New insights into the Quaternary evolution of the Bristol Channel, UK

A synthesis of new publically available borehole and bathymetric data, combined with a wealth of other existing disparate data sources, reveals new insights into the Quaternary history of the Bristol Channel area. Sediment boreholes throughout the Bristol Channel confirm the area was glaciated in the Pleistocene. Till is present below marine deposits and, in some areas, is visible morphologically as submerged moraines. In the central and eastern Bristol Channel the submerged valley course of the palaeo-Severn is very clear in new high-resolution bathymetric surveys. This former river course and associated tributaries cross-cut through glacial sediments in the Bristol Channel. At least three phases of glaciation are recorded in the Bristol Channel, one related to the southern limits of a Late Devensian Substage (∼Marine Isotope Stage (MIS) 2) Welsh Ice Cap which reached into Swansea Bay, an earlier Devensian (MIS 4–3) glaciation associated with Irish Sea ice, and another older glaciation that is associated with ice that filled the entire outer and central Bristol Channel. The age of the older Bristol Channel glaciation is still open, although it pre-dates the Devensian (Late Pleistocene) and must date to the Middle Pleistocene. It is therefore evident that Pleistocene glacial and fluvial activity, combined with subsequent post-glacial sea transgression, directly account for current morphometries of the Bristol Channel and Severn Estuary, and the current geography of the SW British Isles.