Fossil Sponge from Radwinter
Saffron Walden Museum offers a free identification service for objects from north-west Essex. People often find interesting stones in their garden or in fields. Many of these are flint nodules. Flint is a hard rock that comes from chalk, a soft white limestone that is 200 metres thick in north Essex and Cambridgeshire. Chalk was formed as a limy mud on the floor of a tropical sea that once covered most of Britain and north-west Europe during the Cretaceous period 65-145 million years ago. The sea water contained dissolved quartz, or silica, originating from the skeletons of tiny sponges. As the mud was compressed into chalk the silica became concentrated as nodules or layers of flint. When the chalk became exposed as dry land, erosion by rivers released the flint and redeposited it as thick layers of gravel. Flint is often found as brown, iron-stained pebbles. Unweathered nodules, fresh from chalk rock, are black with a white outer surface.
Fossil Sponge, Radwinter
Occasionally flints contain fossils of sea urchins or cockle shells. You can see some of these fossils in the Museum’s geology gallery – The Earth Beneath Your Feet. This circular stone from Radwinter is a sponge called Porosphaera globularis which is fossilised in flint rock. The animal lived in the Chalk Sea that covered Essex 80 million years ago, during the age of the dinosaurs. The flint has been stained brown by iron in the soil. Circular flint nodules are often fossilised sponges, or they have formed around the nucleus of a sponge. The size can vary, from flints the size of musket balls to nodules the size of cannon balls.