Chalk

Chalk is a limestone, and an unusually pure one: 95 per cent or more calcium carbonate, laid down in warm shallow seas in the Late Cretaceous — a period named after it, from creta, the Latin for chalk.

What makes it chalk rather than any other limestone is what the grains are. Almost the whole rock is coccoliths: interlocking calcite plates a few micrometres across, secreted by single-celled algae and shed continuously as they lived and died. A chalk cliff is a hundred metres of the skeletons of organisms too small to see, accumulated over tens of millions of years at a few centimetres a millennium.

It is soft, very porous, and permeable enough to be a major aquifer — the chalk under southeast England supplies a large share of its water, and the same porosity is why chalk landscapes have dry valleys and few surface streams.

The most useful thing to know about chalk is that the stick sold as chalk is not chalk.

Blackboard chalk has been made from gypsum — calcium sulfate — since the nineteenth century, because gypsum can be cast into sticks from a slurry and writes with less dust. Sidewalk chalk is the same. So a schoolroom association that seems to be about a rock is about a different mineral entirely, and the words have simply come apart.

Genuine chalk in the ground is unmistakable: it is white, it marks your hand, it is soft enough to scratch with a fingernail, and it fizzes vigorously in dilute acid, which gypsum does not.

The flint is the other tell. Chalk beds carry nodules and bands of flint through them — silica from sponge spicules dissolved and reprecipitated in the sediment — and a white cliff with dark knobbly seams running horizontally through it is chalk and nothing else.

Ground and purified, chalk becomes whiting, and whiting is one of the most widely used industrial fillers there is: in paint as an extender, in paper as both filler and coating, in PVC and rubber to add bulk and stiffness, in putty, in toothpaste as the mild abrasive, and as the calcium supplement in animal feed.

Burned, it is a source of lime in the same way any limestone is, and chalk-derived lime went into mortar across the regions where chalk is what there is.

Agriculturally it is spread to raise soil pH, which is the oldest recorded use of any rock as a soil amendment.

And its structural role is as an aquifer rather than as a material: the chalk of the Paris and London basins stores and transmits water on a scale that shapes where the water in those cities comes from.

The White Cliffs of Dover are chalk, and so is the corresponding coast at Cap Blanc-Nez across the Channel — the same bed, cut through by the breach that separated Britain from the continent, which is why the two coasts look alike.

Because chalk is soft and the turf over it is thin, cutting a figure through the turf exposes brilliant white underneath and leaves a hill figure that lasts as long as somebody keeps scouring it. The Uffington White Horse is Bronze Age and has been maintained for something like three thousand years, which makes it one of the longest continuously performed maintenance tasks anywhere.