vanished Miocene deposits). Bone fragments and ear-bones of whales, such as Balaena affinis, have also been found. (Note that Walton-on-the-Naze is also a lovely place to take the whole family!)

‘Boulder clay’, or ‘till’, referred to above, is a material laid down by melting ice sheets as they retreated at the end of the ice age. In simple terms, as glaciers formed, the spreading ice sheets scraped up surface material and carried this with them as they advanced. When the glaciers retreated, this material was deposited where it had been transported to, which may have been far from its original source.

Jurassic fossils are found in the boulder clay exposed at many locations in central and north Essex, particularly at High Ongar, where fragments of belemnites, ammonites and specimens of the mollusc genus Gryphaea have been recovered. In particular, some of the boulder clay laid down in Essex contains Oxford Clay fossils transported from the north. These are some of the oldest fossils found in Essex, dating from the Jurassic, some 160myrs ago.

Frequently encountered throughout the county are deposits of sands and gravels that were laid down during the Pleistocene ice ages. An ancestor of the River Thames deposited these sediments on top of the London Clay, into which the river frequently cut channels. These channels were then filled with sediments during the warmer, inter-glacial stages and these have proved to be rich in Pleistocene, ice age mammal remains, many of which have been recovered by amateur collectors.

For example, mammoths, straight-tusked elephants (Fig. 15) , wild oxen, wild horses (FIg. 16), woolly rhino, bison, hyena and cave bear have all been found.

Erosion and human extraction have exposed these deposits and, where the underlying London Clay is also exposed, the channelling features can be observed and the deposition of the more recent sediments can be examined in detail. These gravel and sand deposits are frequently used for commercial purposes and their excavation has made available much fossil material. I have many such fossils in my own collection, retrieved from the site at Roper’s Farm. Of particular interest is a fragment of woolly rhino shoulder blade on which ‘butcher’ marks can be clearly seen, indicating the activities of early man.

The Aveley site, referred to above, achieved fame in 1964 when amateur collector, John Hesketh, and his mother found the physically adjacent skeletons of a woolly mammoth (Mammuthus primigenius)and a straight-tusked elephant (Paleoloxodon antiquus) in river sediments overlying the London Clay. These were subsequently recovered and displayed at the Natural History Museum in London.

So, the evidence is out there! It just needs to be collected and/or recorded. I hope I have shown that the work of amateurs is valuable to the earth sciences in this respect. The word ‘amateur’ does not have to imply unimportance. It is derived from the Latin, amare (‘to love’) and this indicates a passion for the subject and not any inadequacy. In fact, amateurs really can contribute to scientific knowledge and have frequently done so in the past.

Crustaceans found at the sites discussed in this article (Fig. 17)
Crabs: Zanthopsis leachii, Zanthopsis unispinosa, Xanthilites bowerbanki, Goniochele angulata, Portunites incerta, Portunites stintoni, Mithracia libinoides, Dromilites bucklandi, Dromilites simplex, Cyclocorystes pulchellus, Campylostoma matutiforme, Glyphithyreus wetherelli, Laeviranina gottschei, Londinimola williamsi, Sharnia burnhamensis and Panticarcinus maylandiensis.
Lobsters: Glyphea scabra, Hoploparia gammaroides, Homarus morrisi, Linuparus scyllariformis, Linuparus eocenicus, Archaeocarabus bowerbanki and Scyllarides tuberculatus.
This list, together with the addition of a new lobster species that is currently being described, constitutes almost all of the London Clay crustacean fauna that has been found in Essex.

References

Cooper, J. 1974. Report of Field Meeting to High Ongar, Essex. Tertiary Times 2:18-22.

Cooper, J. 1974. Report of Field Meeting to Brentwood Pit, Essex. Tertiary Times, 2: 102-105.

Darmon, C. 2009. p4. ‘Down to Earth‘. Geo Supplies, Chapeltown, Sheffield. S35 2XE. 

George, W. & Vincent, S. 1978. Notes on the London Clay of the Ockendon Clay Plant, South Ockendon, Essex. Tertiary Research. 2: 5-8.

King, C. 1981. Stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper. No. 6.

Kirby, R. 1974b. Report of project meeting and field meeting to Aveley, Essex. Tertiary Times 2: 53-67.

Kirby, R. 1975. Report of Field Meeting to Brentwood Pit, Essex. Tertiary Times 2, 102-105.

Williams, R. J. 2002. Observations on the London Clay excavation at Aveley, Essex. Tertiary Research, 21(1-4), 95-112.

Further reading

Essex Rock. A look beneath the Essex Landscape. Author G. Lucy. Published by the Essex Rock and Mineral Society. Currently out of print.

London Clay Fossils of Kent and Essex. Authors David Rayner, Tony Mitchell, Martin Rayner and Fred Clouter. Published by the Essex Rock and Mineral Society.

Ward, G. 1978. London Clay Fossils from the M11 Motorway, Essex. Tertiary Research 2: 17-21.