---
title: "Stones, Pebbles and Sand"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 5
exercises: 10
source: https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 5 — Stones, Pebbles and Sand

Tom is on holiday by a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). On the [gravel](#def-g3-stones-pebbles-sand-pebble) bank, where the water is shallow, he picks up a [pebble](#def-g3-stones-pebbles-sand-pebble). It is smooth and round, almost like an egg, and it fits in his hand. Last week, walking with his parents on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) path high up the same [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), he saw stones of the same colour lying under a rocky slope — but those stones were sharp, with edges and corners.

How did a sharp stone from the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) become a smooth, round [pebble](#def-g3-stones-pebbles-sand-pebble) down in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform)? To find out, let us start with the [rock](#def-g3-stones-pebbles-sand-rock) itself.

![Round pebbles under the clear water of a stream.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/img-2ee0414190cf.jpg)

*Round [pebbles](#def-g3-stones-pebbles-sand-pebble) under the clear water of a stream.*

**You already know.**

In a [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) there are [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that flow from the high land down to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([Chapter 2](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#ch-g1-landscapes-around-us)). The top layer of the ground, where plants grow, is the soil: the biology books tell how living things make it.

## 5.1 Rock: the hard ground under our feet

Dig a deep hole in a garden. First you find dark, soft soil with roots in it. Below, the soil becomes lighter and stonier. And if you could dig deep enough, you would always reach something hard that a spade cannot cut: [rock](#def-g3-stones-pebbles-sand-rock).

**Definition 5.1 (Rock and bedrock).**

A *rock* is a hard, natural material that the solid Earth is made of. The *bedrock* is the solid rock that lies under the soil everywhere on the land, and under the [sand](#def-g3-stones-pebbles-sand-sand) and the [mud](#def-g3-stones-pebbles-sand-sand) of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor.

In most places the [bedrock](#def-g3-stones-pebbles-sand-rock) is hidden by the soil, the grass and the trees. We see it where there is no soil to cover it: in the steep sides of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), in cliffs by the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and along roads that have been cut through a [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

![A cut through the ground. Under the soil there is always the solid bedrock. On the steep hill side, on the right, the soil is missing and the rock shows at the surface.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/fig-af1cfd2a8790.svg)

*A cut through the ground. Under the soil there is always the solid [bedrock](#def-g3-stones-pebbles-sand-rock). On the steep [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side, on the right, the soil is missing and the [rock](#def-g3-stones-pebbles-sand-rock) shows at the surface.*

## 5.2 From rock to pebbles

[Rock](#def-g3-stones-pebbles-sand-rock) is hard, but it does not last for ever. In winter, water gets into the cracks of a [rock](#def-g3-stones-pebbles-sand-rock), freezes and swells, and pushes the cracks wider. Roots grow into cracks too. Little by little, pieces break off and fall: that is where the sharp stones under the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) slopes come from.

When a sharp stone falls into a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) stream, the water rolls it along the bottom, knocks it against other stones, and drags it over [sand](#def-g3-stones-pebbles-sand-sand). Its corners are the first to wear away. The farther it travels, the rounder and smoother it becomes. By the time it reaches the lower [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), it has become a [pebble](#def-g3-stones-pebbles-sand-pebble).

**Definition 5.2 (Pebble and gravel).**

A *pebble* is a small piece of [rock](#def-g3-stones-pebbles-sand-rock) that has been rounded and smoothed by water rolling it along, in a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or on a beach. *Gravel* is a mass of small stones and pebbles lying together, with grains bigger than grains of [sand](#def-g3-stones-pebbles-sand-sand).

![The journey of a stone in a river: sharp when it falls from the rock, round and smooth after a long ride downstream.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/fig-e4ee0a3be0b8.svg)

*The journey of a stone in a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): sharp when it falls from the [rock](#def-g3-stones-pebbles-sand-rock), round and smooth after a long ride downstream.*

**Remark 5.3 (Reading the shape of a stone).**

The shape of a stone tells us its story. A sharp stone has not travelled far from the [rock](#def-g3-stones-pebbles-sand-rock) it broke from. A round, smooth [pebble](#def-g3-stones-pebbles-sand-pebble) has been rolled for a long time by water, in a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or by the waves of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

## 5.3 Sand and mud

The stones and [pebbles](#def-g3-stones-pebbles-sand-pebble) that bump into one another also chip off tiny pieces. Over a very long time, these tiny pieces become [sand](#def-g3-stones-pebbles-sand-sand). Look at [sand](#def-g3-stones-pebbles-sand-sand) through a magnifying glass: each grain is a little piece of [rock](#def-g3-stones-pebbles-sand-rock), or of a shell, often rounded too.

**Definition 5.4 (Sand and mud).**

*Sand* is made of loose grains of broken [rock](#def-g3-stones-pebbles-sand-rock) smaller than 2 millimetres across: we can still see each grain. *Mud* is made of grains so small that we cannot see them one by one; wet, it is soft and sticky.

Geologists sort broken pieces of [rock](#def-g3-stones-pebbles-sand-rock) by size, from the biggest to the smallest: boulders, [pebbles](#def-g3-stones-pebbles-sand-pebble) and [gravel](#def-g3-stones-pebbles-sand-pebble), [sand](#def-g3-stones-pebbles-sand-sand), and [mud](#def-g3-stones-pebbles-sand-sand).

![Grains of broken rock from the smallest to the biggest. Mud, sand, gravel and pebbles are drawn at their real size: compare with a ruler. A boulder can be as big as a chair, or as a house.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/fig-aea260417e00.svg)

*Grains of broken [rock](#def-g3-stones-pebbles-sand-rock) from the smallest to the biggest. [Mud](#def-g3-stones-pebbles-sand-sand), [sand](#def-g3-stones-pebbles-sand-sand), [gravel](#def-g3-stones-pebbles-sand-pebble) and [pebbles](#def-g3-stones-pebbles-sand-pebble) are drawn at their real size: compare with a ruler. A boulder can be as big as a chair, or as a house.*

![Beach sand seen through a magnifying glass: tiny grains of rock and pieces of shell.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/img-9dec813f38c4.jpg)

*Beach [sand](#def-g3-stones-pebbles-sand-sand) seen through a magnifying glass: tiny grains of [rock](#def-g3-stones-pebbles-sand-rock) and pieces of shell.*

**Method 5.5 (Sorting broken rock by size).**

To name a piece of broken [rock](#def-g3-stones-pebbles-sand-rock), look at its size:

1. if you cannot see the grains, even close up, it is [mud](#def-g3-stones-pebbles-sand-sand) ;
2. if you can see each grain, but each one is smaller than 2 millimetres (the thickness of a coin), it is [sand](#def-g3-stones-pebbles-sand-sand) ;
3. if the pieces are bigger than that, but you can hold one in your hand, they are [gravel](#def-g3-stones-pebbles-sand-pebble) and [pebbles](#def-g3-stones-pebbles-sand-pebble) ;
4. if a piece is too heavy to lift, it is a boulder.

**Example 5.6 (Where do we find each size?).**

Fast water can carry big stones; slow water drops them and carries only small grains. So boulders and sharp stones lie in the steep [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) streams; [pebbles](#def-g3-stones-pebbles-sand-pebble) and [gravel](#def-g3-stones-pebbles-sand-pebble) lie on the banks of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform); [sand](#def-g3-stones-pebbles-sand-sand) gathers on beaches and in quiet bends of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water); and [mud](#def-g3-stones-pebbles-sand-sand) settles where the water is almost still, in ponds, [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and the mouths of [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

## 5.4 Hard rocks and soft rocks

Not all [rocks](#def-g3-stones-pebbles-sand-rock) are equally hard. Some, like granite, are so hard that they scratch glass and last for thousands of years in a wall. Others crumble in the hand, like chalk, which leaves a white mark on a board. Some [rocks](#def-g3-stones-pebbles-sand-rock) are made of grains that we can see; others are smooth and look all the same from one side to the other.

**Example 5.7 (Three rocks compared).**

*Granite* is speckled with pink, white and black grains and is very hard. *Sandstone* is made of grains of [sand](#def-g3-stones-pebbles-sand-sand) stuck together; rubbed with a finger, it sometimes lets a few grains go. *Chalk* is white, soft and powdery. You will learn in a later year how each of them was formed.

![Three rocks seen through a magnifying glass (drawn). The grains of granite and of sandstone can be seen; chalk looks the same everywhere.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/fig-13dda042f754.svg)

*Three [rocks](#def-g3-stones-pebbles-sand-rock) seen through a magnifying glass (drawn). The grains of granite and of sandstone can be seen; chalk looks the same everywhere.*

**Method 5.8 (Looking closely at a stone).**

To describe a stone, look at it and touch it, without breaking it:

1. its colour, or its colours;
2. its shape: sharp with corners, or round and smooth;
3. its grains: can you see them? are they all the same, or of several colours?
4. its feel: hard, or soft enough to leave a mark on paper?

Write what you see in a table, one line for each stone.

**Example 5.9 (A table of three stones).**

Mia describes three stones from a box at school.

| Stone | Colour | Shape | Grains |
| --- | --- | --- | --- |
| A | pink, white and black | round | big, three colours |
| B | yellow-brown | sharp | small, all alike |
| C | white | sharp | none to see |

Stone C also leaves a white mark on paper. From the example above, stone A looks like granite, stone B like sandstone and stone C like chalk. Stone A is round: water has rolled it for a long time.

## 5.5 The stones we build with

For thousands of years, people have built with the [rock](#def-g3-stones-pebbles-sand-rock) around them. In the old villages of a region, the walls are often all of the same colour, because they were built with the stone of the nearest [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

**Definition 5.10 (Building stone).**

A *building stone* is a [rock](#def-g3-stones-pebbles-sand-rock) that is cut or shaped to build walls, roofs, steps, bridges or pavements.

Each [rock](#def-g3-stones-pebbles-sand-rock) is good for something. Hard granite makes walls, steps and kerbs that last for centuries. Some [rocks](#def-g3-stones-pebbles-sand-rock), like slate, split into thin, flat sheets: laid one over the other, they make roofs that keep the rain out. Sandstone and limestone are soft enough to be cut into neat blocks.

![](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/img-7dc29f0e6ff4.jpg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/img-1aa5ff865cd6.jpg)

*A wall of granite blocks, built without mortar. Photograph: Tony Atkin, Geograph (CC BY-SA 2.0).*

*Roofs of thin, flat stone slabs in a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) village of Switzerland. Photograph: Michael C. Rygel (CC BY-SA 3.0).*

![An old village built entirely with the stone of the hill nearby.](https://one-course.com/images/onecourse/chapters/earth-1/g3-stones-pebbles-sand/img-c46adde4b2a1.jpg)

*An old village built entirely with the stone of the [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) nearby.*

Today we also build with bricks, concrete and glass. But a brick is made of baked clay, concrete is made of [sand](#def-g3-stones-pebbles-sand-sand), [gravel](#def-g3-stones-pebbles-sand-pebble) and cement, and glass is made from [sand](#def-g3-stones-pebbles-sand-sand): all of them come from the [rocks](#def-g3-stones-pebbles-sand-rock) of the Earth too.

## 5.6 Exercises

**Exercise 5.1 ★.**

What is the [bedrock](#def-g3-stones-pebbles-sand-rock)? Where can we see it without digging?

**Solution of Exercise 5.1.**

The [bedrock](#def-g3-stones-pebbles-sand-rock) is the solid [rock](#def-g3-stones-pebbles-sand-rock) that lies under the soil everywhere. We see it where there is no soil to hide it: on the steep sides of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), in [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) cliffs, along roads cut through a [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

**Exercise 5.2 ★.**

Put in order of size, from the smallest to the biggest: [pebble](#def-g3-stones-pebbles-sand-pebble), [mud](#def-g3-stones-pebbles-sand-sand), boulder, [sand](#def-g3-stones-pebbles-sand-sand).

**Solution of Exercise 5.2.**

[Mud](#def-g3-stones-pebbles-sand-sand), [sand](#def-g3-stones-pebbles-sand-sand), [pebble](#def-g3-stones-pebbles-sand-pebble), boulder.

**Exercise 5.3 ★.**

A grain is 1 millimetre across. Is it [sand](#def-g3-stones-pebbles-sand-sand) or [gravel](#def-g3-stones-pebbles-sand-pebble)? Another grain is 3 millimetres across: [sand](#def-g3-stones-pebbles-sand-sand) or [gravel](#def-g3-stones-pebbles-sand-pebble)?

**Solution of Exercise 5.3.**

1 millimetre is smaller than 2 millimetres: [sand](#def-g3-stones-pebbles-sand-sand). 3 millimetres is bigger than 2 millimetres: [gravel](#def-g3-stones-pebbles-sand-pebble).

**Exercise 5.4 ★.**

Natural or made by people? A [pebble](#def-g3-stones-pebbles-sand-pebble); a brick; a block of granite; a sheet of glass; [sand](#def-g3-stones-pebbles-sand-sand) on a beach.

**Solution of Exercise 5.4.**

Natural: the [pebble](#def-g3-stones-pebbles-sand-pebble), the block of granite (cut by people, but the [rock](#def-g3-stones-pebbles-sand-rock) is natural), the [sand](#def-g3-stones-pebbles-sand-sand) on the beach. Made by people: the brick and the glass.

**Exercise 5.5 ★.**

Why does a [pebble](#def-g3-stones-pebbles-sand-pebble) from a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) have no sharp corners?

**Solution of Exercise 5.5.**

The water of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) has rolled it along the bottom and knocked it against other stones for a long time: its corners have worn away.

**Exercise 5.6 ★.**

Name two [building stones](#def-g3-stones-pebbles-sand-building-stone) and say what each is used for in the photographs of this chapter.

**Solution of Exercise 5.6.**

Granite, for a wall; thin, flat stone slabs (a [rock](#def-g3-stones-pebbles-sand-rock) like slate), for roofs.

**Exercise 5.7 ★★.**

Tom finds two stones of the same [rock](#def-g3-stones-pebbles-sand-rock): one is sharp, the other is round. Which one has travelled farther? Explain.

**Solution of Exercise 5.7.**

The round one: the longer a stone is rolled by water, the more its corners wear away. The sharp one has not travelled far from the [rock](#def-g3-stones-pebbles-sand-rock) it broke from.

**Exercise 5.8 ★★.**

On the grain-size drawing, how many centimetres long is the biggest [pebble](#def-g3-stones-pebbles-sand-pebble)? How many [sand](#def-g3-stones-pebbles-sand-sand) grains of 2 millimetres would fit side by side along 1 centimetre?

**Solution of Exercise 5.8.**

The biggest [pebble](#def-g3-stones-pebbles-sand-pebble) is 3 centimetres long. One centimetre is 10 millimetres, and $10 \div 2 = 5$: five grains of 2 millimetres fit along 1 centimetre.

**Exercise 5.9 ★★.**

Where would you expect to find [mud](#def-g3-stones-pebbles-sand-sand): in a fast [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) stream or in a quiet pond? Why?

**Solution of Exercise 5.9.**

In the quiet pond. Fast water keeps the tiny grains of [mud](#def-g3-stones-pebbles-sand-sand) moving; only where the water is almost still can they settle to the bottom.

**Exercise 5.10 ★★.**

Bricks, concrete and glass are made by people. Why can we still say that they come from the [rocks](#def-g3-stones-pebbles-sand-rock) of the Earth?

**Solution of Exercise 5.10.**

Because they are made from materials taken from the ground: bricks from clay, concrete from [sand](#def-g3-stones-pebbles-sand-sand), [gravel](#def-g3-stones-pebbles-sand-pebble) and cement, glass from [sand](#def-g3-stones-pebbles-sand-sand). Clay, [sand](#def-g3-stones-pebbles-sand-sand) and [gravel](#def-g3-stones-pebbles-sand-pebble) come from broken [rock](#def-g3-stones-pebbles-sand-rock).