Valley

A valley is a low stretch of land between hills or mountains. Most valleys have a river or stream running through them. Valleys are found on every continent on Earth, and even on the Moon and Mars. They are some of the most common landforms in the world.

Valleys form in two main ways. The first way is through rivers. Over millions of years, a river slowly cuts down into the rock and soil beneath it. The water carries away tiny bits of rock as it flows. Little by little, the river digs a path. River valleys often have a "V" shape when you look at them from the side. The sides slope down to meet the river at the bottom.

The second way is through glaciers. A glacier is a huge, slow-moving river of ice. As a glacier creeps downhill, it scrapes the ground like a giant bulldozer. It pushes rocks and dirt out of the way and grinds the land flat at the bottom. Glacier valleys have a "U" shape, with steep sides and a wide, flat floor. Yosemite Valley in California was carved this way during the last ice age.

Some valleys form in a third way: when the ground itself drops. The Earth's crust is broken into giant pieces called tectonic plates. When these plates pull apart, the land between them sinks. This makes a long, deep crack called a rift valley. The Great Rift Valley in East Africa is the most famous example. It stretches about 4,000 miles, longer than the United States is wide. Some scientists think Africa will one day split apart along this rift, though it would take millions of years.

People have always loved valleys. The flat ground at the bottom is good for farming. The river gives fresh water. The hills on either side can block harsh winds and protect against enemies. Many of the world's oldest civilizations grew up in river valleys. The Egyptians settled in the Nile valley. The first cities in Mesopotamia rose between the Tigris and Euphrates rivers. Ancient China began in the Yellow River valley.

Valleys can be tiny or huge. A small valley behind your house might be a few feet deep. The deepest valley on land is the Yarlung Tsangpo Grand Canyon in Tibet. It is more than three miles deep in places, deeper than the Grand Canyon in Arizona. The longest valleys on Earth are actually under the ocean, hidden in the dark along the seafloor.