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Flint Formed in Chalk

Cliffs at Beachy Head

Q: How is flint formed in Chalk on the southern coast of England?

From Mr Angus Stephen (October 2009)

Reply by Amanda Lane (Chair of Solent Regional group)

Flint is a microcrystalline rock made of silica and is considered to have begun forming soon after the deposition of Chalk. The silica replaces the original Chalk carbonate grain by grain. The carbonate has to be dissolved with silica precipitated in its place.

As Chalk is an alkaline rock an acid has to be generated to dissolve the Chalk in order to enable the silica to precipitate. Bacterial activity in the sediment within the seabed generates hydrogen sulphide which, when migrating towards the seabed/seawater interface, meets dissolved oxygen migrating down in the sediment.

Where the dissolved oxygen and hydrogen sulphide meet, acidic conditions are generated, carbonate is dissolved and silica precipitated to form flint. The silica that formed the flint derives from silica fixed in the skeletal structure of many marine organisms.