The formation of spherical clay balls on the slopes of sandpit quarry, the Rudice-Seč (Czech Republic)

Clays on the steep slopes of a small artificial sedimentary basin (sandpit quarry) alternately desiccate/crack and erode during hot dry and rainy seasons, respectively. Small irregular blocks of clay (IBC) predefined by cracks are released by gully erosion. The IBCs are quickly transported to the slope base and then rolled down the basin bottom by runoff. Due to rotation about a randomly changing axis and under an effect of deformation forces, the IBCs are transformed into spherical clay balls (SCB). The SCBs are transported, in general, towards the depositional base where they are buried by assorted sandy/clayey sediment. As soon as the kinetic energy of the runoff has decreased, the SCBs remain spread along transport path. From their distribution, the shaping process was evaluated. Based on data processing, the dependency,Ψ=1−(1−Ψ0)10−8.9×10−5λdef,relating the clay ball projection sphericity, ψ, to the effective diameter, def, and to the distance from the steep slope base, λ, was found. Based on this equation, the length of clay ball transport can be estimated if the projection sphericity and effective diameter are known.