OSL and micromorphological samples from Rose Cottage Cave

Micromorphological analysis in this first season of the Archer/Schmid excavation project were directed at investigating the formation of deposits associated with the MIS 5 MSA occupation, providing contextual information for their OSL dating by Chantal Tribolo (CNRS Bordeaux) and exploring the potential to build a comprehensive site formation model using archaeological soil micromorphology. Previous efforts to build a site formation model with micromorphological analysis provided mixed results. Lyn Wadley (Evolutionary Studies Institute, University of the Witwatersrand) excavated the site in the 90ies and after finishing removed 9 sediment peels from selected sections at Rose Cottage Cave, covering most of the sequence. Peter Kloos in his master thesis at the University of Tübingen then prepared 15 thin sections from the peels. The impregnation of the peels, however, was uneven and did not penetrate the sediment deeply, restraining and controlling the selection of thin section samples as well as leading to uneven representation of the deposits in the thin sections. On average, the thin section coverage was only about 50% of what was hoped for, allowing only vague interpretations of the site formation.
The aims of the current micromorphological investigation of Rose Cottage Cave is to collect block samples directly from the exposed profiles, to exam and improve on the impregnation process and to target depositional contacts to build a site formation model of the site. In this first season, we focused on the MIS 5 MSA deposits in the so-called Harper trench and respective contacts and took 4 samples, RCC 23 1 to 4. RCC 23 1 to 3 cover the same layers as the OSL samples: LEN, KUA and KUB. The sedimentary sequence shows a clear depositional change here from a dark grey brown colored and locally laminated post-Howiesons Poort (HP) unit and the lower pale brown sand of the MIS 5 MSA. The HP occupation deposit itself is divided between these two sedimentary units, the upper part is situated in the dark grey brown colored and locally laminated unit and lower in the pale brown sand. Analysis of the blocks will center on the question of the depositional change by anthropogenic or environmental processes during the HP. Block samples were removed by Mareike Stahlschmidt (MS) on November 15th, 2023.
After MS left the site, it was decided to collect further samples from potential hearth features – features 1 and 18 - exposed on the surface of the old Wadley excavations. These features consisted of very loose and unconsolidated sediments with dark spots scattered across them, potential micro to macro fragments of charcoal, giving the sediments within the features a dark greyish brown color. Feature 1 had rocks/slabs as its base, the dark greyish brown matrix was observable on top of, within the cracks between and for ~2 mm underneath the slabs/rocks. Feature 18 had no slabs/rocks associated with it. Thomas Beard collected three block samples from two of these features, two from Feature 18 and one from Feature 1 (table 1). The features were revealed to be very thin and ephemeral after excavation, around 1,5 to 2 cm at their thickest, which was closer to the center of the features, and thinner around the edges, around 3 mm. As such, around 3 to 4 cm below the targeted features was included in the samples along with the ~3 to ~7 mm of the feature at the top of the samples (as the samples were taken near the edge of the features to sample the contact between the feature and the underlying layer. Two samples were taken from Feature 18 as the sediments were so unconsolidated that there was a concern that the sample would fall out of the gypsum mold when lifted out. The sample from Feature 1 was taken abutting the rocks/slabs, which are what initially drew attention to this feature. All three samples were from the final MSA of the Wadley excavations.
From the blocks, 1-2 thin sections will be produced by a designated thin section laboratory, Geopraep in Eggenburg, Austria or Mkfactory in Stahnsdorf, Germany. Thin section analysis will be conducted in the Microarchaeology Laboratory of the University of Vienna, using an Evident BX53 light microscope with oblique incident light (OIL), plane- (PPL) and cross-polarised light (XPL).