Natural England - Essex (including Thurrock and Southend-on-Sea)

Essex (including Thurrock and Southend-on-Sea)

The geology of Essex is dominated by three distinct types of deposit; the London Clay (of Tertiary age), boulder clay, sands and gravels deposited during the Ice Age and coastal muds and silts laid down over the past few thousand years.

Much of the Essex coastline, as pictured here on the Stour, is characterised by soft, eroding Tertiary and Quaternary sediments

These rocks and sediments give rise to the distinctive landforms that characterise the Essex landscape.

A broadly flat, but undulating plateau covered by till dominates the northern part of Essex. The southern and eastern edges of the plateau are marked by a shallow wooded ridge, which sweeps round in a curve from Epping Forest to Tiptree. The ridge grades northwards (Chelmsford to beyond Colchester) into heathlands developed on Tertiary sands and gravels and slopes down to a lower-lying area of London Clay, which forms the coastal region to the south-east, and to the south the large urbanised area of London's northern fringe. More recent muds, silts and sands cover the London Clay and give rise to mudflats and saltmarsh all around the Essex coast and the Greater Thames Estuary.

Cretaceous

Cretaceous chalk (90-70 million years old), which was laid down in an extensive shallow tropical sea, is the oldest rock exposed at the surface in Essex. The Chalk is a white limestone comprising over 95% calcium carbonate and contains thin beds of marl and nodules of flint, either scattered or in bands. In Essex, the majority of the Chalk is concealed below Tertiary and Quaternary deposits, to leave only minor outcrops near Saffron Waldon and Grays. The chalk can be seen in quarries in the south of Essex, which represent the remnants of the once thriving Portland cement industry.

Tertiary (comprising Neogene and Palaeogene)

The Upnor, Woolwich and Reading Beds form the earliest Tertiary rocks in Essex. These shallow marine, estuarine and fluvial sands and clays rest on an eroded chalk surface and demonstrate a gradual lowering of sea-level and increased terrestrial influence after the tropical seas of the Cretaceous.

A rise in sea level around 50 million years ago led to the widespread deposition of the London Clay; a uniform bluish-grey clay deposited in a semi-tropical sea. Harder concretions and layers within the Clay, known as Cementstones, have yielded a large number of marine fossils including starfish, crabs, lobsters, fish, turtles, bivalves and gastropods. The youngest part of the London Clay is sandier and is known as the Claygate Beds. These rocks represent the transition between the deeper water London Clay and the succeeding shallow marine to estuarine Bagshot Sand. Outcrops of Bagshot Sand form several ridges ('Bagshot Hills') across the central and southern part of the county (e.g. Danbury-Tiptree and Billericay).

After deposition of the Bagshot Sand there was a significant fall in sea level and what is now Essex became land. The county was reflooded later in the Tertiary and shallow marine deposition continued into the earliest Pleistocene, some 2 million years ago. During this interval the shelly iron-stained sands of the Red Crag were deposited. The Red Crag is exposed along the northern boundary and coast of Essex (e.g. at Walton-on-the-Naze), where it is teeming with fossil marine shells and forms spectacular layers on top of the London Clay.

Quaternary

Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.

Deposits from this time are widespread in Essex and occur typically as river deposits on valley sides or as boulder clay (till). During the early part of the Quaternary, the forerunner of the River Thames flowed north-east across the northern half of Essex. The River Medway was an important southern tributary of this river system, joining it at Clacton-on-Sea. River sediments, laid down as a series of terraces, representing former floodplain levels, are known as the Kesgrave Sands and Gravels and were deposited across parts of Essex by this combined fluvial system. The gravels have great commercial value and are worked in numerous pits between Harlow, Chelmsford and Colchester, which was the route of the ancestral Thames at least 600,000 years ago.

Around 450,000 years ago a severe cold phase known as the Anglian glaciation caused an ice sheet to spread across East Anglia and the north-western half of the county. As the ice advanced it eroded the ground over which it passed, the eroded material was then deposited at the base of the ice to form a sheet of till. The till surfaces one of the dominant landscape features of Essex, the north Essex 'clayland' plateau. Associated with the till are sands and gravels deposited by meltwater streams issuing from the ice front. It was the Anglian ice sheet that pushed the Thames drainage southwards close to its present day course.

When the Anglian ice melted, blocks of ice buried within the till melted to form small lakes known as 'kettle holes'. These lakes filled with sediments during the subsequent warm interglacial phase (400,000 years ago) known as the Hoxnian. Classic exposures of Hoxnian age sediments are preserved at Clacton-on-Sea and East Mercia. During this time Neanderthal man moved northwards from continental Europe taking following the retreating ice.

The greatest and warmest interglacial stage during the whole of the Pleistocene was the Ipswichian, about 120,000 years ago. Ipswichian sediments rich in fossils are exposed at East Mersea. Here they are called the 'hippo gravels' because the hippopotamus was relatively abundant in Essex at this time. The Devensian cold stage followed the Ipswichian and ice spread south again to north Norfolk instigating cold tundra-like conditions in Essex.

In the 10,000 years since the melting of the Devensian ice sheet, sea-level has risen by 30 m and is currently rising at an estimated 2 mm per year. The relative levels of land and sea during this period have dominated the physical development of the Essex coast. The mudflats and saltmarshes of Essex have been created and sustained during this time by mud carried by the sea from the north. Thousands of acres of saltmarsh formed over the last 2,000 years, however, their extent has been greatly reduced in the past few centuries due to human land-claim. The remaining saltmarsh forms part of the Greater Thames Estuary coastal lowlands that stretch north from the Swale Estuary in Kent to the Essex/Suffolk border.

Geological Highlights:

  • The 18th and 19th century London Clay workings of the Harwich cement industry yielded remarkable fossil turtles and mammals. Cementstones occur in the clay and in one of these, in 1856, a workman found the skeleton of Hyracotherium (also known as Eohippus). Hyracotherium was the earliest ancestor of the horse and was no larger than a fox, with toes instead of hooves.

  • London Clay fossils, particularly sharks teeth, turn up all around the Essex coast but the most famous site is at Walton-on-the-Naze where the beach is very popular with collectors. The London Clay here is one of the reasons Walton is a site of international importance, mostly because the London Clay here yields superbly preserved bird fossils, including several species of parrot.

  • The Red Crag contains the teeth of sharks, the most famous of which is Carcharodon megalodon, the largest species of shark that ever lived, weighing in at some 65 tonnes. These teeth, which were derived from rocks of Miocene age and incorporated into the Red Crag, are extremely rare but examples have turned up on the beach over the years.

  • The ancient Thames gravels contain a variety of unusual pebbles, from as far away as North Wales, proving that, at that time, the Thames and its tributaries must have been a huge river system draining the Welsh mountains and bringing their characteristic volcanic rocks into the Thames basin.

  • The Thames gravels contain large boulders of puddingstone and sarsens, which are very hard conglomerates and sandstones. They are believed to be derived from pebble and sand seams in the Reading Beds, which have subsequently become cemented by quartz. They have been put to use by man as ancient way markers at road junctions.

  • The coastal scenery of Essex consists of a maze of winding, shallow creeks, drowned estuaries, mudflats and broad tracts of saltmarsh with sand and shingle beaches along the coast edge. The relatively permanent, meandering creeks, which dissect the saltmarshes, fill and empty with the tide and provide an interesting temporal variation within the marsh landscape. The Essex coastline, populated by a large and varied bird population invokes a strong feeling of remoteness and wilderness.

Local sites

The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.

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