Tutorial:Ditch Fills

SASSA Home Page &#8658; Soils & Sediments Tutorial Home Page &#8658; Anthropogenic Soils, Sediments & Processes &#8658; Ditch & Other Fills

Ditch and Other Fills
Cut features such as ditches, moats, pits, post holes and graves, are often some of the most enduring anthropogenic features on archaeological sites. Even when organic artefacts have decayed and metallic objects have corroded away, the outline of the cut is preserved in the difference between the fill and the surrounding soil material. Bioturbation can blur the outline of the feature, but in some cases the difference in the soil type may still be evident even when the sharp outline is lost.

In most environments cuts into the landsurface such as pits, ditches or post holes if left unfilled, will naturally begin to silt up and fill back in. Anthropogenic deposits are also common, for example cess, and waste deposits, or stored materials such as grain. Often the fill of a primary cut feature is a complex sequence of anthropogenic deposits, the accumulation of natural silts, phases of stabilisation and soil formation, and recut episodes in which the feature is re-excavated. Hence, interpretation of these complex deposits can be difficult.

The identification, form, age, and anthropogenic infills of the features can provide information about land division and management, defense, and waste disposal. Identification of natural silting episodes, as opposed to deliberate anthropogenic fills, can provide information about the local environment (such fills can be valuable sources of palaeoenvironmental information,) and the history of abandonment and reuse of cut features.

Natural processes
Once cut ditches will start to silt up almost immediately as the sides of the ditch and spoil cast up on the surface erode and wash back down during the first rain forming the primary silt (Barker, 2001). Because this "primary silt" consists of essentially the same material as the surrounding soil it can be difficult to identify, but may be siltier and better sorted than the surrounding soil because of water-sorting. Elongated stones and srtefacts such as pottery or flint may tend to be aligned more or less parallel to the cut, and the process of erosion and deposition can destroy the structure of the parent soil producing a relatively dense, structureless deposit. There may be numerous phases of such infills and their shape relative to the cross-section of the cut can relate to the surrounding topography (silts accumulate quickest against the upslope bank, or the presence of a corresponding bank, as again the silts may accumulate most deeply along the side of the ditch with the bank (Barker, 2001). Wind blown-deposits may also form substantial fills in certain environments. These will tend to be  very well sorted, silts or fine sands and may continue beyond the cut feature as a blanket across the immediate landscape.

Anthropogenic processes
Anthropogenic deposits can include fills of anthropogenic materials such as cess or waste materials, or deposits of local soil materials used to deliberately back fill the feature. Identification of anthropogenic materials is usually straight forward because of the density of artefacts and differences in the nature of the soil. Organic rich deposits can also be the focus for earthworm activity resulting in the development of a loose, open crumb structure. Identification of redeposited "natural" soil materials, however, can be extremely difficult. Loss of structure and porosity, the disruption of natural soil horizons and the disruption or inversion of sedimentary features are amongst the indicators of redeposition.

Water-filled features
Deposits in water-filled features tend to be a mixture of washed or thrown in materials and material generated within the water column itself such as precipitates of chemical compounds and organic material. The size and sorting of material will depend on whether the water is flowing or stagnant. The wet anaerobic environment means that organic survival is often good, and deposits may be clearly stratified as they are protected from the effects of earthworms and bioturbation. In calcareous environments deposits of tufa may also be present.

Ditches and pits designed to hold water may also be lined with a layer of fine silt and clay to make the sides impermeable. The clay rich nature of this material, comparison with the local soil materials, the uniformity of depth over all sides of the feature and the strong parallel alignment of the deposit with the cut sides may help to differentiate between this type of deliberate sealing deposit and natural silting.

Stabilisation and soil formation
If there is a period of slow, or no, deposition (natural or anthropogenic) during which the feature is dry, soils may begin to develop. These periods of stabilisation are characterised by a darker mixed, organic/mineral horizon (topsoil) associated with evidence of increased rooting and sometimes the development of an open crumb structure. If the surface is stable for long ebough a sequence of soil horizons will develop in the fill deposits. This can provide information about the local environment and the length of the stable period.