as the "solum", the surface depth of the soil where biologically activity and climate effects drives pedogenesis. The layers below the solum have no collective name but are distinct in that they are noticeably less affected by surface soil-forming processes. C horizon The C horizon is below the solum horizons. This layer is little affected by pedogenesis. Clay illuviation, if present, is not significant. The absence of solum-type development (pedogenesis) is one of the defining attributes. The C horizon forms either in deposits (e.g., loess, flood deposits, landslides) or it formed from weathering of residual bedrock. The C horizon may be enriched with carbonates carried below the solum by leaching. If there is no lithologic discontinuity between the solum and the C horizon and no underlying bedrock present, the C horizon resembles the parent material of the solum. D horizon D horizons are not universally distinguished, but in the Australian system refer to "any soil material below the solum that is unlike the solum in its general character, is not C horizon, and cannot be given reliable horizon designation… [it] may be recognized by the contrast in pedologic organization between it and the overlying horizons" (National Committee on Soil and Terrain, 2009, p. 151). R horizon R horizons denote the layer of partially weathered or unweathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. If there is no lithologic discontinuity between the solum and the R horizon, the R horizon resembles the parent material of the solum. L horizon (not used in the Australian system) L (Limnic) horizons or layers indicate mineral or organic material that has been deposited in water by precipitation or through the actions of aquatic organisms. Included are coprogenous earth (sedimentary peat), diatomaceous earth, and marl; and is usually found as a remnant of past bodies of standing water. Transitional horizons A horizon that combines the characteristics of two horizons is indicated with both capital letters, the dominant one written first. Example: AB and BA. If distinct parts have properties of two kinds of horizons, the horizon symbols are combined using a slash (/). Example: A/B and B/A. Horizon suffixes In addition to the main descriptors above, several modifiers exist to add necessary detail to each horizon. Firstly, each major horizon may be