Grade by Grade
E. Earth and Space Systems · Rocks, Minerals, and Geological Processes

Sedimentary Rocks: Made in Layers

E2.1 E2.2 A1.2 A1.5About 45 minutes
Learning goals
  • I can explain how sedimentary rocks form through weathering, erosion, deposition, compaction and cementation.
  • I can describe and identify sandstone, shale, limestone and conglomerate.
  • I can explain that lower layers are usually older than the layers above them.
  • I can describe the Niagara Escarpment as an Ontario example of sedimentary rock.

Words to know

sediment
Small pieces of rock, sand, mud, shells and plant bits that are moved and dropped by water, wind or ice.
deposition
When moving water, wind or ice drops sediment in a new place.
compaction
When the weight of layers on top squeezes the sediment below tightly together.
cementation
When minerals in the water glue sediment grains together into solid rock.
sedimentary rock
Rock made from layers of sediment that were pressed and glued together over a long time.
escarpment
A long, steep slope or cliff that separates higher land from lower land.

A rock sandwich

Have you ever seen a cliff with stripes, like the layers of a sandwich or a cake? Those stripes are layers of sedimentary rock. Each layer is like a page in a history book. It tells what was happening in that place long ago.

A tall cliff beside a river with horizontal stripes of grey, tan and reddish rock layers.
The stripes in this cliff are layers of sedimentary rock. The layers near the bottom were laid down first, so they are the oldest. AI-generated image

Remember from Grade 3

In Grade 3 you learned that weathering breaks rock into smaller pieces, and erosion takes (moves) those pieces away, by water, wind, ice and gravity. Today you'll find out what happens next to all those bits.

Five steps to make a sedimentary rock

  1. Weathering: Rain, ice, wind and plant roots break rock into small pieces.
  2. Erosion: Water, wind or ice carries the pieces away. These pieces are called sediment. Sediment can also include shells and bits of dead plants.
  3. Deposition: When the water slows down, for example where a river enters a lake or sea, the sediment settles to the bottom. Heavier pieces settle first. Layer after layer builds up.
  4. Compaction: Over a long time, more and more layers pile on top. Their weight squeezes the bottom layers, pushing out water and pressing the grains close together.
  5. Cementation: Water seeping between the grains carries dissolved minerals. The minerals are left behind and glue the grains together, like cement.

This takes a very long time, usually thousands to millions of years.

water with sedimentnewest layer (top)older and olderoldest layer (bottom)
Sediment settles out of water in layers. New layers pile on top, so the layers near the bottom are usually the oldest. The weight of the layers above presses the lower layers together.
RockMade fromWhat it looks and feels like
conglomeraterounded pebbles and sand, cemented togetherlooks like a lumpy mix of pebbles stuck in a natural cement
sandstonesand grains (mostly quartz)feels gritty, like sandpaper; you can often see the grains
shalemud and claysmooth; very fine grains; splits into thin, flat layers
limestonethe mineral calcite, often from shells of sea animalsgrey or tan; often has fossils; fizzes in acid
rock salt and gypsumminerals left behind when salty water dries upsalt rock is made of halite; gypsum is soft (hardness 2)
Common sedimentary rocks

Clues that a rock is sedimentary

  • It has layers (strata).
  • You can see grains of sand or pebbles stuck together.
  • It may have fossils, like shells or leaf prints. (You'll learn more in the fossil lesson.)
  • It is often softer than igneous rock. Some sedimentary rocks crumble or rub off when you scratch them.

The Niagara Escarpment

The Niagara Escarpment is a long ridge with steep cliffs that stretches across southern Ontario, from the Niagara River north to Tobermory on the Bruce Peninsula. It is a UNESCO World Biosphere Reserve, and the Bruce Trail follows it.

The Escarpment is made of layers of sedimentary rock that formed at the bottom of an ancient sea that once covered this area. On top is a layer of hard dolostone (a rock similar to limestone). Underneath are softer rocks like shale.

At Niagara Falls, water pours over the hard top layer. The soft shale underneath wears away faster, leaving the hard rock sticking out like a shelf. In time, the shelf breaks off, and the falls move a little farther upstream. Over thousands of years, this erosion has moved Niagara Falls back by several kilometres.

riverhard dolostonesoft shalewaterfallhard rock sticksout like a shelf
At Niagara Falls, the river flows over a hard top layer of dolostone. The softer shale underneath wears away faster, so the hard layer sticks out like a shelf until it breaks off.
Worked example

Let's read the layers

A cliff has 4 layers. From the bottom up: shale, sandstone, limestone with shell fossils, conglomerate. Which layer is the oldest? Which is the youngest? What was this place like when the limestone formed?

  1. Sediment piles up from the bottom, so the bottom layer formed first. Here that is shale.
  2. The top layer formed last. Here that is conglomerate.
  3. Limestone with shell fossils forms in water where shelled animals lived. So this place was probably covered by a sea or lake at that time.

Oldest: shale (bottom). Youngest: conglomerate (top). When the limestone formed, the area was likely underwater, with sea animals living there. (This works as long as the layers have not been flipped or folded.)

Just for fun

What is a sedimentary rock's favourite dessert?

Layer cake, of course!

Safety

White glue is safe, but keep it out of your eyes and mouth. Wash your hands after the activity. If you collect sand outside, ask a grown-up first and wash your hands afterwards.

Hands-on activity

Make your own sandstone

You need: dry sand (from a sandbox or a craft store), white glue, water, 2 small paper cups, a spoon, a pencil.

  1. Label the cups A and B.
  2. Put 3 spoonfuls of sand in each cup.
  3. Cup A: add 1 spoonful of water only. Mix and press the sand down hard with the spoon (that's compaction).
  4. Cup B: mix 1 spoonful of white glue with 1 spoonful of water, then stir it into the sand. Press it down hard too. The glue is like the minerals that cement real sand grains.
  5. Leave both cups somewhere warm and dry for 2 to 3 days.
  6. Peel away the paper cups. Which one holds together like a rock? Rub each one with your thumb. What happens?
  7. Write a short report: Question, Prediction, What I did, What I saw, What I learned. Use the words compaction and cementation.

Expected result: Cup A crumbles back into sand. Cup B holds together as a "rock". Pressing alone is not enough; the grains also need cement to become sandstone.

Bonus: Layer different coloured sand (or sand and soil) in a clear plastic cup to model rock layers. Which layer did you put in first? Which is "oldest"?

Your turn

1

What is sediment?

2

Put the steps in order: cementation, weathering, deposition, erosion, compaction.

3

What is the difference between compaction and cementation?

4

Which sedimentary rock feels gritty, like sandpaper?

5

Which sedimentary rock splits into thin, flat layers and is made from mud?

6

In a stack of rock layers, which layer is usually the oldest?

7

Name the long ridge of sedimentary rock in southern Ontario.

More practice: warm-up

1

What is sandstone made from?

2

Where does sediment usually settle?

3

Which sedimentary rock often has shell fossils?

4

What does conglomerate look like?

5

True or false: Sedimentary rocks form in a few days.

More practice: on your own

1

In the sandstone activity, why did Cup A crumble?

2

Why do heavier pebbles usually end up at the bottom of a layer?

3

Why do layers of limestone with fossil shells tell us the Niagara area was once underwater?

4

At Niagara Falls, why does the shale wear away faster than the dolostone?

5

Give two clues that a rock might be sedimentary.

Challenge

1

A rock has rounded pebbles stuck in it. What does the roundness tell you about the pebbles' journey?

2

Rock salt forms when salty water dries up. What does finding a thick layer of rock salt deep underground tell you about that place long ago?

3

Explain why Niagara Falls is slowly moving upstream. Use the words erosion, hard and soft.

Check your understanding

Quiz

1.

Pieces of rock, sand and mud moved by water are called...

2.

When the weight of layers squeezes sediment together, that is...

3.

Which rock is made of sand grains glued together?

4.

In layers of rock that have not been flipped, the oldest layer is usually at the...

5.

Which is a clue that a rock is sedimentary?

6.

Why is Niagara Falls slowly moving upstream?

For parents and teachers

Big idea (E2.1, E2.2): Sedimentary rocks form through weathering, erosion, deposition, compaction and cementation. They typically show layers, visible grains and sometimes fossils. In undisturbed layers, lower layers are older (superposition). The Niagara Escarpment is Ontario's best-known sedimentary landform.

How to help: If you live in southern Ontario, look for layered rock in road cuts, river valleys and along the Escarpment (Bruce Trail, conservation areas). Sandstone, limestone and shale are common in building stone and retaining walls. In the sandstone activity, encourage a short written report (A1.5).

Common difficulties:

  • Mixing up weathering and erosion. Weathering breaks; erosion moves.
  • Thinking pressure alone makes rock. Cementation is also needed (Cup A vs Cup B).
  • Thinking the top layer is oldest. Use the idea of a laundry pile: the bottom shirt went in first.

Accuracy note: The top layer at Niagara is commonly called dolostone (dolomite rock), a limestone-like rock containing magnesium. Erosion has moved the falls back several kilometres over roughly the last 12,000 years; the rate today is much slower because water is diverted for hydro power.