ST PAULUS GEOLOGICAL HISTORY
Within Pretoria there are three parallel, east-to-west ridge formations. One of these, the Daspoort ridge formation, runs from the Daspoort Tunnel and through the St Paulus grounds. It continues for another 240 km into Mpumalanga province and ends at Waterval-Boven. St Paulus School is built on the northern slope of a koppie composed of Daspoort quartzite. How did this kind of rock get here? Let’s find out!
Many thousands of years ago, a big river system ran from mountains through here into the sea (Fig 1). Granite Mountains weathered down to form blocks of rock called boulders that were washed down rivers. As these boulders moved over long distances, they would roll and grind against each other and became smaller and smoother like pebbles. Eventually these pebbles became sand grains. Closer to the river mouth, the sand started to sink to the river bed to form sand beds. Shallower muds (clay, mineral and fine soil mixture) and iron-rich silts (very fine, slimy soil) would also sink to the bottom on top of the sand beds. As the sand became deeply buried, it hardened into sandstone. Later the sandstone was buried so deeply that the sand grains cemented together under heat and pressure to form a very hard, glassy rock called quartzite.
Figure 1: Formation of quartzite beds in ancient rivers
Various types of rock layers were buried at great depth (about 10 to 30 km below surface) where processes and pressures deep in the earth’s crust started to bend and deform the layers by folding them (Fig 2). The rock layers started to break forming cracks. These cracks or fracture are called joints. They opened up inside the rock and were usually filled by silica fluids called quartz. These fluids crystallized in the joints to form quartz veins. When the rock beds could no longer bend, they snapped along the joints and moved over or up or down past each other. This process is called faulting.
Figure 2: Rock layers and beds being deformed by folding.
When the faulting happened, fiery molten rock called igneous rock was forced and squeezed in between the different rock layers in the earth’s crust. The molten rock cooled down and formed diorite sills, also called diabase sills. These sills were cutting across the rock layers and made it look like the filling of a sandwich. At that point, we had folded deformed layers of sedimentary rock consisting of quartzites, siltstones, mudstones, ironstones, shales, dolomites and diorites. These mixed-up layers were again pushed upwards towards the surface, and tilted so that the rock beds here at St Paulus dip towards the north. The beds continued to be fractured causing the rocks to crumble and melt into breccia (jigsaw-like fragments of broken-up rock) alongside the fault. Mineral-rich igneous fluids filled the fault zone and fault breccias. The breccias, diorites, siltstones, mudstones and shales are softer rock and erode more easily than quartzites. This is why we see the parallel ridges, hills and valleys in the Pretoria and Tshwane area (Fig 3).
Figure 3: Several fault zones run through the area. The alignment of the various rock layers is disturbed because of faulting.
Figure 3 also shows shearing zones. Shearing is another form of rock-layer movement, where rock plates slide past each other, also causing the formation of breccias. In the St Paulus area, the shearing happened long before the faulting. Whenever shearing or faulting takes place, an earthquake will follow. Two shearing zones and one fault zone is running through the St Paulus school grounds. Fortunately, these zones are stable and no earthquakes should occur for many hundred years to come! Fault and shearing zones are rich in minerals and trees like to grow there. It is at these areas that we also find the commercial mines: let’s start digging for gold!
The geological trail reveals what the rocks tell us about long ago events that created the landscape and grounds of the School.
ST PAULUS GEOTRAIL
Along the trail there are 23 sites of geological interest. At each point there is extra information that explains the very interesting geological features and rock formations you are looking at. For example, at the north-western part of the trail you can read about the existence of a volcano in this area long before the visible sedimentary rock layers and soils were formed.
The geotrail was an initiative of the Grade 7 Environmental Committee of 2013.
It was developed with the assistance of an experienced geologist, Mr Tony Jamison.