The Influence of dust deposition, carbonates and erosion on the formation of Clanwilliam heuweltjies
Heuweltjies (Afrikaans for "little hills") are non-anthropogenic, regularly dispersed earth mounds up to 32 meters in diameter and approximately 1.4 meters in height, that dot about 25% of the land surface of south-western southern Africa. The zoogenic "termite" hypothesis has been widely accepted as a hypothesis of heuweltjie origin. However, the recent "vegetation-patch-erosion" (VPE) hypothesis suggests an equally likely explanation for heuweltjie formation. The aim of this investigation was firstly to determine the influence of dust deposition and carbonates on heuweltjie formation in order to gain insight into the formative process of heuweltjies. The second part of this investigation sought to further test the VPE hypothesis by examining whether erosion alone could give rise to heuweltjies within a reasonable time frame. Soil surveys were conducted on 8 heuweltjies and at 2 inter-heuweltjie areas at depths of up to 0.9m. Textural analysis of soil samples collected from the 8 heuweltjies as well as an additional 11 heuweltjies was conducted. Bulk density was also evaluated for each horizon of each heuweltjie and inter-heuweltjie. Carbonates in all soil samples were determined by acid digestion of soils. A digital elevation model of the Clanwilliam area was constructed and used to obtain heuweltjie area and elevation. There was no difference in average soil texture moving from the centre of a heuweltjie to its periphery (indicating that they formed by layered stratification rather than radially from the centre). There was an apparent difference in texture and carbonates between heuweltjie and inter-heuweltjie zones. Mass of fines (silt and clay) and carbonates were both positively correlated with heuweltjie mass, but were also collinear. Non-parametric regression of heuweltjie mass against both fines and carbonates revealed a clear linear trend. However, the trend between fines and carbonates suggested that fines played a central role in both heuweltjie and carbonate formation and that carbonate took on only a secondary role in heuweltjie formation. Erosion alone would have taken 11000 years to form the heuweltjies. However, some calcretes within heuweltjies have been found to be older than 11000 years, suggesting that erosion has not had a major influence on heuweltjie formation. These findings support the vegetation-patch-erosion hypothesis. However they support dust deposition rather than erosion as a primary contributor to heuweltjie formation.
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