How were the ditches filled? Sedimentological and micromorphological classification of formation processes within graben-like archaeological objects

The ways of infilling archaeological objects are among the most common questions given to geoarchaeologists. Convenient subjects for the study of formation processes of archaeological terrain features (contexts) are V-shaped ditches. Their infilling is usually lithologicaly and texturally considerably variable with regular morphology and special archaeological context. The V-shaped ditches are known only from two chronologically, culturally distinctive periods. In first case there are ditches of “rondels” from the Late Neolithic Period, in the second case we are dealing with the fortification ditches of the Roman temporary camps.On the basis of sedimentological and consequently micromophological study, processes were differentiated in the formation of studied infillings. In Neolithic rondels, two parts of infillings were noted. The lower one typically has straight thin bedded layers, originating due to processes connected with vegetation ingrowth and erosion of the rampart. The upper part of the infilling is usually homogenous, and originated during the human caused grading of the surrounding area. During this phase, remains of rampart constructions were most probably destroyed. The basic type of deposition – especially visible in case of rondels – is lateral planar wash with phases of bioturbation, running pedogenesis on the edges of ditches, or stagnating water. The second main featuring process is mass movement slumping, particularly of upper faces of sloped sides. This process often happens naturally, mainly due to erosion, presence of water, and vegetation. The most distinctive postsedimentary processes determined within the rondel infilling were bioturbation, accumulation of carbonates and movement of clay minerals caused by soil leaching. In the V-shaped ditches of the Roman temporary camps, it was possible to microscopically trace similar records documenting coarse particle sedimentation at the base of ditches, although this layer is not continuous thought the whole width. It indicates the direction from which it was transported. They are remains of intentionally redeposited ramparts. The upper parts are marked by increasing humification and bioturbation as a result of ditch infilling emergence. However, it is possible to trace similar formation processes in both groups of studied V-shaped ditches and to define a basic classification. Prevailing textural and structural features are distinctively different between the groups, due to geological subsoil conditions, hydrological regime and depth of the ditches.