The listed waypoint will take you to a road cutting on the R101
between Pretoria and Polokwane. The rocks that have been pervaded
are lavas of the Rooiberg group. A great thickness of sedimentary
rocks of the Transvaal Supergroup had accumulated on the Kaapvaal
Continent. On this sediment there erupted a succession of lavas,
known as the Rooiberg group. These lavas are of two types. The
first of these were mainly basalt, followed by eruptions
predominantly of rhyolites, which are more extensive and thicker
than the basalt. On weathering, the lavas produce a rock with a
very red surface coloration - hence the name of the hills and
village, Rooiberg, near Bela-Bela/Warmbaths, after which these
rocks were named.
This site will show you how manganese and iron can invade rocks,
giving enough of the relevant minerals, time and the right
conditions. This exposure is particularly interesting and
very beautiful. The manganese minerals (a rare ore mineral) are
generally black, the iron hydroxides red or ochry and rust-colored.
What happens is that ground water, somewhat acidified by
carbon dioxide in the air, dissolves iron and manganese from the
rocks it passes through and in which these metals occur as
extremely lean scatterings or disseminations. Once in
solution, the iron and manganese are moved into areas of lower
pressure, in this instance into every crack in the rock that is
formed by fault- or bedding- and joint-planes. Slowly the fluids
lose their dissolving capabilities and evaporate, leaving the new
minerals, iron and manganese hydroxides, high and dry. And there
they stay, now in equilibrium with their surroundings.
The included pictures (Photos taken on site) amply illustrate the
beautiful effects that result from the passage through rocks of
solutions of manganese (Bottom Photo) and iron (Top Photo).
Acknowledgements:
The Story of Earth and Life (A Southern African perspective on a
4.6-Billion-year journey) Terence McCarthy and Bruce Rubidge.
Geological Journeys (A Travelers Guide to South Africa’s
rocks and land forms) Nick Norman and Gavin Whitfield.
To log this EarthCache e-mail the cache owner the
answers to the following questions:
- Which colors can you identify “painted” in these
rocks at the listed waypoint?
- Estimate the distance of length and height of this road
cutting?
- What is the original color of manganese?
- As a optional request feel free to add photos of some rock art
in this road cutting that you thought were the most
beautiful.