How pots, sand and stone walls helped us date an ancient South African settlement
When walking in the verdant hills of Mpumalanga in northeastern South Africa, you can stumble across some stone walls. Either you poke your toe or appear at about chest height through the grass. These walls guide and interfere with your path.
As you climb these gentle slopes, you will be drawn to circular and linear patterns. From above you can quickly see that these patterns are the remnants of cities – collections of homesteads, traditional households, terraces and streets. These ruins are the remains of the Bokoni community, a region that is home to the southernmost collection of stone terrace acreage in Africa.
Archaeologists are studying the Bokoni sites as they are a marvel of the innovation and ingenuity of urban agriculture.
In our research, we’ve broken new ground in relation to the Bokoni sites and solved a puzzle that has puzzled scientists for decades – when the first sites were built. Our insights were made possible with techniques and technologies normally used in geology.
The search for the beginning
Archaeologists, in collaboration with historians, have defined four phases of occupation for Bokoni. Oral records offer a special glimpse into Phase II, the height of urban growth and planning by the Bokoni when the larger cities were occupied.
For example, in 1936, as part of his research on the seKoni language, the linguist CW Prinsloo mapped the extent of Bokoni in the 19th century and pointed to earlier capitals. The oral traditions of the Pedi, recorded by missionaries in the 1960s, refer to the royal kings of Marateng (Pedi) who lived around 1650 BC. Met seKoni speakers.
However, there are no known historical reports of Phase I. Until now it was not known exactly when the bokoni appeared. However, by turning to material records and archaeological science, our research has solved this enduring puzzle.
We used a technique called luminescence dating to unravel the origins of this tradition. We now know that Bokoni Phase I was built as early as the 15th century – before European colonization or European trade came inland. And that the bokoni farmers continued to flourish for centuries, despite the turbulence that arrived on the nearby banks.
These findings disrupt previous narratives that decipher the presence and ability of African farmers before and during colonization. They also offer new opportunities to understand individual life and family patterns. This research helped us reconstruct when people began building these incredible structures, how long a household was occupied before the task, and how their successors interacted with the structures they left behind.
The four phases identified by researchers as key periods in Bokoni politics are as follows.
Phase I marks the creation of Bokoni (the date of which was previously unknown). In Phase II of the 17th and 18th centuries, Bokoni peaked in urban growth and planning. During this time, most of the residents of Bokoni would have been town farmers, first in and around the capital Moxomatsi and later in the successor capitals Mohlo-Pela and Khutwaneng, which are located in what is now Mpumalanga in South Africa.
Phase III marks the beginning of the upheaval that led to Bokoni’s decline in the 19th century, while phase IV documents the diaspora from the middle of the 19th century. Bokoni broke up in the early to mid-19th century due to regional conflicts.
Given the lack of written or oral tradition from Phase I, we turned to the science of dating in our search for answers.
Only two radiocarbon dates exist for locations from this time and region. This is because radiocarbon dating is not ideal for Bokoni. Radiocarbon dating measures the radioactive carbon isotope in organic remains. The technique provides the date of death by measuring the remaining radiocarbon component of organic remains such as bones or wood. However, under certain conditions, the soil does not retain organic remains.
Read more: Explainer: What Is Radiocarbon Dating And How Does It Work?
Luminescence dating was far more appropriate for the Bokoni site. Optically stimulated luminescence is a dating technique that measures when quartz or feldspar grains in the soil were last exposed to light or heat. This timestamp tells us when these minerals were buried (or trapped in an object like a pot).
When quartz grains are exposed to light, their electrons are excited and leave their correct orbitals. This is called bleaching. At the time of bleaching, the grain is zero age. Once the grain is buried, it uses the radiation in the surrounding soil to return its electrons to their correct orbital.
A GIF showing the process, Ruby-Anne Birin via GIPHY
The scientists then measure the dose absorbed by the grain and divide it by the rate at which that dose was absorbed. This value indicates the date of the last exposure, by means of which we can determine when a material or surface was buried or when a pot was last burned.
Our team used this technique on two homesteads in Komati Gorge Village, a southern town in Bokoni. We already knew that one homestead was older than the other, as many of its stones were redesignated for the newer settlement.
Our results show multiple periods of occupation, abandonment, and rebuilding. The older homestead was already in 1489 BC. Occupied until around 1577 BC. Was abandoned. The builders of the younger homestead used the older one from around 1682 BC. Until 1765 BC Chr. Again. The younger homestead itself was built sometime between 1738 BC. And reused the early 20th century.
Future work that will refine our understanding of the Bokoni occupation may also enable us to better reconstruct the environmental and political landscape in which people lived.