Field Analysis:Is this a colluvial deposit


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[edit] Is this a colluvial deposit?

Colluvial deposits are the loose mixed deposits that are formed by a range of processes including sheet wash, tillage and soil creep at the base of slopes. Some specialists dislike the term colluvium becuase of the range of different slope processes that can be involved, however it is widely used in archaeology and does comprehensively cover an important functional group of deposits. Colluvial deposits are often found burying and interleaved with arhaeological and other sedimentary deposits. The formation of colluvium may also be accelerated by human activities such as agriculture. The ability to identifying colluvial deposits is, therefore, important to interpreting site formation processes. It should be noted, however, that colluvial deposits may also have formed by entirely natural processes. Dating colluvium and assessing its archaeological significance can also be important.

[edit] Indicators of colluvial deposits?

Figure 1: colluvium overlying degraded chalk, Norfolk, UK
Figure 1: colluvium overlying degraded chalk, Norfolk, UK
Figure 2: brown silt loam colluvium with stone lines towards base and a sharp lower boundary
Figure 2: brown silt loam colluvium with stone lines towards base and a sharp lower boundary

When attempting to identify a colluvial deposit in the field a checklist of features that can be looked for/considered are:

  • A location in the landscape that reflects colluviums origin as a slope deposit; i.e. on toe slopes, terraces and benches, floodplain and valley floor margins - anywhere where there is a break of slope.
  • Depending on the process by which the colluvial deposit has formed, the deposit matrix may be slightly siltier and better sorted than the deposits upslope.
  • Any stones or charcoal may have a weak tendency to align in a downslope direction.
  • Any softer inclusions such as pottery and charcoal may tend to be worn and abraded.
  • As colluvial deposits are typically derived from topsoils they will tend to be organo-mineral and have a brownish colour.
  • They should have a similar composition and stone lithology to the soils further upslope.
  • There should be a deepening of the deposit towards the base of the slope, with thinning upslope and away from the break of slope. See Figure 1.
  • A colluvial deposit may have a less well developed granular soil structure than non-disturbed topsoil materials. However, if the rate of accumulation is slow or if there has been a period of stability where there has been no futrther deposition the granular structure may reform.
  • There may be evidence of some internal stratification, particularly stone lines. See Figure 2.
  • The lower boundary may be a sharp or abrupt erosional contact, although again bioturbation and rooting may destroy this evidence depending on the rate and depth of accumulation. See Figure 2.
  • The base of the deposit and underlying materials may contain clay coatings (visible as a slight metallic sheen on void and aggregate walls. These develop because poorly structured soil materials are vulnerable to being washed down profile following rain.

The archaeological significance of colluvium may not always be obvious. The presence of artefacts such as charcoal, pottery, and flint can indicate anthropogenic factors involved in colluvium formation, but there is always the possibility of reworking of older artefacts from upslope. Understanding the relative date of colluvial deposits can be important in assessing their archaeological significance. Where deposits are interleaved between archaeological materials, or have accumulated against field walls, banks etc. this is relatively straightforward.

[edit] Uncertainties in identifying colluvial deposits

Uncertainties in identifying colluvial often arise from:

  • The origin of colluvial deposits as topsoil materials can make it very difficult to distinguish between in-situ and redeposited topsoil materials.
  • Where accumulation rates are slow, processes of soil development can act on the colluval deposits eventually leading to the redevelopment of soil properties, such as structure.
  • Slow accumulation rates and periods of non-deposiution can also lead to the destruction of any stratification or orientation of stones by roots, earthworms and other soil animals.
  • At floodplain margins colluvial deposits can interdigitate with water-lain flood deposits.

In areas where there has been a loss of sedimentary properties the only indication that a deposit may be colluvial in origin may the deepening of deposits towards the base of the slope, and upslope of barriers such as walls or buildings.

[edit] How does the SASSA Field Tool make this interpretation?

A printable recording sheet for SASSA's 'Is this a colluvial deposit?' interpretation tool, can be found here

A high score tends to confirm that this context / deposit is a colluvial deposit. To score highly you need:

  • A suitable landscape position (This question accounts for 20% of the maximum total score.
    • a toeslope, bench or terrace with a flat or hollowed microtopography, or
    • a floodplain a valley floor margin, close to the break of slope.
  • A similar suite of minerals and rock types to the soils upslope. this question accounts for 15% of the maximum total score.
  • An organo-mineral or mineral deposit with the following characteristics:
    • Slightly or moderately stony, with stones showing a tendency to align downslope. This question accounts for 13% of the maximum total score.
    • Evidence of internal sedimentary bedding. This question accounts for 8% of the maximum total score.
    • Evidence of clay coatings in underlying deposits. This question accounts for 5% of the maximum total score.
    • A moderately or well sorted loam or silt loam soil texture. This question accounts for 13% of the maximum total score.
    • A moderately well or well sorted deposit. this question accounts for 26% of the maximum total score.

[edit] Follow-on analysis

Post-excavation laboratory analysis may help to confirm whether this is a colluvial deposit and add to the archaeological and palaeoenvironmental interpretation.

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