Coastal cave settings

A number of caves and rockshelters are located in close proximity to the modern coastline. These include Elands Bay Cave, Diepkloof Rockshelter, the Klipdrift site complex, Blombos Cave, and the site complexes of Klasies River. Our work at these sites includes observations that can be related to sea level changes in the past, including sediment input from local beaches and dunes, salt-induced weathering and contribution of sea spray to secondary mineral formations, occupation of the sites by coastal birds, and human exploitation of coastal resources.

Anthropogenic deposits

We use archaeological micromorphology to document the traces of human activities, which can range from mm-thick surfaces prepared from plant material to middens that are more than one meter thick and many meters in diameter. Our work on pyroarchaeology has applications in all South African sites. We utilize observations of different components in stratigraphic sequence supplemented with molecular analyses in order to determine whether combustion residues are in primary or secondary position. For example, we look for an association of a heated substrate overlain by burned materials such as ash, charcoal and heated bones and shell to indicate the presence of a hearth. In contrast, a mix of heated materials with other not heated components might indicate a secondary context such as an ash dump or midden. Experimental work aids in these interpretations.

Diagenesis

Mild chemical diagenesis of archaeological materials occurs in all caves and rockshelters to some extent, typically expressed in the decomposition of organic components. In limestone caves and rockshelters, archaeological materials such as bones, marine shells, and wood ashes are often well-preserved due to the relatively high pH of the water that enters the site and collects on the cave floor. In caves formed in other types of rock, such as sandstone and quartzite, water pH levels are closer to neutral, and dissolution of ashes and shell is a common process. When caves of any type are also occupied by animals such as bats and birds, fecal materials can accumulate and their breakdown can induce dissolution of archaeological materials and formation of secondary sulfate and phosphate minerals.

We utilize a number of different strategies for documenting the formation of secondary sulfate and phosphate minerals in South African archaeological deposits. In the field, we conduct on-site Fourier transform infrared spectroscopic analyses of loose sediment, bones and mineral nodules that have formed within the site in order to match spectra to our reference libraries of common archaeological minerals. Additional mineral identifications can also be accomplished in the laboratory using x-ray diffraction. Analyses using micro-x-ray fluorescence or scanning electron microscopy aid in understanding the elemental composition and crystal habit of secondary minerals. Finally, archaeological micromorphology contributes observations of the relationship between the primary sediment and archaeological features and the secondary minerals that have formed in place. Combining all of these observations allows us to reconstruct the past micro-environments within the caves and rockshelters.