  We report on a cellular pattern which spontaneously forms at the surface of a
thin layer of a cohesive granular material submitted to in-plane stretching. We
present a simple model in which the mechanism responsible of the instability is
the ``strain softening'' exhibited by humid granular materials above a typical
strain. Our analysis indicates that such type of instability should be observed
in any system presenting a negative stress sensitivity to strain perturbations.
