Bloemfontein is situated on dry grassland at an altitude of 1,395
metres above sea level. The area is generally flat with occasional
hills (koppies in Afrikaans) and the general vegetation is Highveld
grassland.
Most of the Free State Province is underlain by sedimentary
rocks belonging to the Beaufort and Ecca Groups of the Karoo
Supergroup. These constitute sandstone, shale and mudstone, which
have been intruded by dolerite sills and dykes. They are part of
the vast Karoo basin that covers almost two-thirds of South Africa,
and were deposited between 200 and 300 million years ago.
All rocks are divided into three types being igneous,
sedimentary and metamorphic. In this cutting at the listed
coordinates we see Karoo dolerite, which is an igneous rock derived
from deep within the Earth’s crust or upper mantle, where it
rose as molten magma. Molten magma either reaches the surface as
volcanic eruptions, producing extrusive lava which cools very
quickly or, alternatively, settles at a level just below the
surface. If the molten magma reaches a level close to the surface,
it cools relatively quickly, particularly if it intrudes as a thin
body, resulting in an intrusive rock nearly as fine-grained as
surface lava. If, on the other hand a large volume of magma
intrudes at depth where it is insulated from the atmospheric chill,
it has time to grow easily discernible crystals of the rock-forming
minerals, and becomes coarser grained. The most conspicuous
examples of fine-grained intrusive rocks in South Africa are
vertical – or nearly vertical – dykes and horizontal
sills of Karoo dolerite.
In geological timescale this Karoo dolerite intruded
approximately 183 million years ago during the Mesozoic era.
Bloemfontein is situated on one of the biggest dolerite sheets in
these parts and the countryside shows it. The grassland is
punctuated by scrub-covered low hills and ridges that show where
the dolerite has cut through the sediments. This cutting has
exposed the dolerite that is illustrated by blocky jointing.
To log this earthcache
Answer the following questions and email them to me.
Any logs not accompanied by an email will be deleted.
1) Estimate the length of the exposed dolerite in this
cutting.
2) Examine this rock up close and describe from the
“crystal” or grain size whether this rock cooled
quickly or more slowly to produce larger grains.
3) What name appears on the cross nearby to the listed
coordinates?
Acknowledgements and recognitions:
Wikipedia
Geoscience
Geological Journeys – Nick Norman and Gavin
Whitfield