There are three geological elements on Oldenburg:

  • The river flats
  • The lower slopes of Rondekop hill
  • The upper parts of Rondekop hill

The river flats comprise humic sandy alluvium underlain by the river gravels of the Dwars River. These soils are low in trace elements, given their provenance, which is primarily the Table Mountain Sandstones (cemented quartz sands).

The lower slopes of Rondekop comprise eluvial "piedmont" material, i.e. gravity-moved material from upper Rondekop and the mountain above. Hence the soils that constitute Rondekop are primarily sandstone boulders with clasts of the volcanic-sedimentary rocks. A real melange is formed with ferruginous loamy soils and some ferruginous cementation of sandstone pebbles and boulders. This is the concretionarly rusty material one finds in the soils on the western flanks of the hill and across to the dam on Mount Joy (our neighbours), the so called "coffee-rock".  There is very little contribution from the rocks on the hill itself – it is mostly material derived from the upslope (Rainbow’s End, our other neighbours) being clays from the underlying granite and the sandstone material from the mountain above.

The upper parts of Rondekop hill is comprised of what used to be called the Franschhoek Beds. The rocks of the hill are volcanics and sediments which are very rich in iron, manganese and other metallic and non-metallic elements. This is in stark contrast to the granites or the sandstones, each of which contributes very little in the way of metallic elements to the soils derived from them. The reds, pinks, purples and blacks of the hill rocks are evidence of their iron and manganese content, which are reasonably exposed along the Rainbows End / Oldenburg boundary. Identical rocks are exposed in the big cutting halfway up the Hells Hoogte pass, and they contain iron pyrite(fool's gold, iron sulphide) and the oxidised equivalents thereof, generally rusty-colored rock.

Proprietor Adrian Vanderspuy’s father, Peter, is a well regarded geologist and studied at the University of Stellenbosch and the Colorado School of Mines. In his mind the soils of the hill itself, and some of the surrounding slopes below, would have been enriched in the metallic elements mentioned, plus various others, giving the soils a composition which he expects to be richer in metallic elements than normally found in the winelands of the Cape, almost all of which are either on granitic or sandstone terrain.

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