Ice Sand Paper EarthCache
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This cache will show you a glacial striations at the University of
Kwazulu Natal.
The Dwyka Formation, which is 250 – 400 m thick in the Durban area,
overlies the sandstones of the Natal Group unconformably. In a
number of places the surface of the sandstone exhibits glacial
striations as a result of the overriding movements of the
continental Permo-Carboniferous ice sheet which, on melting under
shallow marine ice-shelf conditions, gave rise to what is now the
tillite rock. Silicified varved shale and thin sandstone lenses
occur at places within the main body of the tillite. The Dwyka
tillite is famous as representing part of the original evidence in
favour of the former existence of the southern supercontinent,
Gondwanaland. Similar tillite rocks of approximately the same age
occur in all southern continents, as well as in India. In the
Durban area, the parent ice sheet moved from the northeast.
Evidence of multiple advances of the ice sheet include folding of
shales due to overriding of the ice, and striations on the tillite
near the base of the formation. The Dwyka tillite outcrops in a
number of places in the western parts of Durban.
The unweathered tillite, as exposed in quarry faces, is an
unstratified dark bluish-grey mudstone containing abundant
inclusions of older rock types, the whole being so hard as to
fracture through matrix and inclusions alike. Typical strengths of
such rock types range from 120 to 250 MPa. The matrix is fine
grained and consists of small angular fragments of quartz and rock
embedded in a fine base of rock flour. The glacial inclusions vary
quite markedly in size, from millimetre or so to erratics several
meters in diameter.
The tillite is very well jointed. Preferential weathering along
the joints gives rise to core-stones which crop out as bouldery
masses on hillsides and, on a smaller scale, to spheroidally
weathered material.
Fault zones in the tillites ordinarily contain extensively altered
and kaolinised rock with secondary quartz veins and siliceous
breccia. Single, relatively large displacement faults are uncommon:
most displacement occurred along a number of parallel, small fault
planes.
As it weathers, the tillite passes through various stages: the
first weathering stage comprises a dark, olive-brown hard rock
material known locally as ‘first brown’. In the next stage the
material becomes softer and more yellowish-brown in colour, being
known in this condition as ‘second brown’. The very soft completely
weathered tillite becomes clayey, having a characteristic
yellow-brown colour. Soils developed on the tillite are ordinarily
thin (< 1 m) and comprise ‘hard setting’ greyish-brown silty and
clay sands, with clayey gleyed subsoils. In valley bottom situation
more hydromorphic conditions prevail.
Courtesy
RR Maud Engineering Geology of Durban
Contributions of Eng Geology South African Institute of
Engineering Geology Volume 1
To claim the cache the following must be done:
1. Load a picture
of you with your GPS at the sign board, with your log.
2. Identify two indiginous plants in the bricked up area.
3. Describe how the striations occured.
4. As per the sign at the site, when did the ice sheet begin
to move? (Read the sign very carefully)
Please email me the answers for the above.
Additional Hints
(Decrypt)
Bapr vafvqr gur HQJ tebhaqf ybbx sbe gur ragenapr gb gur cnexvat ng: Gheabss F 29 49.137 R 30 56.602