The depth of the bedrock layer varies significantly throughout the Earth; in some areas, the bedrock layer is exposed, while in others it is hundreds of meters below the surface. Scientists believe that bedrock extends to the base of the planet's crust.
Generally, up to four layers exist above bedrock: weathered rock fragments, subsoil, topsoil and humus. However, not all locations on Earth feature four layers. The upper two layers, humus and topsoil, are typically rich in organic material but poor in mineral material. In contrast, the lower two layers, subsoil and weathered rock fragments, usually are rich in minerals but poor in organic material.