Re: Why are flint and chalk always found together?

The Chalk in England is a fairly pure limestone made of the mineral calcium carbonate (CaCO3). It is Cretaceous in age. The calcium carbonate comes largely from coccoliths. These are tiny plates of calcium carbonate secreted by marine algal phytoplankton. Other organisms living in the sea such as sea urchins and bivalves are often found as intact fossils in the Chalk. Also living in the sea were many species of sponge. These have a skeleton made of silica spicules. When a sponge dies the living material decays and the spicules become incorporated into the sediment at the bottom of the sea. Over millions of years the sediment becomes hardened into rock as all the sedimentary particles become cemented together. The silic becomes dissolved by circulating waters. It becomes deposited elsewhere in the rock as the nodules that we know as flint. The flint is really a form of cryptocrystalline quartz. In the British Chalk the flints are concentrated at specific horizons. As the chalk is weathered and erodes away the more resistant flints become concentrated by wave action on beaches.

References:

Penguin Dictionary of Geology.
Geological History of Britain and Ireland - Edited by Nigel Woodcock and Rob Strachen.