In unconfined parts of the Chalk aquifer in southeast England, permeability generally varies laterally with the lowest permeabilities occurring beneath interfluve areas, and the highest beneath river valleys and dry valleys. Furthermore, the Chalk in the river valleys is normally in excellent hydraulic continuity with the overlying highly permeable Quaternary gravels. However, recent field investigations in the Thames Valley have demonstrated the existence of zones of anomalously low Chalk permeability associated with the development of thin discontinuous confining layers of low permeability ‘putty chalk’ at the gravel-chalk interface. Hitherto putty chalk in the Middle Thames Valley has mostly been reported from interfluve areas where it can occur as a periglacially frost-weathered mantle on the upper surface of the Chalk. The true extent and hydraulic significance of putty chalk in valley bottom positions is only now being realised. Existing models for the lateral variation in Chalk permeability cannot explain these new observations. A new model is therefore proposed in which it is envisaged that, during the Devensian, carbonate dissolution in perennial taliks (unfrozen zones) beneath the major channels of the braided palaeo-Thames caused the high-permeability zones, while permafrost beneath the interfluves restricted dissolution at those sites. Freeze-thaw action in seasonal taliks beneath minor channels would account for the formation of putty chalk at the gravel-chalk interface, and the persistence of permafrost beneath these seasonal taliks would lead to a restriction of dissolution, and thus to a zone of low permeability.