Parent material is the underlying geological material (generally

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...

or a superficial or drift deposit) in which

soil horizon A 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. ...

s form. Soils typically inherit a great deal of structure and

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

material that has undergone some degree of physical or chemical

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...

and the mode by which the materials were most recently transported.

Consolidated

Parent materials that are predominantly composed of consolidated rock are termed residual parent material. The consolidated rocks consist of igneous, sedimentary, and metamorphic rock, etc.

Residual

Soil developed in residual parent material is that which forms in consolidated geologic material.

Unconsolidated

Parent material is classified by its last means of transport. Material that was transported to a location by glacier, then deposited elsewhere by streams, is classified as stream-transported parent material, or glacial

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluv ...

parent material.

Ice transported

Glacial till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

is material dragged with a moving ice sheet. Because it is not transported with liquid water, the material is not sorted by size.

Water transported

Within water transported parent material there are several important types. Parent material transported by streams is called

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

of which there are three main types.

Floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

s are the parts of river valleys that are covered with water during floods. Due to their seasonal nature, floods create stratified layers in which larger particles tend to settle nearer the channel and smaller particles settle nearer the edges of the flooding area. Alluvial fans are sedimentary areas formed by narrow valley streams that suddenly drop to lowlands and widen dramatically. Sedimentary in these types of deposits tend to be larger closer to the uplands and finer near the edges of the fan. Delta deposits, the third of type of alluvium, are finer sediments that are discharged from streams into lakes and eventually settle near the mouth of the river. Lake deposited parent material is called

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

parent material. Beach ridges may be present where glacial lakes once washed up sand. Lacustrine material is well sorted and fine-textured, having finer

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel wh ...

s and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay part ...

s. Soils formed from lacustrine parent material have low permeability in part because of this high clay content. Ocean deposited parent materials, called marine sediments, are collections of material that have been carried by rivers and streams to the ocean and eventually sink to the bottom. Such materials can vary in texture.

Gravity transported

Collections of large rock fragments that have traveled downslope by gravity are called colluvial debris or colluvium.

Wind transported

Parent materials can also be transported by wind, which includes loess and wind-blown (aeolian) sand.

Climate and weathering

Climate is generally considered the most important factor influencing physical and chemical weathering processes. Physical weathering is especially important during the early stages of soil development. Rock can be disintegrated by changes in temperature which produces differential expansion and contraction. Changes in temperature can also cause water to freeze. The forces produced by water freezing can be as great as 2.1 × 10⁵ kPa, which can split rocks apart, wedge rocks upward in the soil, and heave and churn soil material. Chemical weathering: the principal agent is percolating rainwater charged with carbon dioxide from the atmosphere. Parent material becomes hydrolyzed by the acidic solution to produce minerals and to release cations.

References

{{Reflist Pedology