Robin Hood's Bay

THOSE LINES in the foreground of my Art Pen and waterbrush drawing of Robin Hood's Bay (right), are arcing exposures of Jurassic rocks which have been domed upwards by earth movements. Stuart, a local geologist, tells me that those particular beds are the mudstones - some shelly, some sandy - of the Echioceras raricostatum zone. Echioceras raricostatum is an ammonite that gives its name to this particular set of strata.

Shells all over the place

Ammonites were free-swimming creatures, resembling a squid in a shell, like a modern nautilus, so their shells ended up falling to the seabed in a variety of habitats. My animation shows a goniatite which was a forerunner of the ammonites.

The shell could float a long way after the creature had died, as it was divided into chambers, which the creature could fill with water or gas to control buoyancy. Their shells evolved steadily over time so that you can correlate mudstones, limestone, ironstone and sandstones over a wide area because their shells got all over the place.

Peterborough museum has a sauropod vetrebra the size of a cake tin with impression of ammonite shells on its surface. The sauropod, a brontosaurus type of dinosaur, lived on land - and probably wandered along the shore on many occasions. After death, the body or parts of the body must have floated out to sea as the bone was found in an Oxford Clay brick pit.

Pound Stone

Further down the coast, we take a look at a wall constructed in a rustic kind of way from assorted chunks of a shelly limestone from the Hambleton group of Jurassic rocks. These chunks haven't fallen from the cliffs, as I guessed when I saw them, but they're from a local quarry, so the fossils are similar.

Stuart spotted this fossil sea urchin (left, below), called Clypeus from the Latin meaning shield-shaped but it's also known as a pound stone, as the fossil - which you could hold in the palm of your hand - weighed approximately one pound and was used, for example, by dairy maids for weighing butter.

My thanks to Mike Windle for that story; Mike tells me that William Smith (1769 -1839), 'the father of English geology' who was born in Oxfordshire, was familiar with this fossil because he would often have seen it in use at local farms. He became convinced that it was the shell of a living creature, not a meaningless 'sport of nature', which was one of the explanations for fossils at that time.