  The peak effect has been investigated in clean Nb crystals with artificially
corrugated surfaces by measuring the linear surface impedance in the 1kHz-1MHz
frequency range. From a two-mode analysis of the complex spectra, we establish
that vortex dynamics is governed by surface pinning and deduce the associated
vortex slippage length. We demonstrate experimentally and theoretically that
the peak effect is related to a transition from collective to individual
surface pinning. A proper account of the peak-effect anomalies implies
softening of the shear rigidity by disorder-induced lattice deformations. This
leads to a vortex crystal-glass transition induced by surface defects.
