Mountains are landforms higher than 2000 feet or about 600meters above sea level.
Hills are like mountains, but not as tall.
Mountains are formed by forces deep within the Earth andare made of many types of rocks.
Rivers are streams of water that usually begin in mountainsand flow into the sea.
Many early cities were built alongside rivers.
Most of the world's water is in the oceans, which are largebodies of salt water.
The oceans are also an important source of oxygen.
Desserts are areas of land that receive little or norainfall.
The amount of rainfall in some deserts is so low that onlya few plants can survive.
A valley is a low, relatively flat area between hills ormountains, often with a river running through it.
Some valleys are formed by flowing water, and others areformed by glaciers.
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