Pick one before you read on. It is fine to be wrong.
By the end you can
Explain how a fossil forms in sedimentary rock
Make a model fossil by pressing a shell or leaf into clay
A fossil is the remains or the print of a living thing from long ago, kept in rock. It can be a shell, a bone, a leaf or even a footprint. Fossils show us what lived on Earth before people.
Draw it first
Draw how a fossil forms
1 A sea animal dies. Its shell falls to the sea floor. Draw the shell on the ground under the water.
2 Mud and sand cover the shell, one layer after another. Draw the shell with ground on top of it.
3 Over a very long time the layers become rock. Bit by bit, a mineral takes the place of the shell and turns it to stone. Draw the shell with crossed lines.
4 Later the rock is lifted up. Draw an arrow that points up.
5 Wind and water wear the rock above away. The fossil shows at the surface, and someone finds it.
For this to happen, the animal must be buried quickly. If it lies on the surface, other animals eat it or it rots away. Hard parts, such as shells, bones and teeth, become fossils most often. Soft parts rot away.
Shells, bones and leaves can all become fossils.
Most fossils are found in sedimentary rock. Sediment settles in gentle layers and buries animals without crushing them.
Fossils sit inside the layers of sedimentary rock.
See it three ways
As a picture
In words
Igneous rock forms from melted rock, which is far too hot for a living thing to survive. Metamorphic rock is heated and squeezed, which usually destroys fossils. Sedimentary rock keeps them.
Some fossils are not the shell itself. A shell pressed into soft mud leaves a hollow print. The print is a mould. If the hollow fills up with mud or minerals, the filling is a cast. A cast has the shape of the shell.
A mould is a hollow print. A cast is the shape that fills it.
Try it
Make a model fossil
You need: modelling clay or soft clay from the ground, a small shell or a leaf, a flat board.
Be safe: Wash your hands after you handle clay or shells. Never go to cliffs, quarries or riverbanks alone to look for fossils.
Roll the clay into a flat slab on the board.
Press the shell firmly into the clay. Press the whole shell, but do not push it right through.
Lift the shell out slowly.
Look at the print in the clay. You have made a mould.
What you will see: The print has the shape of the shell, with its ridges. A real mould forms in mud in the same way. But real mud needs millions of years to turn into rock.
If you cannot try it, this is what happens.
A wrong idea many people have
A fossil is the real shell or bone, just very old.
Why it feels right: It looks like the shell and it has the same shape.
The test that breaks it: Pick up a fossil shell. It is heavy and hard, like a stone. Minerals took the place of the shell bit by bit.
What is true: In many fossils the original material has been replaced by minerals. The shape stays, and the fossil is stone.
Key words
fossil
the remains or print of a living thing from long ago, kept in rock.
mineral
a solid that forms in nature. Rocks are made of minerals.
mould
a hollow print left in mud or clay by a shell or leaf.
cast
the shape that fills a mould and keeps its form.
Back to your guess
The answer is C: It was buried in mud long ago. The mud became rock, and the rock was lifted up.
A shell can be buried in mud on a sea floor. Over a very long time the mud becomes rock. Later the rock can be lifted high, even into mountains.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What is a fossil?
See the answer
The remains or print of a living thing from long ago, kept in rock
A fossil shows us a living thing from long ago. It can be a shell, a bone, a leaf or a footprint.
If you chose A: A rock can look like a plant by chance. A fossil really was part of a living thing, or was made by one. Go to S3.11.7.
If you chose C: Old stones are not fossils. A fossil must come from a living thing. Go to S3.11.7.
Which part of an animal is most likely to become a fossil?
See the answer
A hard part, such as a shell, a bone or a tooth
Hard parts last longer than soft ones. They are more likely to be buried before they rot.
If you chose A: Soft parts rot away or are eaten before they can be buried. Size does not save them. Go to S3.11.7.
If you chose C: Parts are not equal. Hard parts last long enough to be buried. Soft parts rot first. Go to S3.11.7.
Why are most fossils found in sedimentary rock?
See the answer
Sediment buries animals gently in layers that later become rock.
Gentle layers of sand and mud can cover a shell without breaking it. The layers then harden into rock.
If you chose B: Lava is far too hot. It burns up remains instead of keeping them. Go to S3.11.7.
If you chose C: It is not luck. Melted rock destroys remains, and heat and squeezing usually destroy them. Gentle layers of sediment keep them. Go to S3.11.7.
What must happen to a dead animal first, for a fossil to form?
See the answer
It must be buried quickly by mud or sand.
Burying protects the hard parts from being eaten or rotting away. Then they can slowly turn to stone.
If you chose B: On the surface, it would be eaten or it would rot. It needs to be buried quickly. Go to S3.11.7.
If you chose C: Melting would destroy it. Fossils form in cool layers of sediment, not in melted rock. Go to S3.11.7.
You press a shell into clay and lift it out. What have you made?
See the answer
A mould, which is a hollow in the shape of the shell
The shell leaves a hollow print. That print is a mould.
If you chose A: A cast needs a filling. You must fill the hollow with mud or something else to make one. Go to S3.11.7.
If you chose C: Your print is a model. A real fossil formed over millions of years in rock. Go to S3.11.7.
From an earlier chapter
How does sediment become sedimentary rock?
See the answer
The layers are buried, pressed and glued together.
The weight of new layers presses the old ones, and minerals glue the bits together.
If you chose A: That is how igneous rock forms. Sedimentary rock is made from pressed layers of sediment. Go to S3.11.4.
If you chose C: That makes metamorphic rock. Sedimentary rock forms from buried and pressed sediment. Go to S3.11.4.
Keep it
Copy this into your exercise book.
A fossil is a living thing from long ago, kept in rock. It is buried quickly, the layers become rock, and minerals turn it to stone. Most fossils are in sedimentary rock.
Draw: Draw five small boxes in order. The shell lies on the ground. It is buried. It turns to stone. The rock is lifted. It is found.
