This white rock is virtually pure carbonate. It fizzes with acid. It is fine-grained and poorly cemented, making it soft and low density. It is commonly known as ‘chalk’.

Very uniform cryptocrystalline carbonate. 
Rare larger calcite crystals <0.5mm.

Very fine-grained, so deposited in a very low energy environment. 
Such fine carbonate could have been produced in one of two ways:
1. Tests of calcitic plankton. 
2. Very finely ground shell fragments. Currents alone could not grind the shells this finely. This would require bioerosion – e.g. fish ingesting organisms with carbonate shells, and grinding the carbonate to a fine powder.

limestone
mustone
micrite
chalk

Pale yellow-grey mudstone containing the fossil remains of a small arthropod with a segmented exoskeleton. 
The rock is banded and shows some surface discolouration due to weathering.
Rock fizzes with acid, so contains carbonate. 
This is a soft, though well consolidated limestone (the surface has been scratched).

Moving across the slide, perpendicular to the laminations, there are alternations (gradual transitions) between calcareous mudstone and argillaceous limestone. One band is rich in angular quartz grains (0.1 to 0.2 mm), and contains carbonate shell fragments.

Calcite mudstone – fine-grained mosaic of calcite, small proportion of transparent brown impurity and a few small grains of quartz.

Argillaceous limestone – even finer grained calcite mosaic, more abundant brown material (probably organic) than in the calcite mudstone.

Low energy, aqueous environment.

The fossil remains are those of Archaeoniscus, a small crustacean (which belongs to the arthropods), of supposed freshwater origin. With this extra knowledge it may be inferred that the rock was deposited in fresh water.

argillaceous limestone

Very dark grey to black mudstone. 
Faint, fine-scale laminations visible on cut surface. 
Not fissile - does not readily split along bedding planes.
Contains fossils and fossil imprints. 
Some glinting flakes of mica. 
This rock fizzes gently with acid, indicating that it contains some carbonate. 
Weathered to light brown-grey in places.

Laminations are very clear when the thin-section is held up to the light. They are mainly planar, but some wrap around lighter regions in the rock.

Under the microscope, the rock is brown, with fine dark brown streaks, laminated.
Mainly carbonate, some white mica, rare quartz.
Bioclasts, mainly along bedding surfaces, probably gastropods and bivalves. Some of the shells have been dissolved. The spaces left have been filled by secondary calcite.
Sparry carbonate cement.

Fine-grained, so formed in a very low energy environment. 
Shells are too large to have been swept in by such low energy currents, so the shells are in life position. 
If the shells are indeed gastropods and bivalves the rock could be marine or non-marine. The presence of brachiopods would indicate a marine environment.

calcareous mudstone