GEOLOGY

Geology of the Haenertsburg area. All of the pinks are different kinds of granite, whilst the greens are volcanic rocks. The orange, brown and light green strata in the south make up the Wolkberg strata which are perched on top of the granite-greenstone ‘basement’ of the Haenertsburg surrounds.

The Iron Crown itself is made of Black Reef conglomerate and quartzite which marks the top of the Wolkberg strata and the bottom of the Transvaal Supergroup strata. These rocks extend all the way south to the Blyde river canyon and the Bourke’s luck potholes, and in fact the Blyde river canyon forms a deep incision into the Black Reef and Wolkberg rocks and creates a perfect geological cross-section viewable at the Three Rondavels view site. It is no coincidence that many of South Africa’s famous old gold mining areas such as Sabie, Pilgrims Rest and Haenertsburg occur along the escarpment, because it is here that so much rich geology has been exposed by erosion.

Above the Black Reef conglomerate and quartzites are the Malmani dolomites, and these form the back of the range behind Haenertsburg. Thanks to the calcium carbonate in these rocks, the area above Haenertsburg has numerous caves, as well as tufa falls, which are formed when the calcium carbonate, initially dissolved by acidic groundwater, re-precipitates on cliffs where the water forms waterfalls. These tufa falls are tourist attractions along the escarpment edge, and literally look like waterfalls frozen in time.

Wolkberg-and-Tufa-web-screen.jpg

Photos: Gavin Selfe

Left: Wolkberg quartzite cliffs, south of Haenertsburg. Right: A Tufa waterfall on top of the Wolkberg.

Photos: Mike Strever

Left: Grassland with the Wolkberg strata standing proud in the background. Right: Local cave behind Haenertsburg on top of the Wolkberg

Photos: Gavin Selfe

1's Image: Quartz veining in a dolorite dyke. 2'nd Image: Boudinaged quartz vein. 3'rd Image: Dolerite dykes in Wolkberg quartzite. 4'th Image: Black reef quartzite amongst flowering grass aloes at the Iron Crown.

Geology of the Haenertsburg Grasslands and Surrounds

by Gavin Selfe

The Haenertsburg Nature Reserve sits perched on the edge of the great Transvaal escarpment, at an altitude of around 1,550m. The locals describe us inhabitants as living ‘on the mountain’, but in reality we are around halfway up it. It’s an 800m drop to the tropical lowveld around Tzaneen, following the misty kloof down the edge of the escarpment, but to our south the Iron Crown, the highest peak in Limpopo province, rears up to an altitude of 2,126m.

Between us and the peak of the Iron Crown rear the moody, majestic cliffs and spurs of the Wolkberg (cloudy mountains). Both the geological strata and the mountain range are called the Wolkberg, causing some confusion. Adding to the confusion is the fact that the Wolkberg ‘mountain range’ is really just the name given to an alcove in the northern Transvaal Drakensberg, where it swings around to run east-west for a while, rather than north-south. The rocks making up the cliffs of the Wolkberg range are quite different to those around Haenertsburg itself, being extremely hard quartzite which is very resistant to erosion. The granites around Haenertsburg are much softer and weather easily, which is why they lie at the base of the cliffs on a much flatter ‘foothill’ type area.

A geology map of the area is shown below. All of the shades of pink are various kinds of granite, whereas all of the shades of green are metamorphosed volcanic rocks (ex-lavas) of different kinds. These are all very old, mostly older than 2,700 million years old. Together the granites and volcanic rocks make up the local ‘basement’ while the light green, brown and orange rocks to the south make up the Wolkberg which perches on top of this basement. The Wolkberg strata are 2,650 million years old.

To the south of the Blyde River canyon, the Wolkberg strata eventually peter out leaving only the base of the Transvaal Supergroup to make up the eastern escarpment. This is because the Wolkberg strata occur only locally, having been formed as a small, slightly older sedimentary basin (inland lake) before the main inland sea of the huge Transvaal sedimentary basin started to form. The Wolkberg area is truly unique in South Africa.

The volcanic rocks in the grassland are known as greenstones because they are metamorphosed and altered to green minerals such as talc and chlorite. Where there is a lot of talc in greenstones, these rocks make up the soft ‘soapstone’ for the carvings so commonly bought on the side of the road in Zimbabwe. Greenstones also make up the famous Barberton and Murchison greenstone belts which are well-known gold-mining areas. In fact, the exposed greenstones at the base of the Wolkberg and the Black Reef conglomerate (also gold-rich in places) at the top explain why the Haenertsburg area was originally famed for its gold deposits. The old workings can be seen in Wolkberg conglomerates and quartz veins right at base of the Iron Crown cliff faces. These deposits were never economically viable, being of poor and inconsistent grade, and as such have thankfully never made up major mines. Nonetheless, in their day they allowed many an old digger to make a hard-bitten livelihood, and helped to create Haenertsburg as the truly charismatic frontier town that it originally was.

Photo: Sylvie Kremer-Köhne

Photos: Mike Strever

Left: This rock forms part of a granite outlier (younger rocks encircled by older ones, i.e. greenstones) on the grasslands next to Haenertsburg, south of the school. Limpopo spotted aloe, Aloe longibracteata, in attendance. Right: Greenstone outcrop in the grassland.

Photos: Gavin Selfe

A glimpse into a bygone era. Old gold workings, some enlarged by modern zama-zamas (middle image) and old stone ruins utilized by diggers in the Haenertsburg gold rush during the late 1800's.

Ecologically, the greenstones in the grassland are important because they break up the monotony of the clay-rich, loamy, acidic soils of the granites. Being harder than the granites, they create small rock islands with a localised iron- and magnesium-rich soil. These micro-habitats form havens for certain plant species which prefer the different soil and enjoy the protection of the outcropping rocks. Therefore, as a combined geological terrain, the granite-greenstones make up the perfect arrangement to maximise plant biodiversity in the nature reserve and surrounds.

Added to this, an occasional dolerite dyke (or sill) crosses the area. These different soils also add to the biodiversity. Dolerite dykes are very much younger than the granite-greenstones and the Wolkberg strata, and cut right through all of them in narrow sheets. They are fascinating in their own right, as they fed the huge basalt outpouring at the end of the Jurassic period which makes up the top of the Drakensberg and the ‘Roof of Africa’ in Lesotho. This basalt outpouring marks the break-up period of Gondwanaland, and Africa splitting off from Madagascar and India. The dolerite sills and dykes that you are looking at in the road cuttings leading into Haenertsburg from Moria are a glimpse into the past of this epic event.

Left: Exposed quartzites on the slopes of Serala. Right: Greenstone hill in a sea of gneiss with a Mountain aloe (Aloe marlothii) in the foreground.

© 2019 by Friends of the Haenertsburg Grasslands