Two things are noticeable about the next section of rocks: they are almost black and in very thin continuous layers, a bit like slate.

Watch Dr Mike Simms talk about these rocks

These rocks would have been formed by fine mud settling at the bottom of a sea, where there was little or no oxygen so that iron in the mud was not oxidised.

The mud would have been black, stinking stuff, with a whiff of hydrogen sulphide coming off it, so not much could have lived there.

These rocks also have rusty patches - iron pyrites, better known as fools’ gold.

The presence of fools' gold is typical of rocks formed by mud and clay where there is little oxygen.

Pyrites only really form in a marine environment and so are another clue to the environment these rocks were forming in, over 200m years ago.

There are patches among these rocks where the fools’ gold has not rusted and still has a golden colour.

There are not many fossils here but there are patches where there must have been a little more oxygen for a while which allowed small shellfish to move in.

However it wasn’t long before conditions deteriorated and they died off again.