The World's Foundations; or, Geology for Beginners · Ch. III · Unit 1: Part I: How to read the record
3. Fossils in the rocks
Goal: Your child can explain what a fossil is, name three ways fossils are made, and explain why sea-shell fossils are found on mountains.
You'll need
- The book, open to this chapter
- Modeling clay or salt dough (two lumps)
- A shell, leaf, or small toy with a clear shape
- Five or six notebooks or thin books of different colors
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter III asks how rocks made under the sea came to be on land, and answers that land has slowly sunk and risen again, as the coasts of Sweden and Greenland were doing. It explains fossils and three ways they form, and uses a pile of copy-books to show how tilted layers bring old rocks up to the surface.
The lesson, step by step
A question to start
SaySea-shells have been found on the tops of mountains, thousands of feet high. How could they get there?
Collect the guesses; the chapter gives Giberne's answer.
Read the chapter together
Read Chapter III (pages 20 to 31). Giberne adds a short paragraph saying that she is not writing here about the Flood of Noah; read it as written and go on.
Stop on this definition:
Fossils are the hardened remains of animals and vegetables found in rocks.
Land goes down and up
In plain words: the sea did not rise up to the mountain tops. Instead, pieces of land sank slowly under the sea, layers of mud and shells built up on them, and later the land rose again, carrying the shells with it. Giberne gives proof that land moves: some coasts were rising and others sinking in her own day.
Make your own fossils
The book names three kinds of fossil. Make two of them:
1. Press the shell or leaf firmly into a smooth lump of clay, then lift it out. The print left behind is an impression, or "mould".
2. Press a second lump of clay into the print and peel it off. That copy of the outside is a "cast".
The third kind is when the shell or bone itself slowly turns to stone, like the fossil wood the book describes.
Tilted layers
Try the book's copy-book idea. Stack the notebooks flat. Push the bottom one up at one side so the whole pile tilts, and look at the row of edges.
SayWhich one was put down first? Is it still at the bottom? (Yes.) But now you can see its edge without digging.
The rule of layers
Read:
We might indeed feel sure, as we do now, that the lowest layers were the oldest and the top layers the newest,
Point to the bottom notebook: oldest. Top: newest. Fossils in lower layers lived earlier than fossils in higher ones.
Tell it back
Ask your child to tell the story of one shell: how it lived in the sea, was buried in mud, became a fossil, and ended up on a mountain. Then: What are the three kinds of fossil? Why are there more sea fossils than land fossils?
What we know now
Land really does rise and sink, as Giberne says. We now know the biggest cause: the earth's crust is broken into huge plates that move a few inches a year, and where plates push together, sea-floor rock is lifted into mountains. The top of Mount Everest is made of limestone with fossils of sea creatures in it. Sea level also changes all over the world at once: in the last Ice Age, when much water was locked up in ice, the sea was about 400 feet lower than today.
Afterwards
Let the clay mould and cast dry and label them in the rock box. Notice it this week: look for layers in a road cut, a stone wall, a quarry photo or a riverbank.
Tip
If a museum near you has fossils, this is a good week to visit and look for the three kinds.
Answers to the book's problems
Three kinds of fossil: the shell or bone changed into stone; an impression (mould) pressed into soft mud; and a cast, where mud filled a shell and hardened. Sea animals are preserved whenever the sea floor buries them, but land animals only when they are drowned, caught in bogs or swept out to sea, so sea fossils are far more common.
Worked out for this site from the scan. If a number in your copy differs, trust the book.