Sedimentary rocks

  • Why does the Earth have mountains and valleys?
  • Have mountains always looked like they look now?
  • How come you can sometimes see "layers" in rock which are different colours? How did these layers form?

Chapter 3 deals with one kind of rock - sedimentary rock. This does leave one wondering what other types of rock there are. The main other type of rock is igneous rock. That is rock that has been hot and molten, and pushed up from deep in the mantle. Mostly it hardens under the ground and we see it only millions of years later when erosion has removed the ground over it. Sometimes it breaks through the crust as molten lava, and we have a volcano. The top of the Drakensberg is the remains of a huge outpouring of lava long ago. Nature is always breaking down rock and eroding it, so mountains are always changing. They change so slowly that we cannot notice it in a person's lifetime, but the changes are happening all the time.

We saw in Chapter 2 that the surface of the Earth is made up of rocks and soil. There are different soil types, but did you know that there are also different types of rock? We classify rocks depending on how they were formed.

We are going to look at sedimentary rock in this chapter and find out how it is formed and used.

Formation of sedimentary rock

In Chapter 2, we saw how rocks break up into smaller and smaller pieces, until we have grains of sand. Now we will find out what happens to the sand.

First, rocks break up into smaller pieces, until the pieces are grains of sand. Next, wind and moving water carry the sand and mud away. Then the wind or the water may drop the sand and mud in one place. Finally, the sand grains might get stuck together again over time and make new rock. This new rock is called sedimentary rock.

Erosion and deposition

When wind or water move the pieces away from the rock, we call this erosion. The wind and water erode the rock as they carry away the sand.

A large valley is forming due to soil erosion.
Soil erosion due to water

When the wind and water put the sand grains down, we call this deposition. The wind and water deposit the sand.

The wind is depositing sand in this corner of the school. Deposition is happening here.


When the sand grains collect on top of each other, they form a sediment. Over time, new layers of mud and sand are deposited on the previous layers. Over a very long time, these sediments become compacted and hardened and become a sedimentary rock. This happens because the grains of sand become glued together, and other heavy sediments press down on the grains of sand. Sediments lie on top of each other. We can actually see these layers in sedimentary rock and they are sometimes different colours. Find the sediments in the pictures below.

Sandstone rock in the Cederberg in the Western Cape
Layers of limestone sedimentary rock
Can you see the different coloured layers in this sedimentary rock?
Look at these layers in this sedimentary rock known as shale.

Let's have a look at how sediments are deposited over time. Except, we do not have thousands of years so we are going to pretend each day of the week is actually about 1000 years!

Depositing sediments


  • alarge see-through jar (you can make this from a 2-litre cold-drink bottle)
  • different places for groups to collect sand and soil

For this activity, divide the class up into 5 groups - one for each day of the school week (Monday to Friday). Each group must collect sand or soil from a different place so that the different layers are evident by the end of the week. At the beginning of each lesson for the week, you can ask the group for that day to pour their sediment in and then carry on with the rest of the lesson. By the end of the week you can look at the layers of sediment that each group added. Explain to the learners that you are speeding up the process.


  1. Put the jar in a place where everyone can see it.
  2. Group 1 must collect a large jam-tin full of sand and on Monday they pour their sand into the jar.
  3. Group 2 must collect sand or soil from a different place. On Tuesday, someone from Group 2 pours that sand into the jar.
  4. On Wednesday, group 3 pours in sand or soil from a different place.
  5. By Friday, the jar will have different layers.


Which sand sediment was put in on Tuesday?

The sediment second from the bottom was added on Tuesday.

Which is the oldest sediment?

The bottom sediment is the oldest.

Which sediment is the oldest?

People who dig holes in the river to get to water sometimes see the sand sediments. The Thunderbolt Kids decided to dig a hole down in the river bed just outside their school. Look at the picture below where you can see Sophie's feet standing on the top and the layers of sediment going down below.

This is what you might see if you dig down in a river-bed.

The river is dry now, but last year the river deposited a sediment. This river deposits a sediment every year when the rains come.


  1. Find the sediment of sand that washed down last year. Read the number next to it.
  2. Find the sediment that washed down the year before last year. Read the number next to it.
  3. On the picture, complete the label, Sediment of last year (20__).
  4. On the picture, where must you write Sediment of the year before last (20__)? Write it there.
  5. On the drawing, next to sediment 5, write Iwas __ years old when the river brought this sediment.

Sediment 5 was deposited in 2009. Let the learners count backward to 2009 and work out how old they were.

In sediment 4, we find the bones of a bird. How could a bird get into this sediment? Write or tell a short story about the bird. Explain why we find its bones under four sediments of sand. Work out in what year the bird fell into the mud.

Teacher's note: For example, "The bird died and fell into the water. Then it sank in the water and the mud covered it. This happened five years ago. The next year more sand came down the river and covered it deeper." If Sophie is standing on the 2013 sediment, then the bird fell into the 2010 sediment. We use the example of yearly floods that bring down sand in a river, because the numbers of years is small. Remind the learners that erosion and deposition has been going on not just these few years, but for millions of years. From the time the first rocks formed from volcanoes on the Earth, weathering, erosion and deposition began to happen. That is why we can find sedimentary rocks that are thousands of millions of years old.

What will you find if you dig deeper than sediment 8?

You will find rock. The deep layer of rock is called the bedrock.

Look at the picture below of the Grand Canyon -

Look at the layers in the sedimentary rock in the Grand Canyon.

Show with your finger which sediment of rock is the oldest.

The lowest sediment/layer of rock they can see in the photo is the oldest.

Show where you can find soil, in the picture.

Soil can be found above the top most sediment, where the plants grow.

Sedimentary rocks are also eroded and broken down into grains of sand again.

The sedimentary rock in the Grand Canyon formed a very long time ago. The layers of sediment were once deposited in warm shallow seas and over millions of years they compacted to form rock. The wind and rain have eroded it until it looks like this.

Look at the diagram below which summarises how sedimentary rock is formed, mostly under the sea or lakes and rivers.

The formation of sedimentary rock

Use the diagram above to write a summary paragraph explaining how sedimentary rock is formed.

Rebuild the mountain the way it was


  1. Look at the diagram below; it shows mountains that are being eroded.
  2. The mountains did not always look like this.
  3. Answer the questions below.
This is what mountains look like now. Draw on this picture to show what the mountains looked like millions of years ago.


The rock and sand are being removed from the mountains. How does this happen?

This is due to erosion (rain water and the wind) over many millions of years.

Where does the rock and sand go?

It is washed down the mountain in the river and deposited on the banks lower down or in the sea.

Draw on the diagram to show how the mountain might have looked many years ago, before the rock and sand were eroded.

Learners should draw a higher mountain without valleys and rocks crumbling away and falling down.

Different kinds of sedimentary rock

There are many kinds of sedimentary rock. Here are three kinds:

  1. Sandstone is made from grains of sand that are cemented together.
  2. Shale is made from grains of clay that are cemented together. Shale is quite soft and you can use it to write with, like a piece of chalk.
  3. Limestone is made of layers of shells of sea-animals that died and sank to the bottom of the sea. Other kinds of limestone are made from sea-water evaporating.

Uses of sedimentary rock

You just saw that there are different types of sedimentary rocks. These rock types are used in different ways.


Limestone is a very common sedimentary rock and it has many uses, mostly as building materials.

Limestone is cut into blocks and used in buildings. Look at these pictures below of different buildings made from limestone.

Can you see the blocks of limestone in this building?
This old building is made from limestone blocks and looks like it can withstand anything.

Limestone is crushed and used to make cement. Limestone is often used in sculptures as it can be carved easily.

A sculpture made from limestone

Glass is made from molten sand, and limestone is mixed with the sand to make the glass stronger. Farmers use limestone to improve their soil, if the soil is too acidic.

Limestone is even used in some medicines and cosmetics and as a white pigment in toothpaste, paints and plastics!


Sandstone has been a popular building material since ancient times, especially in houses and cathedrals around the world. This is because it is quite soft and easy to carve. Houses in Lesotho and the Free State were built from sandstone blocks.

A cathedral in England made from sandstone

Sandstone comes in many different colours and so it is often used in decoratively, such as in decorative stones, in fireplaces, in decorative columns and pillars in buildings and cathedrals and to make statues and fountains. Since sandstone is easy to carve, but does not weather, it is often used as paving stones and to make walkways.

Decorative columns made from sandstone in India
Decorative carvings and columns made from sandstone on the front of a building
Paving blocks made from sandstone


Shale is also used in buildings, especially as a raw material to make bricks. Shale also splits very easily into thin sheets and is therefore used as as tiles for floors and roofs. Shale is used for floors in some houses in South Africa.

Shale splits easily into thin tiles which can be used in flooring and roofs.

Cement is also made from shale. The shale is crushed to a powder and heated in a kiln (a kind of stove). Black shale rock is also a very important source of oil and natural gas all over the world.

  • Sedimentary rocks form when small grains of rock, mud and sand form layers and become compacted over a very long time.
  • Rock breaks into small grains through the process called weathering.
  • Sedimentary rock can be identified as it has visible layers.
  • Examples of sedimentary rock are shale, sandstone and limestone.
  • Sedimentary rocks have different uses.

Complete the following sentences using words from the Word box. Write the sentences out completely.

Word box:









Weathering breaks grains of rock off big rocks. _____ and _____ move these grains on top of each other in layers. A layer of rock grains is called a _____.

Wind, water, sediment

Over many years, the _____ become stuck together and we get sedimentary rock. Three types of sedimentary rock are _____, and _____.

grains, sandstone, limestone, shale

Explain how you would identify sedimentary rock in the natural world around you.

Sedimentary rock has visible layers which are often different colours, so look for rock which has these layers in it.

Explain the difference between erosion and deposition. Provide a drawing to accompany your answer.

Erosion is when something, normally rock, is gradually worn away over time by wind, water or other animals. Deposition is when wind or water carries sand along and then drops it (deposits) in another place where it also gradually builds up over time.

Use the space below to draw a series of drawings to show how a rock is broken down into smaller grains over time. Label your drawing to explain the processes that are taking place to break down the rock.

Learners' drawings will vary, but there should be more than one drawing. The first drawing should show a big rock, then subsequent drawings should show smaller and smaller rocks, until there are coarse grains. Labels to include could be: Erosion due to wind, Erosion due to water, Weathering due to wind and water, Weathering due to impact from animals.

Now comes the section I am most excited about - learning about fossils and how they formed! Let's get started!