The World's Foundations; or, Geology for Beginners · Ch. XVII, part 1 · Unit 3: Part II: Reptiles, chalk, mammals, ice and people
18. The age of chalk: chalk and flint
Goal: Your child can explain how chalk was made, and describe the puzzle of the flints found in chalk cliffs.
You'll need
- The book, open to this chapter
- A piece of natural chalk or limestone if you can find one, or a sea-shell
- White vinegar
- A magnifying glass, or a microscope if you have one
- A globe, atlas or free online map
What's in the book
The first part of Chapter XVII (pages 154 to 161) describes the great Chalk Age, when chalk was laid down from Ireland to the Crimea. It tells how deep-sea soundings for the Atlantic telegraph cable brought up mud made of tiny shells, and discusses how flints, sometimes with fossils inside, came to be in the chalk.
The lesson, step by step
A question to start
SayThe white cliffs of Dover are made of chalk. What do you think chalk is made of?
Read to find out.
Read together
Read from the start of Chapter XVII (page 154) to the paragraph ending "much more rapid than it seems to be now", at the top of page 161. Sentences to stop on:
Chalk is a kind of limestone, but it is of a soft loose make, not hard and compact.
But who shall venture to say that the mighty brutes of land and sea were playing a more important part in earth's history than these small animalcules?
How big?
The book says the chalk stretches from the north of Ireland to the Crimea, eleven hundred and forty miles. Find Ireland and the Crimea (on the Black Sea) on a map and trace the line.
Proof from the Atlantic
Explain: people guessed chalk was made of tiny shells that settled in deep water. When the sea floor was sounded before laying the telegraph cable from Ireland to Newfoundland, mud from two miles down was found to be made almost entirely of tiny shells. Ask: Why was that good proof?
Look and test
Look at chalk dust with the magnifying glass (with a microscope, mix a pinch of natural chalk with a drop of water on a slide). Then drop vinegar on it: chalk is a lime rock, so it fizzes.
The flint puzzle
Read:
But how could living animals by any possibility have crept inside the hard flints ?
Giberne's answer is that the flint must have formed around the dead animals. She describes one idea (tiny flint-shelled diatoms gathering together) and then a problem with it: under a microscope, flint looks like flint that was once dissolved.
Tell it back
Ask your child to explain how a cliff of chalk was built, starting with one tiny living creature in the sea.
What we know now
Chalk is now known to be made mostly of coccoliths, tiny chalky plates from single-celled floating algae, mixed with the shells of forams like the ones Giberne describes. Flint formed from silica dissolved out of the glassy skeletons of sea sponges and similar creatures, which hardened again in lumps inside the chalk, so the microscope clue she mentions was the right one. The Chalk Age is now called the Cretaceous, about 145 to 66 million years ago.
Afterwards
Notice it this week: if you have a classroom chalkboard or chalk, test it with vinegar. Look at photos of the white cliffs of Dover and find the dark bands of flint.
Answers to the book's problems
Tiny shelled creatures lived in the sea in vast numbers; when they died their shells sank and piled up on the deep sea floor, slowly forming soft chalk, which was later lifted up into land. The Atlantic mud from two miles down was actually seen to be made of these shells, so it showed the process happening today.
Worked out for this site from the scan. If a number in your copy differs, trust the book.