The break between the flats of Mokopane and the hilly country north
of it is caused by the Zebediela Fault, (the sandstones here at the
site is of the Transvaal Supergroup) which has dropped the whole of
the Karoo Basin, to the south , against older rocks to the north,
so that the basalts abut against folded and upturned Precambrian
formations. The low, hummocky hills just east of this site are made
of the same rock as the low hills on the flats north of Pretoria
(about 150km away).
The scenery here at this fault you are looking at, the ridges built
of the same rock formation as that in Pretoria. However, because
you are going (if travel north on the N1) from inside the
saucer-shaped basin of the Bushveld Complex (see diagram below)
into the Transvaal Supergroup floor rocks (ripple sandstone are
one) on the outside, you cross the geology in the reverse order
now, from younger to older.
After you pass the grey rocks of the Zebediela Fault you get to
this cutting that has quite a different look about it. These are
the sandstones of the Black Reef Formation at the base of the
Transvaal Supergroup and they show extensive bedding surfaces of
ancient ripple marks, the surfaces shaped by material wafting in
the gentlest of currents over 2.5 billion years ago. A few km to
the north as you go up the hill is the Ysterberg Fault. This tilt
happened between the Ysterberg Fault north and higher and the
Zebediela Fault south and lower.
In the diagram below of the Bushveld
Complex, the site is just north of Potgietersrus, new name
Mokopane.
In geology, ripple marks are sedimentary structures (i.e.
bedforms of the lower flow regime) and indicate agitation by water
(current or waves) or wind. The gentlest of currents flow here over
2.5 billion of years ago. (Geological Journeys)
Current ripple marks, unidirectional ripples, or asymmetrical
ripple marks are asymmetrical in profile, with a gentle up-current
slope and a steeper down-current slope. The down-current slope is
the angle of repose, which depends on the shape of the sediment.
These commonly form in fluvial and aeolian depositional
environments, and are a signifier of the lower part of the Lower
Flow Regime.
Definitions
Crest and trough
Crest
The point on a wave with the maximum value or height. It is the
location at the peak of the wave cycle as shown in picture to the
left.
Trough
The opposite of a crest, so the minimum value or height in a wave.
It is the location at the very lowest point of a wave cycle also
shown in picture to left.
Lee
The lee side has a steeper slope than the stoss. The lee is always
on the back side of the ripple, which is also on the opposite side
of where the current flow meets the ripple. The current flows down
the lee side.
Stoss
The stoss is the side of a wave or ripple that has a gentle slope
versus a steeper slope. Current always flows up the stoss side and
down the lee side. This can be used to determine current flow
during the time of ripple formation.
Must involve visitors undertaking some educational task that
relates to the Earth science at the site. This could involve
measuring or estimating the size of some feature or aspect of
the
site, collecting and recording data (such as time of a tidal
bore),
or sending an e-mail to the cache owner with the answer to
Earth
science related questions they obtained by reading an
information
display. While photographs may be requested, they do not take
the
place of other logging requirements. Taking a photograph alone
or
asking people to do internet research does NOT meet these
logging
guidelines. Requests for specific content in the photograph
(must
include the visitor's face, for example) will be considered
an
additional logging requirement and must be optional. Cache
owners
may not delete the cache seeker's log based solely on
optional
tasks.
To Log your earthcache find, please do the
following (answers by email please, not in your log): You can go
ahead and log your find then email the answers to me within 7 days,
if not your log will be deleted.
Click on hennieventer top of the page
and
Send Message
Questions:
1. How does the rock feel when you slip your fingers over
it?
2.How does the Sandstone look texture and coulor comparing to
the other rocks in this area?
3. Examine the ripples on the sandstone rocks at the site. In
which direction did the current flow? (Lee and Stross. This can be
used to determine current flow during the time of ripple
formation.)
4.Looking at the Ripple formation at coordinates, How high
(Crest) and low (Trough) is the average ripple?
5.Opsional: Take a photo with your GPS at Coordinants. Place
photo. with
your log.
I have used sources available to me by visiting the site, the
Polokwane library, internet, research, and asking questions to
get
information for this earth cache.
Acknowledgements:
Geological Journeys (A Travelers Guide to South Africa’s
rocks and land forms) Nick Norman and Gavin Whitfield.
Wikipedia
I am not a geologist. Since Earth caching is educational I
also
leant more through the research. Hope you will enjoy this
Earthcache.
