Hard evidence that heuweltjie earth mounds are relictual features produced by differential erosion

Regularly spaced (over-dispersed, i.e. non-random spacing) non-anthropogenic earth mounds are a surprisingly common occurrence around the world. Their genesis is generally thought to be the consequence of faunal (e.g. fossorial rodents and termites) activities that result in soil translocation towards the mounds. Large (> 20 m diameter) earth mounds in South Africa, called heuweltjies, are commonly attributed to the activities of the termite Microhodotermes viator. In contrast, it was hypothesised that heuweltjies are the remains of an ancient land surface in which the mound soil volume was protected from erosion by roots of regularly spaced patches of woody shrubs. The heuweltjie soil volumes are at least an order of magnitude larger than the largest verifiable termite nests of the Southern African sub-continent. Large (> 2 kg) rocks, that could not have been transported onto the mounds by termites or relatively small rodents, are a common feature both on the surface and distributed through the soil volume of the heuweltjies. Furthermore, the mounds sometimes occur on bedrock, conflicting with the notion that termites displaced soil upwards through the soil profile. Soil particle size analysis indicated no strong changes in sand, silt and clay through the profile of the heuweltjie and soil clay was similar between the heuweltjie and surrounding soils. The mounds also showed evidence of down-slope slumping caused by erosion.In areas where water is a constraining resource, vegetation often exhibits striking patterning. Spatial analysis of patches of woody vegetation (‘bush-clumps’) in the region and further afield demonstrated similar densities, sizes and non-random over dispersions as for heuweltjies. It is suggested here that the heuweltjies originated as bush-clumps that protected soil from erosion and resulted in the formation of hardpans (calcrete) due to increased evapotranspiration and reduced leaching of mound soil. During their development these relictual features of the landscape were likely to become ‘islands of fertility’ and, consequently, hotspots for vegetation and secondary faunal activity.

► Rocks occur in non-randomly distributed earth mounds (heuweltjies) in South Africa.
► Soil particle sizes are homogenous in these mounds, under which there is bedrock.
► Mound sizes/shapes are also not consistent with construction by extant termites.
► The non-random distribution of mounds is consistent with that of bush-clumps.
► Differential erosion of soil around bush-clumps may explain mound genesis.