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Left: The famous
Seven Sisters chalk cliffs
in East Sussex. Right:
A giant chalk ammonite exposed on the foreshore at
Chalk is one of the best known of rocks, recognisable for its white
colouration in striking land features such as the White Cliffs of Dover and
Seven Sisters (pictured above), and familiar
to most in everyday products
such as blackboard chalk. Chalk has been exploited by man for thousands of
years for both its physical and chemical properties and has fascinated
scientists for centuries because of the fossils it contains and the
geological story it tells.
How did the Chalk form?
Chalk is formed from lime mud, which accumulates on the sea floor in the
right conditions. This is then transformed into rock by geological
processes: as more sediment builds up on top, and as the sea floor subsides,
the lime mud is subjected to heat and pressure which removes the water and
compacts the sediment into rock. If chalk is subject to further heat and pressure it becomes
The lime mud is formed from the microscopic skeletons of plankton,
which rain down on the sea floor from the sunlit waters above. The Coccolithophores are the most important group of
chalk forming plankton.
Each miniscule individual has a spherical skeleton called a cocosphere,
formed from a number of calcareous discs called coccoliths. After death,
most coccospheres and coccoliths collapse into their constituent parts.
magnification image of chalk coccoliths. Right: An
Most chalks formed during the Cretaceous period, between 100 and
60 million years ago, and chalks of this age can be found around the
world. The Cretaceous chalks record a period when global
temperatures and sea levels were exceptionally high. This coincided
with the break up of the supercontinent Pangea, which broke apart to
form the continents of today. As continents move apart, an ocean
forms between them, and new ocean-floor is added along the line of
spreading (known as the mid-ocean ridge) by magma which rises from
below. As the continents moved apart in the Cretaceous, a very high
volume of magma rose up to form the new ocean-floor in what is known
as a superplume event. The mid-ocean ridges became swollen, and
large volumes of magma spilled out elsewhere onto the ocean floor,
displacing water onto the continents (causing sea-level to
rise). The volcanic activity also produced greenhouse gases which
raised temperatures, prevented ice from forming at the poles and
hence kept sea levels high. Chalks formed in the sea-ways of the
flooded Cretaceous continents.
Why is chalk white?
Chalk is white because it is formed from the
colourless skeletons of marine plankton. The same is true of many
limestones, so why aren't they all white? The reason is that most
limestones contain impurities, such as clays sourced from the land,
or organics, which give them colouration. The Cretaceous chalk is free
from impurities because sea levels were very high, so there was
little land exposed to supply other sediments, and as the
continental margins were flooded most land was far away. The
Cretaceous sea floor was also very active so any organics were
quickly broken down. The result was a very pure lime mud, formed
almost entirely of planktonic skeletons.
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Where can you find chalk?
Chalk cliffs are a familiar feature of the South and East coasts
of England, and chalk downland runs between Devon and Yorkshire and
across the counties of the South East. The Chalk also extends underground
beneath London, the North Sea and the Channel. In England the Chalk
Group is divided into three formations: the Upper Chalk, Middle
Chalk, and the Lower Chalk.
The chalk was
originally horizontal, but has been folded along with the other
strata of southern England by continental movements over the last 30
million years. Throughout this period Africa has been pushing into
Europe and creating the Alps. Britain is far from the site of
impact, so the deformation is expressed more subtly by large, gentle
folds, slowly uplifted and quickly eroded. The North and South
Downs represent opposing sides of a great dome-like fold, whose roof
has been eroded away to expose the older rocks of the Weald within.
A quick rise in sea level after the last ice age flooded the broad
valleys south and east of Britain, creating the North Sea and
Channel, and incising into the downland to create the chalk cliffs
What fossils might you find?
Fossils found in the Chalk Group record life on the Cretaceous sea
floor. The chalk is very thick and deposition spanned 35 million
years. The fossils found at the base of the Chalk Group (the oldest)
differ from those at the top (the youngest) because of the many
evolutionary and environmental changes that took place.
composed of planktonic skeletons and is therefore made of
micro-fossils. In fact, the coccolithophores that comprise chalk
are small even by planktonic standards and are therefore termed nanno-fossils.
Chalk is an excellent material for fossil
collecting and palaeontological studies. The rock is hard enough to
preserve fossils in their original three dimensions, but soft enough
to allow palaeontologists and collectors to carefully expose
specimens from within the matrix. Among the most commonly found
fossils within the chalk are bi-valves, echinoids, ammonites,
bryozoans and sponges. Provided with access to a reasonable sized
chalk exposure, a good picture of the range and volume of former
life can be built in just a few hours of searching. Typically, only
small-medium sized durable fossils are found, comprised of a single
skeletal part (e.g. brachiopod shell, echinoid spine). Soft parts
are never preserved.
Occasionally chalk sediment was transported downslope and buried the inhabitants of the sea floor alive. Rare
but spectacular fossils of exceptionally preserved fish, starfish,
echinoids, crinoids and crustaceans record these events. Other
scarce fossils include pterosaurs, ichthyosaurs, plesiosaurs and
turtles. Very, very rarely, the remains of dinosaurs were carried
out to sea.
Left: Chalk fish
(Hoplopteryx lewesiensis). Right: Chalk
A vast amount of chalk was quarried in England in the 19th
Century, typically by hand. This was the heyday for chalk fossil
collecting, as the blossoming of scientific study coincided with the
industrial revolution and the demand for chalk. It was fashionable
for gentleman scholars of the Victorian era to establish large
fossil collections, and quarrymen were rewarded for any significant
finds. Most important museum collections were established during
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What can Chalk be used for?
Chalk has a great many uses to mankind, some familiar, some
surprising. Many of you will use chalk all the time, either for
chalking-up pool cues, drawing on blackboards or creating artwork.
Sports-people such as gymnasts, athletes and mountain climbers use
chalk on their hands and feet to provide grip.
Various uses of Chalk (from left):
drawing, writing on blackboards, snooker cue, athletics.
Chalk has been used as a building stone, and chalk rubble is
often used in road construction. When heated chalk becomes lime,
which has a great many applications. Lime is used in the production
of Steel, Aluminium, Glass, paper, sugar, cement and fertilizer.
Various commercial uses of chalk
(from left): construction and building stones, lime for steel
production and agriculture.
The chalk strata itself is widely used as an aquifer, its
network of fractures making it highly permeable. Abandoned
quarries are being developed as retail centres.
Chalk landmarks besides the famous White Cliffs of Dover, a
great number of natural and man-made landmarks are found on the
chalk coast and downland.
From left: Seven Sisters, East Sussex; Beachy
Head, East Sussex; Old Harry Rocks, Dorset and The Needles on the Isle of
From left: Wesbury Horse, Wiltshire;
Uffington Horse, Oxfordshire; Long Man of Wilminton, E.Sussex and the Cern
Giant in Dorset.
Join us on a fossil hunt
Left: A birthday party with
a twist - fossil hunting at
Right: A family hold their prized ammonite at Beachy Head.
Discovering Fossils guided fossil hunts reveal evidence of life that existed
millions of years ago. Whether it's your first time fossil hunting or you're
looking to expand your subject knowledge, our fossil hunts provide an
enjoyable and educational experience for all. To find out more