The World's Foundations; or, Geology for Beginners · Ch. II · Unit 1: Part I: How to read the record
2. Water-built rocks
Goal: Your child can explain how running water wears land away, carries it off and drops it in layers, and why those layers are called stratified or sedimentary rocks.
You'll need
- The book, open to this chapter
- A clear jar with a lid (a pint or quart jar)
- Garden soil with some small pebbles and sand mixed in
- Water
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter II divides rocks into stratified and unstratified, explains crystals (with ice as an example), and describes the sea and rivers always wearing land away. Using a shaken medicine bottle, it shows how mud and sand settle as sediment, and how settled sediment becomes the layered rocks under our feet.
The lesson, step by step
A question to start
SayWhen a stream turns brown after a storm, what makes it brown? Where does that mud go?
Listen to the guess, then read to find out.
Read the chapter together
Read Chapter II (pages 10 to 19). Two sentences to stop on:
The earth-crust is a scene of perpetual change, of perpetual struggle, of perpetual building up, of perpetual wearing away.
For the land's loss here is the ocean's gain.
Two kinds of rock
Explain in plain words: Unstratified rocks were once melted by great heat and then cooled and hardened, forming crystals, like water freezing into ice. Stratified rocks were built up slowly, grain by grain, in layers ("strata"), like the pages of a book or bricks in a wall.
Giberne even calls ice a rock. If it is cold enough this week, look at the needle-like crystals on a freezing puddle, as she suggests.
Make a sediment jar
This is the book's own medicine-bottle idea. Fill the jar about one-third full of soil with pebbles and sand. Add water almost to the top, put the lid on tightly, and shake hard for a minute. Set it down and watch.
Within seconds the pebbles drop, then the sand. Fine mud settles over the next hour, and the water may stay cloudy until tomorrow. Draw the layers in the rock journal now and again tomorrow.
Why the order?
Read the line the jar just showed:
The finer this sediment, the slower it is in settling.
Then reread the part about the mountain torrent: fast water carries stones, slower water drops gravel, then sand, and only the slowest water lets the fine mud sink. The jar did the same thing in a minute.
New words
Detritus: worn-off bits of rock carried by water. Sediment: what settles to the bottom. Sedimentary rocks: rocks made of sediment pressed hard over long ages. Have your child write the three words with a one-line meaning each.
Tell it back
Ask your child to explain the sediment jar to someone else in the family, using the words stratified and sediment.
Check questions: Why can a fast stream carry bigger stones than a slow one? What is another name for stratified rocks, and why?
What we know now
Geologists today call Giberne's unstratified rocks igneous rocks (from the Latin for fire) and her stratified rocks sedimentary rocks. Her account of how rivers sort and drop sediment is still how it is explained today.
Afterwards
Leave the jar standing for a few days and see whether a thin layer of very fine clay forms on top. Notice it this week: look at a puddle, gutter or stream after rain. Is the water muddy? Where has sand been dropped?
Answers to the book's problems
Faster water pushes harder, so it can hold up and roll heavier stones; when it slows down it drops the heaviest things first. Stratified rocks are also called sedimentary rocks, because they were made from sediment that settled on the sea bottom and hardened.
Worked out for this site from the scan. If a number in your copy differs, trust the book.