Interpreting alluvial archives: sedimentological factors in the British Holocene fluvial record

A Holocene alluvial archive of 506 dated units for Great Britain is analysed in terms of the sedimentation styles involved. The database is classified by sedimentation unit (channel sediments, palaeochannel fills, floodplain surface sediments, floodbasins and colluvial deposits) and alluvial ensemble (fans and cones, upland gullies and streams, braided systems and active/inactive meandering and anastomosing systems). Floodplain, palaeochannel and floodbasin sediments dominate the record, mostly from meandering/anastomosing systems. Different sediment units show varied potential for recording environmental change: some are relatively poorly dated with respect to the volumes of material involved, whilst many dates ‘float’ in periods of depositional activity which lasted for extended time periods. Much dated activity is autogenic and of relatively little value in interpreting external environmental influence. Ensembles and units are similar in age patterns in some respects (commonly they exhibit sets of short-term peaks in sedimentation activity) but differ in others. Floodbasins and, to an extent, braided systems feature in the earlier Holocene, whilst dated fan deposition is more evident in the late Holocene. There is a general bias towards sediment units of late Holocene age, which probably reflects preservation factors. Previous British Holocene alluviation models are reviewed: it is suggested that the majority of earlier models reflect partial views imposed by limited regional coverage and available dating.