Irregular blocks of cohesive clay (IBC) are transformed into spherical clay balls (SCB) during transport across a sedimentary basin bed by runoff. This transformation is controlled by rotation of the IBC about a randomly changing axis. During this process, deformation dominates; abrasion is less significant. Based on a theoretical model, the equation describing SCB formation, was derived in terms of the SCB instantaneous sphericity, Y, the IBC initial sphericity, Y₀, the transport length, l, and the effective diameter, d_ef. The equation was validated with the new data set obtained from IBC/SCB distribution along the transport path on the bed of an artificial basin (sandpit quarry). The coefficient k was found to be ... for the shaping of two kinds of clays, respectively. Based on the proposed model, the length of transport can be estimated if sphericity and effective diameter are known. Despite the verification in the artificial setting, the presented model is valid also for the natural environment of various clay rich cliffs or scarps.