Soil the mineral components of soil; it consists of rock fragments, primarily quartz particles, ranging in size from in diameter. Silt ranges in size from . Clay cannot be resolved by optical microscopes as its particles are or less in diameter and a thickness of only 10 angstroms (10 m). In medium-textured soils, clay is often washed downward through the soil profile (a process called eluviation) and accumulates in the subsoil (a process called illuviation). There is no clear relationship between the size of soil mineral components and their mineralogical nature: sand and silt particles can be calcareous as well as
Parent rock Parent rock Parent rock, also referred to as substratum, refers to the original rock from which something else was formed. It is mainly used in the context of soil formation where the parent rock (or parent material) normally has a large influence on the nature of the resulting soil. For example,clay soil is derived from shale while sandy soil comes from the weathering of sandstones. The term is also used in the context of metamorphic rocks where again the parent rock or protolith refers to the original rock before metamorphism takes place. Parent rocks can be sedimentary, igneous or metamorphic.