Late Pleistocene landscape in the Outer Bristol Channel: a preliminary report

Lower sea levels exposed the floor of the Bristol Channel during the Pleistocene glaciations and during these phases the River Severn extended its course over 110 km westwards. The palaeochannel of the Late Pleistocene River Severn is clearly preserved in the bathymetry of the central and inner Bristol Channel/Severn Estuary. However in the Outer Bristol Channel its former course is buried beneath Holocene marine sands. The former British-Irish ice sheet extended up the Outer Bristol Channel multiple times during the Pleistocene as indicated by till and glacially sculpted topography on the sea-floor, as well as on Lundy. The presence of ice would have inhibited westward drainage, forming an ice-dammed lake to form in the Bristol Channel valley. The drainage route of the palaeo-Severn catchment during glaciations remains unresolved. One hypothesis is that the palaeo-Severn system drained via an ice-free southerly route between Lundy and mainland Devon. Subsurface evidence revealed in geophysical surveys suggests that a series of palaeochannels and associated deposits occur in Barnstaple Bay. These are laterally-extensive, some including basal gravels underlying marine sands and spread over several kilometres, suggesting that an extensive braidplain could occur associated with a palaeo-fluvial system. However, these subsurface channels and spreads of gravels may be also associated with a palaeo-Torridge and Taw fluvial system draining from the southwest peninsula. Further research is required to establish the palaeodrainage of the Outer Bristol Channel.