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What is the Cedarberg Formation?
The impressive mountains of the southwestern Cape comprise
one of the thickest expanses of sand in the world. But the geology of the Table
Mountain Sandstones is not quite that simple. The beds actually comprise two
huge sandstone beds:
- The Peninsula formation (1800 m thick) was laid down some 500 million years ago in a situation similar to the east USA coast where strong currents washed out finer material leaving behind only coarse sand.
- The Nardouw formation (1100 m thick) formed in the same environment as the Peninsula, and ended 220 million years ago when strong rivers formed deltas over the inland sea (forming the Bokkeveld formation).
Separating the two formations is a layer of shale, known as the Cedarberg formation (140 m thick). It formed when glaciers scoured the land masses surrounding the sea and formed an ice-sheet turning the area into a freshwater sea. Quite a few fossils may be found in this formation (trilobites, crinoids and brachiopods).
Anyone who has looked at the mountains in the region should have noticed the Cedarberg formation. For instance Table Mountain at Cape Town comprises only the Peninsula formation. The flat top is where the Cedarberg formation used to occur - it and the Nardouw formation has long since been eroded from Table Mountain. The Cedarberg Formation is also very obvious in the Cedarberg, Langeberg and Swartberg Mountain ranges, where is takes one of two forms depending on whether the rock strata are horizontal or vertical. It is most easily recognized in situ by having clay soils. But even from afar, it can be seen as a relatively narrow band of soft rock, with gentle curves, forming platforms or valleys within the sandstone ramparts.
The significance of the Cedarberg formation is that it is home to a whole suite of plant (and protea) species. Some are entirely confined to the shale band, others also occur on the sandstone below the shaleband, where sufficient clay occurs in the soils. Protea cordata is only one example. The problem is that we do not really know (although we have a rough idea) which species are restricted or confined to the Cedarberg shale band. However, from the data collected by atlassers such patterns of edaphic (soil-geology) dependence will become clear. So watch out for the Cedarberg formation - you should notice that different species from those on the neighbouring sandstone-derived soils occur on it. Note that its different soil, slope, geology and landform codes will require you to fill out a separate record locality!
The Cedarberg formation is quite unique: unlike the shales of the Karoo and Bokkeveld formations, it usually lies at high altitudes and receives a high rainfall. Is it possible that in the past, under much higher rainfall conditions than at present, the species now confined to the Cedarberg formation once extended across the shales of the Little and Great Karoo and the Great Escarpment?
Tony Rebelo, Bellville
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