Which horizon is an accumulation of fine material that has moved downward, forming a dense layer?

1. The soil profile has four distinct layers, the O, A, B and C horizons. 2. The O horizon carries freshly decomposing organic matter at its surface and decomposed vegetation at its base. 3. The A horizon mixes organic material with inorganic products of weathering and is the beginning of true mineral soil. 4. The B horizon, or subsoil, is an accumulation of mostly fine material that has moved downward, giving a dense layer. 5. That downward movement is fed by rainwater percolating through the A horizon and carrying material from the surface. 6. The C horizon, or soil base, holds the parent material, and bedrock lies beneath it. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 31 "Soil and Plant Nutrition", section 31.2 The Soil_