Flint

Exposure in chalk pits, roadside and railway cuttings and cliffs, show that the chalk is a bedded rock. It consists of a mass of parallel layers (strata) and splits along these layers in flat slabs; otherwise it smashes when broken into shapeless lumps.

Chalk is a form of calcite, a mineral which consists of calcium carbonate. Touched with a weak acid, it 'fizzes', giving off bubbles of carbon dioxide. When strongly heated, it parts with its carbon dioxide and becomes quicklime (calcium oxide), so corrosive a substance that in medieval times, it was used as a weapon. Water added to quicklime converts it into slaked lime (calcium hydroxide), generating a heat so fierce that it throws off clouds of steam.

Slaked lime is used in making mortar and cement, and can also serve as a fertiliser, restoring calcium to the soil. Lime is also used in the manufacture of steel, as a flux in metallurgy, and in neutralising acids.

Powdered and examined under the microscope, chalk reveals itself to be consisting of white grains, mixed with fragments of marine shells, and even with complete shells of miniature creatures.

The chalk, in its hundreds of feet of thickness, varies in texture and composition. It contains belts of 'chalk rock' and other material hard enough to be used for building. Towards its base, it is grey rather than white and contains patches of marl (limy clay).

FLINT

Most chalk-pits contain nodules (rounded lumps) of Flint. Although most of these form layers parallel to the bedding of the chalk, the two minerals are altogether different in appearance and composition. The flint is very hard and, except on its surface, it is black or very dark grey. It breaks neither into jagged lumps nor along flat surfaces, but in gentle curves like those of an oyster shell; this is called conchoidal (shell-like) fracture. Broken into thin flakes, it is translucent and may have very sharp edges.

Flint is a form of the very common mineral silica (silicon dioxide). Like the chalk, it is derived from material in the bodies of sea-living creatures. At one time it was supposed that this has accumulated on the sea floor seperately from the calcite which forms the chalk; it is now thought more probable that its particles were originally widely scattered through the chalk, but that they gradually 'grew' together by a process resembling crystallisation.

The surface of most flints consists of a porous layer of particles of silica so small that by 'scattering' the light they appear white. Broken pieces of flint may 'weather' to a delicate shade of blue.

Before Humphrey Davey invented the safety lamp, coal mines were illuminated by the fitful light of a 'Steel Mill', in which a revolving wheel struck a shower of sparks from a flint edge; these were not hot enough to ignite the dangerous 'fire-damp'.

Though its modern uses are limited, flint was for ages, the chief raw material used in industry, and was almost essential to human life. Not only did it serve to make fire, sparks being struck either from two pieces of flint, or from flint and ironstone; it was also used to make almost all tools and weapons. Indeed, for the greater part of human history, such implements consisted not of steel, not of iron, not of bronze, not even of copper, but of flint!

The wood with which these tools were hafted, and on which it was used, has long since perished; the most durable flint implements are exhibited in the museums, and may be collected from some river gravels. They include knives with edges keen enough to serve as razors, saws with well cut teeth, scrapers for dressing hides, chisels, sharp-pointed borers and awls and spear and axe heads (arrow heads being a comparatively recent invention). For these purposes, flint was systematically mined.

Many chalk pits also contain smaller nodules with a brown, rusty looking surface: spheres or cylinders with spherical ends. When broken, they reveal a silvery interior with a radial structure, resembling needles tightly packed, with their points meeting at the centre; unfortunately, however, the beautiful silver-like surface soon rusts into a reddish-brown. These nodules, which are sometimes called 'thunderbolts', consist of iron sulphide, either marcasite or a form of iron pyrites.