SedimentarySoil pH usually 7.5–8.3

Chalk

Plankton skeletons, kilometres from any land that might have muddied the water. Soft enough to cut with a spade, yet it stands as cliffs.

How it forms

In a warm, wide, shallow sea, plankton bloom and die in unimaginable numbers, and their carbonate plates rain down continuously. With no mud or sand arriving to dilute them, the result is white, uniform and hundreds of metres thick.

Sedimentary — laid down in layers

Sediment settling through water in layers, then buried and compacted into rockwaterweight above squeezes the water out and cements the grains

Rock built from the wreckage of older rock, or from the remains of things that lived — sand, mud, shell and skeleton, settled in flat beds and buried until the weight turned them to stone. The only rocks that hold fossils.

What it is
An exceptionally pure limestone, built almost entirely from the microscopic calcite plates of single-celled algae. Nothing coarse got in because there was no land close enough to supply it.
How to know it
White, soft enough to mark with, and light. Often studded with nodules of dark flint or chert.
How it wears
Dissolves readily, but it is permeable rather than slippery, so it does not slump — which is how something you can scratch with a fingernail forms vertical cliffs.
What it means above ground
Thin, dry, very sweet soil with flint in it, over an enormous groundwater store. Chalk country is dry at the surface and wet underneath.
Where you stand on it
The Selma and Austin chalk belts of Alabama, Mississippi and Texas, and the Niobrara of Kansas and Nebraska.
What lives on it
A distinctive lime-loving grassland flora and very little else. The Texas Blackland Prairie is chalk country.
What it's used for
Cement, agricultural lime, whiting in paint and putty.
What it does to the soil
Sweetens the soil above it — raises pH, and grows a lime-loving flora.

Next to it in the cycle

  • Limestone

    the wider family it belongs to

  • Marl

    chalk with clay mixed in

Is this what you're standing on?

Find the rock under your town

Enter a town or postal code and we'll show you the rock beneath it and how old it is, the soil it makes, and the chain of rivers your water joins on the way to the sea.

General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.