Erosion can produce heterogeneous distributions of cultural materials of differing densities, weights, and shapes within an archaeological site. A large, provenienced surface collection from a preceramic site in highland Peru is examined for evidence of downslope movement as an explanation for observed heterogeneity in artifact distribution. Statistical methods of spatial analysis are used to define the extent of movements of surface materials in this site. For this particular area of Peru, critical angles for downslope movement will limit the site areas in which analysis of horizontal distributions can be expected to produce culturally significant information.