Holocene alluvial sedimentation and valley floor development: the River Swale, Catterick, North Yorkshire, UK

SUMMARY Holocene river channel and floodplain response to environmental change is examined using field and laboratory data collected from the valley of the River Swale at Catterick. Multiple methods including the study of archaeological evidence, EDM survey, geochemical analysis, geomorphological mapping, ground penetrating radar (GPR), logging of cut-bank exposures and trenches, and 14 C dating have been employed to date alluvial units. Archaeological excavations of the 12th century St Giles Hospital on the bank of the Swale at Catterick reveal sedimentary sequences dating from the late Neolithic – early Bronze Age overlain by fine-grained alluvium, colluvium and an Iron Age palaeosol. A series of terraces and multiple palaeochannels inset below the St Giles hospital, show the channel has been laterally and vertically active in the post Roman period. GPR images across these lower gravel terraces reveal a series of broad channels (up to 100 m wide) into which a series of smaller channels (c.50–70 m wide) have been cut. Charcoal from fine sandy deposits overlying one of these channels has been 14 C dated to the 15th century. In addition, geochemical analysis of fine-grained sediment demonstrates variations in heavy metal concentrations, in particular Pb, which can be related to metalliferous mining upstream in Swaledale. Much of the lower Yorkshire Ouse basin has aggraded primarily as a consequence of Holocene sea-level rise and an increased fine sediment supply. In contrast, the River Swale at Catterick remains confined between Late Pleistocene bluffs, has coarser bed-load, and a steeper channel gradient than downstream reaches. Consequently, the river has continued to rework and incise its valley floor, depositing alluvial sequences which are predominantly of late Holocene age.