The soil profile layers are called horizons. They are designated with letters, starting with O at the surface and going down:

* O horizon: The uppermost layer, made of organic matter like leaves, twigs, and decomposing plant material.

* A horizon: The topsoil, which is rich in organic matter and minerals.

* E horizon: A layer that has been leached of minerals and organic matter, leaving behind a lighter color. This is not always present.

* B horizon: The subsoil, which is rich in minerals and clay.

* C horizon: The parent material, from which the soil is formed.

* R horizon: Bedrock, the unweathered rock beneath the soil.