ESSEX ROCK

GEOLOGY BENEATH THE LANDSCAPE

Ian Mercer & Ros Mercer

Illustrations and design by Trevor Johnson

Pelagic Publishing
Contents
Preface to the second edition 5
Preface to the first edition 6
Foreword 7
Acknowledgements 8

1 Reconstructing Essex 12
Geology: the great detective science 14
Geological time 16
The importance of Essex geology 19

2 The rocks of Essex 22

Seeing Essex rocks 24
The land beneath the soil: geological maps 26
The shaping of the Essex landscape 28
Digging into Essex 30
Naming rock layers: decoding the jargon 32
Time gaps 36

3 The deep history of Essex 38

Hidden history: the oldest rocks beneath Essex 40
Dinosaur island 47

4 The geological structure of Essex 48

Down to the basement 50
A story of tilting and squeezing: the London Basin 60

5 The drowning of the island 66

A rise in world sea levels 68
The Chalk Sea 70
The Chalk in Essex 76
Life in the Chalk Sea 88
The origin of flint 98

6 Seashores and swamps 104

A great extinction 107
Edgeland Essex 109
The Lambeth Group 116
Pebble beds and shell banks 134

7 Palm trees and crocodiles 136

Subtropical Essex 138
Harwich Formation 141
The London Clay 145
Shallow seas to deltas 168
4 Essex Rock

8 Giant sharks and shell banks 174

A time gap and a cooling climate 176
Crag seas and shell banks 184
Red Crag in Essex 186
Chillesford Sand and River Thames 195

9 Ice age Essex 198

Time and change 200
Ice age Essex in three episodes 202
Climate change and ice age geology 205
Ice age River Thames: the Pre-Anglian Kesgrave Sand and Gravel 208
River terraces 216
Glaciers and ice sheets: the Anglian Glaciation across Essex 228
Around the ice edge 236
The Post-Anglian: a major diversion 254

10 Looking into the Essex landscape 280

After the ice 282
Geology, soil and farming 292
The dynamic landscape 295
Landslips 301
Earthquakes 313
Stories in the Essex landscape: looking back into time 317
Caring for the evidence: geoconservation and Essex geological sites 329

11 Uncovering Essex geology 330

The Essex geology detectives 332

12 Rock and people 346

Economic geology: what we take out of the ground 348
Development of water supplies in Essex 374

13 The future of Essex rock 380

Climate, rock and recycling 382
Sea defences and the future: the Naze example 388
The next glaciation 394
Beyond the ice 395

Sites and views of Essex 398

Geological collections and displays 403
Index 404
Maps and charts 412
About the authors 416
 2

Ice age gravels revealed

by cliff regrading works
at Holland-on-Sea
in 2018.
These gravels were
laid down by a great
river formed by the
combined Thames
and Medway.

The Age of Essex

500 million years 400 300 200 100 Now
Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene
The rocks of Essex
24 Essex Rock

Seeing Essex rocks

Perhaps you have looked at the scenery while
travelling around Essex and wondered why its ‘lumps
and bumps’ and its flat areas are where they are;
or maybe you notice a hill or slope as you cycle or
walk. There is something to spot wherever you go,
regardless of farmland, woodland or urban cover.

Sarsen stones by a farm track

near Gestingthorpe.

Flint and quartzite

in a cobble wall,
Little Baddow church.
Ancient walls
were constructed
of whatever rocks were
available locally
and can therefore tell
us much about the
local geology.

London Clay exposed

on the foreshore at
Walton-on-the-Naze.
It is soft – but it is
Essex rock.
 The rocks of Essex 25

When you are out walking, maybe in a country

park or along a footpath, do you see the colour of
the soil, a selection of pebbles, an area where the soil
has worn away and different colours of sediment are
revealed? Perhaps you notice ‘odd rocks’ on village
greens, at crossroads or by trackways; or you spy an
ancient church and wonder what the walls are made
of. What are those layers of soft rock and fossils at the
seaside? How old are they? Such observations help
to reveal the deep history of this county and indeed
anywhere we might find ourselves.

A footpath worn into the geology beneath: pale,

sandy Claygate Beds at Thorndon Country Park.
26 Essex Rock

The land beneath the soil: geological maps

The surface geology of much of Essex consists of a The geology of Essex has been mapped by the
thin veneer of sands, gravels and clays, together called British Geological Survey (BGS) over many decades.
‘superficial’ deposits. They have been left behind by Their maps show the rocks that occur at the surface.
rivers, ice and storms during the ice age. These layers, Maps, borehole records and much else can be
rarely much thicker than 40 metres in total, could accessed from the BGS website. Paper maps are also
be regarded as the icing on the geological cake – a available for Essex at a scale of 1:50,000. With these
good analogy, as they contain some remarkable resources it is possible to predict which types of rock,
fossils and exotic rocks yielding evidence of our most and therefore fossils, might be found at any place in
recent geological past. These superficial layers, shown Essex. They also help provide an insight into the rocks
in the map below, underlie wide areas of farmland occurring at depth.
and urban sprawl, yet river erosion is carrying them
away to the sea, leaving a messy-looking picture with
varied scenery. The map opposite reveals the layers
that would be exposed if all the superficial deposits
were removed. This shows the pattern of older rocks
across Essex. Geological map of Essex – ‘superficial’.
These coloured areas show where
the ice age superficial layers lie.

• Ipswich
• Sudbury
• Saffron Walden

• Harwich

• Colchester
• Bishop's Stortford • Braintree
• Walton-on-the-Naze

• Witham • Clacton-on-Sea
• Harlow
• Chelmsford • Maldon

Superficial layers:
• Brentwood Alluvium
• Finchley
• Hornchurch • Basildon Brickearth/coversand
• Ilford • Southend-on-Sea Post-Anglian river deposits
Anglian: glacial outwash
• Grays Anglian: glacial till
• Dartford Pre-Anglian river deposits
Crag group
Chalk bedrock seen in a quarry face Geological map of Essex – ‘bedrock’.
at Chafford near Grays in south Essex. These coloured areas show the underlying layers,
all laid down before the ice age.

• Ipswich
• Sudbury
• Saffron Walden

• Harwich

• Colchester
• Bishop's Stortford • Braintree
• Walton-on-the-Naze

• Witham • Clacton-on-Sea
• Harlow
• Chelmsford • Maldon

Bedrock layers:
• Brentwood
• Finchley Bagshot Sand
• Hornchurch • Basildon Claygate Beds
• Ilford • Southend-on-Sea London Clay
Lower London Tertiaries
• Grays Chalk
• Dartford Rocks older than Chalk
28 Essex Rock

The shaping of the Essex landscape

The surface features of the landscape are largely broad southern Thames terraces, and the coastal areas
controlled by the underlying geology. Although and river estuaries. Each landscape area has its own
anyone might notice a change from the rolling distinct character and the underlying rocks influence
countryside of north-west Essex to the flat coastal the vegetation, slopes, wildlife, land use, building and
areas in the east, there are distinct types of landscape infrastructure, even the architecture.
across the whole county: the Chalk areas, the glacial
till plateau, the London Clay vale, the Claygate Beds The rolling chalk landscape of north-west Essex.
The hills of this part of Essex are an extension
and Bagshot Sand ridges and hills with high-level
of the Chiltern Hills to the west.
gravel hilltops, an enclosed bowl of fenland, the Here, bedrock is making the landscape.

East Horndon church nestled within

the Essex landscape.
Landscape types
The types of landscape across Essex have been with the Essex landscape character map, you can
assessed to help planners establish policies for conser- appreciate the strong influence of the geology upon
vation and development. A landscape map shows the landscape.
geographical areas each with a characteristic pattern
of landscape. When you compare the geological map, Essex landscape character areas.
which shows which rock layers are at the surface, Based on Essex County Council information.
 • Ipswich
• Sudbury
• Saffron Walden

• Harwich

• Colchester
• Bishop's Stortford • Braintree
• Walton-on-the-Naze

• Witham • Clacton-on-Sea
• Harlow
• Chelmsford • Maldon

Chalk Uplands
Coastal Landscapes
• Brentwood
• Finchley Glacial Till Plateau
• Hornchurch • Basildon London Clay Landscapes
• Ilford • Southend-on-Sea River Valley Landscapes
Sandy Coverloam
• Grays Urban Landscapes
• Dartford Wooded Hills / Ridges
30 Essex Rock

Digging into Essex

The soft sedimentary rocks of Essex are easily disin- for example when a pit is dug, a major new road is
tegrated by rain and frost and become covered with constructed or a quarry is opened, that a window into
vegetation. Consequently, the only natural rock the past is created. Any excavation that is sufficiently
exposures to be found in Essex are on the coast, deep to penetrate the topsoil will expose the rocks
where the sea is continually eroding the rocks. Inland, beneath.
it is only when the blanket of vegetation is removed,

Essex rocks beneath

the landscape:
the gravel bed
of the former River
Thames revealed
at Birch Quarry
near Colchester.

Digging a pit
to investigate
ice age layers near
the clifftop at
Walton-on-the-Naze.
 Dig down a third of a kilometre (1,000 ft)
Geological time Rock layers
Soil / Alluvium
Holocene Loess / Head
Quaternary Pleistocene 11,700y Glacial Till
2.58My Superficial
Thames Gravel deposits
Neogene Pliocene Red Crag

Bagshot Sand
Claygate Beds

Thames Group
Paleogene
Cenozoic

Eocene
London Clay

Harwich Beds
Oldhaven Beds
Woolwich / Reading Beds Lambeth
56My Upnor Beds Group
Thanet Sand Montrose
Palaeocene Group
58My
80My

White Chalk
Mesozoic
Cretaceous

Chalk Group
Late

Grey Chalk

97My

Early Gault Clay

100My
Devonian Old Red
Sandstone
Paleozoic

380My
Slate
Silurian Basement
430My
= Time gap
My = million years
32 Essex Rock

Naming rock layers: decoding the jargon

The sediments laid down in the past may have ways depending on the types of information available;
hardened (e.g. into limestone) or remain relatively for example, in biostratigraphy (life stratigraphy) fossils
soft (e.g. gravel, clay), but these are all called ‘rock’ by are used for correlation and in chronostratigraphy
geologists. Sedimentary rocks are usually deposited (time stratigraphy) the age of the rock is used.
in layers or strata. Sometimes a layer is referred to Lithostratigraphy is fundamental to most geological
informally as a ‘bed’. To build up a more complete studies. Sets of rock strata are assigned into units
geological picture, the layers need to be correlated so that they are easier to talk about and to show
from place to place. The description, definition and on geological maps. For instance, the Woolwich
naming of rock layers is termed lithostratigraphy (rock Formation are a set of sandy, shelly and clay layers
stratigraphy). The layers can also be described in other that are put together into one unit.
Harwich Formation
Thames Group

Wrabness
Member

Harwich
Conglomerate
Bed

Upper Shelly
Beds
Member
Woolwich Formation

Upper Mottled
Lambeth Group

Beds
Member

Sand
Channel
Bed

Laminated Beds Rock layers in the

Member Woolwich Formation
revealed in the
Thames Tideway
Project, a super sewer
constructed beneath
London.
Tim Newman
 The rocks of Essex 33

The relationship of each unit in the whole rock biostratigraphy is extremely valuable, sometimes
record is set within a formal hierarchy: Supergroup, enabling correlation of rocks across great distances.
Group, Formation, Member and Bed. The layers of The formal units used are Eon, Era, Period, Epoch, Stage,
the Woolwich beds, for instance, are together called Biozone and Bed. This approach is also particularly
the Woolwich Formation. This, in turn, is part of the useful where the rock type is broadly similar over a
Lambeth Group. The units are usually named after long time period, such as during the Upper Cretaceous
a geographical locality, typically the place where when chalk was the dominant rock type; for example,
exposures were first described. Another rock layer the Micraster coranguinum (a type of sea urchin) zone
in Essex is referred to as the Thanet Sand Formation, enables a particular layer of Chalk to be correlated in
even though Thanet is an area in Kent. A layer with a a thick and confusingly uniform pile of layers.
place-name keeps the same name wherever it occurs,
under Essex or elsewhere.
The Formation is the basic rock unit for mapping
purposes, as in the London Clay Formation. A Group
is an assemblage of related and adjacent Formations
– so the London Clay Formation is part of the Thames
Group. A Member is a subdivision of a Formation, for
instance the Claygate Member is part of the London
Clay Formation. The terms ‘Bed’ and ‘Band’ are very
often used in an informal way to aid description, such
as with the Bullhead Bed and the Harwich Stone Band.
For some sedimentary rocks a useful correlation is
one based on the fossils they contain – the biostra-
tigraphy method. Because animals evolve over time,
the species present may be different in each layer. A
rock layer containing the same fossil species is of the
The sea urchin Micraster coranguinum
same age wherever it is found, even if it has a different is a zone fossil in the Chalk.
lithology (rock type) in different locations. Therefore, Richard Hubbard

Age dating of rocks

These two methods of correlation, lithostratigraphy rate over time. However, such isotopes are not always
and biostratigraphy, provide only the relative ages present in a particular rock and other methods have
for rocks and fossils; they merely help us put the rock to be used in combination. Palaeomagnetism may
layers into a time order, but without us knowing how be used with rocks that contain magnetic minerals –
old they actually are. The absolute dating of rocks, to layers may be correlated using their ‘locked-in’ record
give an age in millions of years, depends on the occur- of the direction of the Earth’s magnetic field when
rence of radioactive isotopes that decay at a known the rock formed.
34 Essex Rock

Tephrostratigraphy, the correlation of volcanic ash century have been more specifically assigned and
layers, has enabled the phases of rifting during the new names have been introduced to accord with
opening of the North Atlantic Ocean to be linked to national and international standards. So, the names
ash layers in rocks across the north-west European familiar to those of us who have been studying Essex
continental shelf. geology for more than 30 years have been added to
In recent years, major revisions of the naming and changed. The study of geology is an ever-evolving
and grouping of rock units have taken place. This is scene. What a rock layer is called should not be a
due to a more rigorous approach, with international barrier to discovering how it was formed and the
collaboration and a much greater knowledge of the contribution it makes to the understanding of the
rocks themselves. Subsurface data from oil and gas story of our county through deep time.
exploration offshore and large-scale civil engineering
projects have helped considerably. Names first intro- Volcanic ash in pale bands within the Harwich
duced by Sir Joseph Prestwich in the nineteenth Formation at Wrabness.
 Coastal erosion reveals
an ice age deposit at
Wrabness. Correlation
is based on the fossils
it contains.

•Gestingthorpe

•Bulmer Brick & Tile Co.

A geological map of
a small area of Essex
Bedrock: Superficial layers: near Sudbury, showing
London Clay Alluvium – clay, silt, sand & gravel
the bedrock and the
superficial rock layers
Lower London Tertiaries Solifluction head – silt, sand & gravel
that appear at the
Chalk River terrace – sand & gravel
surface. All types of
Anglian glacial outwash – clay, silt, sand & gravel
stratigraphy are used
Anglian glacial till in making and revising
Kesgrave Thames sand & gravel such maps.
36 Essex Rock

Time gaps
Sediments are not deposited continuously over Essex lies within an ever-changing edgeland of the
geological time. The rocks we see were formed in European continent. It was ‘separated’ from mainland
various geological settings. The potential for sediments Europe whenever a shallow sea occupied the area of
to be preserved and then turned into rock depends the subsiding North Sea Basin. As a consequence, over
on the type of environment, such as whether the area millions of years, the surface of Essex has fluctuated
was sinking and a sediment layer was quickly buried by between low land and shallow sea. Thus, sedimentary
the next influx of material, rather than being washed layers have only occasionally accumulated and some
away. For large expanses of geological time there may of these layers have been eroded away subsequently,
be no record in the sediments. Instead there will be resulting in a discontinuous rock record with many
time gaps. The rock layers beneath Essex illustrate time gaps.
this remarkably well.
The larger time gaps usually represent periods The layers of rock beneath Essex are shown
when an area was above sea level and the land was on this timescale to reveal the time gaps.

worn away by erosion. Other time gaps were caused

when sediment was not laid down or preserved. The Geological research has revealed that there is a
50-million-year time gap between this dark grey
next layer of sediment then covered an area where London Clay and the overlying shelly Red Crag
the rock record was missing, leaving a time gap. layer along the cliffs at Walton-on-the-Naze.

Time Gap
 More gap than record: Essex rocks in geological time
Era Period Epoch
Now Ice age deposits
Quaternary Pleistocene
Norwich & Red Crags
Pliocene
Ice a
warmgpe and
5
eriods
10
Neogene
Miocene
15

20

25 48 milli
o
time gnayear
Oligocene p Uplift, folding and erosion
Cenozoic

30 of Paleogene sediments

35
Paleogene

40

45
Eocene

50
Million years

Bagsho
t Sand Claygate Beds
Harwich Beds
London
55 Clay Woolwich
1 million year time gap Beds
60 Reading Upnor
Paleocene Thanet Beds Beds
S and Thanet Sea floods over
65 the eroded Chalk

70
25 milli
o
75 time gnayear
p
Uplift, folding and erosion
of the Chalk
Cretaceous

80
Mesozoic

85

90 Gault and Chalk sea

White C floods over the
h alk
Grey Ch Anglo-Brabant Massif
95 alk
Gault C
lay
100 400 mil
lion yea Uplift, folding and erosion
time ga r of Devonian / Silurian rocks
p