The small stone content of mima-like mounds in the south african cape region: Implications for mound origin

Abstract

Mima-type earth mounds known locally as heuweltjies occur in many parts of the western and southern Cape Province of South Africa. The active hypotheses of their origin are biological, and involve soil translocation by (i) termites, (ii) bathyergid mole rats, or (iii) both of these animal groups. We sampled the small stone composition of mound and intermound soils at two locations, Robertson and Clanwilliam, to determine if stones in the size range that mole rats can move are abundant in the mounds. Gravel (8–15 mm) and pebbles (15–50 mm) were common in the surface soil of mounds, but were only about one-fourth to one-seventh their concentration in intermound soils. The concentration of small stones increased with depth in the mounds, however, and in the basal strata of mounds these stones became incorporated in calcrete nodules or layers. The total abundance of small stones in a column from the surface to the base of the soil system was greater on mound tops than in intermounds, as expected if mole rats contribute substantially to mound formation by lateral translocation of soil. Increase in small stone density with depth is interpreted as resulting from the movement of soil fines to the surface by termites. Additional data on small stone concentration in deep mound layers are needed to determine if mole rat translocation can fully account for mound formation.