There are 3 main layers of rock in Golden Gate Highlands National Park, the characteristics of which have lead to the famous cliff formations present in the park today.
The red layer was created 200 million years ago when swampy rivers deposited the mud-like sediment. 196 million years ago, the area dried up and became a desert, resulting in the yellow sandstone deposits. From 160-190 million years ago, volcanic activity capped the area with a basalt layer.
The rivers and streams in the park have carved the rocks into their present formations. The name ’Golden Gate’ originates from the two cliffs that face each other on either side of the road: at sunset, the yellow sandstone becomes a rich gold colour.
Formation: Many meters of silt and mud were laid down when the area was a wetland/flood plain around 200 million years ago. When rainfall in the area dropped, the mud dried out and over time has turned into rock.
Formation: This rock was formed when the area was a desert 190-196 million years ago. Sand dunes built up over time and the sand particles were squeezed together to form a weak rock.
Formation: Over the years, water moving through the sandstone has caused a buildup of calcium carbonate in some layers. The calcium carbonate acts as cement and holds the rock particles together making the rock harder.
Formation: The heat and pressure of the molten lava ‘cooked’ the sandstone layer underneath it and turned it into quartzite—a metamorphic rock. The sand grains in the quartzite have melted together making the rock much tougher than normal sandstone.
Formation: 183 million years ago, the area was volcanically active. Sheets of molten rock came out of the volcanoes and spread over the land before solidifying into basalt. This is a very hard rock, and does not erode easily.
The rocks of Golden Gate show variations in toughness. Caves and overhangs are formed when an area of soft rock is eroded/weathered more quickly than the hard rock around it. This can be caused by wind, water and salt crystallization
Dykes and Sills
When pressure built up beneath the earth’s surface, liquid magma was forced through the cracks in the rocks. These ancient magma-filled cracks, common in the park are called dykes and sills. They can be seen as lines of dolerite rock within the sandstone. When the magma was forced through cracks to the surface, the vertical sheet is called a dyke. Sometimes magma is injected in a horizontal sheet, especially where there are flat-lying beds of rock. The horizontal sheet of dolerite is called a sill.
An example of a dyke can be seen at Rooidraai, where you can see the dolerite dyke and the effect of the heat on the surrounding rock.
And those black marks? All around the park you can see black stripes on the rock, even long after it has rained. These are areas where water seeps out of the rocks. Minerals from the top basalt layer (manganese dioxide) are carried in the water and these stain the rocks black. The water enables organisms like algae and moss to live on the rock as well.