Field Analysis:Why Texture

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Why record soil texture?
Soil texture is determined by the proportion of sand, silt and clay it contains. Texture can provide information about the nature of the parent material, and about the depositional environment.



Natural processes such as wind and water erosion sort sediments according to their grain size and the energy available to entrain and carry them. The greater the energy the larger the particle that can be carried, so a fast flowing river can transport larger particles than a slow flowing river. Likewise, the fast flowing centre of the river channel is characterised by coarse lag deposits, which may only be moved during the highest flows. Whilst, away from the channel the overbank deposits that form the floodplain usually consist of silt and clay.

Gravity driven processes, such as slope mass movements and rock falls, tend to result in poorly sorted deposits. Glacial deposits can contain a mixture of grain sizes from clay to boulders, although reworking by water can produce better sorted deposits of sand and gravel. Wind eroded deposits are generally very well sorted; loess usually consist of silts and very fine sands, whilst dune sands can be composed of larger grain sizes as saltation is the main method of transport.

Rounding
Abrasion during transport can result in the rounding and polishing of grains; both water and wind erosion can result in rounding. Hence the degree of rounding can give information about the mode of erosion and the distance it has been transported. Artefacts can also become rounded as a result of water and wind erosion.

All textural properties can be inherited from the geological parent materials, this can be used to help identify the source of parent materials for a particular deposit. However, it also means that texture alone is usually insufficient to interpret depositional environment and other evidence such as the presence of characteristic sedimentary structures should be sought.

Where the depositional environment is in question texture is a crucial property and should be recorded along with sorting, rounding of sand grains, and evidence of any fining sequences within the deposit, or in alluvial or wind-blown deposits through the profile as a whole. Texture can also provide information about the source of materials in a deposit. More information on recording texture can be found here.

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