Earth Systems and Geology

Sedimentary Rocks are rocks that are made from tiny pieces of other rocks, sand, mud, or even bits of plants and animals.

These tiny pieces are called sediment. Sediment can be very small, like dust, or bigger, like pebbles.

Over a long, long time, layers of sediment pile up, get pressed together, and turn into rock. That rock is called sedimentary rock.

Let’s learn how this happens step by step.

How Sedimentary Rocks Form

1. Weathering: Big rocks break into smaller pieces.
2. Erosion: Water, wind, or ice moves the small pieces.
3. Deposition: The pieces settle down in a new place.
4. Compaction: More and more layers pile on top and press down.
5. Cementation: Minerals act like glue and stick the pieces together.

1. Weathering

Weathering is when rock breaks apart into smaller pieces.

This can happen when rain, wind, ice, or plant roots crack and break rocks. A big rock can slowly turn into little rocks, sand, or mud.

Weathering does not mean the rock moves to a new place yet. It just means the rock is breaking apart.

2. Erosion

Erosion is when the broken pieces of rock are moved from one place to another.

Water can carry sand down a stream. Wind can blow dust across the land. Ice can push rocks along slowly.

Weathering breaks rocks. Erosion moves the pieces.

3. Deposition

Deposition happens when the moving sediment stops and falls into a new place.

For example, a river may slow down and drop sand and mud at the bottom. This makes a layer of sediment.

Over time, more and more layers can build up.

4. Compaction

Compaction means the layers get squeezed together.

When new layers of sediment land on top of older layers, the bottom layers get pressed down. They become flatter and tighter.

You can think of it like pushing down on a pile of soft crumbs. The crumbs get packed closer together.

5. Cementation

Cementation is when minerals in water act like glue.

These minerals fill the spaces between the pieces of sediment and help stick them together. After a long time, the sediment becomes solid rock.

Why Sedimentary Rocks Have Layers

Sedimentary rocks often have layers because the sediment was dropped one layer at a time.

A river may bring mud one day and sand another day. Over many years, these layers stack up. When they turn into rock, you can still see the layers.

These layers are like pages in a book. They can tell us about what happened long ago.

What Sedimentary Rocks Can Hold

Some sedimentary rocks can hold fossils. Fossils are the remains or marks of plants and animals from long ago.

This can happen when a plant leaf, shell, or animal track gets covered by sediment. Later, the sediment turns into rock and keeps the mark safe.

Examples of Sedimentary Rocks

• Sandstone is made from sand.
• Shale is made from mud or clay.
• Conglomerate is made from rounded pebbles stuck together.

Clues That a Rock Is Sedimentary

You can look for these clues:

• It has layers.
• You can see pieces of sand, mud, or pebbles.
• It may have a fossil.

Worked Example 1

A big rock cracks into tiny pieces after many rainy days and cold nights. What part of sedimentary rock making is this?

Answer: This is weathering.

Why: The rock is breaking into smaller pieces. It is not moving yet.

Worked Example 2

Sand is carried by a river and then settles at the bottom when the water slows down. What two steps are happening?

Answer: First is erosion, then deposition.

Why: The river moves the sand, which is erosion. Then the sand drops and settles, which is deposition.

Worked Example 3

Many layers of mud sit on top of older layers. The bottom layers get squeezed tight. What step is this?

Answer: This is compaction.

Why: The weight of the top layers presses down on the bottom layers.

Worked Example 4

Tiny pieces of sand are packed together. Minerals in water stick the pieces together like glue. What step is this?

Answer: This is cementation.

Why: The minerals help glue the sediment together to make rock.

Easy Way to Remember the Steps

1. Rocks break = weathering
2. Pieces move = erosion
3. Pieces drop = deposition
4. Layers squeeze = compaction
5. Pieces stick = cementation

Let’s Compare

• Weathering: breaking
• Erosion: moving
• Deposition: dropping
• Compaction: pressing
• Cementation: gluing

Summary

Sedimentary rocks form from sediment, which is made of tiny pieces of rock, sand, mud, or once-living things.

First, rocks break apart by weathering. Next, the pieces move by erosion. Then they settle by deposition.

After that, the layers are squeezed by compaction. Finally, the pieces are glued together by cementation.

This is why sedimentary rocks often have layers and sometimes fossils. Sedimentary rocks help us learn about Earth’s past.

Erosion and Deposition are two ways Earth’s surface changes over time.

Erosion means small pieces of rock and soil are moved from one place to another.

Deposition means those pieces of rock and soil are dropped in a new place.

These small pieces are called sediment. Sediment can be tiny bits of sand, soil, or rock.

Earth is always changing. Wind, water, ice, and gravity can all move sediment. First, sediment is often moved by erosion. Then, when it slows down or stops, the sediment is dropped by deposition.

Think of it like this: erosion is take away, and deposition is put down.

What causes erosion?

• Water can carry soil, sand, and small rocks.
• Wind can blow sand and dust to new places.
• Ice can push and carry rocks as it moves.
• Gravity can pull rocks and soil downhill.

1. Water

Water is one of the biggest causes of erosion. Rain can wash dirt down a hill. Rivers and streams can carry sand and rocks as they flow.

Fast-moving water can pick up more sediment. Slow-moving water drops more sediment.

When a river reaches a lake or ocean, it often slows down. Then it drops the sediment it was carrying. Over time, this can build a landform called a delta.

A delta is a piece of land made when a river drops sediment near its mouth, where the river meets a bigger body of water.

2. Wind

Wind can move tiny pieces of sand and dust. In dry places, strong winds can blow sediment far away.

When the wind slows down, it drops the sand. Over time, the sand can pile up into hills called dunes.

A dune is a hill of sand made by wind deposition.

3. Ice

Ice can also cause erosion. Big moving ice, called glaciers, can scrape the ground and carry rocks with it.

When the ice melts, it drops the rocks and soil it was carrying. That is deposition.

4. Gravity

Gravity pulls things downward. It can make rocks, sand, and soil move down a hill or mountain.

When the moving earth comes to rest at the bottom, that is deposition.

How are erosion and deposition connected?

Erosion and deposition are a team. First, sediment is moved. Later, sediment is dropped.

For example, a stream may erode soil from a riverbank. Then the stream may deposit that soil farther downstream.

Clues you can look for

• If soil or sand is being carried away, that is erosion.
• If soil or sand is being dropped and piled up, that is deposition.

Worked Example 1

It rains on a hill. Water washes soil down the hill.

Question: Is this erosion or deposition?

Answer: This is erosion because the soil is being moved.

Worked Example 2

A river carries mud and sand. When the river reaches the ocean, the water slows down. The mud and sand settle at the river’s mouth.

Question: What is happening at the river’s mouth?

Answer: This is deposition because the sediment is being dropped.

Worked Example 3

Wind blows sand across a beach. Later, the wind becomes weaker, and the sand piles up into a small hill.

Question: Which part is erosion, and which part is deposition?

Answer:

• Wind blowing the sand is erosion.
• Sand piling up into a hill is deposition.

Worked Example 4

Some rocks tumble down a steep hill because of gravity. The rocks stop at the bottom.

Question: Where do we see erosion? Where do we see deposition?

Answer:

• The rocks moving downhill is erosion.
• The rocks resting at the bottom is deposition.

Let’s compare

• Erosion = sediment moves
• Deposition = sediment drops

Examples of erosion

• Rain washing dirt away
• A river carrying sand
• Wind blowing dust
• Ice moving rocks
• Gravity pulling soil downhill

Examples of deposition

• A river dropping mud near the ocean
• Wind making a dune
• Melting ice leaving rocks behind
• Rocks collecting at the bottom of a hill

Why does this matter?

Erosion and deposition help shape Earth’s surface. They can change hills, rivers, beaches, and deserts.

They also make new landforms. Deltas and dunes are made by deposition.

Quick way to remember

• Erosion: Earth material goes away.
• Deposition: Earth material goes down in a new place.

Summary

Erosion is when wind, water, ice, or gravity moves sediment from one place to another. Deposition is when that sediment is dropped in a new place.

Water can make deltas, and wind can make dunes. If sediment is moving, it is erosion. If sediment is settling, it is deposition.

Soil Horizons are the different layers of soil in the ground. If we could cut the ground like a cake and look at the side, we would see layers. Each layer is a little different.

These soil layers help plants grow. They also hold water, air, tiny living things, and pieces of rock. Learning about soil horizons helps us understand what is under our feet.

The main soil horizons we will learn are topsoil, subsoil, and bedrock.

1. Topsoil

Topsoil is the top layer of soil. It is the layer closest to the ground surface.

Topsoil is usually dark. It has small pieces of dead plants and animals mixed in. This helps make the soil rich and good for plants.

Many roots grow in topsoil. Worms, bugs, and tiny living things live there too. Topsoil often has the most food for plants.

2. Subsoil

Subsoil is the layer under topsoil. It is usually lighter in color.

Subsoil has less plant material in it. It may have more clay and more small pieces of rock. Some plant roots can grow down into subsoil to find water.

Subsoil is important because it helps hold minerals and water. Even though it has less food for plants than topsoil, it still helps plants live.

3. Bedrock

Bedrock is the hard rock under the soil. It is the bottom layer in this lesson.

Bedrock is not soft like soil. It is solid rock. Soil can form slowly from rock breaking into smaller pieces over a very long time.

Sometimes there may be loose, broken rock above the bedrock. But bedrock itself is the hard rock layer underneath.

How soil horizons form

Soil horizons form slowly over time. Wind, water, plant roots, heat, and cold can break rocks into smaller pieces.

Then dead leaves and other plant parts mix into the top layer. Little by little, different layers form. The top layer gets more dark plant material, while lower layers have more rock and less plant material.

Why soil horizons matter

• They help plants grow. Topsoil gives plants space, water, air, and food.
• They hold water. Some layers hold more water than others.
• They are home to living things. Worms, insects, and tiny living things live in soil.
• They tell us about the land. Soil layers can show what the ground is like below the surface.

Think of soil like a layer cake

A cake has layers stacked on top of each other. Soil does too. The top layer is topsoil, the middle layer is subsoil, and the hard bottom layer is bedrock.

But soil is not food, of course. This is just a way to help us picture the layers.

From top to bottom

1. Topsoil — dark, rich, good for many roots
2. Subsoil — lighter, more clay and rock pieces
3. Bedrock — hard solid rock

Worked Example 1

A child digs a small hole in a garden. First the child sees dark soil with roots and worms. What layer is this?

Answer: This is topsoil.

Why? Topsoil is the dark top layer. It has many roots and living things.

Worked Example 2

A deeper part of the hole has lighter soil with fewer roots and more small rock pieces. What layer is this?

Answer: This is subsoil.

Why? Subsoil is under topsoil. It usually has less plant material and more rock pieces.

Worked Example 3

A builder digs very deep and reaches hard, solid rock that is not soft like soil. What layer is this?

Answer: This is bedrock.

Why? Bedrock is the hard rock under the soil layers.

Worked Example 4

Put these layers in order from top to bottom: bedrock, topsoil, subsoil.

Answer: Topsoil, subsoil, bedrock.

Why? Topsoil is at the surface, subsoil is below it, and bedrock is deepest.

Helpful clues for each layer

• Topsoil: dark, roots, worms, rich soil
• Subsoil: lighter color, fewer roots, more clay or rock pieces
• Bedrock: hard, solid rock

Let’s remember

When you look at the ground, there is more than just one kind of dirt. There are layers. These layers are called soil horizons.

The top layer is topsoil. Under that is subsoil. Under the soil is bedrock.

Each layer is important. Together, they help make Earth’s surface a place where plants and animals can live.