Very cool. There's some info at "What is chalk".

Chalk is white because it is formed from the colourless skeletons of marine plankton.

and

Most chalks formed during the Cretaceous period, between 100 and 60 million years ago, and chalks of this age can be found around the world. The Cretaceous chalks record a period when global temperatures and sea levels were exceptionally high. This coincided with the break up of the supercontinent Pangea, which broke apart to form the continents of today. As continents move apart, an ocean forms between them, and new ocean-floor is added along the line of spreading (known as the mid-ocean ridge) by magma which rises from below. As the continents moved apart in the Cretaceous, a very high volume of magma rose up to form the new ocean-floor in what is known as a superplume event. The mid-ocean ridges became swollen, and large volumes of magma spilled out elsewhere onto the ocean floor, displacing water onto the continents (causing sea-level to rise). The volcanic activity also produced greenhouse gases which raised temperatures, prevented ice from forming at the poles and hence kept sea levels high. Chalks formed in the sea-ways of the flooded Cretaceous continents.