Lime mines and fossil sites in South Africa

One of the well-known features of Plio-Pleistocene early hominid sites in South Africa is that they are all associated with historic lime mines – think of Taung, Sterkfontein, Makapansgat, and others. As mentioned in earlier posts, this association is one of the critical factors that we used to develop our method and implement this field project, and thus has great practical value to us in the field – find the old lime mines, and we might also find some nice fossils!

However, there is also a very interesting historical component to this association. Mining, primarily of gold and other precious metals, was a principal factor supporting the economic development of the early South African state in the late 19th Century. The lure of vast riches associated with mining attracted migrants from many parts of the Western world. Lime mining was important for many industrial uses, literally providing the mortar and cement for building new towns and factories, and it was also an important component in the extraction process for gold and other economically important metals.

Cemetery at Pilgrim’s Rest, Mpumulanga. The gravestone on the left (pointed pillar) is for a chap from East Dulwich, London.

Speaking of gold, there were several early ‘gold rush’ events in South Africa, including Pilgrim’s Rest (1873), Ottoshoop (1879), Barberton (1881), Kaapsche Hoop (1882), and finally the Reef goldfields on the Witwatersrand (1886) which led to the establishment of the city of Johannesburg – and to conflicts that contributed to the start of the second Anglo-Boer War (1899-1902). We have covered some of these geographic regions in our survey activities, and read a great deal of information about their history in our archive work.

Photo of a gold rig in the Ottoshoop area, from ‘Developments on the Malmani Goldfields’, The Mining and Industrial Magazine, Nov 8 1933.

The people we have met in all of these areas are very knowledgeable about their local history, and of the historical importance of these events, and we have also located additional documentation about mining history through our archive work at the Wits University Historical Papers Library, Council for Geosciences, and other sources.

One example is the lime kilns near Ngodwana in Mupumulanga Province. We located them through a reference in a geological textbook1, and later determined that there was an old lime mine nearby. Our local contacts informed us that the kiln site (see photos) had been an important distribution centre for lime extracted from many mines in the area prior to 1899, and after processing the lime was shipped out to other areas by railroad. With this information, we were able to locate six additional lime mines within a few kilometers of the kiln site, and there are likely to be even more! (While we did not find fossil deposits at any of these mines, we did find in situ clastic cave sediments, or ‘rock breccia’, and some very thick remnant speleothem deposits. Definitely worth some future sampling!).

The Ngodwana lime kilns, closed down around 1899, but used later as a citrus packing plant when lime mining was no longer lucrative in the region.

As it turns out, the geological formations in which the metallic ores occur – including gold, iron, and other mined resources – are closely related geographically to the dolomite formations of the Transvaal Supergroup. Thus, in the areas of our survey we have walked across other rock units in the Transvaal Supergroup such as quartzites and Banded Iron Formations (BIFS) [see previous posts], and encountered mines for resources such as gold, iron, and asbestos. In fact, some alluvial sources (i.e., those that have eroded out from original geological context and were redeposited elsewhere) actually occur in younger sedimentary context in dolomite regions. In both a geological sense, and historically, our observations in the field have led to a strong connection between the mines and our quest to locate new fossil localities in the dolomite formations. In learning more about mining history and practice in our study areas (focusing on lime mines, but requiring some understanding of more general issues), we believe that we will be able to refine our field methods to identify additional areas that hold strong potential to locate fossil sites. This is not a simple task, however, and will continue to be a focus of the research programme in the long term.

-KL Kuykendall

 

We think we found a fossil site – now what?

When we locate a potential site locality, we begin a process of exploration and evaluation to determine whether there is potential for further research at that locality. Our basic tools are a handheld GPS, a rock hammer, and a panga (or machete).

Field crew, 2nd trip: Andy with the GPS, Morris with the rock hammer, Kevin with the machete: Did the timer go off? is the camera working? did anyone hear a ‘click’?

The main objective of our project is to locate historic mining sites, and to evaluate whether they contain deposits and other features that are useful to us as palaeoanthropologists. We are looking for a variety of in situ (i.e., undisturbed) deposits including:

  • speleothem or cave formations, which can be sampled to obtain geological dates, and for stable isotopes which tell us about past climatic and environmental conditions;
  • cave infill sediments (derived from soil and other debris from both outside and inside the cave), which can tell us how the cave filled in, and also may contain evidence of the outside environment such as pollen;
  • fossil bone, including those of reptiles, birds, and rodents, as well as of larger animals including bovids, carnivores, primates, and of course, our own ancestors (hominids).

For some sites, the presence of such evidence can be quite obvious, especially if there are dense fossil deposits just waiting to be excavated. However, at other sites, it can take considerable effort to determine whether this evidence is present. Some sites are small and the deposits easily located, and others – especially when the mining activity was extensive – can be complex and more difficult to define.

One of the underlying objectives of this project is to consider what evidence at such sites can be used to answer questions of interest to palaeoanthropologists even in the absence of fossils. After all, when we find fossils (and especially those of hominids), we then use a battery of sampling techniques such as those above to obtain information about the temporal, environmental, and depositional context of that fossil. But since fossils are relatively rare in the geological record, we may be missing out on valuable information about past environmental conditions by passing over deposits in which hominid or other fossils are rare or even absent. This information could help us to refine our models about environmental and adaptive change during hominid evolution.

So, after that extensive preamble, what do we do when we think we have located a site? There are three basic tasks to complete: defining the limits and features of the site; mapping the locality; and evaluating the site’s potential for future research.

Defining the limits of the site involves exploration of the site’s geographical and geological features. We walk up and down hills, climb rock outcrops, and (carefully!) climb into mine shafts and trenches, looking for speleothem deposits, breccia and cave infill sediments, and fossils.

A miner’s trench exposing a lot of geological formation: Can anyone see any breccia up there?

One of the challenges in this work is being able to see the ground clearly in order to find features, breccias blocks, and of course, fossils. This is where the panga comes in handy!

Like most arid regions, many of the plants here have thorns. This site is overgrown with acacia trees and brambles. Keep repeating to self: ‘This is better than being stuck behind my desk at the universiity!’ (There is actually a nice cave infill deposit in this view… just to the right of the machete!)

The miner’s dumps are a great source of information about the deposits on the site. We comb the hills of rock, searching for fossils and other evidence of the nature of the underground deposits. Depending on what we find, we then have to search the mine trenches and chambers to determine where the blocks in the dump came from.

Fossils can be difficult to ID in the field. Observe the fossil braincase (below the knife to the right), and a vertebra (to the right on the edge of the block) of un-named mammals in this breccia block.

Finally, all of the features of the site are mapped using the handheld GPS, which locates each data point taken to within approximately 5 meters. For our initial fieldwork survey, this is good enough, but if we were to return for sampling or excavation we would use total station survey equipment and map the site with more accuracy.

Andy enters data points on the Trimble GPS at a small locality near Gondolin, North West Province.

The initial survey and exploration of a new locality can take several days, and also includes extensive photographic and written documentation; we also record information about the site on databases on the handheld GPS and a table PC. The final task is to sit in the shade and discuss what we have observed on site, and evaluate the potential for further work in the form of sampling or excavation.

Once we have completed this process of site survey and evaluation, we go back to the maps, head out to a new area, and begin the process again. All in a day’s work!

– KL Kuykendall

Geology maps: where is that dolomite?

Apart from the two historic mining maps we have acquired, a substantial amount of our information has come from 1:250,000 maps of the bedrock geology. These provide the geological characteristics, and the location, of the underlying rocks in any area of South Africa. Because the types of caves we are looking for form in dolomite, locating areas where this rock is present is one of the main factors in determining our survey strategy.

The geology maps have been treated in the same way as the historic mining maps, utilising only the information relevant to the project and converting that information into a usable digital format. However, unlike the mining maps, on which the digital information is characterised by a point, the information we require from the geology maps relates to the region (including the size and shape) occupied by a lithological unit of bedrock.

Geology base map. An example section of a 1:250000 geological map showing areas of dolomite in different shades of blue (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

 

In order to convert this map information from Raster to Vector formats, we trace around the edges of the bedrock formations we are interested in, save them as a polygon (instead of a point), and then add the lithological information as an attribute of the feature. Not only does this kind of digitisation help in establishing areas to investigate but it also has a number of other advantages.

Geology map with polygons.  In this figure, the blue areas denoting a dolomitic lithological unit have been overlain by a traced polygon using ArcGIS and are now represented by different colours to indicate different attributes the rock unit might have, such as including interbedded chert, quartzite, or shale (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

The first advantage is the reduction in size of the file from a large scanned map which may be as much as 390 Megabytes per file to a few Kilobytes of data. This is helpful not only when trying to use the data on a handheld computer but also for storage on the device, since we can fit many more polygons than scanned maps on a 4 Gigabyte memory card. The second advantage of digitising in this way is that we can select the only data that are  relevant to the project and its aims. We can ignore all the other geological formations which do not hold the appropriate characteristics and only digitise those that we need. This again saves space on the memory cards but also saves time, both in the tracing of the geology and in locating regions appropriate to the project in the field.

Geology polygons. This figure shows how the digitisation of the geological formation looks when the area is loaded onto the handheld computers. The pink and orange colours represent the (dolomite) geology and the green background represents the Vector outline of the main provinces of South Africa.

The “Identify” function on the GIS software (ArcPad) we use on the hand-held computers can be used to call up the attributes of any of the Vector layers  once defined. The GPS function on the handheld also enables us to determine our current field position relative to the geological formations (polygons) and to the mines (points). This helps us to avoid spending a lot of time driving around searching in the field!

This is the final step in the digitisation process to prepare the maps for the field. The next blog in this series will deal with some of the hardware and software we are using in more detail.

– A Reid

Malmani Dolomites

To a large degree, our project is a field survey of the geological formation known as the Malmani Dolomite Subgroup, of the Chuniespoort Group of the Transvaal Supergroup. The Transvaal Supergroup is an extensive geological sedimentary rock sequence extending across much of the northern part of South Africa and into Botswana (Ericksson & Altermann, 1998). The Malmani Dolomites are just one rock formation in this sequence, and they are of interest to us because it is in the dolomites that we find the historic lime mines, and thus palaeocave fossil deposits. On the map below, the Transvaal Supergroup, including the Malmani dolomites, is indicated by the light blue-shaded areas in the north & central regions of South Africa. Geological dates for these rocks indicate that they were formed between 2.6-2.5 billion years ago, making them one of the oldest dolomite formations known.

Simplified Geology of South Africa: The Malmani Dolomites in blue in the north and central parts of the country – unfortunately, NOT at the coast!

Dolomite is a type of limestone rock that forms in warm, shallow seas from the slow accumulation of the remains of marine microorganisms and fine-grained sediment. It differs from other limestone rocks in having a higher magnesium content, and these dolomites are also characterised by fossils of algae formations that are known as stromatolites (see picture below). These materials contain high levels of calcium carbonate, and thus such rock formations are often referred to as carbonates.

Photo of stromatolite fossils (the clusters of concentric structures) in the roof of Sudwala Cave, Mpumalanga. These represent ancient algae formations in the sea at the time that the dolomite rock sediments were accumulating, roughly 2.6 billion years ago. 

One important quality of dolomite is that, over long periods of time, this rock is water soluble – ground water is a weak acid that will erode away the calcium carbonate matrix as it percolates through cracks and fissures within dolomite rock layers. This process eventually results in the occurrence of underground solution cavities, and when these break through to the surface due to erosion or collapse they form caves and sinkholes (see schematic below). Further percolation of groundwater through these solution cavities results in the precipitation of calcium carbonate to form various kinds of speleothem – the beautiful cave formations known as stalactites, stalagmites, and other structures. In addition, once the cave or sinkhole is open to the surface, it will begin to fill in with sediment, and if occupied by carnivores the infill will include animal bone. In South Africa, the ancient caves of this kind were open to the surface, and filling in with sediment and bone, between roughly 3-1 million years ago.

Idealised model of cave formation in dolomites (Martini, et al. 2003). The brick-like symbol indicates dolomite, and the differently-stippled symbols represent separate infill deposits over time.

Thus, over a period of millions of years solution cavities will form in the dolomites, break through to the surface as caves and sinkholes, and then fill in again with sediment and bone. The constant percolation of groundwater through the deposits will cement everything together in a calcium-carbonate matrix, and fossilise the bones. Finally, lime miners in the early 20th century blasted and excavated into the dolomite formations to retrieve the extensive cave formations (calcium carbonate, or ‘lime’), and in so doing often exposed the infill deposits containing fossilised bone. This attracted the attention of palaeoanthropologists who were interested in the fossil record of human ancestors and other mammals.

And that is why we are in South Africa surveying the Malmani Dolomite formation, looking for ancient cave deposits!

– KL Kuykendall