How can a rock tell us about a time long before people?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain why lower rock layers are usually older
Explain what fossils tell us about life long ago
A cliff by the sea often shows stripes of rock. So does the side of a road cut through a hill. Each stripe is a layer. The layers are like the pages of a book about Earth. A geologist is a scientist who reads that book.
Draw it first
Draw the layers as they formed
1 Draw the first layer, at the bottom. It formed first, so it is the oldest.
2 Draw the second layer on top of it. It formed later.
3 Draw the third layer on top. It formed last, so it is the newest.
Layers form one on top of another. A new layer cannot settle under one that is not there yet. So the lowest layer is usually the oldest, and the top layer is usually the newest.
Sometimes mountains build and layers are tipped or even turned over. Then the order may be wrong. Fossils help scientists check.
The layers also give us time. Here is an example. In one quiet lake, mud settles about 2 mm each year. A layer of mud 30 cm thick holds 300 mm of mud. It took about 150 years to build.
A thicker layer took more years to build. Other places build up faster or more slowly.
Fossils are evidence of the past. Evidence is a fact that helps us find out what is true. A fossil shows that a living thing was once in that place.
See it three ways
As a picture
In words
Read the cliff from the bottom up. First there was water, with fish. Then shells settled in the sea. At the top the layers hold a fern, a land plant. The place changed from water to land.
A fossil tells us what the place was like when the animal or plant was alive.
Shell fossils are found on some high mountains. This tells us that the rock was once sea floor. Plates pushed it up over millions of years.
Some fossils are of animals that no longer live anywhere. We say they are extinct. The dinosaurs are one example.
The Earth is about 4.6 billion years old. People have lived on it for only a tiny part of that time.
The dates are rounded. Scientists find them from rocks and fossils. Life may be older than its oldest fossils.The whole bar is 4600 million years. People fit in a line thinner than a pencil mark.
Try it
Build layers in a jar
You need: a clear plastic jar, dry sand, dry soil, dry rice, a spoon, a ruler.
Be safe: Wash your hands after you touch soil. Do not taste anything in the jar.
Pour a layer of sand about 2 cm thick into the jar. Level it with the spoon.
Pour a layer of soil about 2 cm thick on top of the sand.
Pour a layer of rice about 2 cm thick on top of the soil.
Look at the jar from the side. Which layer did you pour first? Which did you pour last?
What you will see: The sand, poured first, is at the bottom. The rice, poured last, is on top. The lowest layer is the oldest, just as in a cliff.
If you cannot try it, this is what happens.
A wrong idea many people have
The top layer of rock is always the oldest.
Why it feels right: The top is what we see first, and old things seem to be on the surface.
The test that breaks it: Pour sand, soil and rice into a jar in that order. The sand was poured first, and it is at the bottom.
What is true: Layers settle on top of each other, so the lowest is usually the oldest. Only if layers are turned over is the order reversed.
Key words
geologist
a scientist who studies rocks and the history of the Earth.
evidence
a fact that helps us find out what is true.
extinct
no longer living anywhere on Earth.
Back to your guess
The answer is A: Layers show which rock is older, and fossils show what lived there and what the place was like.
Layers of rock are laid down in order, so lower layers are usually older. Fossils in the layers show what lived there long ago.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
In a cliff of layers that have not been turned over, which layer is usually the oldest?
See the answer
The bottom layer
The bottom layer settled first. Later layers settled on top of it.
If you chose A: The top layer was laid down last, so it is usually the newest. Go to S3.11.8.
If you chose B: The number of fossils does not tell the age. The order of the layers does. Go to S3.11.8.
Why is the bottom layer usually the oldest?
See the answer
It was laid down first, and the later layers settled on top.
A layer cannot settle under one that is not there yet. So the order of the layers is the order of time.
If you chose B: Layers of rock do not sink through each other. Each new layer simply settles on top of the one before. Go to S3.11.8.
If you chose C: Weight does not decide age. The bottom layer is older because it was laid down before the others. Go to S3.11.8.
A shell fossil is found in rock high on a mountain. What does this suggest?
See the answer
The place was under the sea long ago.
Shell fossils are evidence of the sea. Plates pushed the rock up over millions of years.
If you chose A: Sea animals cannot climb mountains. The rock was once sea floor, and plates lifted it. Go to S3.11.8.
If you chose B: The shell is inside the rock, not lying on it. It was buried when the rock formed. Go to S3.11.8.
What can a fossil fern tell us about a place long ago?
See the answer
Plants grew there, on land with enough water.
Ferns grow on land that is damp. A fern fossil is evidence of that kind of place.
If you chose A: A fern is a land plant. It tells us about land, not deep ocean. Go to S3.11.8.
If you chose B: Ferns need damp ground. A dry desert would not have grown them. Go to S3.11.8.
Mountain building can turn layers over. What does this mean when we read layers?
See the answer
The oldest layer may end up on top, so fossils help check the order.
Turned layers break the usual order. Scientists use fossils to find out which is which.
If you chose B: Layers still hold evidence. Fossils and other clues show whether the order has been turned. Go to S3.11.8.
If you chose C: That is only usually true. If layers are turned over, the top one may be the oldest. Go to S3.11.8.
From an earlier chapter
A shell is pressed into clay and lifted out, leaving a hollow print. What is the print called?
See the answer
A mould
A mould is the hollow print a shell leaves. A cast is what fills it.
If you chose A: A cast is the shape that fills a mould. The hollow print itself is the mould. Go to S3.11.7.
If you chose C: A mineral is a solid that forms in nature. The hollow print is called a mould. Go to S3.11.7.
In one quiet lake, mud settles 2 mm each year. A layer of mud 30 cm thick took how many years to build? There are 10 mm in 1 cm.
See the answer
150 years
30 cm × 10 = 300 mm. 300 mm ÷ 2 mm a year = 150 years.
The Earth is about 4600 million years old. The dinosaurs ended 66 million years ago. About how many million years passed from the start of the Earth to the end of the dinosaurs?
See the answer
4534 million years
4600 − 66 = 4534. About 4534 million years passed.
Keep it
Copy this into your exercise book.
Layers settle one on another, so the lowest is usually the oldest. Fossils are evidence of what lived there. The Earth is about 4.6 billion years old.
Draw: Draw a cliff of three layers. Mark the oldest at the bottom and the newest at the top. Put a fossil in one layer.
