The White Cliffs of Dover are primarily made of chalk, a soft, white, porous rock formed from the fossilized remains of microscopic marine organisms called coccolithophores. These organisms lived in a shallow, warm sea that covered much of Europe during the Late Cretaceous Period, about 100 million years ago.

Here's a breakdown:

* Chalk: The main rock type, responsible for the iconic white color.

* Coccolithophores: Tiny, single-celled algae that created the calcium carbonate shells that form chalk.

* Late Cretaceous Period: The geological era when the chalk deposits formed.

While chalk is the dominant rock type, there are also smaller layers of other sedimentary rocks within the cliffs, such as:

* Flint: Dark, hard, and brittle rock that formed within the chalk.

* Clay: Fine-grained sediment that can be found in thin layers.

These other rock types contribute to the diverse appearance of the cliffs, with flint nodules creating darker bands within the white chalk.