Residuum (geology)

Residuum is commonly used to refer to the soil and subsoil that forms as the result of long weathering over carbonate rocks (limestone and dolomite) bedrock. The term is more specific than regolith, which refers to all unconsolidated material, including transported sediments such as sand or alluvium, but less specific than saprolite which commonly refers to weathered material retaining details or structural features of the bedrock. In limestone terrains the boundary between bedrock and residuum is commonly very sharp, but may be highly irregular, defining pinnacles and even isolated blocks of fresh bedrock surrounded by reddish residual silts and clays derived from its decomposition. Resistant materials such as chert, silicified fossils, or beds of sandstone remain and may concentrate on the surface as float.