  We present a theoretical and experimental study of the peak effect in the
surface pinning of vortices. It is associated with a sharp transition in the
vortex slippage length which we relate to a crossover from a weakly disordered
crystal to a surface glass state. Experiments are performed on ion-beam etched
Nb crystals. The slippage length is deduced from 1kHz-1MHz linear AC
penetration depth measurements.
