horizon

A distinct layer within a soil profile, visible when a vertical section is cut or exposed. The major horizons are designated by letter: O (organic material on the surface), A (topsoil — dark, rich, where most biological activity occurs), B (subsoil — where minerals leached from above accumulate), C (weathered parent material), and R (bedrock). Each horizon tells a chapter of the soil's history — what fell on it, what grew in it, what washed through it, what it was made from. A road cut or river bank that exposes a soil profile is a cross-section of time.