It seems that, around 200 million years ago, there was a big continent called Gondwana which was subject to terrific forces that ripped the it apart. The various pieces of it became the southern continents, including Antarctica, Australia, South America, Africa, Madagascar and India, which drifted off in their own directions.
The really fascinating discovery, for me, was that the Falkland Islands started out in life adjacent to what became the coast of KZN. They were really one piece of land, with the Falklands plateau shearing away and moving off into the distance along the Agulhas/Falkland Fracture Zone. It is actually still moving away from us at a pretty nippy 6cm a year, thus steadily widening the Atlantic Ocean.
With the newly formed Indian Ocean now along the KZN coast, the next major event from 120-60 million years ago, was the Cretaceous marine transgression, when the sea level rose to as much as 400m above present levels. The sediment deposited by the sea during that period eventually turned to rock and, although much of it has been eroded on the surface, layers of it still underlie parts of Durban, including the city centre, Bluff and the beachfront. The high sea levels of the Cretaceous gave way to the Tertiary Marine Regression when the sea level slowly began to fall due mainly to to cooling of the oceanic crust. Since the last million years or so sea levels have been up and down at least 5 times due to the Pleistocene ice ages. During the height of an Ice age thick ice capes develop at the poles lowering sea levels.
During the most recent Ice Age about 18 000 years ago, sea levels reached about 120m below current levels, meaning that Durban, if it had been around, would have been about 15km inland. During this time, the features we know today as the Bluff and Berea, were formed from sand dunes which became dune, or aeolianite rock, through a process of lithification. This happened when rainwater percolated through the sand, dissolving calcium carbonate shell fragments which then dried into a type of cement and stuck the sand particles together. The red sand, found all over Durban, also known as Berea-type sand, is the result of the weathering and oxidation of dune rock. Durban is pretty solidly grounded on a geological basement consisting of layers of rock which are known as the Karoo Supergroup. These rocks formed on the continent of Gondwana before it split up. Around Durban one can find the Ecca Group, which consists of dark grey shales formed in an inland sea about 350 million years ago. Underlying the Ecca shales are tillite of the the Dwyka Group, which formed from rock material deposited by glaciers 300 million years ago. The tillite forms the huge cliffs on either side of the Umgeni River and is quarried around Durban for aggregate.
The Karoo Supergroup is cut by a number of faults. These include the Springfield fault, which runs slap-bang across Jan Smuts Avenue at 45th Cutting. It was apparently active during the breakup of Gondwana but the good news is that it, and the other faults in our area, are inactive. The area has been geologically stable for 30 million years, and thank goodness for that.
(Acknowledgements – Facts About Durban)
So being underwater, having neighbours such as the Falklands, sand dunes, glaciers, mud flats, and dinosaurs roaming the area were all ‘par for the course” in this region.
In order to substantiate your visit and be able to claim this Earthcache, a number of questions need to be answered and submitted to the cache developer, via e-mail.
You could also post a photograph on the site with your log of your GPSr near GZ, or an interesting geological POI you have observed in the city. {Optional}.
1. As you drive to the Point – describe the topography and how the underlying geology may have influenced it (i.e. where is it hilly – why? – where is it flat – why?).
2. Are the rocks that the Durban area mainly sedimentary, metamorphic or igneous? Why do you say that?
3. Describe the Bluff (ridge under the lighthouse across the bay entrance) and some clues about how it may have come to be formed in this shape. Why can houses be built on it (with a firm foundation) considering it’s origin?
4. List 2 rocks in the region that could have been depositied in (a) much colder climates and (b) much hotter/drier climates.
NOTE: You may log your visit prior to approval, but e-mail submissions that do not meet the above criteria will be deleted.
Free counters