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Primary Science · Year 6Chapter 5 · Fossils and living things over time
Year 6 Science · Fossils and living things over time · Lesson 5.2

Year 6 Science: How fossils form

Part of the Year 6 Science fossils and living things over time chapter. Chapter overview · whole chapter on one page

5.2

How fossils form

Year 6
EV1: fossils provide information about living things that inhabited the Earth millions of years ago; how fossils are formed
Story. Mo sees a rock wall with lots of thin layers. “How did these layers form?” he asks. Mr Patel says, “They are layers of sediment, which built up over a very long time. Some of them trapped living things, and those became fossils.”

Sediment is tiny pieces of rock, sand and mud that settle in layers, usually at the bottom of water. Fossils usually form like this:

  1. an animal or plant dies in water, and is quickly buried by sediment;
  2. more and more layers of sediment build up on top, over a very long time;
  3. the layers are pressed and cemented together to form sedimentary rock;
  4. minerals from water slowly replace the hard parts, and erosion later exposes the fossil.
1 buried2 layers build up3 rock formsrock4 fossil exposederosion
Four panels showing how a fossil forms: an animal is buried, layers build up, the layers become rock, and erosion exposes the fossil.

The steps are shown in order below.

StepWhat happens
1An animal or plant dies in water, and is buried by sediment.
2Layers of sediment build up on top, over a very long time.
3The layers are pressed and cemented together, and turn into sedimentary rock.
4Minerals from water slowly replace the hard parts, and the fossil is exposed by erosion.

Key ideas

  • Fossils usually form when remains are buried quickly in sediment.
  • Layers of sediment build up over a very long time, and form sedimentary rock.
  • Minerals from water can slowly replace the hard parts of the remains.
  • Burial protects the remains from decay and from scavengers.
Worked example 5.2.1
The table in section 5.2 gives four steps. Which step comes first? Write the number.

Method. Read the steps in order.

  1. Read step 1. Step 1 is the animal or plant dying and being buried.
    buried
  2. Write the number. So the first step is number 1.
    1
  3. Write the answer.
    1

Check. Layers build up after the burial. ✓

Worked example 5.2.2
Which process presses and cements the sediment together to form rock? Write the word.

Method. Find the step about rock forming.

  1. Read step 3. The layers are pressed and cemented together to form sedimentary rock.
    pressed
  2. Write the word. The process is called compaction and cementation.
    compaction
  3. Write the answer.
    compaction

Check. Pressing squeezes the layers together. ✓

Worked example 5.2.3
How many steps are shown in the fossil formation diagram? Write the number.

Method. Count the panels.

  1. Count the panels. The diagram has four panels, labelled 1 to 4.
    four panels
  2. Write the number. So there are four steps.
    4
  3. Write the answer.
    4

Check. Each panel shows one step. ✓

Watch out

Watch out: a fossil does not form in a few days. It takes a very long time for layers to build up and for minerals to replace the remains.

Exam tip

Exam tip. When you describe fossil formation, give the steps in order: buried, layers build up, rock forms, minerals replace the remains.

Try it (with your teacher, or a grown-up at home). Put a layer of sand in a clear jar, then a layer of soil, then a layer of small stones. Tap the jar gently to settle the layers. Notice how the layers are in order. Which layer was added first? Clear up the sand and soil afterwards, and wash your hands.

Exercise 5B  How fossils form

1
FWhich word means tiny pieces of rock, sand and mud that settle in layers? Write the word.
2
FWhat is the name of the rock made when layers of sediment are pressed and cemented together? Write the words.
3
FWhich step comes first in the formation of a fossil? Write the number.
4
FWhich step comes last in the table in section 5.2? Write the number.
5
FMinerals from water can replace the hard parts of a dead animal. Is this true? Write yes or no.
6
FWhat is the name for the process that protects remains from decay, by burying them quickly? Write one word.
7
FLayers of sediment build up on top of each other. Which layer is older: the one at the top, or the one at the bottom? Write top or bottom.
8
PSAli says a fossil forms in a few days after an animal dies. Is Ali right? Write yes or no.
9
PSAn animal is buried in mud quickly after it dies. Explain why this helps it to become a fossil. Write one sentence.
10
PSPut these events in order, starting with the first: 2 layers build up; 1 the animal is buried; 3 rock forms. Write the three numbers in order.
11
RExplain why a fossil is rarely made from an animal left on the ground in the open. Write one sentence.
12
RExplain why hard parts are more likely to form fossils than soft parts. Write one sentence.
13
RChallenge: explain why a fossil is likely to be found in sedimentary rock and not in a rock formed from cooled melted rock. Write one sentence.

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