Crater morphology is investigated after femtosecond laser ablation of wide bandgap insulators (BaF2 and CaF2). Experiments performed at a weak laser ablation rate show a particular elastic, sound wave-like structure, frozen on the surface, with a periodicity in the order of some microns and a modulation depth up to 0.5 mum. When superimposed, a periodic fine structure (ripples) is generated, the origin of which cannot be explained in the classical ripple model. We observe a periodicity ranging from 100 to 500 nm, dependent on the laser intensity rather than on the laser wavelength. Based on our recent results, demonstrating an explosive surface decomposition, we assume self-organizing relaxation of a non-equilibrium surface to be responsible for the structures. (C) 2002 Elsevier Science B.V. All rights reserved.