Abstract

Excavations at Blombos Cave have produced early evidence for advanced cognitive behaviour associated with the Middle Stone Age (MSA) in South Africa. An important question posed at this site was whether these significant artefacts may perhaps represent intrusions from the younger Later Stone Age (LSA) levels. Here it is demonstrated how optically stimulated luminescence (OSL) measurements of individual quartz grains can provide experimental data from which mixing between adjacent sedimentary layers can be assessed. By association, the likelihood of movement of artefacts through these sedimentary layers can be inferred. The stratigraphic sequence at Blombos Cave is ideal for such a study, since a thick archaeologically sterile aeolian dune layer separates the c. 2 Ka old LSA from the c. 70 Ka old MSA occupation layers. Based on the known age estimates from the LSA and MSA, an artificial sediment mixture was produced and measured in the laboratory to simulate what measured De values for grains from the dune layer would look like if mixing of LSA and MSA sediments had occurred. This mixture is then used as a guideline against which the measured distribution of De values from the natural sediment sample obtained from the dune layer itself can be compared. OSL measurements on a large number of quartz grains from the dune layer found no evidence for mixing and confirmed the stratigraphic integrity of the MSA artefacts.

Résumé

La fouille de la grotte de Blombos, en Afrique du Sud, a permis la découverte, dans des couches du Middle Stone Age, de preuves particulièrement anciennes de comportements cognitifs modernes. Une question cruciale concernant ce site est de savoir si les objets permettant de proposer l'hypothèse d'une origine précoce des comportements modernes pourraient en fait représenter des intrusions des niveaux plus récents, datés du Later Stone Age (LSA). Nous montrons dans cette contribution de quelle façon les mesures faites par luminescence stimulée optiquement (OSL) sur des grains de quartz singuliers peuvent fournir des données expérimentales permettant d’évaluer le degré de mélange entre couches sédimentaires adjacentes. Par analogie, nous pouvons déduire la probabilité que des objets se soient déplacés à travers les couches archéologiques.