Rock Layers Tell Time
- Earth & Space
- Ages 8–11 · Grades 3–5
What you’ll find out
- Rock layers form as sediment settles and is pressed into rock over millions of years.
- In undisturbed layers, the lowest are oldest and the highest are youngest.
- Fossils in each layer show how a place’s environment changed over time.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Layer on layer
If you look at a cliff or the walls of a canyon, you can often see stripes in the rock. Each stripe is a layer. Long ago, mud, sand and tiny shells settled at the bottom of seas, lakes and rivers. More material kept piling on top, and over time the weight pressed the lower layers into solid rock. The bottom layers were laid down first, so the lowest layers are the oldest and the top layers are the youngest.
Reading the clues
Rock layers are like the pages of a history book, and scientists read them from the bottom up. The fossils in each layer show what lived there when it formed. A layer full of sea shells tells us the area was once under the sea, while a layer above it with fern fossils shows the land later became a warm, wet swamp. In this way, rock layers show how a place changed over a very long time.
Curriculum links: NGSS 4-ESS1-1 · England KS2: Rocks · Cambridge Primary Stages 4–6: Earth and space · NCERT Environmental Studies, Classes 3–5