  We investigate the depinning transition in a dirty periodic medium
considering a model of layered charge density waves as a prototype system. We
find that depinning from strong disorder occurs via a two stage process, where,
first, the pinned system experiences a continuous transition into a plastically
sliding state and undergoes a second sharp hysteretic transition into a
coherently moving 3D state at higher drives. In the weakly disordered system
the depinning into a coherently sliding state remains continuous.
