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A visit to Tswaing

text and images by Robert Groess (ASSA Johannesburg Centre)

For all the technology we have mustered, and all the power it commands, there are places on this Earth that present a doleful reminder that Nature's forces pales our handiwork into insignificance. This is part of the sheer allure that presents itself, at a lesser known locale, barely 40km North of the Capital City of Tshwane (Pretoria). The Tswana people have given it the name "Place of Salt" – or in their language, "Tswaing". This is the place, where two hundred thousand years ago, you would have found yourself squeezing every last drop out of the cliché: finding yourself between a rock and a hard place. This is the site of the Tswaing meteorite crater.

(above)  View of the Tswaing Crater salt pan basin from the north-eastern rim. Photograph © 2005 Robert Groess.

220 000 years ago, a remnant from our primordial solar system had a rendezvous with terra firma. While our atmosphere does a great job of protecting us from small menacing stones pelting down on us every now and again, it was hard-pressed to bleed off a 50m space rock's destructive energy. There is no remnant of the original impactor – it was completely destroyed during impact.

I was fortunate to have been invited to go along on a tour organized by Trevor Gould of the Johannesburg Centre of ASSA, one weekend in early September, 2005. Our goal: A reconnaissance of ground zero. A greater contrast it could not have been, from the scene of destruction that must have been evident just after impact. Today, the site is tranquil and serene. Approaching the impact site from the south on foot, there is nothing but a small hillock which juts out to add relief to the otherwise flat Transvaal Supergroup plane. A gentle hike up a pseudo-contour soon presents itself with the telltale sign of human activity. Ruins. Or more precisely – remnants of a foundation. From the foundation, the concept of a solitary hill is immediately transformed, and you realize you're standing at the threshold of a larger structure – on the rim of an impact crater! The opposite rim wall, an impressive 100m high, is reflected in a pool of water below.

(above)  Reflection of the opposite crater rim which lies approximately 1.3km away (the crater diameter) and at its highest extends about 100m from the crater basin. (The basin is lower than the surrounding landscape outside of the crater.) Photograph © 2005 Robert Groess.

The bottom of the crater harbours a salt pan, earning it the name given by the Tswana people. The foundation on the southerly crater wall was once the house of the manager of a salt mining operation. This mining activity continued for 44 years until 1956 at which time much of the salt and soda-ash mined from Tswaing had been exhausted. During its tenure of operation, the salt had to be bleached to remove discolouration which is endemic to the deposits found at Tswaing. While this sort of practice would be utterly unacceptable by today's standards, in the first half of the 1900's, it was acceptable to sell salt that just had to look white!

 

(above, left) Close-up of the south-eastern shoreline. Evident is how much the water level has dropped after a typically dry winter.
(above, right) Fresh footprints left in the salty sediment on a minor protuberance jutting towards the salt pan centre. The surroundings are imbued with a distinct odour due to bacterial growth in the pan. Photographs © 2005 Robert Groess.

Salt mining activities notwithstanding, Tswaing remains a relatively unspoiled vestige of the past it bears witness too. (Compared with the comparable Barringer Crater, near Flagstaff, Arizona – which sees enough tourists to enforce a ban on walking down to the crater basin.) A healthy bird population inhabits the area, with indigenous flora typical of the region, having well settled on both sides of the crater rim – only to be broken at the basin where the saline content of the pan acts as a natural boundary. A definitive causeway juts towards the centre from the north-western side of the crater basin – used in the past for surveying the geological content beneath the surface and determining that there is no impactor body embedded below, but confirming the structure to be of impact origin. The causeway also features (quite humerously) a borehole water tap – drawing from the same sub-surface spring that replenishes the water in the pan.

 

The total reconnaissance involved a 7.5km hike from the car park – back to the car park, and ended off by inspecting the ruins of salt-beds which were used to evaporate the water pumped up out of the crater, during its operational phase. If ever you do get an opportunity to visit the northern parts of Pretoria, I warmly recommend that you pay a visit to our national heritage at Tswaing.

 

(above)  The landing at the end of the causeway was once used to extract a core sample to investigate the sub-surface geology. The salt pan water level is lower than usual - look at the large "shoreline" to the right, as well as numerous sand banks or little islands that jut out of the water. Photograph © 2005 Robert Groess.

 

 

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