Soil wetting and drying processes influence stone artefact distribution in clay‐rich soils: A case study from Middle Gidley Island in Murujuga, northwest Western Australia

Soils that contain swelling clay minerals (e.g., montmorillonite) expand and contract during wetting and drying, causing movement within the soil profile. This process, known as argilliturbation, can alter artefact distributions, destroy stratigraphy and complicate the interpretation of archaeological deposits. Our study investigates the influence of argilliturbation on stone artefact distribution in a clay‐rich sediment basin on Middle Gidley Island in the Murujuga rock art province, northwest Australia. We report on an experiment that quantified the movement of artefacts following wetting and drying treatments using the local soil and other contrasting soil textures. Six weekly wetting and drying cycles were conducted on 27 soil experiment containers, which contained one of three soil types and embedded (non‐cultural) stone artefacts. The soils used were the Middle Gidley clay‐rich soil (MG‐Soil), a pure quartz sand (S‐Soil), and a mixed soil (X‐Soil; 25 wt% MG‐Soil and 75 wt% S‐Soil). The containers were imaged using computed tomography scanning before and after the experiment, and the movement of artefacts was calculated using 3D image correlation. The results demonstrated that wetting and drying processes can cause movement of artefacts and disrupt stratigraphy in a variety of soils, most notably in the vertical direction. Argilliturbation caused upward movement of all artefacts within the MG‐Soil (average = 4.3 mm). Artefacts also moved upwards in the X‐Soil (average = 1.2 mm) but was less pronounced. In contrast, all but one artefact within the S‐Soil moved downwards (average = 2.0 mm), indicating slumping or sinking within the sand matrix. The archaeological assemblage at Middle Gidley Island was interpreted considering the experimental results, which indicated that artefacts found on the surface of the sediment basin likely reflect artefacts deposited at different times, which had been pushed upwards and concentrated at the surface by argilliturbation. We discuss the wider implications of these post‐depositional processes for archaeological sites.