The London Clay
The environment of the region was now changing dramatically, leading to the deposition of the London Clay, a mud laid down on the floor of a subtropical sea some 50 million years ago. The London Clay has yielded fine fossils of the sea’s inhabitants such as molluscs, lobsters, crabs and sharks. There are also fossilised fruits, seeds and twigs which provide us with valuable information about the rain forest vegetation which existed on the land at this time.
Most remarkable of all are probably the fossil turtles and mammals from the workings of the Harwich cement industry in the 18th and 19th centuries. The cement was made from hard limestone nodules that occur in the clay and in one of these, in 1856, a workman found the skeleton of Hyracotherium (also known as Eohippus). Hyracotherium was the earliest ancestor of the horse and was no larger than a fox, with toes instead of hooves. This animal lived on the river banks and its bones must have been washed down a river into the sea. This was a time of rapid evolution of mammals following extinction of the dinosaurs.
London Clay fossils, particularly sharks’ teeth, turn up all around the Essex coast but the most famous site is at Walton-on-the-Naze where the beach is very popular with collectors. The London Clay here is one of the reasons Walton is a site of international importance, mostly because in it are the best preserved bird fossils of Tertiary age to be found anywhere in the world. The London Clay also contains layers of volcanic ash which may have originated in Scotland where there were active volcanoes around this time.
Lying on top of the London Clay is a sandy clay called the Claygate Beds. Above this is a delightful, fine-grained yellow sand called Bagshot Sand which indicates a shallowing of this sea. It formerly covered the whole region but erosion has now reduced it to isolated patches on hill tops in central Essex. There are virtually no visible exposures of Bagshot Sand in Essex and GeoEssex is currently hunting for any long-lost pits where a section might be preserved.