Soil horizonA soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. These may be described both in absolute terms (particle size distribution for texture, for instance) and in terms relative to the surrounding material, i.e. 'coarser' or 'sandier' than the horizons above and below. The identified horizons are indicated with symbols, which are mostly used in a hierarchical way.
Negative selection (natural selection)In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random mutations. Purging of deleterious alleles can be achieved on the population genetics level, with as little as a single point mutation being the unit of selection. In such a case, carriers of the harmful point mutation have fewer offspring each generation, reducing the frequency of the mutation in the gene pool.
Earthworks (engineering)Earthworks are engineering works created through the processing of parts of the earth's surface involving quantities of soil or unformed rock. An incomplete list of possible temporary or permanent geotechnical shoring structures that may be designed and utilised as part of earthworks: Mechanically stabilized earth Earth anchor Cliff stabilization Grout curtain Retaining wall Slurry wall Soil nailing Tieback (geotechnical) Trench shoring Caisson Dam Gabion Ground freezing File:Mechanically stabilized earth diagram.
DiggingDigging, also referred to as excavation, is the process of using some implement such as claws, hands, manual tools or heavy equipment, to remove material from a solid surface, usually soil, sand or rock on the surface of Earth. Digging is actually the combination of two processes, the first being the breaking or cutting of the surface, and the second being the removal and relocation of the material found there.
