Bucks Earth Heritage Group

Chalk

Chalk forms the central backbone of Buckinghamshire, running from east to west and forming the scarp face of the Chiltern Hills. This rock is some 300m thick across the county, and is a distinctive soft, white limestone. The basal unit (Grey Chalk Subgroup, or Lower Chalk) has a higher clay content and contains no flint. The higher unit (White Chalk Subgroup, or Middle and Upper Chalk) is nearly pure, 98% Calcium Carbonate, and flint-bearing.

This pure limestone was laid down during a period of global warming. Sea levels rose to an estimated 300+m above present levels, the areas of land receded and the amount of river-borne sediment being brought into the area diminished, explaining its purity. Chalk is almost exclusively formed from the minute remains of fossil algae, called Coccolithophores. The calcite skeletons of these minute organisms invariably disintegrated on death, and descended to the sea-floor, as a soft ooze. Depositional rates must have been slow during this period. The high global temperatures and high nutrient levels in the sea meant that life was abundant. Although the chalk has selectively preserved some organisms over others (for example, calcite shells are preserved whereas aragonite has dissolved) fossils are fairly common, and include the single celled Foraminifera, Ostracods, Brachiopods, Sponges, Bryozoans, Bivalves, fish and even marine reptiles like turtles and plesiosaurs. Other molluscs such as ammonites and gastropods were undoubtedly present, but their shell composition meant that fossilization potential was low, and they are only found at certain levels where preservation was possible. The Palaeontological Association has produced a book called The fossils of the Chalk.

The sketch on the right is 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.

Bivalves such as Plagiostoma are common in the Chalk

 

 

 

 

Spondylus - another well preserved bivalve. Bivalves usually have matching valves, each a mirror image of the other. The bivalve Spondylus, has unequal valves, the larger being covered in long spines which anchored or supported it on the soft sea floor.

 

 

 

Sea urchins are common fossils. Some urchins are heart-shaped, like this Micraster, which are suited for a burrowing life-style.

 

 

The sea urchin to the left is an internal cast, in which the original chalk infill has been replaced by flint.

 

 

 

 

Other sea urchins had a more regular, circular, shape and lived on the sea floor. The specimen on the left had large spines which afforded protection and support.

 

 

 

 

Sponges are common in the Chalk although their fossilized remains can be difficult to recognise. They may appear as shadowy forms within Chalk but are more commonly seen in flint nodules which have nucleated around them. Silica formed the skeleton of these sponges and after death and burial it dissolved and was reprecipitated to form flint.

 

 

This Chalk exposure forms a roadside cutting on the old A41 at Tring Hill.