Field Analysis:Is this the natural

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Is this the ‘natural’?
The ‘natural’ essentially refers to the pre-occupation deposits. It is important to identify this point as it usually marks the end of useful archaeological excavation; environmental reconstruction, however, may demand further exploration. Pre-occupation soils are sometimes viewed as ‘natural’, but in the interpretation tool we exclude soils as although they may be pre-occupation this doesn’t mean that they haven’t been affected by human activity such as cultivation or clearance.

'''It is important to note that this tool will only help you identify whether a deposit matches what you expect from the local natural, archaeological and archaeologically valuable deposits can be buried by or interleaved with wholly natural deposits such as alluvium or wind blown deposits. '''

Indicators for the 'natural'
Identification generally relies on comparison of the deposit with the expected geological or soil derived parent material.

A checklist of features that would be looked for are:
 * An absence of anthropogenic artefacts – artefacts may be embedded in the upper few centimetres of the parent geology by the action of soil fauna, roots and freeze-thaw cracking.
 * Mineral composition – though in areas of colluvial deposition it may be organo-mineral.
 * Sedimentary features and soil features such as iron pans, should generally not be strongly disturbed as this indicates redistribution. However, certain natural redistribution processes such as slumping can have similar effects.
 * Should contain few fine roots as these are generally confined to topsoils.
 * Should be structureless, or have a blocky, prismatic or columnar structure. Granular structures are rare.
 * Colour, texture, mineralogy and lithology should match the expected parent material. As well as the solid geology consideration should be given to whether this might also include natural sedimentary deposits such as till, alluvium, colluvium and solifluction deposits etc.
 * To ascertain the archaeological potential of these deposits requires local knowledge and advice should be sought in cases of uncertainty.

Uncertainties in identifying the 'natural'
Uncertainties in identifying pre-occupation deposits often arise from:
 * Anthropogenic redistribution of geological materials which can be used for example in the reclamation of land at river edges and for site levelling (made ground).
 * Redistribution of archaeological artefacts by natural processes, for example the occurrence of Palaeolithic stone tools within fluvio-glacial sands and gravels.
 * Interleaving of anthropogenic and natural deposits in depositional environments, for example on alluvial floodplains, or within colluvial deposits.
 * Similarities in characteristics between certain anthropogenic and natural deposits, for example, primary fills and water-lain deposits in the base of ditches and pits.
 * Patterning that can develop in natural deposits, for example free-thaw liquefaction features in glacial and periglacial deposits.

How SASSA makes this interpretation
A printable recording sheet for SASSA's 'Is this the natural?' interpretation tool can be found [[media:Is this the natural.pdf|here]]

A high score tends to confirm that this context / deposit is the "natural". To score highly you need:
 * A mineral deposit. This question accounts for 20% of the maximum total score
 * The material should be structureless, or possibly columnar or prismatic. This question accounts for 20% of the maximum total score
 * Few or no roots. This question accounts for 20% of the maximum total score
 * No anthropogenic artefacts, or if there are a few they should be confined to the uppermost few centimetres. If the deposit contains no artefacts, but artefacts are also uncommon throughout the overlying sequence of deposits the weighting for this particular question will be inconclusive. This question accounts for 20% of the maximum total score

The final 20% of the score comes from the comparison of this deposit with what is expected to be the local "natural". In situations where you are very confident of what the local natural is likely to be you may wish to give this set of answers a higher weighting.
 * If you are expecting the natural to be water-lain you should have:
 * Evidence of intact sedimentary bedding. This question accounts for 6.5% of the maximum total score.
 * Well sorted grain sizes. This question accounts for 7% of the maximum total score.
 * Rounded sand grains and stones. This question accounts for 6.5% of the maximum total score.
 * If you are expecting the natural to be wind-lain you should have:
 * Evidence of intact sedimentary bedding. This question accounts for 10% of the maximum total score.
 * Very well sorted grain sizes (This question accounts for 10% of the maximum total score), with:
 * A silt or sand dominated texture. This question accounts for 5% of the maximum total score.
 * A stone free or slightly stony deposit. 5% This question accounts for 5% of the maximum total score.
 * If you are expecting the natural to be a slope deposit you should have:
 * A toe-slope, terrace or bench landscape position with a predominant downslope alignment to any stones it contains. These questions account for 20% of the maximum total score.
 * If you are expecting the natural to be a glacial deposit you should have:
 * Angular or sub-angular stones that tend to align in one direction. These questions account for 10% of the total maximum score.
 * A poorly sorted deposit. This question accounts for 10% of the total maximum score.

You may also wish to try the deposit-type decision tree most appropriate to the expected "natural" for more detailed confirmation.

Follow-on analyses
Identification of the natural can be made more reliable using post-excavation analyses. However, the need to define 'the natural' is usually to inform excavation, hence time-intensive laboratory analysis may be inappropriate. Field analysis and quick labroatory techniques that can be used include:
 * Coring
 * Magnetic susceptibility
 * Particle size analysis

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