Grinding grounds: function and distribution of grinding stones from an open site in the Pilbara, Western Australia

Grinding stones are sometimes found as isolated artefacts but more commonly as minor components of archaeological sites, defined in site registers as lithic scatters or lithic concentrations that are dominated by flakes and cores. Here we describe a case study where lithic technology on an open site is almost exclusively dominated by grinding and pounding. In a functional study of 48 stones with macroscopic traces of surface smoothing, we identify 46 grinding stones, including eight near-complete implements and 38 fragments, found on flood-prone land (about 2.8 km2) in the Pilbara, northwestern Australia. A variety of flat, concave and convex grinding surfaces were recorded on ironstone (n = 44) and sandstone (n = 2) artefacts. Analysis shows that the dominant wear is typical of grinding seeds, from locally available food sources. However, microscopic residues from five grinding stones indicate both animal (collagen and bone) and plant (cellulose, starch, phytoliths) processing. Specific plant taxa indicated by the residue study are: Chloridoid and Panicoid grasses (including Triodia spinifex), Acacia, Marsilea nardoo and tubers (most likely Ipomoea costata, native bush potato, or possibly Vigna lanceolata pencil yam). We discuss site function; other evidence for a proposed new open site type-grinding grounds; possible rates of visitation; and associated tasks. We conclude that grinding grounds, either in the form of scattered grinding stones and fragments or bedrock grinding patches, provide a significant source of information about past settlement, subsistence and resource-use