Which soil horizon consists of weathered rock?

The C horizon, which is the correct answer, comprises weathered rock and partially disintegrated minerals. This layer is crucial as it serves as a transition between the unweathered bedrock (often referred to as the R horizon) beneath it and the upper soil layers. In essence, the C horizon is often characterized by its greater amounts of rock fragments and less biological activity compared to the more fertile A and O horizons, which are rich in organic material.

The A horizon, or topsoil, is where most biological activity occurs and is enriched with organic matter, making it vital for plant growth. The O horizon is primarily composed of organic matter, including decomposed leaves and other plant material, contributing to soil fertility. Meanwhile, the R horizon refers to solid bedrock, which lies below the C horizon and lacks the weathering characteristic that defines the C horizon itself. Thus, the presence of weathered rock in the C horizon highlights its unique role in soil formation and development.