正文：

Mountains are landforms higher than 2000 feet, or about 600 meters, above sea level.

山脉是海拔2000英尺（约600米）以上的地貌。

Hills are like mountains, but not as tall.

丘陵就像高山，却不高。

Mountains are formed by forces deep within the Earth and are made of many types of rocks.

山脉是由地球深处的力量形成的，由多种类型的岩石构成。

Rivers are streams of water that usually begin in mountains and flow into the sea.

河流是水流，通常始于山区，然后流入海中。

Many early cities were built alongside rivers.

许多早期的城市都建在河边。

Most of the world's water is in the oceans, which are large bodies of salt water.

世界上大部分的水都在海洋里，海洋是大的含盐水体。

The oceans are also an important source of oxygen.

海洋也是氧气的重要来源。

Deserts are areas of land that receive little or no rainfall.

沙漠区是很少或没有降雨的地方。

The amount of rainfall in some deserts is so low that only a few plants can survive.

在一些沙漠中，雨量太少了，只有少数植物可以生存。

A valley is a low, relatively flat area between hills or mountains, often with a river running through it.

山谷是丘陵或山脉之间的低洼、相对平坦的区域，通常有一条河流贯穿其中。

Some valleys are formed by flowing water, and others are formed by glaciers.

有些山谷是由流动的水形成的，而另一些则是由冰川形成的。