Field Analysis:Why Soil Features

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Why record soil features
Features such as crystals, concentrations, nodules, iron pans, coatings and infillings are formed by processes of soil formation and can provide information about environmental conditions and in some cases anthropogenic influence over soil processes. These features may relate to pre-occupation as well as occupation phases, they may be inherited from soil type parent materials or they may be formed by post-depositional or post-burial taphonomic processes.

A soil or deposit may contain multiple features from multiple phases of development superimposed over each other. The survival of features from earlier phases depends on the nature of the original feature and on subsequent soil conditions. Soil features from later phases of development can imprint over existing sedimentary boundaries complicating the stratigraphic record.

Where there is any suggestion of post-depositional changes to the stratigraphy record of the presence, frequency and distribution of soil features can be invaluable in untangling post-depositional overprinting from the primary sedimentary sequence. They are also important in the identification of buried soils and in the assessment of profile truncation.

More information on recording soil features can be found here.

Crystals
Crystals may develop as a result of soil formation in particular climates or of chemical transformations following burial. Hence, the type, size and distribution of crystals in a deposit can provide evidence of past climates or of the nature of the burial environment. Halite precipitates in the surface of saline soils in hot, arid climates. Gypsum and calcite may be deposited from base rich ground waters in calcic or gypsic horizons in soils in environments where evaporation exceeds precipitation. They may also be precipitated from groundwater in the burial environment or from percolating rainwater where overlying deposits are very calcareous. Vivianite and sulphides form in anoxic organic-rich burial environments.

More information about identifying crystals can be found here.

Concentrations and nodules
Concentrations and nodules can form in soils or buried deposits, and hence provide information about former soil development and about the nature of the burial environment. Gleying (temporary waterlogging) in soils and the burial environment is the commonest reason for the formation of iron concentrations and nodules and can lead to the development of mottled colour and the formation of iron nodules around voids, organic matter, and roots. These colour changes can be visually very dominant and overprint the depositional stratigraphy. Reduction (grey) colours may be reversed on exposure to air, but strongly impregnated iron nodules are very resistant and can be inherited.

Pans
Iron pans form in soils through the processes of podzolisation - the down profile movement of organic matter and iron and aluminium oxides. In buried soils, therefore, iron pans can be a good indication of former conditions of soil development as they are resistant to change. However, iron pans also frequently form in buried archaeological deposits. In buried soils, for example under burial mounds, iron pans frequently develop in the turf at the old ground surface. These pans may persist even after the mound has been ploughed out. Iron pans also often form along context boundaries where the physical and chemical contrast between two deposits causes iron in solution to be deposited. However, care should be taken as they are rarely faithful to the original boundary and some appear to migrate through the profile over time.

Coatings
Coatings may be formed of mineral particles (clay and silt or occasionally sand), amorphous organic matter, or mineral deposits such as iron and manganese oxides or calcium carbonate. Textural coatings of clay and silt form from the washing down of material from above. This translocated material is deposited as coatings on peds, pore walls, sand grains and capping stones. Clay coatings in the upper part of a buried soil are a good indicator of truncation as these features can only develop in the lower portion of the A horizon or more usually within the B horizon, they may also indicate post-burial alteration. Iron and manganese coatings on sand grains may be inherited from the parent material, for example, grains derived from Devonian Old Red Sandstone may be coated by pinkish hematite which formed in hot arid conditions, whilst ped coatings may be the result of waterlogging (gleying).

Infillings
Channel voids infilled by material from an adjoining context are a good indication of bioturbation and the mixing of deposits.

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