Explosion of oysters!
This fossil consists of a mass of oysters from the strangely named ‘Cinder Bed’. This rock layer lies within the Purbeck Beds, a complex succession of limestone and clay rocks that formed in lakes and lagoons at the start of the Cretaceous period. The Victorian geologists thought that the Cinder Bed resembled cinders from a fire, but they also knew that it is actually composed of millions of oyster shells fused together.
The Cinder Bed formed in a time when sea levels were marginally higher, and salty water flooded the area. Geologists know this because, unlike nearly all of the other layers in the Purbeck Bed, the Cinder Bed can be seen wherever the Purbeck Beds are exposed: from Durlston Bay to Portland. Most of the other layers cannot be traced very far because they formed in lakes and lagoons that were not large. In other words, different environments existed across the Purbeck landscape at the same time, and different rocks formed in them. But when sea levels rose slightly, conditions were perfect for this oyster to grow across the whole area.
Layers of rock like this, formed from one single type of animal, are the result of extreme conditions in which only that one animal may be able to survive. Without competition or predators, their numbers explode into rock-forming proportions.
More about bivalves on the molluscs pages.