Microstratigraphic Analysis of the Sterkfontein Cave Site
by antonio kuilan
Sterkfontein is an extremely complicated plio-pliestocene site. The cave formation process climaxed ca 25 Ma as the sea that it was submerged in retreated. Avens on the surface washed in debris that later cemented into breccia. In the late 1800s, an Italian miner dynamited the surface of Sterkfontein in search of lime which led to discovery of cave formations and put Sterkfontein on the map of the scientific world. Primate fossils appeared in the mid-30s in discarded breccia by the miners and this generated more interest that led to the discovery of Australopithecus by the late Robert Broom. This ignited archaeological interest that evolved an excavation program that continues to the present day. The site has produced thousands of fauna fossils and stone tools (including Oldowan and Acheulian industries), and hundreds of hominid remains and fragments of fossilized wood with notable hominins: Australopithecus, A. africanus, Paranthropus, P. robustus, and early Homo. Understanding the complex stratigraphy is still a work in progress and recent dating methods that includes magnetostratigraphy and uranium dating are arriving at more precise dates. One controversy was the discovery of blood residue of stone tools but studies by three researchers suggest that the clay-rich environment of Sterkfontein slows down the decay process that will give residues the appearance of being preserved tens of thousands of years later.
Sterkfontein Cave Site
To begin to understand the complexities of the Sterkfontein Cave site, one must start back at the beginning to have a working geological knowledge of its formation. The Plio-Pleistocene site of Sterkfontein is located in the Gauteng region about 50km northwest of Johannesburg, South Africa. Around 2.5 billion years ago, this region was submerged under a warm shallow sea that provided the conditions for the formation of sediments that later solidified as dolomitic limestone rock. Through hundreds of millions of years, extensive formations of quartzite and shale formed from other sediments that covered the dolomite as it rested beneath the seabed. Circa 25 million years ago, the sea retreated and geological processes altered the attitude of the strata to dip in a northwestern manner (Figure 1).
Around 50 m deep under the water table, cavern formations commenced with the calcium carbonate being dissolved out of the dolomite across horizontal planes by the weakly acidic ground water. Later, these cavern formations became sealed air-filled cavities as the water table dropped (Figure 2).
Slow seeping acidic groundwater continued to work the dolomite, dissolving minerals, and when each drop reached the cave’s ceiling, it precipitated back to limestone, forming elongated stalactites (figure 2) – a process that has taken thousands of years. Eventually, avens opened on the surface above, resulting from the prolonged dissolution of the dolomite along vertical cracks or joints (figure 3).
Some joints became exposed due to the surface being subjected to erosional processes, which also ended the sealed state of the air-filled cavities (Figure 3). Over thousands of years, earth and rock washed into the cave forming talus deposits that eventually became cemented by dripping lime. Some of the breccia, formed from talus, collapsed from the higher, mature, cave formations above, with some of it returning to returning to its natural, soft state, due to decalcification (Figure 4).
The excavation of these ancient infills is what yields hominid remains, flora, fauna, and stone artifacts .
In the 1890s, it was Italian miner Guglielmo Martinaglia that first started dynamiting the Sterkfontein surface to exploit the stalactites and stalagmites below for its lime.
Around the same time, Geologist David Draper from the South African Geological Society attempted to convince the miners to stop mining and preserve the main cave due to its rich geologic content but was unsuccessful. Mining continued throughout the decades, with the area becoming a local attraction. It was the initial blasting by Martinaglia that paved the path to later scientific exploration at Sterkfontein.
Various studies have shown that the Sterkfontein caves with its brecciated infills, talus deposits, surface debris deposited through avens, orientation of deposits, the dissolution of limestone, and the decalcification of lime-consolidated breccias, make the site extremely complex and its stratigraphy difficult to decipher.
Figure 5 shows an overall diagram of the complexity of the Sterkfontein Caves. The site is divided into the Lincoln Cave, Silberberg Grotto, Jakovec Cavern, Name Chamber, Elephant Chamber and Milner Hall. There are 6 members at Sterkfontein. Geologist Tim Partridge performed detailed studies on the stratigraphy of Sterkfontein and identified six geological divisions of the Sterkfontein Formation that he described as Members 1, 2, 3, 4, 5, and 6.
These members are shown in a simplified drawing by Ron Clarke in Figure 6; Members 4, 5, and 6 are based on the surface and Members 1, 2, and 3 are underground.
This Member naming structure continues to be used in scientific literature.
Archaeological Evolution and the Significant Finds at Sterkfontein
The mining at Sterkfontein since Martinaglia blasted the area in the 1890s changed evolved throughout the decades and by the early 1930s large amounts of blasted breccia removed by the lime workers started to reveal primate bones, from which Trever Jones, a student of Raymond Dart, obtained fossil monkeys in 1935 and published the first scientific study of fossils form the site in 1936. Interest kept increasing at Sterkfontein. Other students of Dart introduced Paleontologist Robert Broom, who was 70 years old at the time and searching for ape-man remains at nearby Pretoria, to Sterkfontein in August 1936. A few days later, George Barlow, the lime quarry manager at the time, handed Broom the first hominid fossil from the breccia, Australopithecus. This led to many other discoveries including the most complete skull of Australopithecus africanus in 1947.
Interest at Sterkfontein was at its maximum and in 1956 Charles K Brain and Anthony B.A. Brink discovered the first early Acheulian hominid stone stools in the breccia dumped by lime workers. After this discovery, archaeologist Revil Mason conducted the first test excavation with Charles Brain in 1956. John Robinson (the late Robert Broom’s trusted assistant) excavated Sterkfontein from 1957-1958 revealing an early Acheulian industry that was later analyzed by Revil Mason. Palaeoanthropologist Philip V Tobias created the full-time excavation program in 1966 at Sterkfontein that has run to the present day without interruptions. Ian Watt surveyed the Sterkfontein Caves and created a greatly extended grid system necessary to record the positions of finds at the site; this grid system was implemented in Tobias excavation program. Alun Hughes headed the Tobias excavation program from 1966 to 1991 and during this time the discovery of hundreds of Australopithecus fossils were credited to him.
Palaeoanthropologist Ronald Clarke has headed the excavation program since 1991, a few years later he was searching through stored boxes containing faunal remains that were excavated in early 1980 from the Silberberg Grotto section of Sterkfontein, discovering leg/foot bones that belonged to one individual. He predicted the rest were embedded in the breccia of that section and based on the cataloged information, he instructed this assistants to search the Member 2 breccia. A short time later in 1997, the rest of the bones (radius, lower legs, left humerus) were discovered, including the first complete skull ever found of Australopithecus, which was later dubbed “Little Foot.” Its species designation is an ongoing controversy.
Over 9,000 stone tools/flakes, thousands of faunal fossils, hundreds of fragments of fossil wood, and nearly 500 hominid fossils (skulls, jaws, teeth, and other skeletal remains) have been found in the entirety of the excavations at Sterkfontein. Members 1 through 3 are mostly partial remains of monkeys and carnivores. Analysis on fossilized wood fragments from the breccia in Member 4 reveal that the area was surrounded by tropical forest trees ca. 2.8 Ma. Australopithecus has been found in the breccias of Member 2 & 4 and in the lower cave, Jacovec Cavern. A. africanus is found in Member 4. Early Homo has been found in StW 53 (an infill between Members 4 and 5) and in Member 5. Paranthropus and P. robustus are also found in Member 5. Member 6 is dated < 200ka and produces primates of modern form. Oldowan and Acheulian tools are found in Member 5.
One Controversy at Member 5
In 1997, T.H. Loy claimed that he detected blood residues on Oldowan stone tools from the stratigraphic Member 5. This became controversial. It was stated that researchers are unable to describe the mechanism of residue decays, leading analysts who record data to make interpretations that are oversimplified or inaccurate. Peta Jane Jones (2009) conducted a study that directly assessed Loy’s blood residue claim. Jones performed an indepth microstratigraphic investigation into the Member 5 geological matrix that housed the stone tools from Loy’s claims. Jones conducted a battery of chemical tests, including petrographic analysis and pH analysis on the composition of the breccia burial matrix. Jones study suggested that a cave environment with a well-cemented, clay-rich burial matrix, a favorable alkaline pH, and a high soil cation exchange capacity, favors residue preservations over extreme timeframes. Jones summarized that:
- Stone tools are used multiple times to butcher, leaving thick layer of blood residues on them; as it dries, it secretes into microcracks.
- Through transport, residues later come in contact with clay and leads to a degree of binding that creates hydrophobic proteins.
- The artifacts and sediment are transported, and may be deposited through avens, slopes, or other entrances.
- Organic-mineral complexes are encouraged by large negatively charged surface area and swelling ability of clay sediment; the accumulated proteins with clay are more resistant to degradation.
- Leaching occurs by rainfall releasing calcium ions from dolomitic rock on top and the solution filters through sediments below as calcite which cements deposits into brecciated infill.
- Organic material absorbs calcium ions, weak bonds form between calcite and clay surfaces, and the precipitated calcite creates a well-cemented clay/calcite matrix that encloses and protects artifacts and blood residue.
- A regulated environment is maintained by the breccia matrix that “…is not directly exposed to surficial water and was possibly buried with further compaction from overlying brecciated deposits, protected from detrimental conditions.”
Geeske Langejans (2010) took a step further and performed experimental archaeology by leaving stone tools at 3 different sites for a year to test preservation. Sterkfontein was one site, along with Sibudu, and Zelhem, a site in the Netherlands that act as a control. At Sterkfontein, he left 80 samples; 40 samples inside the cave and 40 samples outside the cave (see Figure 7).
The 40 samples to be placed inside the cave were further divided: 20 left buried and 20 left on top of sediment. Langejans retrieved the residue samples in the cave 1 year later and found that those that were buried preserved best. Langejans stated that different residues preserve differentially, some better than others; though residue always undergo decomposition, those that are in a dry zone bioactivity undergo slow decay that can take tens of thousands of years, give residues the appearance of being permanently preserved – as long as the environment remains stable. This experiment and the work of Jones and Hopley provide evidence of preservation at Sterkfontein, but can also be used to make predictions of residue preservation at other and future sites.
Early dating of the Sterkfontein Caves was based on faunal correlation based on changes in dentition and anatomy, followed by later cosmogenic isotope burial ages, uranium-lead dating of speleothems, ESR, and magnetostratigraphy.
Pickering (2010) performed dating on young carbonates from open exposures at Sterkfontein, on boreholes that were drilled twenty-five years earlier by Partridge and Watt, and on underground deposits from the caves. Figure 8 shows updated stratigraphic logs with black arrows pointing to the uranium dating. Uranium dating gave the following estimates to the given members:
- Member 1 = >2.8 Ma
- Member 2 = 2.6 – 2.8 Ma
- Member 3 = 2.6 – 2.0 Ma
- Member 4 = 2.6 – 2.0 Ma
- Member 5 = < 2.0 Ma
Interesting to note that Pickering argued for a revised stratigraphy at Sterkfontein, that Pickering “believe” no clastic sedimentation exist between Members 2 and 4 but instead separated by a massive flowstone layer. Most strongly, Pickering claims that there is no evidence that the present cave “…was ever wholly filled up with sediments.” Pickering also note that Sterkfontein is complex including the problem of past mining activity removing material, noting that his interpretation is not absolute, but that stratigraphy at Sterkfontein is “…likely to remain a contentious issue.”
One recent palaeomagnetic analysis was published by Herries and Shaw in based on analysis from speleothems in Members 1 through 5. Figure 9 (Herries) depict the location of paleomagnetic samples based on the composite stratigraphy compiled by Partridge in 2000. Two methods of sampling were executed at Sterkfontein by Herries: block sampling and drill coring.
Figure 10 (Herries) illustrates the magnetostratigraphy for the Sterkfontein Cave. Palaeolatitude (degrees) versus depth (meters) are plotted against: archaeology, hominin fossils, polarity, Members, and the geomagnetic polarity timescale. Key: O = Olduvai event, R = Reunion event, HR = Huckleberry Ridge event.
Herries (2011) proposed a chart with suggested age ranges based on a combination of uranium, ESR, and magnetostratigraphy data (Figure 11).
The Sterkfontein Cave is an extremely complicated site. Although continuing work has been conducted since the late 30s at the site, there is much debate about its stratigraphy. Pickering even implied that the Member naming scheme may be limiting since he attempted to revise the stratigraphy without distorting the Member system. The complex infills and other deposition combined with collapsing cave mechanics and chemical weathering, leave the guts of Sterkfontein intertwined. Because of the topographic complexity, it had made earlier to recent dating slightly difficult. The accumulative work of researchers over the decades seem to be revealing precise dates but what is going on inside the cave still continues to be the subject of long arduous work. The site still continue to produce countless finds – perhaps the next one will add another clue to our evolutionary past.
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