Witwatersrand Contact EarthCache
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Located in a small park in Cyrildene. This is a lovely park and is accessible from the top and the bottom. I would suggest using St George’s Street access at the top rather, although Da Gama Street is just as good.
Dr Edward Thomas Mellor – the geologist who first mapped the Witwatersrand – as a result of his pioneering mapping work of the Witwatersrand goldfields came to the conclusion that the Ventersdorp Supergroup lies on top of the sediments of the Witwatersrand Supergroup. The layering in these two Supergroups is parallel to each other – and this is why he made the conclusion. He also concluded that this line of contact would be along the foot of the Klipriviersberg mountain range south of Johannesburg.
This was proved slightly inaccurate by more recent investigations, which have indicated that the northern contact between these two Supergroups may be faulted and lie far closer to the city than originally thought.
A wedge of rock that was down faulted to bring the Ventersdorp lavas in contact with the lower formations of the West Rand Group. The ripple marked quartzite of the West Rand Group here is in contact with the Ventersdorp Supergroup which is represented here by a diamictite that grades upwards into a greywacke (yep – these are genuine geological names of rocks! Aren’t you glad you didn’t do geology as a full time job ). Greywacke is a rock that consists of quartz, feldspar and other dark materials. The diamictite is a type of conglomerate or scree containing particles from the size of large boulders of quartzite and shale – down to a matrix of finer material. The boulders are not sorted according to size (like the Main Reef conglomerates which were laid down in water) and many of them are not well rounded (again unlike the Main Reef conglomerates). The nature of the boulders tends to indicate that they were derived from the adjacent Witwatersrand rocks. The presence of shale indicates that these boulders did not move very far from their original source, as they are soft and would have broken into much smaller particles, or been completely destroyed if they had moved very far from the original source. The ripple-marked quartzite dips away at about 30° towards the south (not clearly evident at all exposures at this site due to the angle of exposure) and the Ventersdorp strata at a considerably steeper angle. In addition the Ventersdorp rocks are tightly folded, and careful examination shows some of these folds being visible.
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
Try look for freshly exposed pieces of rock - rather than the older weather material on the outside of the rocks. There are plenty of pieces that visiting geologists have broken off here.
Although this park is fairly clean and muggle free – always be careful when caching and make sure that you look for vagrants or dodgy types of characters. Rather safe than sorry – the cache will still be here next time you come.
In order to qualify to log this cache, you need to answer the following questions and email the cache owner. Any logs not accompanied by an email will be deleted.
1) Take a photo of you and your GPSr at this spot with the outcrop visible in the background. [Optional]
2) Look at what you think is the exposed contact zone. Describe what you see and why you think that this was the contact zone (hint – look for a zone where rock colours and type change dramatically in a small space).
3) Look carefully at the darker of the rocks. Is this quartzite? Explain your answer.
4) What is a fault?
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!