Palmiet Nature Reserve Geology EarthCache

The cliffs and gorge walls of the Palmiet Nature Reserve is comprised of Natal Group sandstone, formed about 600 million years ago. At that time, the Palmiet area must have resembled a cold and arid desert, lacking plants (other than algae) and animals. There were no soils and vegetation to retain water.

According to Prof. Dunlevey (2006), the area "must have resembled a long, flat-bottomed, sand-filled valley stretching from a major lake or inland sea near Port Shepstone, north-eastwards to a mountainous area in what is now southern Swaziland. Along the western edge of this valley (by Pietermaritzburg) barren rolling hills rose and stretched away to the far distance. On the eastern margin, which lay out past the present-day coast, similar hills stretched several hundreds of kilometres to the area that is now the Antarctic. A large river drained the highlands to the north east and smaller tributaries flowed in from both margins. However, as there was no soil or vegetation to form wetlands and retain water, the flow rate and river level was highly variable. When rains fell in the highland or hilly margins there was rapid runoff causing flash-floods to sweep down the valley, scouring out new river channels during peak discharge and depositing sheets of sandy alluvium after the flood."

He added that "these layers of sand, now converted to sandstone, preserve the ripple marks and cross bedding caused by the flowing water during their deposition. During major floods the mass of water also caused movement in the layers of sand in the river bed, distorting the fine structures and sometimes folding and deforming the bedding."

He explains that "the sequence of events during each flood followed a very similar pattern. First the water would sweep down the almost dry, or slowly flowing, river bed washing away obstacles and often cutting new channels. As the power of the flood waters waned, first the coarsest sediment (pebbles and coarse sand), then progressively finer and finer material was deposited, so that in most sandstone beds the grain size systematically decreases from bottom to top. Although most of the material carried by the rivers was sand and gravel, after the floods some pools of muddy water remained. The mud settled in these ponds to form lens-shaped bodies of mudstone and siltstone while some ponds evaporated completely producing mudflakes, just like those formed when modern puddles dry out. In most cases the next flood eroded and destroyed the mud layers before depositing another bed of sand, but occasionally a mudstone layer was preserved, or the mudflakes survived long enough to be incorporated into sandstone beds. Often river channels filled with sand during repeated minor floods, so that during the stronger floods the river would overflow and cut new channels through the layers of sand."

Prof Dunlevey (2006) added that "this environment existed for many millions of years, with movement along fault lines at each side of the valley allowing the base of the valley to sink so that the several hundreds of metres of sand and pebbles accumulated, and were converted into rock by burial. However, during this time continental drift was slowly moving the supercontintent Gondwanaland towards the South Pole. The climatic change generated glaciers in the highlands, which steadily grew until about 350 million years ago Arctic conditions prevailed. During the cooling and initial glaciation much of the Natal Group was eroded as the debris-rich ice sheets scoured the country side. This erosive phase formed glacial pavements, such as the one in the University of KwaZulu-Natal, Westville Campus (GC1XWW1), and the scratch marks made by boulders frozen in the base of the glacier clearly indicate the direction of ice movement."

As a consequence of the continental drift, Prof. Dunlevey (2006) says that the area was finally "carried past the pole and into slightly warmer climates where the eroded debris could accumulate at the base of the glacier. In some parts of KwaZulu-Natal there is evidence that this glacial debris, termed the Dwyka Group, was reworked by flowing water, but in the Palmiet Nature Reserve area the mass of blue-grey rock containing many exotic pebbles and boulders is unsorted."

He concludes that "the geological history of the Palmiet Nature Reserve during the last 10 million years is one of erosion and the development of the various African Land Surfaces. However, in the most recent 50 000 years there have been dramatic changes in sea level related to the Ice Age. The massive sea level drop to about 120 m below the present level approximately 20 000 years ago caused the rivers meandering across the flat land surface to cut down in to the plain. The sudden increase in gradient, due to the change in sea level, combined with higher than present rainfall caused the rivers to cut down into their existing channels creating a pattern of incised meanders, and not the straighter lines associated with young or high gradient drainage patterns."

Reference:
Dunlevey, J, 2007. Palmiet Nature Reserve Geology. Accessed from http://www.palmiet.za.net/geology.htm. Last accessed 15 July 2011.
Whitmore, G. Meth, D. and Uken, R. 200. Geology of KwaZulu-Natal. Accessed from http://www.geology.ukzn.ac.za/GEM/kzngeol/kzngeol.htm. Last accessed 15 July 2011.

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QUESTIONS:
1) Describe the river in relation to the valley walls observable.
2) Describe the rocks that make up the valley walls.
3) How many layers of rock can you see at GZ?
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