Why are fossils from Charmouth in Wareham Museum?

The answer is that we wanted to make the connection between the oil under Poole Harbour (the Wytch Farm oilfield), and the rocks from which the oil originated. The oil comes from organic material trapped in rock. These rocks formed on a stagnant sea floor where the organic material, mostly plankton, rained down onto the sea bed and became buried in the sediment.

At Charmouth, you can see the layer of dark clay rocks that the oil comes from, but you can’t see this rock at the other end of the Jurassic Coast. This is because the coast dips to the east, so much so that in Purbeck and Poole Harbour, the rocks are buried underground. In fact, they are deep enough for heat and pressure to ‘cook’ the organic material into oil and gas. The petroleum industry calls these places ‘kitchens’. Charmouth doesn’t produce oil because the rocks are not buried deep enough, but around the world there are thousands of places where underground organic-rich rocks do form oil – and our entire economy is based on it. Who says geology is dull or unimportant!


These lovely ammonites come from Charmouth. Find out more about ammonites on the molluscs pages.

Common name


Scientific name

Asteroceras obtusum


Molluscs > Cephalopods


Black Ven Marl Charmouth Mudstone Formation

Time period

Lower Jurassic


195 million years

Where found


Found by



Wareham Museum

Accession number

WARE 0003