Low-permeable marine geological formation
Eroding London Clay cliffs at
The Naze
in Essex
Oxshott Heath, where the overlying sand and the London Clay layers are exposed as a sand escarpment, rising approximately 25 metres (82 feet)
The
London Clay Formation
is a
marine
geological formation
of
Ypresian
(early
Eocene
Epoch, c. 54-50 million years ago)
[1]
age which
crops out
in the southeast of
England
. The London Clay is well known for its
fossil
content. The fossils from the
lower Eocene
rocks indicate a moderately warm climate, the
tropical
or
subtropical
flora. Though
sea levels
changed during the deposition of the clay, the
habitat
was generally a lush forest ? perhaps like in
Indonesia
or
East Africa
today ? bordering a warm, shallow ocean.
The London Clay is a stiff bluish
clay
which becomes brown when weathered and oxidized. Nodular lumps of
pyrite
are frequently found in the clay layers. Pyrite was produced by microbial activity (
sulfate reducing bacteria
) during clay sedimentation. Once clay is exposed to atmospheric oxygen, framboidal pyrite with a great
specific surface
is rapidly oxidized.
Pyrite oxidation
produces insoluble brown
iron oxyhydroxide
(
FeOOH
) and
sulfuric acid
leading to the formation of relatively soluble
gypsum
(CaSO
4
·2H
2
O,
calcium sulfate
dihydrate). This latter is more soluble and mobile than
iron oxides
and can further recrystallize to form larger crystals sometimes called
selenite
(coming from the
moon
, but not related to
selenium
, although the
etymology
is the same), or "waterstones".
Large
septarian concretions
, produced by microbial activity (oxidation of
organic matter
) in the ancient seafloor during clay early
diagenesis
, are also common. These have been used in the past for making cement. They were once dug for this purpose at Sheppey, near
Sittingbourne
, and at
Harwich
, and also dredged, off the
Hampshire
coast. The clay is still used commercially for making
bricks
, tiles, and coarse pottery in places such as
Michelmersh
in Hampshire.
Distribution and geology
[
edit
]
The London Clay is well developed in the
London Basin
, where it thins westwards from around 150 metres (492 feet) in Essex and north Kent to around 4.6 metres (15 feet) in
Wiltshire
.
[2]
It is not frequently exposed as it is to a great extent covered by more recent
Neogene
sediments and
Pleistocene
gravel deposits. One location of particular interest is
Oxshott Heath
, where the overlying sand and the London Clay layers are exposed as a sand escarpment, rising approximately 25 metres (82 feet). This supported a thriving brick industry in the area until the 1960s. The London Clay is also well developed in the
Hampshire Basin
, where an exposure 91 metres (299 ft) thick occurs at
Whitecliff Bay
on the
Isle of Wight
and around 101 metres (331 ft) is spread along 6 kilometres (4 miles) of foreshore at
Bognor Regis
,
West Sussex
.
[3]
The clay was deposited in a sea up to 200 metres (660 ft) deep at the eastern end. Up to five cycles of deposition (representing
transgression
followed by shallowing of the sea) have been found, most markedly at the shallower, western end. Each cycle begins with coarser material (sometimes including rounded
flint
pebbles), followed by clay which becomes increasingly sandy. The final cycle ends with the Claygate Beds.
[2]
Claygate Beds
[
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]
The youngest part of the London Clay, known as the
Claygate Beds
or
Claygate Member
forms a transition between the clay and the sandier
Bagshot Beds
above. This is shown separately on many geological maps, and often caps hills. It is up to 15 metres (49 ft) thick at
Claygate
, Surrey.
[2]
It is now believed to be
diachronous
, with the formation at Claygate for example being the same age as the end of the fourth cycle of deposition further east.
[4]
Engineering
[
edit
]
The presence of a thick layer of London Clay underneath
London
itself, providing a soft yet stable environment for
tunnelling
, was instrumental in the early development of the
London Underground
, although this is also the reason why London had no true
skyscraper
buildings, at least to the same degree as many other cities throughout the world. Erecting tall buildings in London required very deep, large and costly piled foundations. This has changed in recent decades due to the development of 'plunge piles'. London's skyscrapers float on rafts embedded in the clay.
London Clay is highly susceptible to volumetric changes depending upon its moisture content.
[5]
During exceptionally dry periods or where the moisture is extracted by tree root activity, the clay can become desiccated and shrink in volume, and conversely swell again when the moisture content is restored. This can lead to many problems near the ground surface, including structural movement and fracturing of buildings, fractured sewers and service pipes/ducts and uneven and damaged road surfaces and pavings. Such damage is recognised to be covered by the interpretation of
subsidence
in buildings insurance policies, and the periods of dry weather in 1976/77 and 1988/92, in particular, led to a host of insurance claims. As a result, many insurance companies have now increased the cost of premiums for buildings located in the most susceptible areas where damage occurred, where the clay is close to the surface.
London Clay is also used to line exhausted quarries. This is because old quarry holes are generally refilled with waste material and by lining it with London Clay (which is virtually impermeable) it prevents waste and hazardous substances from entering the groundwater.
Tunnels in London Clay
[
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]
London Clay is an ideal medium for boring tunnels, which is one reason why the
London Underground
railway network expanded very quickly north of the
River Thames
. However, south of the Thames, the stratum at tube level is composed of water-bearing sand and gravel (not good for tunnelling) with London Clay below, which partly explains why there are very few tube tunnels south of the Thames. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost
waterproof
, resulting in virtually no seepage of
groundwater
into the tunnel. It is over-consolidated, which means that it was once subject to an overburden pressure higher than it is subjected to today, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground.
Uses
[
edit
]
Due to its impermeability especially when exposed by ploughing, London Clay does not make good agricultural soil.
[
citation needed
]
In
Middlesex
, ploughing London Clay land so deep that it brings up clay has historically been called "ploughing up poison".
[6]
It was also disliked for building houses on, though inevitably most of
Greater London
is built on it.
Jane Ellen Panton
in her
Suburban Residences and How To Circumvent Them
(1896),
[7]
remarks: "I do not believe clay is or even can be fit for anyone to reside upon ... though roses flourished magnificently children didn't, and coughs and colds [lasted through autumn and winter, this at
Shortlands
".
[8]
Many London buildings have ultimately been constructed with London Clay. When compressed and burnt, London Clay can be fashioned into brick known as
London stock
, identifiable by its yellowish brown hue.
[9]
Fossil flora and fauna
[
edit
]
Plant fossils, especially seeds and fruits, are found in abundance and have been collected from the London Clay for almost 300 years.
[10]
Some 350 named species of plant have been found, making the London Clay flora one of the world's most diverse for fossil seeds and fruits.
[11]
The flora includes plant types found today in tropical forests of Asia and demonstrates the much warmer climate of the
Eocene epoch
, with plants such as
Nypa
(Nipah palms) and other
palms
being frequently encountered.
Notable coastal exposures from which fossils can be collected are on the
Isle of Sheppey
in
Kent
and
Walton-on-the-Naze
,
Essex
, in the London Basin, and
Bognor Regis
in the Hampshire Basin.
Fruit and seed fossils from the Isle of Sheppey
[
edit
]
See also
[
edit
]
References
[
edit
]
- ^
a
b
Friedman, Matt; Beckett, Hermione T.; Close, Roger A.; Johanson, Zerina (2016).
"The English Chalk and London Clay: two remarkable British bony fish Lagerstatten"
.
Geological Society, London, Special Publications
.
430
(1): 165?200.
doi
:
10.1144/SP430.18
.
ISSN
0305-8719
.
- ^
a
b
c
Sumbler, M. G. (1996).
London and the Thames Valley
. British Regional Geology series (4th ed.).
British Geological Survey
.
ISBN
0-11-884522-5
.
- ^
Melville, R. V. & E. C. Freshney (1992).
The Hampshire Basin and adjoining areas
. British Regional Geology series (4th ed.).
British Geological Survey
.
ISBN
0-11-884203-X
.
- ^
Ellison, R. A.; et al. (2004).
Geology of London: Special Memoir for 1:50,000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales)
.
British Geological Survey
.
ISBN
0-85272-478-0
.
- ^
Buildings on Clay
The effects of geology, climate and vegetation on heave and settlement, Derek Clarke and Joel Smethurst
- ^
View of the Agriculture of Middlesex: With Observations on the Means of Its Improvement, and Several Essays on Agriculture in General
. By Board of Agriculture (Great Britain), John Middleton. Published by G. and W. Nicol, second edition, 1807, p. 20.
- ^
Jane Ellen Panton (16 August 2012).
Suburban Residences and How to Circumvent Them
. Cambridge University Press.
ISBN
978-1-108-05320-4
.
- ^
Quoted in
Aslet, Clive
and
Powers, Alan
,
The National Trust book of the English House
, p. 174, Penguin/Viking, 1985,
ISBN
0670801755
- ^
Ackroyd, Peter (2000).
London: The Biography
. London: Vintage. p. 9.
ISBN
0099422581
.
- ^
Chandler, M. E. J.
1961. The lower Tertiary floras of southern England I. Palaeocene floras, London Clay flora. London: British Museum (Natural History).
- ^
Collinson, M. (1983).
Fossil plants of the London Clay
. The Palaeontological Association.