Why do sediments form layers? | 11-14 years

In this lesson, students interpret annotated diagrams of layers of sedimentary rocks to suggest a possible sequence of events which could have led to the sequence of strata.

Diagrams are used as a prop to help focus thinking and discussion about the formation of sedimentary rocks.

Learning objectives

Students will appreciate:

• That sedimentary layers are the result of distinct episodes of sedimentation over a variety of timescales.

Sequence of activities

Introduction

1. Show an image, using a data projector if available, of a sequence of sedimentary rock layers and share the learning objective with students.
2. Explain that they are going to use the evidence of sedimentary layers to suggest what conditions might have been like when the layers were formed.
3. Use questions to help students formulate their ideas:
   • What words or phrases could you use to describe conditions that led to these layers?
   • How would you know that organisms lived in the water?

Activity

• Arrange students into pairs.
• Give one student in the pair ’Rock sediments A’ and the other student ‘Rock sediments B’. Give each student a copy of ‘Activity sheet 1’ and ‘Activity sheet 2’.
• Guide students through the following tasks by displaying a list and/or offering help verbally at relevant stages. Circulate and support while students:
  1. Write, on ‘Activity sheet 1’, a short story describing a sequence of conditions that could have caused the rock sediments shown on their sheet. Ask another student with the same sheet for help, if they have difficulty completing the story.
  2. Exchange their stories with the other member of the pair.
  3. Draw, on ‘Activity sheet 2’, an annotated diagram of strata based on the story written by the other student. Again, if necessary, ask for the help of another student who started off with a copy of the same rock sediment sheet as themselves.
  4. Compare the original copy of ‘Rock sediments A’ with the annotated diagram drawn by the student.
  5. Identify similarities and any differences and discuss what might have caused the differences.
  6. Do the same thing with ‘Rock sediments B’.
  7. Write down on their partner’s ‘Activity sheet 1’, what were the good points in the story that helped them draw a diagram that was similar to the original copy and what else could have been added that would have helped them even more.
  8. Give back the copy of ‘Activity sheet 1’ and check that the comments about their stories are fair and helpful.
  9. Use the comments from their partner to make a list on ’Activity sheet 1’ of the ideas they need to look at again to be sure that they understand why sediments form layers.

Feedback

Take in ‘Activity sheet 1’ and write comments on what students have already achieved. Direct them to where they can find the information they need to consolidate their understanding.

Commentary

An initial still image enables a clear explanation of the learning objective.

The assessment mechanism, ie the exchange of diagrams and the critique of each others’ work, is intense in this activity. This provokes a strong stimulus for the students when they are thinking creatively and ordering their thoughts.

Written feedback based on their work and their partner’s comments is used to identify the student’s next steps.

Stories to match sediment strata

Phrases and words that students might find useful in writing their stories include:

• a very long time, a short time, longer than, shorter than
• soon after, after some time
• rivers receded, water dried up
• salty water
• living organisms

The kind of stories that match the strata pictures are, in reverse chronological order:

Rock sediments A

• The rivers stopped flowing again and the rock became hard.
• Much later, rivers covered the Earth’s surface again and they wore away the rocks.
• The rivers stopped flowing and the rock became hard.
• Rivers covered the Earth’s surface.
• Sometimes lakes formed and then dried up to form salt layers.
• Rivers covered the Earth’s surface.
• The salty water receded.
• The Earth’s surface was covered by salty water in which there were living organisms.
• A very, very long time ago.

Rock sediments B

• The salty water receded again and the rocks became hard.
• There were living organisms in the salty water.
• Much later, the Earth’s surface was covered by salty water again which wore away the rocks.
• The salty water receded and the rocks became hard.
• There were living organisms in the salty water.
• The Earth’s surface was covered by salty water.
• The rivers receded.
• Rivers covered the Earth’s surface.
• Sometimes lakes formed and then dried up to form salt layers.
• Rivers covered the Earth’s surface.
• A very, very long time ago.