Chalk in the Cretaceous of Kansas, USA.

These are chalk outcrops near Castle Rock, a famous cluster of chalk pillars south of Quinter, Kansas. Chalk is a biogenic, calcitic, marine sedimentary rock composed of numerous coccolith microfossils (see: <a href="http://www.soes.soton.ac.uk/staff/tt/eh/pics/lith2.gif" rel="nofollow">www.soes.soton.ac.uk/staff/tt/eh/pics/lith2.gif</a>). Coccoliths are individual calcareous plates that covered a single-celled, photosynthesizing marine organism called a coccolithophorid (a.k.a. coccolithophore; "coccolithophorid" is not an adjective, contra Wikipedia) (see: <a href="https://upload.wikimedia.org/wikipedia/commons/d/d9/Emiliania_huxleyi_coccolithophore_(PLoS).png" rel="nofollow">upload.wikimedia.org/wikipedia/commons/d/d9/Emiliania_hux...</a>).

Most chalks are Cretaceous in age ("creta" means "chalk"). The most famous example is the White Cliffs of Dover, along the southern shore of Britain.

Weathered chalks in western Kansas range in color from white to pale yellowish to pale grayish.

Stratigraphy: Smoky Hill Chalk Member (a.k.a. Smoky Hills Member), Niobrara Formation, Upper Cretaceous