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.