Which horizon represents unaltered bedrock in a typical soil profile?

• A. R horizon represents unaltered bedrock.
• B. O horizon represents surface litter.
• C. A horizon represents topsoil.
• D. C horizon represents partially weathered parent material.

Answer: (A)

In a soil profile, the layer that consists of unaltered bedrock is the one at the bottom of the mineral horizons. This horizon, often called the R horizon, is where rock remains in its original, largely unchanged form and has not been transformed into soil by weathering processes. Surrounding layers reflect progression from organic matter on the surface (O horizon) to the mineral topsoil (A horizon), and down through weathered and partially weathered material (C horizon) before you reach the bedrock. The O horizon is surface litter, the A horizon is topsoil, and the C horizon is partially weathered parent material. Therefore, unaltered bedrock is represented by the layer below these weathered zones—the R horizon.