How Fossils Are Formed

1 - Death 

Having died, the ammonite slowly sinks to the sea floor.  Scavengers feed on the fleshy body of the creature and the rest decays. After several weeks all that remains is the shell.

2 - Deposition

The shell gradually becomes covered with silt and sand.  These layers build up, providing a shield around the shell and protecting it from damage.  As time passes more and more layers are deposited.  After a few hundred years the shell is several meters beneath the surface.

Time continues to pass, 1,000 years, 10,000 years and more...

3 - Permineralization

Gradually the chemicals in the shell go through a series of changes.  As the shell slowly decays, water infused with minerals passes through it. Minerals such as Calcite, Iron or Silica replace the chemicals in the shell.  This process is scientifically known as 'permineralization'.

Over millions of years the shell is completely replaced by the minerals and what remains is a rock-like copy of the original shell.  The fossil has the same shape as the original object, but is actually rock, so the shell’s original colour is lost. 

4 - Erosion

Over millions of years the seabed is forced above the surface by the movement of the earth’s plates.  The new landscape holds the fossilized remains of the ammonite.

The rock is eroded by the sea, wind, rain, ice and the sun, slowly but surely the fossil is exposed once more.