Chert and flint
What are chert and flint?
Chert is a mineral formed of micro-cystalline siica, which is extremely small
crystals of quartz (silica) tightly packed together.
Flint is a type of chert that formed in one particular time and place, in the
soft white limestone rock called the Chalk during the Late Cretaceous
period, 100 to 65 million years ago, towards the end of the time of the
How and when was flint formed?
The Chalk rock was formed in a sea which covered most of Northern and
Central Europe and Britain. Tiny plankton called coccolithophores lived in
the water column and over millions of years their accumulated calcium
carbonate skeletons formed the Chalk.
Simple filter-feeding animals called sponges were also very common on
the sea floor Natural bath sponges are the skeletons of sponges composed
of flexible protein material containing no mineral matter. Many sponges
have a different skeleton formed of tiny needles of silica called spicules.
After death, siliceous sponges were buried by later sediments, and the
silica spicules dissolved under the higher pressure and temperature of
burial. Chemical conditions later caused this dissolved silica to be
redeposited. The redeposited silica formed the flint that replaced the Chalk
rock in the layers (tabular flint) and rows of scattered lumps (nodular flint)
that can be seen in white Chalk cliffs such as those at Dover.
Chalk crops out at the surface in a swathe across Britain from Dover to the
Yorkshire coast, but flint from the Chalk can be found all over Britain. Flint
is hard and insoluble, so that when the Chalk has been dissolved or
washed away by weathering the flint remains behind. Flint nodules are
broken up by frost, stream or wave action, and the last two of these wear
the fragments into round pebbles. Flint has been transported all over the
country by the sea, rivers, glaciers in the past and by people.
Characteristics of flint
Left: Flint showing conchoidal fracture and shiny fresh
Flint is dark, shiny and smooth when freshly
fractured but weathers to a dull surface that
can be white, yellow or brown and has a
Flint fractures with a conchoidal fracture, which means a curved surface like
a concave rainbow.
Fossils in flint
Some flints preserve fossils or the impression left by fossils next to the
flint, common examples are the shells of sea-urchins.
Above: The echinoid Echinocorys scutata preserved in flint.
Pseudofossils is a term for anything that is mistaken for a fossil but isn’t
one. Flint nodules sometimes have fantastic shapes and are mistaken for
fossils such as claws, teeth, bones and even fossil heads!
Left: Flint nodules mistaken for fossil
footprint (left) and fossil shark’s tooth (right).
These may be branching examples of burrow
infills (see below).
Flint burrow infills
Many of the flint nodules that are mistaken for fossils do actually have a
different kind of fossil origin. Many elongate, tapering flint nodules, which
may also be branching, have that shape because silica gel flowed into the
burrows of animals such as small shrimp in the ancient seabed. This silica
gel later hardened to form flint nodules in the shape of the burrows. These
are a type of trace fossil, meaning fossils that preserve the behaviour of
creatures (such as footprints or burrows) rather than body parts. Flint
burrow infills are commonly mistaken for fossil teeth and claws!
Left: A tapering flint nodule that may be a burrow
infill, which was brought in for identification as a
suspected fossil tooth.
Sponges in flint
Some flint nodules can be very spherical, and these are often mistaken for
fossil eggs! Many of the spherical flints have a fossil origin, having formed
around sponges. Fossil sponges in flint can also be other shapes such as
elongate or club-shaped. The fossil sponge can sometimes be seen in tact
within the flint nodule, but often the sponge breaks down to leave a
hollow with a distinctive “fluffy” texture. This is known as a “rotten” sponge
in flint. If a flint nodule has a hollow centre, you can sometimes hear loose
material in the centre rattle when you shake it!
Above: Spherical flint showing fossil sponge centre Above: Hollow left where sponge
has broken down.
Sometimes after a sponge has broken down, flint nodules have later
mineral growth within the hollows. This happens when the flint lies in
water with dissolved mineral content. Over long periods of time the water
penetrates the flint and chemical conditions cause the deposition of the
dissolved mineral within the water around the hollow inside. This forms a
geode, a term for a crystal lined cavity. The crystals in flint are usually the
common mineral quartz.
Left: Flint where hollow left by broken
down sponge has been filled by quartz
crystals to form a flint geode.
Some flint show a series of roughly parallel lines, either as three
dimensional ridges or a surface pattern. This lines vary in how far apart
they are and how straight or curved and they may change direction or end
abruptly. These are banded flints. They are often mistaken for fossils.
The cause of banded flint is still not known for certain. One theory is that
in some flints the silica was originally deposited rhythmically in bands that
differed in their original water content, which caused areas of the flint to
weather differently to create this shape. Other theories relate to the
movement of water through flints after their original formation changing
Above: two banded flints.
Find out more:
Shepherd, W. 1972. Flint: Its Origin, Properties and Uses. London, Faber
Smith, A. B. and D. J. Batten (eds). 2002. Fossils of the Chalk.
Palaeontological Association Field Guide to Fossils: Number 2. London, The
Luanne Faulknall, Earth Sciences Advisor
Identification and Advisory Service
Call: 020 7942 5045
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