The Pleistocene deposits of the Lower Thames Valley have been studied in detail from exposures and borehole sections, the location of which are shown in Fig. 1. The deposits have been subdivided following the International Stratigraphic Guide (Hedberg, 1976) in which aggradations are defined and delimited as far as possible as formal units from stratotype localities. These units are considered to be of member status because this is the most appropriate hierarchical level and is compatible with neighbouring areas, particularly the Middle Thames Valley (Gibbard, 1985, 1988a, 1989). This is contrary to the proposal by Bridgland (1988a, b) that aggradations should be assigned formation rather than member status, because the former is the ‘primary unit (in lithostratigraphy)’. Whether this argument is theoretically correct or not is not in dispute. The real problem stems from the concepts of compatibility and practicality (Gibbard, 1988a). The present author considers the formation to be a large-scale unit, as applied in neighbouring areas (e.g. Kesgrave Formation, Lowestoft Formation, North Sea Drift Formation, Red Crag Formation), and therefore in this text the term formation is retained to refer to a collection of formally defined members with broadly unified characteristics, as in earlier publications.

A similar argument applies to the next unit in the lithostratigraphical hierarchy, the Group. This term is considered too large scale for use in Pleistocene stratigraphy, by comparison with Group status units in other parts of the geological column. No units of this rank will be used or defined here.