A revised lithostratigraphy and palaeoenvironmental history of Anglian sediments in Hertfordshire and west Essex is based on field observations and laboratory analysis of samples from 79 sites within the Lea basin and adjacent areas. Analyses included particle size, acid solubility and small clast lithology of tills, pebble counts of gravels, small clast lithology of sands, till macrofabric and palaeocurrent determinations of sands and gravels. Multivariate analyses of the petrographic properties showed that the till samples could be divided into four main groups. Similarity networks, augmented by field relations, identified four till lithostratigraphic units, which merge north-eastwards. Four gravel lithostratigraphic units were also identified from multivariate analysis of petrographic data. The oldest unit, the Westmill Lower Gravel Member, was deposited by the proto-Thames, and can be followed from the middle Thames region into the mid-Essex depression. Eastward flow was disrupted by the advance from the north-east of ice depositing the first (Ware) till. This caused the formation of ice marginal lakes. A temporary southward spillway was initiated in the lower Lea valley from the Watton Road Laminated Silt lake, but permanent diversion of the Thames was effected by a spillway from the Moor Mill Laminated Clay lake, dammed by Ware Till ice near its southwesterly maximum in the Vale of St. Albans. Three readvances followed, depositing lodgement tills named the Stortford, Ugley and Westmill Till Members. Of these, the Stortford Till is the most extensive, reaching the southern margin of glaciation at Finchley and Hornchurch. Gravels deposited during or between these three advances are the Smug Oak Gravel in the Colne Basin, and the Westmill Upper Gravel in the Lea basin. The latter is divided into the chalk-poor Hoddesdon Gravel Bed and the chalky Ugley Gravel Bed.