Distribution of artefacts in the Mertenhof sequence.

<p>Non-silcrete artefacts (filled grey circles) and silcrete artefacts (white circles) are shown from squares 4 and 6 only (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0131824#pone.0131824.s004" target="_blank">S2 Text</a> for square locations). Unifacial points (nested triangles), backed artefacts (black circles), bifacial points and thinning flakes (inverted black triangles) are shown from all excavated squares, including the deep sounding square 3. The single nested circle represents a core with Nubian characteristics. Frequency histograms for all artefacts (grey line and scale) and silcrete artefacts (black line and scale) are generated using arbitrary 20 mm horizontal bins.