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Lesson · Grade 7 · Science

Relative Dating: Reading Rock Layers

  • Grade 7
  • Science
  • 1 min 19 s
  • British Columbia
  • BC7-SC-34

Sediment settles under water and hardens into five rock layers that build up one at a time, burying a shell, a fish and a fern leaf as they form, so the bottom layer is tagged oldest and the top layer youngest with an arrow of time between them. A working panel then puts the three fossils in order from oldest to youngest, names this relative dating, notes that it gives the order but not an age in years, and links the different fossils in different layers to living things changing over time.

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In one sentence

The question this lesson answers

Here is a place where sand and mud settle under water. How can we tell which rock layers are older?

Sand, mud and the remains of living things settle on the bottom, and slowly harden into a layer of rock.

Transcript

What the lesson says

Here is a place where sand and mud settle under water. How can we tell which rock layers are older?

Sand, mud and the remains of living things settle on the bottom, and slowly harden into a layer of rock.

Over a very long time, each new layer forms on top of the one before.

Layer after layer builds up, over millions of years.

So the layer at the bottom is the oldest. It was there first.

The layer at the top is the youngest. It formed last.

In rock that has not been disturbed, older layers are below and younger layers are above. This rule is called superposition.

A fossil is as old as the layer it is buried in.

So the shell is older than the fish, and the fish is older than the leaf.

Putting rocks and fossils in order from oldest to youngest is called relative dating.

It tells us the order, but not the age in years.

Different layers hold different fossils. That shows that living things on Earth have changed over time.

Remember: lower layers are older, and so are the fossils inside them.

Curriculum

Where this fits

British Columbia, Grade 7

Science, Earth science: the fossil record provides evidence for changes in biodiversity over geological timeBC7-SC-34

Previous: The Geologic Time Scale: Reading Earth's History in Rock LayersNext: Absolute dating: radioactive decay is a clock for rocks

The outcomes and strands follow the British Columbia curriculum.

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