A DIFFUSION MODEL OF THE DEGRADATION OF THE EARTHEN WALLS AT THE HOPEWELL SITE (33RO27), CHILLICOTHE, OHIO

Diffusion models have been used to describe the topographic degradation of landforms in a variety of geomorphic terrains. Although such models are extended to estimate the degradation of engineered landforms, the approach has not been applied to ancient earthworks in an archaeological context. Because soil creep and slopewash processes cause earthworks to widen and flatten over time, they provide an example of topographic smoothing that is amenable to diffusion modeling. Stratigraphic and topographic data from an earthen wall and ditch at the Hopewell site, constructed ca. 1800 years B.P., are used to apply a diffusion model in an archaeological context. The 1.5 m high and 8 m wide portion of the wall used for this study is constructed from the diamicton and surface soil excavated to create the adjacent ditch of similar dimensions. A comparison of the earthwork dimensions to historical records indicates that the study portion of the wall and ditch have remained largely undisturbed by human activities of the last few centuries. Therefore, the current topographic profile across the study area provides the final condition of the diffusion model, and the stratigraphy identified in three test pits at the wall crest, wall slope, and center of the ditch is used to constrain the amount of erosion and subsequent infilling since construction. As suggested by the diffusion model, slope degradation processes are manifested in increasing depths of colluvium from the slope to the base of the ditch. Evidence of the pre-construction soil 1.5 m under the wall crest, and the lack thereof on the mid slope, suggests that the original wall form was likely steeper than the smoothed convex-hull shape apparent today. Using a value for the topographic diffusivity of approximately 5 x 10 4 m2/y, the model predicts an initial condition of the earthworks necessary to create the current topography and stratigraphy. The results of this initial survey provide data that can be used to model topographic degradation in similar geological and archaeological contexts, and insight into a process that adds to the stratigraphic complexity apparent in many sites.