Dynamics of poly(ethylene oxide)-polystyrene (PEO-PS) brushes with shorter anchoring PEO blocks adsorbed on a flat surface are considered theoretically. Three relaxation modes are identified: a fast cooperative (hydrodynamic) mode, a slower drag mode, and the slowest anchor-sliding mode. it is shown that the anchor mode is likely to be dominant and that it is strongly coupled to a two;dimensional microdomain structure, which is presumably formed by PEG. A scaling picture of pattern formation on the adsorbing surface is developed. The theory yields an interpretation of existing evanescent-wave dynamic light-scattering data. The PEO pattern gives rise to high scattering intensity and slow dynamics at high wavenumbers.