Mountains, Rivers and Valleys

The shapes of the land

Look at a map or out of a window in the countryside and you will see that the land is not flat. There are tall mountains, winding rivers and low valleys in between. These shapes are called landforms, and each one has a story about how the Earth made it. Some took millions of years to appear, and they are all still slowly changing today.

How mountains rise

A mountain is a piece of land that rises high above everything around it. But where do they come from?

The Earth's hard outer shell, the crust, is cracked into giant pieces called tectonic plates. These plates float on the hot, slowly moving rock below and they are always drifting, only a few centimetres a year.

When two plates push toward each other, the crust between them has nowhere to go but up. It crumples and folds, like a rug that wrinkles when you push it across the floor. Over millions of years these folds grow into mountains.

The mighty Himalayas in Asia, home to Mount Everest, formed this way when India slowly crashed into the rest of Asia. The crash is still happening, so the Himalayas are still rising. Some mountains form a different way, when melted rock erupts and builds up into a volcano — you can read more in volcanoes and earthquakes.

How rivers flow

A river is a natural stream of water that flows across the land. Every river has a beginning and an end.

A river begins at its source, which is usually high up in hills or mountains. The source might be a bubbling spring, a melting patch of snow or water draining out of a lake. Because the source is high up, gravity pulls the water downhill.

As the water travels down, smaller streams join it and the river grows bigger and wider. Finally it reaches its mouth, the place where it empties into a lake or, most often, the sea.

The Nile in Africa and the Amazon in South America are two of the longest rivers in the world, each flowing for thousands of kilometres from highland sources all the way to the ocean. The water then evaporates, falls again as rain, and the journey starts over in the water cycle.

How valleys are carved

A valley is a low area of land between hills or mountains. Most valleys are made by rivers, and the secret is erosion.

As a river flows, the moving water picks up sand, grit and small stones. These act like sandpaper, scraping away the rock and soil of the riverbed. Bit by bit, over thousands of years, the river cuts down into the land, carving a valley. Because the river cuts mostly downward in a narrow line, river valleys are usually shaped like the letter V.

There is another valley-maker: glaciers. A glacier is a huge, slow-moving river of ice. Glaciers are very wide and incredibly heavy, so instead of cutting a narrow V, they grind out a broad valley shaped like the letter U. Many beautiful U-shaped valleys in places like the Alps and Norway were carved by ice during the Ice Age.

Always changing

It is easy to think that a mountain or a valley will look the same forever, but the Earth never stops working.

• Mountains slowly rise where plates collide, and slowly wear down from wind, rain and ice.
• Rivers keep cutting their valleys deeper and carrying soil to the sea.
• The pieces of worn-away rock become sediment, which can one day form new rock.

These changes are far too slow for us to watch in a single day, but over millions of years they completely reshape the world.

Why it matters

Mountains, rivers and valleys are not just scenery. They are important to all living things:

• Rivers give fresh water to drink, water for crops and homes for fish and birds.
• Valleys often have rich soil, so people build farms and towns there.
• Mountains catch rain and snow that feed the rivers below.

Understanding these landforms helps us plan where to live, grow food and protect nature.

Try it yourself: make a river valley

You can watch erosion happen in just a few minutes. Do this outside or over a large tray.

1. Build a slope using a tray of damp sand, soil or even a pile of flour. Tilt one end up to make a "mountain".
2. Slowly pour water from a cup or jug at the top of the slope.
3. Watch carefully. The water will run downhill and cut a little channel — your own tiny river. The dip it carves is a valley.
4. Notice where the water drops the sand it carries, near the bottom. That is how rivers build flat land near the sea.

Try pouring faster or making the slope steeper. You will see exactly how moving water shapes the land, using the same forces that carved the Grand Canyon over millions of years.