• Quick retrieval from earlier learning (How fossils form). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. In how a fossil forms, what happens straight after "an animal dies"? (minerals slowly replace the bones over millions of years / the rock is worn away and the fossil is uncovered / the soft parts rot away / it is quickly buried by mud or sand) → it is quickly buried by mud or sandPrompt: Only the hard parts usually become fossils.
2. A fossil is found in a deep rock layer. Another is found in a layer near the top. Which is probably older? (the fossil near the top / the fossil in the deeper layer / they are the same age / it is impossible to say anything) → the fossil in the deeper layerPrompt: Layers build up one on top of another.
3. In how a sea shell becomes a fossil, what happens straight after "its shell sinks to the sea floor"? (the sediment slowly turns into rock / the fossil is found in the rock millions of years later / a sea creature dies / layers of sediment cover it) → layers of sediment cover itPrompt: Fossils form very slowly, buried in rock.

• Pass round different rocks and ask how each one might have formed.
• Share the objective: Rock layers: reading time from the bottom up. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: At the bottom of a lake, mud builds up about 2 cm every 100 years. A layer of mud is 22 cm thick. About how many years did it take to form?
• Step 1: 22 ÷ 2 = 11
• Step 2: 11 × 100 = 1100
• Step 3: So the answer is: 1100
• Answer: 1100
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. At the bottom of a lake, mud builds up about 1 cm every 100 years. A layer of mud is 12 cm thick. About how many years did it take to form? → 1200Prompt: How many groups of 1 cm are in 12 cm?
2. A river has cut a canyon and you can see many layers in the walls. What do the layers show? (the river's colour / one year of weather / paint added by people / a record of how the area changed over a very long time) → a record of how the area changed over a very long timePrompt: Each layer formed in different conditions.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. A rock layer contains rounded pebbles and fossil river snails. What was the environment when it formed? (the deep ocean floor / a river / a desert with no water / a glacier) → a riverPrompt: What rounds pebbles, and where do these snails live?
2. At the bottom of a lake, mud builds up about 2 cm every 100 years. A layer of mud is 12 cm thick. About how many years did it take to form? → 600Prompt: How many groups of 2 cm are in 12 cm?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-4.