The White Cliffs of Dover are made of trillions of dead things too small to see

Chalk is not a mineral deposit in the way I'd assumed. It's a graveyard.

It's made almost entirely of coccoliths — microscopic calcium carbonate plates that were grown by single-celled algae called coccolithophores. Each organism builds a spherical armour of these plates around itself. They live in the surface ocean in enormous numbers. When they die, the plates sink, and over an extraordinary span of time they accumulate on the seafloor as ooze, which compacts into chalk.

The English chalk was laid down in the Cretaceous, roughly a hundred million years ago, in a warm shallow sea. The period is literally named for it — creta is Latin for chalk.

The numbers are the part that stops working in your head. A single cubic centimetre of chalk contains on the order of hundreds of millions to billions of individual coccoliths. The cliffs are hundreds of feet high and run for miles. Every particle was manufactured by an organism, one at a time.

Coccolithophores are still alive. Blooms of them are visible from orbit as milky turquoise swirls in the ocean. They are, right now, making more of the stuff, and it's settling, and in a hundred million years it'll be rock.

Two consequences I hadn't connected. Chalk is porous and holds water, which is why chalk downland has the specific dry, thin-soiled ecology it does, and why chalk aquifers supply drinking water to a lot of southern England. And the same biological carbonate story explains most limestone, which means the cathedrals built of it are built out of compressed ancient sea life.

Blackboard chalk is mostly gypsum now, incidentally, not real chalk. That switch happened a long time ago.

I have a piece of raw chalk from a beach, which cost nothing, and I keep it near the expensive things.

It's the least impressive object on the shelf. It's also the one where, if you actually look at it and let the scale land, the floor tilts a bit.