Which horizon is not considered a regolith horizon?

• A. O horizon
• B. A horizon
• C. B horizon
• D. Bedrock

Answer: (D)

Regolith is the loose, weathered material that sits on top of unweathered bedrock. It includes the organic-rich surface layer (O horizon) and the mineral horizons formed by weathering and soil development (A and B horizons, and often C if it's weathered). Bedrock, by contrast, is solid rock that lies beneath the regolith and has not become loose, weathered material. So the horizon that is not regolith is the solid bedrock below the weathered zone.