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2017-09-01 05:23:18   •   ID: 1657

Silkrete MSA Artifact from Tankwa Karoo (South Africa)

Figure 1
This is a MSA truncated blade with lateral retouches 4 cm long and made of heated Silcrete, an isolated surface finding, found at Tankwa Karoo (South Africa).  It has the characteristic lustrous red patina, which can only produced by heat treatment-a innovative transformative technique which was invented in South Africa during the Post-MIS5 MSA.

Figure 2
Similar pieces are known from the the Howiesons Poort material of Klipdrift complex (Rock Shelter and cave)  in the De Hoop Nature Reserve, southern Cape, South Africa.

Today, Tankwa Karoo National Park is a Scientific National Park in South Africa. The park lies about 70 km due west of Sutherland near the border of the Northern Cape and Western Cape, in one of the most arid regions of South Africa, with areas receiving less than 100 mm of average annual precipitation and is classified as semi-arid desert. 

Figure 3
Moisture-bearing clouds from the Atlantic Ocean are largely stopped by the Cederberg. It encompasses the Succulent Karoo Biome – an internationally recognized hotspot and the world’s only arid hotspot – which stretches 116 000 km2 from the southwestern Cape into southern Namibia. 

Tankwa’s landscape offers vivid seasonal contrasts of colored wild owers and stark desert, set against the backdrop of the Roggeveld Escarpment to the east, Klein Roggeveled to the south and the Cederberg to the west. Consequently, the landscape is sparsely vegetated, featuring succulents endemic to the Karoo biome.

Figure 4
The most prominent MSA open air site  in Tankwa Karoo, Tweefontein contained a very large Middle Stone Age unifacial point Ensemble.

The specific preferential Levallois strategy used for point production, together with the unusually high use of silcrete, marks this as a site of importance for our understanding of Middle Stone Age adaptations to an arid, marginal environment. The ensemble could be post-Howiesons Poort (HP).

Silcrete is a pedogenic silica rock available along South Afri- ca’s west and south coasts. In its raw form never has the lustrous red and gray coloring seen after intentionally heat-treating. When Silcrete is heated, it undergoes several physical and chemical transformations.

From 250°C upward, chemically bound ‘water’ (SiOH) is lost from the structure, allowing for the formation of new Si-O-Si bonds that transform the mechanical properties of the rocks. These modifications of the material properties produce a tool-stone that can be worked more easily, as a result of various altered fracture properties including decreased fracture toughness.

After heat treatment, the fracture behaviour of silcrete becomes closer to the one of finer grained silica rocks like chert and flint. Untreated Silcrete is coarse-grained rock is great for making large flakes, but it is difficult to shape into small, refined tools, such as the typical backed Lunates during the HP-phase. Such artifacts can only be produced after pretreatment, as experimentally demonstrated. Heat treatment of Silcrete has been documented at several southern African sites.

Figure 4
At Pinnacle Point, (Figure 4) on South Africa’s south coast, the majority of the silcrete from between 71 and 60 k.a. was heated. This time interval corresponds with the production of microlithic technologies at the site which are similar to Howiesons Poort occurrences elsewhere.

During the two so far analysed occupation phases in the HP at Diepkloof Rock Shelter, on South Africa’s west coast, heat treatment was a ubiquitous technique, applied to almost all silcrete before knapping.  

At Blombos Cave, heat treatment was applied to the tips of some of the bifacial points from the Still- Bay (SB) phase, dating ~74–71 k.a.Here the tips were  prepared by heat treating for final retouch with a pressure flaking technique.

Documentation of silcrete heat treatment in the HP and SB reflects the fact that both of these periods are unusually silcrete-rich in the context of the broader MSA; in most sites through most of the MSA outside of the HP and SB silcrete is a marginal assemblage component which has received less archaeological attention, but it is not unknown during the Post-HP phase (for example at Sibudu - a rock shelter in a sandstone cliff in northern KwaZulu-Natal, South Africa). Heat treatment of Silcrete was also detected through the Howiesons Poort and post-Howiesons Poort of the rock shelter site Mertenhof, located in the Western Cape of South Africa.

The site is known to contain high proportions of a diversity of fine grained rocks including  hornfels and chert at various points through the sequence. The excavators found a strong inverse correlation between frequency of heat treatment in silcrete and prevalence of chert in the assemblage, and a generally positive correlation with the proportion of locally available rock. This could mean that, at Mertenhof at least, heat treatment may have been used to improve the fracture properties of silcrete at times when other finer grained rocks were less readily available. As such, heat treatment appears to have been a component of the MSA behavioral flexible adaptive repertoire.

This does not exclude a symbolic meaning of the red color or of the transformative process itself. The systematic evaluation of heat treatment, not known from MSA sites in East or North-Africa contributes to a growing body of evidence that Homo Sapiens-MSA populations in South Africa were capable of far more sophisticated behaviour than previously realised.

Anyhow, we should very careful  to call this technique an indicator of (self-referential) "modernity".

Transformative techniques were also invented by Neanderthals in Europe and clearly earlier than during MIS4. A Micoquian camp of Inden-Altdorf near Jülich in the Rhineland (Germany) has been securely dated to OIS 5e. Here birch pitch residues were found on tools and offers evidence for the production of synthetic pitch for the use of composite tool technology from the Neanderthal world. Two flakes with birch tar residues from Campitello, Central Italy, dated before OIS 6 are the earliest indication for this technology so far.

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