The results of a linear and a non-linear stability analysis are presented for the system composed of a cohesive, frictional overburden resting on a viscous substratum of lower density. Stability depends on the in situ stress and not solely on the density contrast, as for visco-elastic models. It is shown that an unstable system evolves towards a stable fold whose finite amplitude is a function of the tectonic force. However, if faulting occurs in the overburden during the fold development, the structure then collapses in a chevron mode.