Paraglacial adjustment of the fluvial system to Late Pleistocene deglaciation: the Milfield Basin, northern England

Abstract

AbstractLandform–sediment assemblages in the middle reaches of the River Till in the Milfield Basin, northern England, provide a comparatively rare example of a fluvial system emerging from drainage of a Late Pleistocene ice-dammed lake. This paper reviews the chronology and sequencing of Late Pleistocene lake drainage and early Holocene valley-floor development using new geomorphological, palaeoenvironmental and radiocarbon data, and considers the results in the context of paraglacial models of landscape response. The balance of currently available evidence suggests drainage of the proglacial lake occurred some time between the end of the Dimlington Stadial and the relatively mild climate of the Windermere Interstadial. Fluvial downcutting through glaciodeltaic and glaciolacustrine sediments was associated with recoupling of the fluvial sediment system to lower reaches of the Till and paraglacial development of inset fluvial terraces and valley widening prior to establishment of early Holocene channel systems at least 13–15 m below the equivalent glaciodeltaic surface. This short-lived phase of high paraglacial sediment yield was followed by relatively abrupt relaxation of coarse-sediment reworking as Holocene channels became largely decoupled from Late Pleistocene sand and gravel terraces in the basin. The combination of a bedrock barrier at the basin outlet and relatively gentle valley gradients in the basin has promoted a tendency towards Holocene floodplain alluviation (in central parts of the basin) with little net change in channel elevation until recent historic times. Paraglacial landscape modifications will have continued to exert an influence on the fluvial system during the Holocene, especially with regard to fine sediment yields from localized erosion of glaciolacustrine deposits, but this has most probably diminished considerably with time as sediment supplies to the basin increasingly reflect the impact of anthropogenic catchment disturbance and reworking of Holocene valley-floor deposits.