Field Analysis:Is this water deposited?
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[edit] Is this a water-lain deposit?
Water-lain deposits can be incredibly diverse in their exact mode of deposition, hence broad characteristic properties can be difficult to identify. Material can be deposited in moving or stagnant water and this affects the sorting and rounding of deposits. Water-lain deposits may build up vertically in more or less parallel horizontal units as on a flood plain, or they may accumulate laterally in a series of overlapping inclinded deposits (cross bedding) typical of fans and migrating channel deposits.
The archaeological importance of water-lain deposits is for the identification of site formation processes. For example, is this an anthropogenic deposit or a natural deposit. Water-lain deposit, also often remain waterlogged resulting in good preservation conditions for organic materials and palaeoenevironmental evidence. The identification of whether it was lain in moving or still water can also be important, when considering whetehr material has been transported onto site.
[edit] Indicators of water-lain deposits?
When attempting to identify a water-lain deposit in the field a checklist of features that can be looked for/considered are:
- Location in the landscape; i.e. on floodplains, depressions, channels, gully and rill depressions.
- Water-lain deposits usually exhibit some degree of size sorting, whether this is expressed as lateral or vertical fining or as single units consisting of grains all of a very similar size.
- Rounded stones - Stones and other materials transported in water are subjected to physical abrasion resulting in rounding. The degree of rounding depends on the hardness of the stone and the distance it has been transported. Hence, stones deposited in stagnant water will not show rounding nor will particles transported only short distances.
- Bedding - Water-lain deposits ususally exhibit some degree of bedding. Overbank floodplain deposits tend to contain very fine internal laminae or 'flood couplets' consisting of thin (few mm's) alternating layers of fine sand or silt, and silty clay. Fans and channel deposits often show evidence of cross bedding, and even in stagnant water upwards fining and seasonality can lead to the development of clear bedding.
[edit] Deposition in moving or stagnant water
Distinguishing between particles deposited in moving water and those in stagnant water relies on:
- Evidence of size sorting - In moving water particles below a certain size can be kept in suspension (depending on the speed of the water). this means that the particles that drop out of suspension and are deposited also tend to be of a characteristic size. In stagnant water there isn't the same degree of size sorting. Particles falling into stagnant water will settle at different rates depending on their size so their may be upwards fining but all particle sizes can be expected to be represented at any one location.
- Scour and flute marks can form in river beds where water currents or objects carried in suspension groove, mark or scour the surface of the river bed.
- Palaeoenvironmental study of diatoms can provide valuable information on water conditions.
[edit] Uncertainties in identifying water-lain deposits
Uncertainties in identifying water-lain deposits often arise from:
- One of the biggest uncertainties in identifying water-lain deposits is the range of possible processes this encompasses including floodplain, channel, pond, moat, overland flow or beach. This means that water-lain deposits can vary in size from clay particlaes to boulders, and extend over a few centimetres or many miles.
- Reworking and redeposition of water-lain material can be difficult to identify as the reworked material still retains many of the properties of the water-lain material. The coherence of any internal bedding and fining sequences may be the only clues. If material of different origin has become mixed sorting and the development of a bimodal particle size distribution can also provide evidence of reworking. In some cases way up criteria such a flute and scour marks which are negative depressions in the surface of deposits can also provide evidence of reworking though these are quite rare to find.
- Stabilisation and soil formation in water-lain deposits involves bioturbation and rooting, which can disturb internal bedding. Looking for evidence of bedding deeper within the section where there is less evidence of root traces can help.
- Gleyic overprinting can be common in water-lain deposits as they frequently remain waterlogged. Waterlogging causes iron in the sediment to be reduced to its ferrous (Fe2+)more mobile form. The loss of iron from the sediment leads to a grey colour with reddish mottling around roots, and channel voids where oxygen penetrates and iron is oxidised. These characteristic 'redoximorphic colour patterns' can hide depositional boundaries and superimpose a pseudostratigraphy of their own.
More information on redoximorphic colour patterns can be found here
[edit] How does the SASSA Field Tool make this interpretation?
A printable recording sheet for SASSA's 'Is this a water-lain deposit?' interpretation tool, can be found here
A high score tends to confirm that this context / deposit is a water-lain deposit. To score highly you need:
- A suitable landscape position (This question accounts for 20% of the maximum total score.
- a floodplain, or
- a basin, hollow or channel.
- A well or very well sorted deposit. This accounts for a maximum of 20% of the total score.
- Rounded or sub-sounded stones. This accounts for a maximum of 20% of the total score.
- A deposit showing evidence of internal bedding and fining. This accounts for a maximum of 20% of the total score.
[edit] Follow-on analysis
Post-excavation laboratory analysis may help to confirm whether this is a water-lain deposit and add to the archaeological and palaeoenvironmental interpretation.
