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The Chalk

The Chalk is exposed in chalk pits on the South Downs and also outcrops eastwards along the coast from Felpham. Originally divided into the Lower, Middle and Upper Chalk, it is now separated into the Grey Chalk Group and the White Chalk Group. These are further divided into a number of Formations that are characterised by relatively distinctive variations in the chalk sediment. Many chalk pits, like the one shown in the photograph are on private land, but there are numerous smaller pits that can be reached from public footpaths.

 

For more on the Chalk, listen to our expert Rory Mortimore by following this link and selecting the Chalk audio programme.

 

Chalk was formerly dug for lime-making and the remains of lime kilns can still be found on the South Downs. There are large ones to be seen at the Working Museum at Amberley. Chalk was also once used as a building stone and many churches, such as at North Stoke, have made extensive use of the stone. Flint also comes from the Chalk and the traces of ancient flint mines can be seen on the hilltops  The mines at Cissbury (photograph) are particularly well known.

Formation of flint

There are many incorrect statements on how flint is formed, but the chemistry is now well understood. The Chalk seas thrived with life, including sponges and microscopic organisms that built their skeletons from silica dissolved in the sea water.

On death and burial in the chalk mud, the silica slowly re-dissolved to provide silica-rich water within the sediment. There then follows some complex chemistry in which the silica is precipitated from solution at the boundary of the oxygen-rich sediments immediately beneath the sea floor and the low oxygen sediments below about 10 metres beneath the sea floor.

Flint is the end product from this precipitation of silica. It often forms around fossils sponges and the shells of sea urchins (echinoids), which provide a nucleus for the flint to form. However, most flint is formed in ancient burrow systems that provided easier passage for the mineral-rich waters.

Flint nodules in Chalk in a beach exposure at Seaford. The nodules have formed within burrows excavated by creatures that lived in the soft ooze of the Chalk sea floor. The partial preservation of burrows is the reason why flint nodules tend to be knobbly.