age). However, paleosols are difficult to recognize in the geologic record. Indications that a sedimentary bed is a paleosol include a gradational lower boundary and sharp upper boundary, the presence of much clay, poor sorting with few sedimentary structures, rip-up clasts in overlying beds, and desiccation cracks containing material from higher beds. The degree of weathering of a soil can be expressed as the chemical index of alteration, defined as . This varies from 47 for unweathered upper crust rock to 100 for fully weathered material. Weathering of non-geological materials Wood can be physically and chemically weathered by hydrolysis and other processes relevant to minerals, but in addition, wood is highly susceptible to weathering induced by ultraviolet radiation from sunlight. This induces photochemical reactions that degrade the wood surface. Photochemical reactions are also significant in the weathering of paint and plastics. Gallery See also References Other links Geological processes Soil Climate forcing Climatology Pedology Geomorphology Earth sciences