Bucks Earth Heritage Group

Gault Clay

The Gault Clay is Lower Cretaceous (Albian) in age, and spans from around 105 to 95 million years ago. This was a time when the world was heating up, a global warming was beginning and sea levels were rising rapidly - the climax of which is seen in the Chalk overlying the Gault. This clay is grey in colour and is highly fossiliferous in places. The fossils occur in two forms: original shells (very fragile) and phosphatised (very robust). The phosphate fossils occur in discrete bands and represent a time when the sediment did not reach this offshore area of the sea-floor.

 

An ammonite. Small ammonites were common in the Gault sea. They evolved very rapidly and hence can be used to date many marine rocks to within a million years. (To a geologist plus or minus one million is excellent accuracy!)

 

 

 

 

 

Ammonite with a bivalve Birostrina sulcata embedded in the centre.

Note the sutures (wiggly lines) on the ammonite. These are where the internal walls join the shell and hence, to be able to see them the outer shell has to be worn away.

 

 

 

 

Gault ammonite called Euhoplites.

 

 

 

 

 

 

 

 

 

 

 

Ammonite from the Lower Gault Hoplites bonarelli. Note the white original shell and the black interior which is phosphatised clay.

 

 

 

 

 

 

 

 

These bivalves are Nucula pectinata and they are shallow burrowers. The internal cavity has filled with mud which has become phosphatised (hence the black colour). The shell has then be worn away, leaving just tiny white remnants 

 

Bivalve Birostrina concentricus. Note the smooth shell and concentric lines that give its name.

Bivalve Birostrina sulcata. This organism evolved over several million years from the smooth 'concentricus' to this strong-ribbed form. Presumably the selective pressure that forced this change was predation (as the environment was very gentle).

 

A little scaphopod mollusc called Dentalium. This calcareous tube was inhabited by a small animal with a filter feeding arm which could protrude from the shell.

 

 

 

 

 

 

 

 

 

Belemnites such as these Neohibolites were teeming in the Gault sea. These are the internal skeletons of squid like animals (see below).

 

 

 

 

What the belemnites used to look like within the Cretaceous sea

 

 

 

 

 

Although corals on the whole do not do well on muddy sea floors, these solitary corals existed in the Gault sea.

 

 

 

Fish vertebra. Fish and shark vertebrae are reasonably common, as well as scales and occasionally whole fish.

 

 

 

Gault clay with fossils. This is commonly how the clay appears, together with a miriad of small fossils. When wet these fossils can be cut out in small blocks with a penknife.