A mark, or set of marks, representing a particular textural feature or rock type. For example, conventionally a sandstone is represented by stipple, limestone by a ‘brick’ emblem. However, as there is not a complete range of generally accepted lithologic symbols, where they are used (e.g. in columnar sections) it is customary to give a key-panel explaining the meaning of the symbols. Symbols may be idiosyncratic to particular authors but they are often standardized, especially within large oil companies.