Soils are comprised generally of three horizons, the A, the B and the C. They are labeled as they are encountered when you dig a pit.The C Horizon is most like the parent material that the soil developed on. It is regolith. Eventually mosses, lichens, and other non-vascular plants colonize and prepare the substrate for the establishment of vascular plants by improving soil, organic matter and freeing up some nutrients somewhat. This creates the A horizon eventually, as vascular plants colonize, creating a thicker A Horizon. Over time, the B horizon forms from the bottom of the A, and though any leaching or deposition from the A.

While it is the second horizon encountered, it is the last formed!