The World's Foundations; or, Geology for Beginners · Ch. VIII, part 1 · Unit 1: Part I: How to read the record
8. Ice-work: rain-prints, ripples and wandering stones
Goal: Your child can explain what fossil rain-prints and ripple marks tell us, and describe the puzzle of the scratched stones and erratic boulders.
You'll need
- The book, open to this chapter
- A shallow tray or baking pan of damp sand or mud
- A cup of water
- A notebook for a "rock journal" and a pencil
What's in the book
The first half of Chapter VIII (pages 70 to 76) describes rain-drop pits and ripple marks hardened in sandstone, then the strange scratched stones, the stony clay called till or drift, and huge "erratic" blocks carried far from their home rocks. It explains why a flood of water could not have done this work.
The lesson, step by step
A question to start
SayCould a rain shower from millions of years ago leave a mark we can still see today?
Let your child think about it, then read.
Read together
Read pages 70 to 76, from the start of Chapter VIII up to where the author turns to the snow on high mountains. A question from the book:
Did you never note how the retreating tide leaves often upon the sand tiny ridges and hollows side by side, which the next tide smooths away only that fresh ones may be formed ?
Make rain-prints and ripples
Smooth the damp sand in the tray. Flick drops of water onto one end with wet fingers: little pits, just like the book's rain-prints.
At the other end, pour in a little water and gently rock the tray back and forth many times. Ridges form, like ripple marks on a beach. Leave the tray in the sun to dry and harden a little.
Why they last
Explain: usually the next tide or shower smooths these marks away. But sometimes the sand dried, was gently covered by more sand, and hardened into rock. Then the marks were kept for us to find.
The puzzle of the wandering stones
In Scotland and other northern lands there is a stiff clay full of jumbled stones, called till or boulder-clay. Many of its stones are scratched in neat lines along their length. And huge blocks, some as big as a cottage, sit on mountains far from any rock like them.
SayWhat could carry a stone as big as a house fifty miles and up a mountain, scratching it as it went?
Write your child's best guess in the rock journal. The book gives its answer in the next lesson.
Water could not do it
Giberne mentions that some people once thought a great flood did it, and that Noah's Flood comes to mind. She then explains why she thinks water alone could not: water rounds and rubs stones, but does not scratch neat lines on them, and cannot lift huge blocks up mountains.
Tell it back
Ask your child to explain how a ripple mark could end up inside a rock. Then: What is an erratic? Why did the author think water could not have carried them?
Afterwards
Notice it this week: look for ripples on a beach, a sandbox after rain, or the bottom of a shallow stream. Look at large boulders in parks or fields: do they match the rock around them?
Answers to the book's problems
The marks were made in soft sand, which dried and was covered by more sand before it could be washed smooth, and in time hardened into rock. An erratic is a large block of stone found far from the rock it came from. Water rounds stones but does not scratch them in neat lines, and it cannot carry huge blocks uphill.
Worked out for this site from the scan. If a number in your copy differs, trust the book.