Reddish brown rock. Medium sand grade.
Round to sub-round grains of quartz, often with a thin rim of red hematite. Mainly reddish and pink-brown grains, fewer white grains and very few black grains. Well sorted and well cemented.
No fizzing with acid, so does not contain significant carbonate. 

Laminations can be observed in sample L326b. There is an inter-layering of coarser and finer laminae.

Fine laminations can be seen when the thin-section is held up to the light. 
Moderately well rounded quartz grains with thin rims of hematite, and overgrowths of quartz in optical continuity with the original grain. 
Occasional grains of very fine quartzite, microcline, plagioclase, mica and decomposed fine-grained igneous rocks. 
Siliceous cement and the secondary quartz overgrowths hold this rock together.

The grains are finer as a whole in L326b, and less well rounded.

Probably deposited in an aeolian environment, since it almost entirely comprises well sorted and well rounded quartz grains.

ferruginous sandstone
ferruginous arenite

Mainly medium sand (to coarse sand in the case of L327a). 
Yellow-brown iron stained quartz, cemented by the same yellow brown oxide. A few larger quartz grains up to 5 mm long. Grains are angular to sub-rounded, and poorly sorted. 
Fizzing with acid, indicating that there is a carbonate cement.

Angular to sub-angular grains of quartz, displaying undulose extinction. 
Most have thin brown rims of iron oxide. 
A few grains of feldspar. 
Minor carbonate cement (sparite).

Deposited in an aqueous environment, since it is not clean enough to be aeolian.

Iron staining 
Hematite (Fe₂O₃) → red (blood-red, cherry-red, brown-red) iron-staining 
Limonite (FeO(OH)•nH₂O) → yellow or brown iron-staining

The yellow colour of this rock is probably produced by limonite.

ferruginous sandstone

Dark grey rock, mainly fine sand.
Does not react with acid, probably mainly quartz. 
Some glints of mica. 
Cement reacts with acid, so it is at least partially carbonate. 
Band of brown-grey material is due to weathering.

The thin-sections are only cut from dark grey, unweathered region of the rock. They are best viewed under high magnification.

Sub-angular to sub-rounded quartz grains. Mainly fine sand, with a significant proportion of silt. >15% clay, so this is a wacke.
A few white mica flakes and opaque grains.
The main cement is carbonate, though the grains are also partially cemented by the clay grade grains.

Wackes are commonly produced by turbidity currents, though we cannot be sure without seeing graded bedding.

sandstone
wacke

L340a 
- Homogeneous texture apart from one hairline quartz vein. Mainly mud sized grains too small to be distinguished, but probably clay minerals. Some silt sized grains of quartz, plagioclase and white mica. Patches of minor red-brown iron staining.

L340b 
- Hold up to light to see discontinuous, wavy laminations. Mud sized grains of quartz, feldspar, white mica and opaque minerals. Red iron-staining.

The dark purple-red of L340b indicates an oxidising environment during diagenesis, while the grey colour of L340a indicates a reducing environment.

Non-marine environments are more likely to be oxidising than marine environments where organic matter tends to promote a reducing environment during diagenesis. Reducing environments are associated with grey, black or greenish rocks.

Iron staining 
Hematite (Fe2O3) → red (blood-red, cherry-red, brown-red) iron-staining 
Limonite (FeO(OH)•nH2O) → yellow or brown iron-staining

The red / red-brown colours of these rocks is probably produced by hematite.

mudstone

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

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 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 buff coloured, crystalline rock. 
Numerous cavities, thought to have been formed by shrinkage during alteration of primary calcite (CaCO₃) to secondary dolomite (CaMg[CO₃]₂). There is a weak reaction to dilute acid, which suggests that there may be some calcite remaining. 
Colourless crystalline material infilling holes.

Almost entirely made up of very fine grains of dolomite (which tends to show more rhomb shaped crystals than calcite).
Irregular cavities into which well-developed rhomb crystals of dolomite have grown. Note that one should distinguish between the cavities in the rock (with rhombs of dolomite) and those due to the preparation of the sections.

It is thought that this rock was initially made-up of calcite. The alteration from calcite to dolomite, with its consequent increase in specific gravity and decrease in volume, accounts for the formation of the cavities.

dolmitic limestone
dolomite
magnesian limestone

Grey-buff colour rock made up of ooids, mainly medium grained and spherical. 
Some shell and lithic fragments. 
Well compacted and cemented by calcite, which fizzes with acid.

The ooids look brown. They are made up of calcite grains much smaller than the thickness of the slide (which is 30μm). These small grains refract, diffract and reflect light travelling through the slide producing the brown appearance of the ooids. Most of the ooids were formed around a central nucleus (usually an angular quartz grain, sometimes a shell fragment).The calcite forming the ooids is mostly too fine-grained to distinguish individual crystals. 
Only some of the ooids appear to be touching in thin-section, but taking account of their 3D relationships, the rock is probably clast supported.
Rounded shell fragments including aragonite bivalve moulds and silicified brachiopods. 
Occasional quartz grains.
Clean, colourless calcite cement.

This is a packstone (Dunham naming system) or oo-biosparite (Folk naming system).

Deposited in a moderate energy, marine environment.

oolitic limestone
packstone
oo-biosparite
oolite

Carbonate fossil fragment in a carbonate cement (sparite). The laminated structure of the shells is visible in places. 
Cement contains some hematite (brown regions). 
There are a few epidote grains and some opaque areas.
This is a rudstone (Dunham naming system) or biosparite (Folk naming system).

The shells are not in life position – they have been transported. 
Deposited in moderate to high energy environment. 

shelly limestone