On the ancient basement rocks that surround Hoedspruit sit slightly younger rocks that make up the great escarpment, or Mpumalanga Drakensberg. These mountains were formed by sediments laid down on a very different early earth. One of these layers formed primarily on the shallow edges of an ancient ocean, and gave us rocks known as Dolomite and Limestone. Timeous
When you drive to Hoedspruit through the Abel Erasmus pass, after Ohrigstad, and before the tunnel, you pass through a long section of these layered grey and black rocks; the picture shows a good example. Look out for them the next time you travel that route.
At the time this rock was being formed, there was hardly any oxygen in the earth’s atmosphere. As living organisms emerged and developed, the dominant life forms for an extremely long time, were vast communities of microorganisms, primarily blue-green algae. These grew as thin sticky mats, at and just below the shallow edges of the sea. They were photosynthesising, taking carbon dioxide out of the water and releasing oxygen. More and more oxygen was produced on a huge scale, and the atmosphere gradually changed, enabling the emergence of more and more complex life, including humans. We can thank the humble blue-green algae, or cyanobacteria, as we take our next oxygen-rich breath.
When cyanobacteria take in carbon dioxide, calcium carbonate precipitates out of the water and settles on the sticky algal layer. They respond by growing on top of this mineral deposit, and gradually a series of layers develops. Over time, these layers of calcium carbonate become Limestone. If this calcium is replaced by magnesium, we get a rock called Dolomite.
A distinctive feature in these rocks are structures that show how many of the blue-green algal communities formed layered domes as they grew. The rounded rock in the picture clearly shows some of these fossil structures. They are called Stromatolites. This rock was found in the Olifants River near Mica, and would have travelled down the river from higher up on the escarpment.
Because of the minerals in these rocks, they produce soils that are very distinct. This allows unusual plants to develop, and there are some that grow only in limestone soils. The little known Aloe fouriei is one example – it is only found in this part of the great escarpment.
Just before you drive through the tunnel on the Abel Erasmus pass, on your way towards Hoedspruit, you can see a famous and mysterious waterfall, on your left, flowing out of the rocks and falling into the gulley below. Most waterfalls wear away the rock they pass over, but this one is actually growing forwards. As it’s water travels through the Limestone and Dolomite rocks it picks up calcium, which is then deposited at the point where the water flows out. Layers of new calcium-rich rock, formed in this way, are called Tufa.
So the mystery of this waterfall is that the exit point of the water from the underground source, (that what we see as the waterfall), is actually “moving forward” very slowly, year on year due to the deposit of calcium from the water.
John has a continuous curiosity and an abiding passion for sharing the stories of South Africa’s remarkable biodiversity, rocks and landscapes. Based in KZN, he offers geological learning adventures throughout South Africa. Check out www.johnroff.co.za.
Cover image: Gavin Whitfield, Geological Journeys – Dolomite layers.
