Tiny sea organisms, specifically the calcium carbonate shells of microscopic creatures called foraminifera, diatoms, and coccolithophores, accumulate over millions of years to form chalk.

Here's a breakdown:

* Foraminifera: These single-celled organisms have shells made of calcium carbonate.

* Diatoms: These algae have silica (glass-like) shells.

* Coccolithophores: These algae have calcium carbonate plates called coccoliths.

As these organisms die, their shells sink to the ocean floor and accumulate. Over vast spans of time, these accumulations can become compacted and cemented together, forming chalk.

Chalk is a soft, white sedimentary rock, often used in writing, painting, and other applications. The famous White Cliffs of Dover are a spectacular example of chalk cliffs formed from the remains of these tiny sea creatures.