The Wonder of Limestone Part 2 – Chalk

One of the most common types of limestone in the UK is chalk. It is a very famous rock type in general, due to its extremely useful drawing property, and connection with some certain white cliffs. As the song goes…..

“There’ll be bluebirds over the White Cliffs of Dover…..”

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 So what is chalk? Well, essentially it is a giant composite fossil! While the main makeup of chalk is the mineral calcium carbonate, that mineral itself comes from marine creatures’ shells! Tiny marine creatures (plankton) called ‘coccolithophores’(kok-o-lith-o-fores) have tiny shells made out of calcium carbonate. Add these shells in their billions, and you get a chalk deposit!

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A coccolithophore under a microscope

 This makeup is also what makes chalk so useful. When you draw with a piece of chalk, some of the tiny plankton pieces get left behind on the blackboard, leaving the white streak that you see! This very loose and porous design of chalk makes it useful in other ways as well.

 Next time you are in a supermarket, look at all the bottled water. Many of them will say that they have been filtered through chalk. This is because chalk is a very good purifier. It will filter and clean natural water as it seeps through it, making it a lot safer to drink.

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Chalk hills that purify water.

 Chalk is also full of much larger fossils! Creatures that got caught up in the plankton while the chalk was forming, then buried very quickly, can turn into stone: fossils! Many chalk fossils are preserved in another type of rock that chalk produces, flint. This is all the silica that has been ‘squashed’ out of the plankton shells, and run down until it has filled up any airholes and fossil moulds.

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Flint fossils

 So be grateful for chalk, as it affects our lives in the UK quite a bit!