Chalk (calcium carbonate CaCO₃)

The Origin of Chalk

The Chalk of Bell Hill (and Southern Britain generally) was formed around 70 - 90 million years ago, in the Upper Cretaceous period. Chalk is composed of the skeletal remains of the bodies of minute marine animals, that can only accumulate uncontimated by muds and other sediments in the deep ocean, away from land (It is though the ocean was between 100 and 300 metres deep at this time, with a temperature around 20 degrees centigrade, deduced from comparisons of the micro fauna found fossilised in the chalk with those of today). Research has revealed the chalk is almost entirely uncontaminated by other deposits, with the only exception being ocassional tiny quartz grains, which are believed to have either blown into the ocean from surrounding dessertified land, or possibly come from volcanic origins.
The thickness of the chalk layer varies greatly, but in general is about 215 m in total depth. The chalk layer is divided into the Lower Chalk, averaging 60 m in thickness, the Middle Chalk, averaging 60 m in thickness, and the Upper Chalk, averaging 95 m in thickness.

Uses of Chalk

Chalk is used for a number of purposes. It is used extensively in the building industry to make lime, cement, quicklime, putty, plaster,and mortar, essential elements in building both now and historically, and the last local commercial chalk quarry closed only recently. As a raw material for building chalk is very limited, having little resistance to water and frost, and degrading rapidly. Nevertheless, it can frequently be found as in fill, covered by protective layers, inside walls and buildings. St Aldhelm Church has chalk on the inside of older parts of the structure and as in fill in non-structural later internal additions and renovations. At nearby Woolland Hill a large estate boundary is partially encircled by a flint faced wall in filled mainly with chalk.
Amongst the other uses for chalk are liming agricultural land to sweeten the soil, as a surfacing material for tracks, yards etc (The Saxon inhabitants of nearby Okeford Fitzpaine are thought to have used packed chalk for flooring their pigsties). The obvious use as artists chalk and blackboard chalk, though blackboard chalk is often made from gypsum (calcium sulphate), and talc (magnesium silicate) is used for tailors chalk as it is softer, so less scratchy. Gymnasts and rock climbers use powdered chalk on their hands to aid grip on equipment and/or rockface. Builders use a chalk line (string covered in chalk) to mark out a straight line. When showing a dog in a show ring that has white fur, rubbing chalk power in the fur can make it whiter.
Other older uses for chalk include it being used for medicinal purposes in the fight against scurvy during the Middle Ages, and in the mid to late 14th century Welsh market traders used to try to pass chalk off as a hard cheese on unsuspecting customers, hence the popular term 'chalk & cheese'.

Flint

Chalk has a small number of associated minerals. By far the most significant of these to man is flint. Flint is comprised of Silica, and comes in many shades, predominately white through orange and grey to black, depending on it's formation and contamination by other minerals, though many different colours can be found. Some flint is almost translucent, particularly in thin sections.
Flint is thought to have originated from the decomposition of sponges and other sea plants on the bottom of the ancient ocean, creating deposits of Silica from their remains. The process isn't fully understood, but essentially involved the accumulation of the silica in nodules, or sometimes bands, within the chalk, and undergoing solidification, probably over a considerable time. In some circumstances the silica may have dissolved again and been re deposited within the chalk. Flint often replaces the minerals of sea urchin fossils, thus making them far more durable and prolific than most chalk fossils.

Uses of Flint

Man has used flint from the very earliest times. Prehistoric tools were knapped from it, and it's importance at this time cannot be overstated, as a huge variety of tools were crafted from it with great skill. The best tools came from certain types of flint, which often had to be mined. It seems either the blackest or whitest flint was best for tools, as most tools we have found are at one end of the spectrum or the other. Flint tools are extremely sharp, and can be up to four times sharper than a surgeons scalpel.
In later times flint was used extensively for building, providing a very resiliant building material. Many buildings in Belchalwell have some flint in their construction, including the oldest parts of the church. Whilst these were the primary uses for flint, other uses included the knapping of flints for flintlock firearms, for road building, and in the making of stoneware.

Pyrite and Marcasite

Minerals from the chalk - Iron Pyrites and Marcasite nodules

Iron Pyrite and Marcasite are two other commonly found minerals in the chalk, though they are essentially two forms of the same substance, Iron Sulphide (FeS₂), crystallised under differing conditions. They are frequently deposited within hydrothermal veins, though can be produced in many other conditions, and both are frequently altered to other forms and minerals.
Pyrite is the most numerous form on Bell Hill, with some nodules up to 100 mm across, and in varying shapes from spherical (see picture) to downright rude!.
The Marcasite is in smaller nodules, and often of a form known as Cock's Comb Marcasite, due to its Cock's Comb shaped crystal form. Many deposits contain differing forms of both Marcasite and Pyrite in the same 'nodule', whilst at the other end of the scale one nodule approx. 50 mm diameter was composed of near perfect Marcasite crystals (see picture).

Uses of Pyrite and Marcasite

Native Americans used to carve and polish pyrite for use in mirrors, though there is no evidence of this in prehistoric Britain. The primary use for Pyrite was for the production of Sulphuric Acid. Pyrite can also be used to strike sparks when struck with iron.

Clay with flints

Clay with flints describes a number of deposits resulting from the decomposition and erosion of the chalk, with remnants of Palaeogene sediments. The deposits are composed of reddish brown silty or sandy clays with flint, and lie on top of the chalk on the higher and flatter ground.

Fossils from the Chalk

Fossil sea urchins from the fields on the downs

Ironically for a mineral made from animal remains, relatively few identifiable fossils are found in the chalk of Bell Hill, as with many chalk areas. Many remain just as impressions in the chalk, and many are crushed and distorted beyond recognition.
The main exceptions to this are sea urchin fossils, which, the fossil being replaced by durable flint, have been eroded from the chalk over time and brought to the surface by the plough. Many species of fossil sea urchin are represented from the top of Bell Hill.

Other fossils we have found on Bell Hill include ...

• Belemnite - Hibolites (Belemnites were pen-shaped molluscs)
  
• Scaphoda - Dentaliidae (Shaped like a minature elephants tusk, essentially a linear mollusc with striations along the slightly curved shell)
  
• Ammonite - Oppeliidae
  
• Inoceramus - (A bivalve mollusc)
  
• Fish (Species unknown, but several small fish, scale patterns and individual scales, bones and tiny teeth identifiable but mixed)

Bivalve fossilised in Marcasite

Chalk Springs - 'The Petrifying Spring'

Where Hibbets Coppice, in the south-east of the parish, meets the scarp slope of Bell Hill, several springs rise in close proximity to one another, within 100 metres or so. One would expect the water in these springs to be of a similar quality, but this is not the case. Of the five main identifiable sources, two appear as normal clear chalk springs. The next two are heavily contaminated with Iron, the stream bottom stained a deep orange brown. The fifth spring we have nicknamed 'The Petrifying Spring'. The water is clear, but every object in contact with it is encrusted in a heavy layer of calcium carbonate - essentially limescale. Stones, leaves and twigs become rapidly covered in this chalky deposit, even fragile leaves which would normally decompose quickly are preserved by this rapidly growing layer. Presumably the water arising from this spring is essentially super-saturated with calcium carbonate dissolved from the chalk, and once it escapes from the chalk into the open, evaporation and other chemical changes cause it to deposit it's load rapidly.