Folds within layered rock systems are critical for comprehending the historical processes of deformation and the rheological behaviour of rocks. The current study employs finite element modelling to investigate the development of folds in a layered rock system, with a particular focus on the impact of thinner layers on the folding of adjacent thicker layers and their subsequent interactions. The analysis indicates that harmonic folds can evolve into polyharmonic or disharmonic configurations because of the intricate interactions occurring within the contact strain zone of the thinner layer. Our numerical findings demonstrate that the geometry of folds is significantly affected by the reciprocal interactions between thinner and thicker layers, initiated by the folding of the thinner units and their consequent influence on the thicker layers, and vice versa. This dynamic interplay, however, may frequently diverge from predictions made by more simplistic models, as suggested by earlier studies. Furthermore, this research highlights the potential of utilizing higher-order fold geometries to estimate the relative viscosity between the layers and the embedded medium.