O horizon: An organic layer of fresh or decaying plant residue. A horizon: The mineral horizon at or near the surface in which an accumulation of humified organic matter is mixed with the mineral material. E horizon: The mineral horizon in which the main feature is the loss of silicate clay, iron, aluminum, or some combination of these. B horizon: The mineral horizon below an O, A, or E horizon and above the C horizon. Usually displaying soil structure and/or accumulatio of clay, sesquioxides, humus, or a combination of these. C horizon: The mineral horizon or layer excluding indurated bedrock, that has been little affected by soil-forming processes. R layer: Hard, consolidated bedrock beneath the soil.