In this topic you will learn about how mountains are formed and worn away.

As the crust moves, the rocks of the crust can change. They can change position. They can move up, down, or sideways. Rocks can also be bent, squeezed, twisted, or broken. These changes may cause mountains to be made.

The most common type of mountain is made mostly of rock layers folded by being squeezed together. This is called a fold mountain. The Appalachian Mountains are fold mountains.

A fault is a large crack in rocks along which there is movement. A mountain made by huge tilted blocks of rocks separated from surrounding rocks by faults is called a fault-block mountain. The Grand Tetons of Wyoming are fault-block mountains.

At the same time that forces are building up the crust, other forces are breaking down the crust. The two main forces breaking down the crust are weathering and erosion. The breaking down of rocks into smaller pieces by natural processes is called weathering. The picking up and removal of rock fragments and other particles is called erosion. These forces work as a kind of balancing act. As land is lifted up, it is being broken down. Land gets higher in elevation when it lifts up faster than it gets broken down.

The end result of weathering is soil. Soil is a mixture of weathered rock, decayed plant and animal matter, living things, air, and water. Soil can support the growth of rooted plants. Soil takes thousands of years to form. Plants and animals live in and on the soil. When they die, their remains enrich the soil. Rocks in the soil break down into smaller and smaller pieces. Rainfall seeps through the soil. Slowly the soil develops distinct layers. Any of the layers of soil from the surface to the bedrock below are called soil horizons.

Below the soil is weathered rock and rock layers. Soil and the layers below can act like a sponge when rain falls on the surface. Water that soaks into soil and rock by collecting in the spaces between the rock particles is called groundwater.

Soil can easily lose valuable minerals. Poor soil can be improved by adding fertilizers. It can also be improved by adding humus. Humus is material in soil produced by the breakdown of plant and animal material. Humus helps soil absorb and retain water.