| Where weathering products gradually form or accumulate at the surface of the earth, they form a fine-grained substance that may support plant-life. This 'soil' commonly displays horizontal zones called 'soil horizons' which, together, constitute a 'soil profile.' | Some soil horizons are rich in organic debris. Others are depleted or enriched in certain chemical elements: rain percolating downward through the soil selectively dissolves elements from some layers and precipitates them in other layers. The soil may grade downward into the unweathered rock from which it was derived. |