Which horizon in a soil profile consists of bedrock?

The story behind the answer

R is correct because the R horizon is the unweathered bedrock at the base of a soil profile—the solid rock from which soil may eventually form. It contrasts with higher horizons: O is the organic layer (leaf litter and humus), B is the subsoil where minerals accumulate, and D isn’t a standard horizon in most soil classifications.

Context: Soil forms as rock weathers and organic matter accumulates, creating distinct layers (horizons) that tell a story of climate, organisms, and time. Memory tip: R for “Rock” (bedRock), O for “Organic,” B for “Buildup” of minerals. The composition of the R horizon (the parent material) strongly influences soil pH, fertility, and the types of plants that can thrive above it.