Geology of Thabaphaswa and Mehlotlo (Blinkkop / Wellington Dome) climbing areas.
Dr Roger Diamond (Geologist, Lecturer at University of Pretoria and Rock Climber. )
The geology of the Mokopane area is extremely complex by world standards. This is mainly because the rocks are very old, which means they have undergone many events and have younger rocks intruded into, or deposited onto them. When travelling north on the N1, one goes from relatively young Karoo age rocks of 200 Ma at Mookgophong, through the Bushveld Igneous Complex rocks at Mokopane coming in at around 2000 Ma, to the rocks at Thabaphaswa which knock around the 3000 Ma mark. This is old. The earth is only 4560 Ma old, so these are more than half the age of the earth. Given that most old rocks get destroyed or covered by younger ones, there is a mere few percent of the earth that has rocks this old at surface, where we can see them.
The full version of the geology of the Mokopane region would take a short book to describe, but if we focus just on the hills that we hike and rock climb at Thabaphaswa and at Mehlotlo (Wellington Dome), the story is fortunately much simpler. These are nearly the oldest rocks in the region, and geologists use the term basement for the lowermost rocks of an area, upon which everything else has been deposited or intruded into. These hills are made of a variety of granitic rocks, which means they are broadly-speaking granites, but often are slight variations away from true granite, and include rock types like monzonite, syenite, gneiss and pegmatite. These terms are used because the granite-like rocks have slightly different chemistry, minerals or textures.
The granites have been grouped into various types based on these and other geological differences, and have the names Turfloop Granite, Lunsklip Granite and Uitloop Granite, and are grouped into the Randian era. The first of these is what makes up Blinkkop (Wellington Dome) and for the geologists in the audience, is described as a “fine to medium grained grey and pink biotite granite with relicts of migmatite and banded biotite granite-gneiss, sometimes porphyritic and pegmatitic”. The second of these is what is climbed at Thabaphaswa and has the somewhat easier description of “medium to coarse grained pink granite” (Nylstroom 1:250 000 geological map, Geological Survey, 1978).
The minerals found in these rocks include:
> quartz (greyish, translucent with poor shape)
> plagioclase feldspar (whitish, well shaped)
> orthoclase feldspar (yellowish, pinkish, orangish, larger crystals)
> biotite mica (black, small flakes)
> muscovite mica (silvery, flaky)
> amphiboles (black, hard, small crystals).
From a climbing perspective, these minerals form good friction overall and the large crystals sometimes make for good edges. As the minerals weather (breakdown) at different rates, the rock can be crumbly. There are also some very coarse grained sections on Mehlotlo (Wellington Dome), known as pegmatite – the crystals in these pegmatites are fist sized and bigger and reveal the internal crystal structure of the minerals – a good thing to take one’s mind off the shaking leg and sweaty palm as one leads out onto the face.
So next time you climb in the Thabaphaswa area, be aware that you are on some of the oldest crags in the world, from a geological perspective. When these rocks were crystallizing, multicellular life was not even a thought (never mind dinosaurs), oxygen was still a trace gas in the atmosphere and a year was much shorter than it is now. All good things to consider when kicking back after a solid day of climbing, watching the hills go dark and stars come out, and finding your place in the universe.