What is "heuweltjie"

Heuweltjies are a class of soil surface feature that occurs widely in the south-western Cape of South Africa. They may be recognised as large mounds above or near the surface of the landscape. Like other similar-looking phenomena, such as Mima mounds and fairy circles, from which they have been poorly distinguished, particularly in early literature, they have been the subject of a wide range of speculation and of debate that has not yet been settled conclusively.