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 dinosaurs.
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 surface.
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 rougher texture.
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
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.
(Diameter 3cm).
Flint geodes
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.
Banded flint
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 the shape.
Above: two banded flints.
Find out more:
http://www.discoveringfossils.co.uk/chalk_formation_fossils.htm
http://www.chalk.discoveringfossils.co.uk/Default.htm
Shepherd, W. 1972. Flint: Its Origin, Properties and Uses. London, Faber and Faber.
Smith, A. B. and D. J. Batten (eds). 2002. Fossils of the Chalk. Palaeontological Association Field Guide to Fossils: Number 2. London, The Palaeontological Association.
Luanne Faulknall, Earth Sciences Advisor
Identification and Advisory Service
Call: 020 7942 5045
Email:
Visit our forum: http://www.nhm.ac.uk/natureplus/community/identification
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