Mountains are landforms higher than 2,000 feet，or about 600 meters， above sea level.
Hills are like mountains, but not as tall.
Mountains are formed by forces deep within the Earth and are made of many types of rocks.
Rivers are streams of water that usually begin in mountains and flow into the sea.
Many early cities were built alongside rivers.
Most of the world's water is in the oceans, which are large bodies of salt water.
The oceans are also an important source of oxygen.
Desserts are areas of land that receive little or no rainfall.
The amount of rainfall in some deserts is so low that only a few plants can survive.
A valley is a low, relatively flat area between hills or mountains, often with a river running through it.
Some valleys are formed by flowing water, and others are formed by glaciers.
（Many early civilizations developed near rivers.）
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