The paper examines the relations of the Chalky Boulder Clay Drift along the northern margin of the Vale of St. Albans and in the several Chiltern valleys entering it from the north-west. The diversion of the Mimram by a drift-plug in its former valley at Codicote was demonstrated by R. L. Sherlock during the re-survey of the Hertford area. This episode is shown to have been part of a more complicated series of temporary glacial diversions, associated with pro-glacial lakes in the Lea and Harpenden Valleys. The evidence indicates two separate periods of ice-advance probably associated with the two distinct sheets of Chalky Boulder Clay known in the Vale of St. Albans. The phenomena described are noteworthy in that they are associated with the ‘Older Drift’; most of the classic examples of pro-glacial drainage in Britain are of later date.