RECONSTRUCTING GEOMORPHOLOGY AT A STONE AGE ARCHAEOLOGICAL SITE

The Bestwood Farm in Kathu, South Africa, consists of a sand-filled valley bound by two ironstone slopes littered with stone-age artifacts. The sand, blown in from the Kalahari desert, is believed to cover the topography present during stone-age human habitation. Imaging the paleogeography is crucial for an understanding of how early humans interacted with their environment in this area. We applied various geophysical methods to image the sand-ironstone boundary. GPR radargrams show a clear boundary which can be traced to the outcrop of the ironstone on the slopes. The thickness of sand can be mapped by defining this boundary at depth from multiple radargrams crossing the valley. However, in areas where sand thickness exceeds 11m, signal attenuation prohibits further mapping using GPR. Magnetics data shows wide variations in total field strength and rapid variance even within short distances (1000’s of nT within 10’s of meters). Localized mean magnetic gradients across the study site (based on numerous walking surveys) vary with the thickness of the sand determined from radargrams. We therefore are able to extrapolate the map of sand thickness to sectors where GPR did not provide useful results and thus cover a 600x1000 m large area. Seismic P-wave refraction tomography clearly defines two layers marked by a transition between them. This transition may be attributed to the fact that ironstone cobbles derived from the fractured bedrock are lodged in sand above the solid underlying ironstone. Our maps show a sinuous depression running north-south through the valley flanked by plateau-like features of lesser sand thickness before grading into ironstone slopes. Based on evidence from the maps, radargram imagery, and exposed sediments uncovered through digging of test pits this feature has been interpreted to be a meandering river contained within the paleovalley system.