Skip to content

Rock cycleSedimentaryChalk

Sedimentary / Specimen 06

Chalk.

How to identify it

Very soft, fine, powdery — easily scratched with a fingernail

Explore the sedimentary family ↗

Pale chalk specimen with a fine, powdery-looking surface — Nessa Eull · CC0 · Image source ↗

01 / Origin story

How does chalk form?

Chalk is a soft form of limestone made almost entirely from the microscopic calcite shells of ancient marine plankton (coccolithophores), accumulated in huge quantities on ancient sea floors.

02 / Read the clues

How can you identify chalk?

Use several clues together.
One photograph shows one specimen, not every variation.

Enlarged crop of the Chalk specimen — Texture study · enlarged crop of this specimen

01Very soft, fine, powdery — easily scratched with a fingernail
02White to pale gray
03Also reacts with acid, like limestone (since it's chemically the same mineral, calcite)

03 / Commonly confused with

Similar at first glance.

Compare the materials side by side.
Photographs are not shown to the same scale.

Chalk specimen for comparison — Chalk ↗
Very soft, fine, powdery — easily scratched with a fingernail

Limestone specimen for comparison — Limestone ↗
Usually light-colored: white, gray, tan, sometimes yellowish

Limestone generally — chalk is really just an especially soft, fine, pure form of it. The powdery softness (you can write with it) is the giveaway.

04 / Out in the world

Where it’s found.

The White Cliffs of Dover are the most famous example; similar deposits exist in parts of the central and southern U.S.

Continue exploring

More from this family.

Sandstone specimen with fine grains and curved tan and reddish bands

Sandstone

Visibly grainy texture, feels gritty

Shale weathering into many thin gray fragments

Shale

Very fine-grained, smooth to the touch

Conglomerate with rounded pebbles embedded in a finer matrix

Conglomerate

Visibly made of rounded pebbles/clasts embedded in a finer matrix