  We show the relevance of the dispersive analogue of the shock waves in the
FPU dynamics. In particular we give strict numerical evidences that metastable
states emerging from low frequency initial excitations, are indeed constituted
by dispersive shock waves travelling through the chain. Relevant
characteristics of the metastable states, such as their frequency extension and
their time scale of formation, are correctly obtained within this framework,
using the underlying continuum model, the KdV equation.
