Bucks Earth Heritage Group

Chalk

The Chalk is a very pure limestone laid down during a massive global warming occurring between 95 and 65 million years ago. Sea levels rose to well above 300 metres above present levels and Buckinghamshire was completely covered by the warm waters. Life was abundant and the seas teemed with ammonites, belemnites, bivalves, sea urchins, sharks and sponges, and later on, the swimming reptiles called the Mosasaurs who ate ammonites and other animals. The Palaeontological Association has produced a book called The fossils of the Chalk.

Bivalves such as Plagiostoma are common in the Chalk

 

 

 

 

Chlamys - another well preserved bivalve. Bivalves usually have asymmetric valves, both a mirror image of each other. Some bivalves, like Chlamys, have symmetrically shaped valves.

 

 

 

Sea urchins are common fossils. Some urchins are heart-shaped, like this Micraster, others are round. The former are a streamlined shape for a burrowing life-style; the latter are surface dwellers and grazed on algae.

 

 

The urchin above has preserved its original shell and has been infilled by the chalk. This urchin has been faithfully replaced by flint and is much harder as a result. (Flint is a form of quartz.)

 

 

 

 

These round shaped sea-urchins are known as 'regular' urchins and grazed on algae present on the seafloor. They are therefore only found where the chalk had become hardened. The heart urchins in contrast would only be found where the chalk had remained a soft mud.

 

 

 

 

Sponges are incredibly common fossils, but people tend not to notice them. They are present in virtually every flint nodule which has a cavity within it. Next time you look inside a flint look for the tell-tale holes and texture of the sponges. They can be many shapes ranging from spherical to finger-like to very irregular shapes.

 

 

You will not see this fossil - or only indirectly anyway. This is a sketch of a Coccolith - a tiny alga composed of spherical plates. Many hundreds of them will fit onto a pin head! When you rub chalk and get the white powder onto your fingers you will have just smeared thousands of them between your fingers.

A typical Chalk exposure seen as this roadside cutting on the A41 Aylesbury to Tring road showing well preserved Chalk.